physics.space-ph

0

astro-ph.SR 2026-07-03

Clavius report caps 1567 solar radius at modern value

by Hisashi Hayakawa, Mitsuru Sôma +5 more

Analyses on Christoph Clavius' Reports of Total Solar Eclipses in 1560 and 1567: Key References for the Centennial Variations of the Earth's Rotation Speed and the Solar Radius

Revised Delta T bounds from 1560 and 1567 eclipses exclude linear shrinkage but allow oscillations

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Variations in solar radius (hereafter R_Sun) is a key reference for solar magnetic activity in time. The sunlight amount may have varied with R_Sun and had an effect on the Earth's climate in the past. Eclipse observations offer a unique opportunity to measure the absolute R_Sun value before modern direct observations. The scientific community has discussed a possible long-term R_Sun variability from 1715 onward. Prior to their coverage, Clavius' eclipse reports had been subjected to qualitative debates regarding the local eclipse visibility and a possible secular R_Sun trend. This study leverages the recent dramatic developments of lunar topography data and ephemeris data to provide an effective resolution of this debate. Clavius' eclipse reports described an explicit totality in 1560 at Coimbra and a "slender circle" around the eclipsing Moon in 1567 at Rome. Our study revised the {\Delta}T constraints of -492 s =< {\Delta}T =< 200 s in 1560 and 140 s =< {\Delta}T =< 151 s in 1567 to satisfy Clavius' descriptions, considering the lunar limb profile and assuming Auwers' canonical R_Sun. This study constrains the R_Sun margin of 1567, utilising three scenarios to interpret Clavius' account. The local totality requires an upper R_Sun limit of 1567 as R_Sun =< 696200 km in absolute size (959.92" in angular size), indicating no linear secular R_Sun shrinkage but possible R_Sun oscillations on a centennial timescale. Conversely, the annularity scenario is considered unlikely because it requires an R_Sun decrease of 7.5" within 3 centuries, even beyond the capacity of extreme shrinking-Sun hypotheses.

0

eess.SY 2026-07-03

Closed-form bounds certify safe approach to tumbling targets

by Omer Burak Iskender, Keck Voon Ling +2 more

Reachability-Based Safe-Start Regions for Approach to a Tumbling Target with Rotating LOS Constraints

Two conservative criteria run 250 times faster than Hamilton-Jacobi reachability while retaining 0.91 recall on 500 feasibility cases.

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This paper presents a reachability-aware guidance architecture for autonomous approach to a tumbling, uncooperative target under a rotating line-of-sight (LOS) docking corridor. The LOS admissible set rotates with the target body frame, producing time-varying polyhedral constraints in the chaser's relative coordinates. A safe-start region is constructed via two conservative criteria: (i) directional per-constraint erosion, the margin consumed by rotation-induced drift before thrust can arrest it, and (ii) a synchronization range bound $r < 2a_{\max}/\omega_t^2$ ensuring the chaser can cancel the apparent rotational velocity without overshooting the hold point. Closed-loop guidance uses a receding-horizon MPC controller with Clohessy-Wiltshire-Hill (CWH) prediction dynamics and explicit LOS corridor constraints in the quadratic program. Truth propagation uses the exact discrete CWH state-transition matrix with sub-stepping, so feasibility claims are physically honest: no reference blending or state projection is applied. A three-regime tracking law manages the transition from long-range inertial approach to body-frame co-rotation and synchronized hold. The analytical safe-start region is benchmarked against four standard reachability engines (backward and forward polytopic reachable sets, Hamilton-Jacobi level sets, and closed-loop Monte Carlo): the closed-form criteria are 250x faster than Hamilton-Jacobi reachability while predicting closed-loop feasibility with precision 0.80 and recall 0.91 on a 500-case sweep. The residual 6% false-positive rate and the IoU gap against Hamilton-Jacobi quantify a structural property: the synchronization set (reach and co-rotate) is a strict subset of the positional reachable set, the gap widening with tumble rate. The analytical bound is thus a sound inner certificate for onboard go/no-go decisions where Hamilton-Jacobi is prohibitively expensive.

0

physics.space-ph 2026-07-03

Four Solar Orbiter shocks show energetic particles exceeding thermal plus magnetic pressur

by D. Trotta, D. Lario +11 more

Energetic particle-mediated interplanetary shocks observed by Solar Orbiter

The events are strong and fast, with particle-dominated regions extending up to 100000 ion inertial lengths upstream.

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Context: In collisionless shocks, energetic particles can carry sufficient pressure to modify the upstream plasma and the shock structure itself, a regime often invoked in theories of cosmic-ray acceleration but rarely observed in the heliosphere. Aims: We find and characterize {interplanetary} IP shocks where energetic particles dynamically dominate the upstream pressure. Methods: We analyze IP shocks observed by Solar Orbiter inside 1 au and compute the energetic particle pressure $P_{EP}$ from proton measurements above 10\,keV, comparing it with the upstream thermal $P_{Th}$ and magnetic $P_{B}$ pressures. Results: We identify four shocks for which $P_{EP} \geq P_{Th} + P_B $. These events correspond to strong and fast shocks in the high-Mach-number tail of the Solar Orbiter shock population. In several cases the $P_{EP}$ increase coincides with a decreasing upstream bulk flow speed in the shock frame, and the resulting particle-mediated foreshocks extend up to $\sim10^5$ {ion inertial lengths} $d_i$. The extent of such energetic particle dominated region depends on shock geometry. Conclusions: These observations provide evidence that accelerated particles can dynamically modify interplanetary shocks. They highlight the importance of the coupling between energetic particles, upstream fluctuations, and shock structure for understanding particle acceleration at collisionless shocks.

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astro-ph.SR 2026-07-02

Optimal PFSS source surface radius grows in solar cycle ascent

by Shiouhe Wang, Fang Shen +3 more

Optimization Algorithm for Determining the Source Surface Radius Based on Parker Solar Probe in situ Measurements from Encounters 1 to 19

Tuning to Parker Solar Probe data improves open flux agreement while holding polarity accuracy steady

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The Potential Field Source Surface (PFSS) extrapolation is a method for estimating the large scale coronal magnetic field from photospheric magnetograms. The source surface serves as the outer boundary of its solution domain, and is typically a spherical surface. An appropriate source surface radius ($R_{ss}$) enables more accurate identification of the coronal magnetic field topology and estimation of the open flux, thereby potentially enhancing the accuracy of space weather modeling. We prove the well-posedness of the PFSS forward problem and establish the existence and uniqueness of the optimal source surface by combining compactness of the admissible set with continuity of the objective functional. The objective functional is the mean squared error (MSE) between PFSS extrapolation and Parker Solar Probe (PSP) radial magnetic field measurements after Parker spiral backmapping and radial scaling for Encounters 1-19. The optimization algorithm is validated with an analytical solution, and Advanced Composition Explorer (ACE) in situ measurements are used as an independent cross-validation dataset. Additional evaluation metrics and Pareto analysis are used to identify the dominant metrics between open flux and polarity prediction accuracy. Our results show that the optimal $R_{ss}$ derived from the algorithm generally increase from solar minimum into the ascending phase of solar cycle 25. The optimized solution improves open flux agreement while preserving or improving polarity prediction accuracy relative to $2.5R_{s}$. The Pareto frontiers show a transition for dominant metrics from open flux during solar minimum to polarity prediction accuracy during the ascending phase.

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physics.space-ph 2026-07-02

VLBI sessions refine deep space orbit tracking over two years

by Oliver James White, Guifre Molera Calves +3 more

VLBI Tracking of the JUICE Mission: Two Years of Cruise Phase Operations and Performance Analysis

More than 100 observations add geometric diversity for better position accuracy and spacecraft diagnostics during cruise.

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The JUpiter ICy moons Explorer (JUICE) mission, launched by the European Space Agency (ESA) in April 2023, represents one of the most ambitious deep space exploration endeavours targeting Jupiter's icy moons. This paper presents results from the Very Long Baseline Interferometry (VLBI) radio telescope tracking conducted by the University of Tasmania during the first two years of JUICE's cruise phase operations. We have conducted over 100 tracking sessions capturing the spacecraft across different orbital regimes as JUICE progresses through its complex cruise trajectory towards Jupiter. Our analysis focuses on three key areas: Doppler residual characterisation, mission performance indicator extraction, and solar wind scintillation pattern analysis (including space weather forecasting). UTAS measurements demonstrate the enhanced capabilities that VLBI networks provide for deep space mission support, particularly for precision orbit determination and spacecraft health diagnosis. The results showcase the UTAS VLBI array as a valuable complement to traditional tracking infrastructure, offering Southern Hemisphere coverage and enhanced geometric diversity for deep space missions.

0

physics.space-ph 2026-07-01

Electron cooling sets ion regimes near comet nucleus

by Victor Steinwand, Peter Stephenson +4 more

Cometary ion dynamics at a weakly outgassing comet

The cooling exobase aligns with Rosetta density data and explains when ions co-move with neutrals.

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The ESA/Rosetta mission escorted comet 67P/Churyumov-Gerasimenko for two years, exploring its plasma environment across diverse outgassing conditions. Plasma density observations from the Rosetta Plasma Consortium (RPC) are broadly categorized into two regimes for the ion dynamics, linked to the presence of a diamagnetic cavity at Rosetta's location. With a diamagnetic cavity present, ions detected by Rosetta are accelerated with respect to the neutral coma. Without a diamagnetic cavity present, at lower outgassing, and nearer the nucleus, ions co-move with the neutrals. We examine the transition between regimes following Rosetta's last detection of the cavity in February 2016. During this transition, global 3D plasma models of the cometary ionosphere underestimate plasma densities. To investigate this underestimation, we assess the sensitivity of cometary ion densities to different parameters using a 3D collisional ion test particle model, driven by electromagnetic fields from hybrid modeling. We show that considering cometary electron cooling is necessary to model cometary ion dynamics within 100 km of the surface. Electron temperatures derived from collisional electron modeling affect ion dynamics via the ambipolar electric field, increasing ion number densities. We further show that the cometary electron cooling exobase organizes Rosetta plasma density observations; different ion dynamics regimes are linked to the position of Rosetta relative to the exobase. These findings demonstrate that Rosetta was below this exobase for much of the post-perihelion period. They justify the absence of ion acceleration in plasma density assessments and the use of uniform electron-impact ionization frequencies between Rosetta and the surface during post-perihelion.

0

astro-ph.SR 2026-07-01

SKA to enable radio measurements of CME magnetic fields

by Devojyoti Kansabanik, Surajit Mondal +14 more

Role of SKA in Advancing Remote Measurements of Magnetic Fields of Solar Coronal Mass Ejections

Wider bandwidth and higher sensitivity remove current limits on remote diagnostics of solar eruption fields.

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Coronal Mass Ejections (CMEs) are large expulsions of magnetized plasma from the Sun into interplanetary space and are the primary drivers of extreme space weather variations. The strength and topology of CME magnetic fields largely determine their impact on Earth. Although visible-light coronagraphs routinely observe CMEs and provide their geometric and kinematic properties, they cannot directly measure CME vector magnetic fields. These fields evolve from initiation through the inner heliosphere due to interactions with other CMEs, coronal structures, and the ambient solar wind, leading to significant structural deformation. Such evolution complicates predictions of the CME magnetic field at Earth. Accurate measurements of CME magnetic fields in the corona and heliosphere are therefore essential for advancing space weather forecasting. Radio observations spanning MHz to GHz frequencies provide a powerful remote-sensing approach for measuring CME magnetic fields from the ground. Recent observations with Square Kilometre Array (SKA) precursors and pathfinder instruments, as well as other new-generation facilities, have demonstrated the potential of these radio techniques for CME magnetic-field diagnostics. At the same time, these studies have highlighted several limitations of current instruments. The higher sensitivity, wider instantaneous bandwidth, and broader frequency coverage of the SKA will open a new observational window, enabling these techniques to be fully exploited for constraining SpWx models and improving predictive accuracy. However, such observations are non-standard and require special consideration in scheduling, calibration, and imaging. Developments achieved with SKA precursors and pathfinders are paving the way for robust CME magnetic-field measurements with the SKA.

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physics.space-ph 2026-07-01

Proxy-driven model forecasts galactic cosmic-ray fluxes over decades

by David Pelosi, Fernando Barão +4 more

A new model for long-term forecasting of Galactic cosmic rays

A one-dimensional transport equation with solar-proxy parameters matches existing data and projects intensities for future mission planning.

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The modulation of galactic cosmic rays, driven by the evolution of the heliospheric magnetic field, strongly influences the intensity of cosmic rays reaching near-Earth space. Characterizing this process is crucial both for advancing our understanding of cosmic-ray transport and for assessing radiation exposure and related hazards in space environments. Here we present a newly developed forecasting framework built on a numerical description of charged particle transport in the heliosphere and its dependence on solar activity, designed for the long-term forecasting of galactic cosmic-ray fluxes. It solves a one-dimensional, spherically symmetric form of the Parker transport equation, including diffusion, solar-wind advection, and adiabatic energy losses. The model has been validated using multi-species flux measurements from space-based experiments: PAMELA, AMS-02, and ACE. Its strategy is based on Hilbert-Huang transform filtering and cross-correlation between delayed solar proxies and effective model parameters. Our charge-sign- and rigidity-dependent parametric description of the diffusion-advection processes yields good overall agreement with the data, as shown by the reconstruction uncertainty. The robustness of this approach is validated across a broad set of multichannel datasets covering different particle species, energy ranges, and phases of solar activity, supporting its applicability to space radiation monitoring and forecasting. Furthermore, when coupled with solar-proxy forecasting models, it enables decadal-scale predictions of galactic cosmic-ray fluxes, thereby supporting long-term planning and radiation-risk assessment for future space missions.

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cs.LG 2026-06-29

30-second TEC model hits 91% accuracy during solar activity

by Stephen Tete, Carl Shneider +4 more

t-STEP: An interpretable model for Total Electron Content predictions and irregularities estimations

t-STEP derives irregularity indicators from high-cadence predictions and beats LSTM baselines on storm data from one station.

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Earth system infrastructures relying on satellite-based technologies, such as Global Positioning System (GPS) communications, are affected by ionospheric Total Electron Content (TEC) gradients. Modeling these gradients under physical constraints remains challenging due to their dynamic and transient nature. While existing machine learning (ML) models can predict hourly TEC variations, it remains unclear whether their temporal resolution is sufficient to preserve small-scale TEC irregularities within predicted signals. To address this gap, we introduce an interpretable ML-based model, t-STEP, designed to predict TEC at a 30-second resolution and estimate irregularity signatures from the modeled signals. This high cadence enables the derivation of Rate of TEC changes (ROT) and the ROT Index (ROTI) as diagnostic indicators of ionospheric variability. The model is developed using GPS observations from solar cycle 24 at a station located at 5.49{\deg}S, 47.49{\deg}W. A multi-metric evaluation framework, including dynamic time warping, is used for robustness assessment, while SHAP (SHapley Additive exPlanations) provides insight into feature contributions. The 30-second TEC predictions achieve 91% accuracy with a mean absolute error (MAE) of 4.38 TECU during high solar activity (2015). Compared with the International Reference Ionosphere (IRI-2020), the hourly model improves accuracy by 35%, reduces absolute errors by 57%, and increases prediction skill by 54%. More importantly, the 30-second model captures TEC irregularity dynamics and morphologies during geomagnetic storms of different intensities, outperforming an attention-based Long Short-Term Memory model under the same experimental conditions. This study demonstrates the potential of a single TEC prediction framework for scalable irregularity monitoring without requiring separate models for individual transient events.

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physics.space-ph 2026-06-29

Switchbacks boost transverse magnetic power beyond geometry

by Kyung-Eun Choi, Oleksiy V. Agapitov +10 more

Wave Activity at MHD-ion Scales Associated with Switchbacks

Matched-angle analysis shows intrinsic amplification linked to higher proton temperatures in the near-Sun wind.

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Magnetic switchbacks (SB) -- the localized magnetic structures with magnetic field direction inclined at an angle $\theta$ relative to the background $B_0$ -- in the young solar wind have been associated with enhanced ion-scale wave activity and local plasma heating. It remains debated whether the apparent wave-power increase is intrinsic or mainly caused by sampling geometry. In this work, we analyze magnetic and electric field fluctuations measured by Parker Solar Probe, focusing on the 0.1--3~$f_{cp}$ frequency band that spans the transition from the MHD inertial range to ion-kinetic scales. By decomposing magnetic fluctuations into field-aligned and transverse components and comparing SB and non-SB intervals at the same local magnetic field angle, we test whether SBs sample an anisotropic cascade from different viewing angles or host intrinsically amplified wave activity. We find that the transverse magnetic power $\delta B_{\perp}$ is systematically enhanced inside switchbacks across a wide range of magnetic field rotation angles $\theta$. The enhancement persists even at small and intermediate deflections, where geometric projection alone predicts weak power, indicating an intrinsic origin beyond sampling geometry. The inertial-range spectral indices also remain similar between SB and non-SB intervals despite the enhanced wave power inside SBs, suggesting that the underlying turbulence cascade is largely preserved. This excess $\delta B_{\perp}$ coincides with elevated proton temperatures and enhanced electric-field fluctuations, supporting the interpretation that SBs act as localized sites of cross-scale energy transfer and ion-scale dissipation in the near-Sun solar wind.

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physics.plasm-ph 2026-06-29

Recurrent model recovers solar-wind fields from sparse probes

by Maryam Reza, Farbod Faraji

Inferring solar-wind plasma structures from sparse probe trajectories using recurrent reduced-order learning

Time histories from a few virtual probes yield reconstructed 2D velocity and density maps in meridional and equatorial planes.

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In space plasma studies, spacecraft measurements often provide time histories of the solar-wind plasma. However, many heliospheric plasma processes are organized over spatial scales that cannot be directly resolved by limited local sampling. This creates a persistent challenge: how to use limited probe measurements to recover the spatial plasma distributions needed to interpret evolving solar-wind structures. In this work, we present a recurrent reduced-order learning framework to address this challenge. The method is demonstrated using WSA-ENLIL solar-wind simulation data to reconstruct two-dimensional meridional and equatorial fields from a small number of virtual probes, with radial velocity and plasma density considered as target quantities on both planes. From sparse temporal probe signals as inputs, the model recovers the dominant radial and latitudinal variations in the meridional plane and the spiral-shaped organization of the equatorial solar wind. It is also able to reconstruct spatial distributions of dynamically coupled plasma fields not directly sensed. Sensitivity studies are performed to assess the dependence of reconstruction accuracy on key parameters of the machine-learning framework: modal rank, number of probes, and input-history length. The outcomes underline the methodology's promise as a practical route for extracting spatial plasma-state information from spacecraft measurements in support of studies on the underlying physics of space and solar-wind plasmas.

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physics.space-ph 2026-06-26

Radar echoes recover auroral electric-field spectrum with -5/3 index

by Magnus F Ivarsen, Kaili Song +2 more

Excursion-set structure factor of the auroral electric field

Structure factor of threshold exceedances matches in-situ observations in co-moving frames

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We treat coherent radar echoes from aurorae as a finite point process and measure its structure factor $S(k)$ from pairwise echo separations. Backscatter requires electron drifts to exceed the ion-acoustic speed, making the echoes a threshold (excursion-set) sample of the ionospheric electric field, and $|S-1|$ is that field's spectrum, to leading order. We test this against in-situ observations: in co-moving frames, the radar spectrum is scale-free with a spectral index near -5/3, matching the in-situ indices. The auroral electric field is thus imaged by its excursion set, a point process of Farley-Buneman threshold exceedances.

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physics.space-ph 2026-06-26

Local normalization flags magnetic cloud boundaries

by Ziwei Huang, Zhenjun Zhou +3 more

Magnetic Cloud Boundary Identification Using a Local-Normalized Magnetic Field Parameter

The parameter tracks field deviations from a five-minute median and is supported by spectral differences inside versus outside seventy-six c

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Due to the lack of quantitative and reproducible criteria for identifying magnetic cloud (MC) boundaries, we propose a parameter that characterizes short-timescale variability in magnetic field strength. The parameter, referred to as the Local-Normalized Magnetic field parameter (LNM), is defined as $\mathrm{LNM}(t)=\log_{10}\left(B(t)/\langle B \rangle_{\mathrm{5m\text{-}med}}(t)\right)$, where $B(t)$ is the total magnetic field strength and $\langle B \rangle_{\mathrm{5m\text{-}med}}(t)$ is its 5-minute running median ending at time $t$. This parameter measures the deviation of the magnetic field magnitude from its local background and reveals a clear contrast between the coherent magnetic structure inside MCs and the more variable ambient solar wind. Based on this parameter, we develop a semi-automated method for MC boundary identification, supported by Time Series Scalogram visualization. We further analyze 76 MC events using power spectral density (PSD) and slab fraction diagnostics. The results show that the dissipation-range spectral index inside MCs ($\sim f^{-2.21}$) is systematically smaller than that outside ($\sim f^{-2.59}$ and $f^{-2.89}$), and the slab fraction is reduced, indicating suppressed small-scale variability and enhanced anisotropy. These results support the applicability of the proposed parameter for MC boundary identification.

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physics.space-ph 2026-06-24

Solar Cycle 25 cost LEO satellites $2.77 billion in lost years

by Scott Shambaugh

The Billion Dollar Surprise: How Solar Cycle 25 Cut Satellite Lifetimes in LEO

Densities ran 2-3 times above forecast, exhausting even conservative drag budgets for thousands of mission years

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Solar Cycle 25 has run far stronger than the 2019 consensus forecast issued by the NOAA/NASA/ISES prediction panel, with densities in low Earth orbit from 2022-2026 holding at 2-3x the predicted levels. The cumulative drag impulse experienced by LEO satellites reached 5-6 standard deviations beyond the forecast's stated uncertainty. This means that even operators who designed conservatively against the two-sigma worst case fell short of their drag budgets. This paper quantifies a lower bound on the economic cost of that misprediction. Starting from the 13,704 payloads on-orbit below 800 km during 2022-2026, we screen to the 1,597 payloads which we validated with high confidence to be both operational and in ballistic freefall. We estimate each satellite's ballistic coefficient and propagate its trajectory under the forecasted atmosphere versus the observed one. A probabilistic cost model assigns each satellite an annualized mission cost based on direct costs (amortized capital costs plus annual operations), stratified by size class, with bespoke estimates for high value missions. Survival and forward cost discounting is applied at a modal 11% per year. We combine the differences in lifetime with the cost model to estimate the total dollar impact. Against the forecast's two-sigma upper bound which we consider to be a standard engineering design target, these satellites lost 688 cumulative mission years valued at \$0.88 billion. Against the nominal forecast, they lost 2,472 mission years worth \$2.77 billion. These estimates are deliberate lower bounds which exclude propulsive satellites, revenue above direct cost, and downstream economic impact. The results give a quantitative case for the value of accurate decadal-scale space weather forecasting, and show that well-calibrated uncertainties are as valuable to a satellite operator end-user as the accuracy of the central prediction itself.

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astro-ph.SR 2026-06-23

Sound speed gradient damps acoustic waves in solar tachocline

by D. Tsiklauri

Phase-mixing of acoustic waves with applications to solar tachocline

Phase-mixing supplies the bulk dissipation missing from classical models of low-ℓ p-modes above 3000 μHz.

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We adapt the \textit{magnetohydrodynamic} wave phase-mixing paradigm [Tsiklauri et al. (2003)] to investigate \textit{acoustic} wave propagation and damping in media where transverse sound speed gradients exist. Using an analytical model, we recover previous harmonic wave and Gaussian pulse evolution solutions now controlled by the spatial gradient of the local \textit{sound speed}. We discover a scaling law governing a Harris current sheet-like pulse evolution: under developed-stage phase-mixing, the peak envelope of such pulse amplitude scales with propagation distance as a new power-law $\max(P_1) \propto x^{-9/2}$. Applying our model to the solar tachocline directly resolves the 26-year-old helioseismic mystery of low-$\ell$ global $p$-mode linewidth anomalies observed by BiSON above $\nu \approx 3000\,\mu\text{Hz}$. We demonstrate that the sound speed gradient forces rapid, non-turbulent energy damping directly in the shear zone, providing the exact high-efficiency bulk dissipation needed to account for the missing energy sink deep within the solar tacholine. Our model provides the exact damping rates that classical, homogeneous models severely underestimated in the past. Finally, our results provide actionable design strategies for engineering compact stealth coatings or meshes to achieve enhanced acoustic signature suppression from moving bodies immersed in a fluid.

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physics.plasm-ph 2026-06-22

Rising current forms expanding magnetic bubble in plasma

by Yang Zhang, Brandon K. Russell +10 more

Plasma Flow Generation and Particle Acceleration from Expanding Magnetic Bubbles

Bubble front advances at Alfvén speed from inner field and outer density, providing a basic route to lab plasma flows and particle accelerat

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Impulsive plasma dynamics in the laboratory are often driven by rising electric currents, yet their quantitative plasma response has not been well established. By means of fully kinetic particle-in-cell simulations and laser-driven capacitor-coil experiments, we show that a rising current expels plasma, forming an expanding magnetic bubble and accelerating particles. The expansion front velocity scales with the Alfv\'en speed determined by the magnetic field at its inner edge and the plasma density at its outer edge. This mechanism establishes impulsive current drive as a fundamental way that generates plasma flows and accelerates particles in laboratory plasmas, with potential relevance to astrophysics.

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physics.plasm-ph 2026-06-22

Reconnection in KH current sheets energizes electrons

by Silvia Ferro, Fabio Bacchini +3 more

Reconnection-induced electron energization in magnetospheric Kelvin-Helmholtz dynamics

2D kinetic simulations tie reconnection activity during vortex breakup to anisotropic heating and nonthermal tails.

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The Kelvin-Helmholtz instability (KHI) is a major driver of multiscale plasma dynamics at velocity shear layers, where it can promote the formation of current sheets and the onset of magnetic reconnection as well as drive plasma energization. While recent kinetic studies have shown efficient electron heating during nonlinear KH evolution, the connection between reconnection dynamics and localized electron energization is still not fully understood. We investigate this link using two-dimensional fully kinetic simulations of KHI developing in a double-periodic system with two velocity shear layers and a uniform guide field, initialized from a finite-Larmor-radius equilibrium. During the nonlinear stage, initially coherent vortices evolve into layers populated by fragmented current sheets displaying reconnection activity. The global energetics reveal species-dependent energization pathways. Ions act as the primary energy reservoir, transferring energy to the electromagnetic fields, while electrons receive the dominant net positive energy input. Electron energization is strongly anisotropic ($T_{\parallel,e} > T_{\perp,e}$) and localized within intermittent current sheets associated with enhanced field-particle energy exchange and elevated agyrotropy. These regions also show the development of suprathermal tails in the electron energy distributions, providing evidence for nonthermal electron energization. Despite opposite vorticity orientations, the two shear layers exhibit similar statistical behavior. Together, these results establish a direct connection between reconnection-associated current structures and localized electron energization in collisionless KHI dynamics.

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astro-ph.SR 2026-06-18

Kappa tails render Spitzer conduction undefined in quiet Sun

by Victor Edmonds

The Quiet-Sun DEM Under Kappa: Diagnostic Degeneracy and the Failure of the Conductive Closure

The conductivity integral diverges for kappa near 2.5, so the standard heat-flux term has no physical form and DEM inversions become degener

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For a plasma whose electrons carry a $\kappa \approx 2.5$ suprathermal tail, the Spitzer-Harm conductive closure does not exist: the conductive flux is the tail-carried third velocity moment, and the local conductivity integral diverges across the entire $\kappa \in [2,3]$ range -- the finite value the closed-form $\kappa$-conductivity returns at $\kappa = 2.5$ is an analytic continuation of a divergent integral, not a physical conductivity. Edmonds (2026a) places the quiet solar corona (QS) in this regime. Taking that as premise, two failures follow for any plasma in the class: the standard EUV-DEM diagnostic cannot resolve such a plasma, and the conductive term of the standard QS energy budget has no valid form. The diagnostic failure is shown end-to-end. A single-T $\kappa = 2.5$ probe, a multi-T $\kappa = 2.5$ source, and a multi-T Maxwellian source, all run through the regularized DEM inversion of Hannah & Kontar (2012), recover $\log T$ widths inside the FWHM distribution the same pipeline returns from 80 real quiet-Sun AIA patches; the pipeline cannot distinguish them. Two structural features also emerge: a Fe XI charge-state crossover and an EUV continuum reversal. The ionization-gated diagnostic structurally returns the tail-weighted effective temperature $T_{\mathrm{eff}}$, while Spitzer-Harm takes the bulk-core $T_{\mathrm{core}} = (\kappa - 3/2)/\kappa \cdot T_{\mathrm{eff}}$ as input. The mismatch invites a temperature substitution yielding a budget reduction -- mechanically correct and physically empty, because the coefficient it corrects has no convergent form: it is the Fourier-law closure itself that fails, not its temperature input. Two QS pillars for impulsive heating -- DEM-width multi-thermality and the conductive-budget gap -- lose their structural assumptions, and the budget question shifts to non-local kinetic transport outside any fluid closure.

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physics.space-ph 2026-06-17

Titanium emissivity drops during oxidation in entry tests

by Andrea Fagnani, Bernd Helber +2 more

Emissivity of oxidizing titanium in simulated atmospheric entry flows

Time-resolved measurements reveal a temperature-linked change missed by pre- and post-test methods.

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The aerothermal demise of titanium components plays a critical role in the uncontrolled re-entry of space debris from low-Earth orbit. Exposure to high temperatures and dissociated oxygen environments promotes rapid oxidation, significantly influencing the material degradation and surface thermal balance. This study presents time-resolved infrared emissivity measurements of both Grade 2 and Grade 5 titanium samples across five wavelength bands during exposure to entry-relevant conditions simulated in the Plasmatron facility at the von Karman Institute for Fluid Dynamics. The results reveal a dynamic evolution of emissivity throughout the test, including a pronounced drop associated with a characteristic surface temperature jump that is not captured in existing literature data. Post-test electron microscopy highlights a diverse oxide layer morphology at the microscale. Although plasma wind tunnel experiments reproduce only a subset of flight-relevant phenomena to space-debris entry, these findings demonstrate that the complex coupling between oxidation and surface radiative behavior is not adequately captured by conventional pre- and post-test analyses, highlighting their limitations in resolving in-situ emissivity evolution.

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physics.space-ph 2026-06-17

Scattering ignores correlation length in guide-field-free anisotropic turbulence

by Daniela Maci, Rony Keppens +1 more

High-energy Particle Transport in Three-dimensional Anisotropic Turbulent Magnetic Fields

Simulations reveal a mechanism unlike pitch-angle diffusion when no background field organizes the fluctuations.

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The understanding and modeling of high-energy particles transport in turbulent magnetic fields is an important open question in space- and astrophysics. The multiscale, nonlinear nature of turbulence, and the high variability of turbulence properties across different environments, make it particularly challenging to reach a full understanding of the interactions between particles and turbulent fluctuations. Using synthetic, realistically looking turbulent magnetic field realizations generated by the BxC toolkit, we investigate how the scattering of particles is affected by anisotropic fluctuations in strongly turbulent fields. We find evidence that, in the absence of a uniform background or guide magnetic field, the scattering process is not governed by the turbulence correlation length. We then further verify this hypothesis by studying particle transport in the presence of a guide field. We find evidence of a different scattering mechanism than the usual pitch-angle diffusion used to describe scattering in strong-guide-field settings.

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physics.space-ph 2026-06-16

MAVEN spots first magnetic reconnection exhausts near Mars

by Chi Zhang, Chuanfei Dong +11 more

Direct Observations of Magnetic Reconnection in the Solar Wind Current Sheets near Mars

Bifurcated fields and Alfvenic outflows appear in solar wind current sheets and exceed their usual thickness.

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Magnetic reconnection is a fundamental and ubiquitous process in astrophysical plasmas that converts magnetic energy into plasma kinetic and thermal energy. Throughout the heliosphere, the solar wind is permeated with current sheets (CSs), providing a natural laboratory for investigating this process. Using measurements from NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we report the first direct observations of magnetic reconnection occurring within the solar wind CSs near Mars. Specifically, MAVEN observed the classic Petschek-type reconnection exhaust regions, evidenced by bifurcated magnetic field signatures and Alfvenic ion outflows. Notably, the observed exhaust region appears to be large-scale, significantly exceeding the typical thickness of solar wind CSs near Mars. This suggests that magnetic reconnection may significantly broaden the CS. Our results underscore the ubiquity of magnetic reconnection across heliocentric distances and may provide new insights into the large-scale evolution of the solar wind and the development of turbulence within it.

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physics.flu-dyn 2026-06-15

Finite-time sets separate rotation from transport in solar supergranules

by Francisco J. Beron-Vera

Quasi-material finite-time rotationally coherent sets in photospheric supergranulation

Combining two diagnostics shows coherent regions can form by contraction rather than persistent vortices in the photosphere.

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Supergranular flows organize transport in the solar photosphere over spatial and temporal scales much larger than granulation. While coherent vortical motions have been identified using objective Lagrangian diagnostics such as the Lagrangian-averaged vorticity deviation (LAVD), rotational coherence captures only one aspect of coherent flow organization. Here we introduce finite-time rotationally coherent sets (FTRCS) by combining the inflated dynamic Laplacian (IDL), which identifies finite-time quasi-material coherent regions, with LAVD-based rotational diagnostics. The IDL extracts coherent structures with finite lifetimes, while LAVD identifies those exhibiting enhanced intrinsic rotation. Application to photospheric velocity fields shows that instantaneous vortical features do not necessarily correspond to finite-time rotationally coherent structures. The analysis also illustrates the effect of compressibility: coherent sets may form through persistent contraction associated with convergent transport, rather than through the persistence of rotating material regions. The combined IDL--LAVD approach separates finite-time transport coherence from intrinsic rotational organization in time-dependent flows.

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physics.space-ph 2026-06-11

Density fluctuations steer Langmuir waves near bow shock

by Daniel B. Graham, Yuri V. Khotyaintsev +3 more

Plasma frequency waves in Earth's electron foreshock

MMS data shows small-scale perturbations shape wave growth and radio emission alongside nonlinear decay.

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At Earth's quasi-perpendicular bow shock, electrons can be reflected and accelerated to high velocities, forming beams. These beams excite Langmuir and beam-mode waves, which can then be converted to radio waves. We aim to understand the properties and evolution of Langmuir waves excited in the electron foreshock region using the Magnetospheric Multiscale (MMS) mission. We use fields and particle data from the four MMS spacecraft to investigate the properties of Langmuir/Z-mode waves in Earth's electron foreshock. MMS provides extended high-resolution snapshots of the three-dimensional electric field, enabling detailed analysis of wave properties. Probability distributions of the electric field are used to investigate the evolution of the waves and the role of density fluctuations. Distinct spectral peaks near the electron plasma frequency are often observed, suggestive of simultaneous observations of beam-mode and Langmuir waves, as well as nonlinear electrostatic decay of Langmuir waves or reflection off density gradients. In addition, the electric fields often have large perpendicular components, consistent with Z-mode waves. The statistical results show that the electric fields are largest near the electron foreshock boundary with the solar wind. Both the parallel and perpendicular components of the electric field exhibit close to log-normal probability distribution functions, consistent with predictions from Stochastic Growth Theory. These results suggest that small-scale density perturbations in the ambient plasma, in addition to nonlinear three-wave decay, are crucial to the evolution of Langmuir waves and the generation of radio waves. These results apply to Langmuir waves in the solar wind, such as in Type II and Type III solar radio burst source regions, where the same density fluctuations are expected and large-amplitude Langmuir waves with similar properties are observed.

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astro-ph.SR 2026-06-11

Transformer detects SIRs at 0.93 AUC and ranks flow deflection

by Prateek Mayank, Yogesh +5 more

Finding Novel Precursors for Solar Wind Stream Interaction Regions with Interpretable Deep Learning

Proton density and magnetic field magnitude lead attributions while transverse velocity contributes 13-17 percent on 102 held-out events.

abstract click to expand

Solar wind stream interaction regions (SIRs) drive recurrent geomagnetic storms, yet most existing catalogs rely on expert inspection and simple thresholds that are subjective and can miss events with complex morphologies. We present SIREN (SIR Encoder Network), a lightweight Transformer based model for per timestep SIR detection from in situ solar wind observations. The model ingests sequences of 11 solar wind parameters, spanning magnetic field, velocity, and thermodynamic properties. With approximately 100,000 trainable parameters in a two layer encoder architecture, SIREN is trained using weighted binary cross entropy loss and a cosine annealing learning rate. Platt scaling is applied to produce well-calibrated detection probabilities. On a held-out test set of 102 events, the calibrated model achieves a ROC-AUC of 0.93, F1 score of 0.78, and true skill statistic of 0.67. Analysis of the self-attention weights confirms that the model concentrates on the SIR, grounding its decisions in the physically relevant portion of each sequence. Integrated Gradients attribution reveals a quantifiable feature hierarchy: proton density (24.3%) and magnetic field magnitude (21.6%) dominate, followed by temperature (13.9%) and bulk speed (12.1%). Notably, the transverse velocity component Vy and east-west flow angle together contribute 13-17%, identifying flow deflection as a consistent but previously under-quantified SIR signature. By producing continuous probabilities rather than binary labels, SIREN enables flexible threshold tuning for operational use and provides a template for compact, interpretable deep-learning systems in space weather.

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physics.plasm-ph 2026-06-11

EMIC waves heat cold ions via secondary instabilities at low amplitudes

by Opal Issan, Patrick Kilian +3 more

Secondary drift-driven instabilities in the presence of a parallel-propagating electromagnetic ion cyclotron wave and cold multi-component ions

Simulations show polarization drifts excite lower-hybrid modes that anisotropically heat protons, oxygen ions and electrons even when EMIC w

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Electromagnetic ion cyclotron (EMIC) waves are commonly observed in Earth's inner magnetosphere, particularly during geomagnetic storms driven by anisotropic ring-current protons. While their role in radiation belt scattering of hot ions is well established, their interaction with the cold (less than 100 eV) plasma remains less understood. This is partly due to limited magnetospheric cold ion observations, as spacecraft charging can prevent cold ions from reaching onboard instruments. It is well-known that the electric field of a parallel-propagating EMIC wave can drive inter-species perpendicular polarization drifts that excite lower-hybrid secondary instabilities. In multi-component plasmas, these include the modified two-stream and the ion-ion cross-field instabilities. In this paper, we study the impact of such secondary instabilities on the parallel-propagating EMIC wave and multi-component plasma via a fully kinetic particle-in-cell simulation and linear theory. We find that the secondary waves persist even at low EMIC amplitudes, provided the cold population remains sufficiently cold. The kinetic simulation demonstrates that these secondary modes produce anisotropic heating of cold protons and singly-charged oxygen ions, primarily in the direction perpendicular to the ambient magnetic field and of electrons in both parallel and perpendicular directions.

0

physics.space-ph 2026-06-11

Alfvén pulse from thinned current sheet drives 330 mV/m ionospheric fields

by Magnus F Ivarsen, Yukinaga Miyashita +6 more

Extreme, transient bursts of energy in the auroral ionosphere. II. A magnetotail dipolarization event

Radar and satellite data link extreme auroral transients to a shear wave launched during magnetotail dipolarization.

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We report ground-based coherent VHF radar observations of extreme turbulent field-structures detected in coincidence with a magnetospheric substorm-associated magnetotail dipolarization. The field-structures are observed by the ICEBEAR radar, in the form of Farley-Buneman (FB) waves in the auroral electrojets, and the field-structures themselves move an order of magnitude faster than the saturation speed of the underlying FB waves, implying transient electric field sources up to 330 mV/m in strength. The field-structures are identified and automatically tracked using an unsupervised clustering & tracking algorithm, applied to clutters of ICEBEAR radar backscatter targets, a method that turns the Doppler radar into a tracking radar capable of measuring the ionospheric ExB-drift by proxy. We place this finding in a coordinated multi-instrument context. Three THEMIS spacecraft observed the dipolarization event in-situ in the near-Earth plasma sheet. In the ionosphere, Swarm A, crossing through the guilty auroral arc at the onset of the dipolarization event, recorded clear signatures of propagating Alfv\'en waves threading the relevant flux tube. We interpret the ICEBEAR transients as the natural ionospheric foot signature of a shear Alfv\'en pulse launched by the bipolar space-charge (Hall) electric field of the thinned current sheet, with amplification along the converging flux tube, partial reflection at the ionospheric boundary, and spatial sharpening by precipitation-produced Pedersen-conductance gradients on the auroral arc edges. A one-dimensional wave-transmission analysis recovers the observations. Our results elucidate a tightly controlled coupling between magnetotail processes and meter-scale auroral plasma turbulence, and demonstrate the capability of ICEBEAR to resolve extreme, transient electric-field enhancements in the ionosphere.

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0

astro-ph.EP 2026-06-10

HDO ice detected for first time in protoplanetary disk

by Alexey Potapov, Piyush Kalambkar +5 more

First detection of HDO ice in a protoplanetary disk

JWST data on 132-1832 shows HDO/H2O upper limit higher than in comets or chondrites, pointing to disk ice processing.

abstract click to expand

Protoplanetary disks are the birthplace of planets and planetary systems. Investigating the molecular inventory of disks is key to linking the chemical evolution of the interstellar medium and the makeup of planets and their atmospheres. In particular, tracing the history of the deuterium enrichment of water along the journey from interstellar clouds through protoplanetary disks to planetary systems provides critical insights into the chemical inheritance. We aim to investigate the chemical composition of ices in protoplanetary disks; specifically, the presence of HDO ice that ought to be present, but has not been detected in disks thus far. We analyzed JWST/NIRSpec observations of the 132-1832 edge-on disk located in the Orion Nebula Cluster using the ENIIGMA fitting tool and unique laboratory data. We report on the first detections of HDO ice in a protoplanetary disk. The estimated upper limit for the HDO/H$_2$O ratio for 132-1832 is much higher, compared to HDO/H$_2$O ratios obtained for chondrites, comets, and embedded young stellar objects. In the disk ices, beyond HDO, we detected H$_2$O, CO$_2$, $^{13}$CO$_2$, CO, OCN$^-$, and OCS, species, whose presence has also been detected in other disks. The HDO ice detection may point to the efficient ice processing in the disk and confirm the findings of laboratory experiments on deuterated ices.

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astro-ph.SR 2026-06-10

Uncombed loops structure solar flare ribbons

by L. P. Chitta, E. R. Priest +34 more

Solar flare ribbons structured by uncombed chromospheric loops

Spectral observations link stable non-flaring chromospheric structures to the spatial pattern of energy deposition in flare ribbons.

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A part of the magnetic energy released during a flare is transported to the lower atmosphere. High-resolution observations show that flare ribbons, sites of energy deposition at the footpoints of flaring loops which appear bright in the chromosphere and transition region, are structured on small spatial scales on the order of 100 km. Based on idealized numerical models of flares it is suggested that the ribbon fine-structures could originate from a tearing instability and the development of plasmoids in current sheets. Here we report on Fe I 5250.6 {\AA} and Mg I b2 5173 {\AA} spectral observations of a solar flare from the Tunable Magnetograph onboard the SUNRISE III balloon-borne mission that reveal an intricate link between the flare ribbon structure and the ambient chromosphere. We identified uncombed chromospheric loops and non-flaring fine-structures that are interspersed among brighter flare ribbon threads. These loops remain stable on timescales of minutes. Spectral lines from these regions show reduced emission or self-reversal in the line core compared with the immediately adjacent flare ribbons. We discuss the potential role of these structures in the onset of a flare. Furthermore, we suggest that irrespective of the complexities in the flaring current sheet, uncombed chromospheric loops and nonflaring fine-structure might play a role in spatially modulating the flare energy deposition in the lower atmosphere.

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physics.space-ph 2026-06-09

Neural surrogate runs Europa MHD in milliseconds instead of hours

by Sachin Alexander Reddy, Abigail R. Azari +7 more

LEAP: A Rapid Neural Surrogate of Multi-Fluid MHD at Europa

Matches parent model at 2.6 nT error, enabling rapid surveys of plasma conditions for ocean detection.

abstract click to expand

Characterizing Europa's subsurface ocean is a key objective of the Europa Clipper and JUICE missions in the search for life beyond Earth. Although the ocean's induced magnetic field provides key constraints on habitability, interpretation is complicated by perturbations arising from Jupiter's plasma interaction with Europa. Physics-based models (e.g. magnetohydrodynamic, MHD) required to characterize these effects are physically comprehensive, but have a prohibitive computational cost. To address this, we introduce Learning Europa's Atmosphere and Plasma (LEAP), a transformer-based surrogate trained on outputs from a state-of-the-art multi-fluid MHD code to predict magnetic field perturbations along spacecraft trajectories. LEAP evaluates in milliseconds on a laptop, whereas MHD takes 12 hrs on a high-performance computer (~40,000x speed-up). The model has test set errors of -/+ 2.6 nT, and for the Galileo E4 and E14 flybys of Europa it matches the parent MHD model in accuracy. Its enhanced speed enables large-scale parameter surveys and probabilistic estimations of plasma conditions, establishing a new framework for accelerated plasma interaction modeling. LEAP can also inform future MHD simulations while learning from them. Beyond Europa, this framework could be expanded to planning future missions or to other high-priority bodies, including Uranus and Neptune.

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physics.plasm-ph 2026-06-09

New estimator eliminates interval dependence in solar wind scales

by Jean C. Perez, Sofiane Bourouaine +1 more

The integral and correlation scales of solar wind turbulence

Standard ACF methods distort long lags, but an ergodicity-based approach gives consistent correlation and integral timescales regardless of

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Many works have attempted to estimate the correlation and integral timescales associated with turbulent fluctuations in the solar wind, which are interpreted as length scales based on Taylor's~Hypothesis. However, accurate estimates of these timescales from spacecraft observations heavily rely on the accurate estimation of autocorrelation functions (ACF), which have been recently shown to depend strongly on the interval length used to estimate them. In this Letter, we show that this dependence on interval length may be artificial because common ACF estimators do not correctly capture the long-lag behavior of the true ACF of the underlying turbulence. We introduce a new ergodicity-based methodology to unambiguously estimate the integral timescale, and a new ACF estimator with better ergodic convergence than current ones. Due to its ergodic properties, the new ACF estimator properly captures the long-lag behavior, and is independent of the interval length. We use this approach to estimate the integral and correlation scales of magnetic fluctuations in the solar wind near $1~{\rm au}$.

0

physics.plasm-ph 2026-06-09

AC drive flips current sheet direction in plasma null

by Sripan Mondal, Abhishekh Kumar Srivastava +1 more

A Numerical Experiment on Oscillatory Magnetic Reconnection in a Laboratory Plasma System Driven by Alternating Currents

Simulations show y-to-x reorientation, Hall flows lagging pressure peaks, and stronger effects at higher amplitudes

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Using the open source MPI-AMRVAC framework, we study oscillatory reconnection in a laboratory plasma, which occurs when a magnetic null is perturbed by incoming fast magnetoacoustic waves driven by an alternating current. The magnetic null region collapses to first form a $y$-directed current sheet that later changes its orientation to the $x$-direction. The $x$-directed current sheet has smaller enhanced thermal pressure and out-of-plane current than the $y$-directed current sheet. The Hall effect produces an out-of-plane plasma flow that evolves with a time lag with respect to the enhanced thermal pressure and out-of-plane current density. Increasing the amplitude of the alternating current produces higher thermal pressure, out-of-plane current density, and out-of-plane plasma flow, while the first peaks of thermal pressure and out-of-plane current density occur earlier.

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astro-ph.SR 2026-06-09

Magnetosonic waves pump mass into spicules at 100 times solar-wind rate

by D. Tsiklauri

Peristaltic Flow in Compressible, Ideal Magnetohydrodynamics: A Mechanism For Solar Spicules

Peristaltic mechanism in compressible MHD tubes yields collimated upward flow under equipartition conditions

abstract click to expand

We present analytical model for peristaltic transport within compressible, ideal magnetohydrodynamics (MHD). By employing small-amplitude perturbation expansion, under thin-tube long-wavelength approximation with a uniform axial background magnetic field, we study non-linear coupling between thermodynamic pressure variations and Maxwell's magnetic tension stresses. The resulting net time-averaged volumetric flow rate $\langle Q \rangle$ is calculated. When applied to solar chromospheric spicules under equipartition constraints ($\beta \sim 1$), where sound speed matches the Alfv{\'e}n speed, we find $\langle Q \rangle = 4\epsilon^2/(M^2-1)$. Because the denominator remains positive across all operational supersonic Mach numbers ($M \approx 2\text{--}10$), upward-propagating mechanical disturbances drive a highly directional, collimated upward flow which we interpret as a spicule. Estimates show that for observationally realistic magnetosonic waves with amplitudes of $\approx 10\%$, the peristaltic mechanism generates a localized mass flux $\approx 100$ times that of solar wind. We propose an explicit observational signature of this mechanism, wherein the launch of individual spicular jets is directly preceded by magnetosonic wave trains detectable as localized intensity modulations. Beyond solar chromospheric application, the model may be applicable to traveling magnetic field pinches in laboratory plasma devices and astrophysical mass-loading processes in stellar winds and inner regions of magnetized accretion disks.

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physics.space-ph 2026-06-09

Parker equation overestimates GCR intensity by 30% at Earth

by J. P. van den Berg, N. E. Engelbrecht +1 more

Are the Parker and Focused Transport Equations Equivalent for Galactic Cosmic Ray Modulation?

Focused transport equation shows lower values because particles stream in over the poles where scattering is weaker.

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The Parker transport equation (TPE) has been the equation of choice for the past 60 years in studies of galactic cosmic ray (GCR) modulation. Conversely, the focused TPE describes the same processes on a more fundamental level than the Parker TPE by modelling an anisotropic distribution rather than an isotropic one. It is usually assumed that the Parker TPE is valid for modelling GCRs, but the two TPEs have not been tested against each other in this context. We conduct a first-of-its-kind comparison of these TPEs without particle drifts to test whether they produce the same results under identical diffusion conditions. A new model for protons during solar minimum conditions is developed to numerically solve the TPEs using stochastic differential equations. The TPEs are designed to be as consistent as possible for diffusion by normalising the pitch-angle-dependent diffusion coefficients (DCs) used in the focused TPE to the isotropic DCs used in the Parker TPE. The Parker TPE overestimates the GCR intensity at Earth's orbit for low energies by ~30%, and by ~40% over the poles. This stems from a small first-order anisotropy caused by particle fluxes over the poles. Particles gain easier access to the inner heliosphere by streaming in over the poles, where pitch-angle scattering is generally weaker, and the magnetic field is typically less wound. The focused TPE also yields nearly identical results for different pitch-angle dependencies of the DCs. The description of particle streaming and weak pitch-angle scattering as effective parallel diffusion in the Parker TPE makes it overly diffusive. This suggests that DCs derived from fitting the Parker TPE to observations are likely underestimated. Furthermore, GCR spectral and anisotropy data alone cannot distinguish between scattering theories with similar mean free paths but different pitch-angle dependencies.

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physics.space-ph 2026-06-08

Replace vague SIR label with specific names for solar-wind structures

by Bruce T. Tsurutani, Rajkumar Hajra +1 more

"Corotating Interaction Regions (CIRs)", "Interaction Regions (IRs)" and "Stream Interaction Regions (SIRs)", which term should be used?

Authors argue broad term blurs distinct phenomena and introduce SCIR for rare strong-field events that drive superstorms.

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We discuss the early history of quasiperiodic ~27-day recurrent geomagnetic activity starting with Maunder (1904, 1905), Chree (1913) and Bartels (1932, 1934), and the Bartels term "M-regions". We show the iconic "interaction region (IR)" schematic of Belcher and Davis (1971) and the further development of Smith and Wolfe (1976) and the term "corotating interaction region (CIR)". We quote the Jian et al. (2006) definition of a "stream interaction region (SIR)". We disagree with Jian et al. (2006) on the use of the term (SIR) to indicate "transient and possibly localized stream interactions" with "poor recurrence" (Gosling et al., 2001). We feel that this description is too vague for use in scientific studies. We suggest, instead identifying the specific known interplanetary phenomena: interplanetary coronal mass ejection (ICME) sheaths, ICMEs (loops, magnetic clouds, filaments), CIRs, high-speed streams (HSSs) and slow streams. All of these various interplanetary phenomena have different solar and interplanetary origins and different plasma and magnetic field properties. The different interplanetary phenomena have been shown to have different geomagnetic effectivenesses. In keeping with this theme of naming specific interplanetary phenomenon, we introduce the term "Super CIR (SCIR)", which describes a CIR associated with magnetic reconnection at the edge of a solar coronal hole with an embedded coronal jet. SCIRs are a new form of a "transient event" and can be identified by exceptionally strong internal magnetic fields and bounded by both forward and reverse shocks. The SCIR on 6-7 April 2000 caused an exceptionally strong SYM-H = -319 nT superstorm, a first detected/reported event of its kind.

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physics.plasm-ph 2026-06-08

Beam instabilities create hammerhead proton distributions

by R. A. López, Shaaban M. Shaaban +5 more

Hybrid simulations of the proton beam instabilities in the young solar wind. The formation of hammerhead-like distributions

Hybrid simulations of young solar wind show relaxation through right-handed waves imprints the observed velocity features.

abstract click to expand

Parker Solar Probe (PSP) observations in the young solar wind reveal new properties of both plasma particle velocity distributions (VDs) and associated electromagnetic (EM) wave fluctuations. The quasilinear (QL) kinetic theory of plasma wave instabilities has recently shown that new hammerhead (HH) proton distributions can be generated by the relaxation of proton beams through the instabilities of right-handed (RH) polarized waves. Such RH waves have indeed been reported in association with HH distributions. In this paper, new results from hybrid simulations of proton-beam-plasma systems with properties typical of those observed to excite EM-RH wave instabilities are presented. From the long-term evolution of these systems, it is found that beam relaxation is driven by instabilities and growing wave fluctuations, leading to HH-type features in the velocity distributions. The production of these features, as well as their prominence, depends on the magnetic power of the waves generated by the instabilities and, therefore, implicitly on the available free energy, quantified by the plasma beta parameter and the relative beam drift. The simulation results capture the self-consistent evolution of the instabilities and their nonlinear development. Linear theory, together with simulations, helps identify the nature of the unstable modes and the plasma conditions under which they arise. The good agreement with quasi-linear (QL) theory further indicates that it can serve as a computationally efficient complementary framework for interpreting the associated wave-particle interactions.

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astro-ph.SR 2026-06-08

Flux rope tension decides MC versus MO signatures

by M. Sangalli, E. K. J. Kilpua +4 more

Radial and angular evolution of magnetic cloud signatures in the turbulent solar wind: virtual spacecraft analysis

Simulations show disordered MO signatures form only when axial field escapes tight magnetic confinement by 1 AU

abstract click to expand

Interplanetary coronal mass ejections (ICMEs) carry magnetic clouds (MCs), large-scale structures with average radial widths about a fifth of an astronomical unit at Earth's orbit. ICMEs display substructures in white light images and reveal rich dynamics across many spatial scales when directly measured by spacecraft. A spacecraft encounter with an ICME can result in smoothly rotating MC intervals or less organised magnetic obstacle (MO) ones. We investigate how the interplay of expansion, turbulence, and internal cloud dynamics affects magnetic cloud properties, which are reflected in the plasma signatures measured by spacecraft. We perform high-resolution 2.5D MHD simulations of a magnetic flux rope cross-section, which is embedded in the turbulent, expanding solar wind with the expanding box model. We probe the local plasma properties, and thus the flux rope signatures and angular coherence, with virtual spacecraft. Our simulations reproduce clear and stable MC signatures when the flux rope core is intercepted by virtual spacecraft. Disordered MO signatures appear at the edges of the flux rope, and are attributed to both expansion and turbulent transport. We vary some key physical parameters of the flux rope and the environment to understand their effect on the observed coherence and signatures. The pace of the expanding flow controls the angular extent of MC signatures, whereas the intensity of interplanetary turbulence controls how asymmetric and distorted the flux rope appears at 1 AU. The geometry of spacecraft encounters determines whether MC or MO signatures are observed. The presence of a magnetic structure which can result in MO signatures is strongly controlled by the flux rope's initial/early magnetic configuration: MO signatures can only be observed when the axial flux rope field is spatially not well confined by the rope's own magnetic tension, and disappear otherwise.

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physics.space-ph 2026-06-08

10 Hz Alfven waves precipitate 125 MeV protons from Van Allen belt

by Snehanshu Maiti, Harishankar Ramachandran

Numerical Study of Alfven Wave-Energetic Particle Interaction in the Inner Van Allen Belt and predictions of Seismic-Related Energetic Proton Bursts for the IITMSAT Mission

Simulations identify distinguishable bursts that could serve as earthquake precursors for satellite detection.

abstract click to expand

The IIT Madras nano-satellite aims to investigate the science of energetic particle precipitation from the inner Van Allen radiation belt into the upper ionosphere as a potential precursor to earthquakes. Precursors in the form of low frequency electromagnetic waves can appear several hours before an earthquake. These waves, captured near the ionosphere magnetosphere transition region, propagate along geomagnetic field lines as Alfven waves and interact resonantly with trapped energetic particles in the radiation belt, causing their precipitation. Such precipitation can be observed by satellites as energetic particle bursts occurring a few hours prior to the earthquake. A numerical study of Alfven wave energetic proton interactions in the inner Van Allen belt is presented here to investigate the energetic proton precipitation and make predictions to support the scientific objective of the IITM satellite mission. A kinetic model of the energetic trapped proton population in the inner belt is developed, yielding a steady-state distribution that reproduces the observed density profile. The Finite Difference Time Domain method is employed to simulate both narrowband seismic event specific emissions and broadband background noise representing magnetohydrodynamic Alfven wave activity in the inner radiation belt. The studies of interactions of narrow-band Alfven wave packets with the energetic protons in the belt reveals that a sharp cyclotron resonance condition arises at a low Alfven frequency 10 Hz, causing substantial precipitation of high energy protons 125 MeV from their stable mirror orbits. This precipitation can be clearly distinguished from background noisy interactions. Based on these results, we predict the optimal satellite orbital altitude for detecting such energetic proton bursts.

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astro-ph.SR 2026-06-05

Flare line widths shrink from disk center to limb

by Andy S. H. To, Alexander J. B. Russell

Nonthermal line broadening at solar flare footpoints is primarily field-aligned

The pattern reveals nonthermal broadening at footpoints is mainly along magnetic fields rather than random or across them.

abstract click to expand

Magnetic reconnection powers solar and stellar flares, but a full understanding of how the released energy is transported and converted within the solar atmosphere remains elusive. One clue lies at solar-flare footpoints, where spectral lines are far broader than the electron temperature alone can explain. Unresolved flows, waves, turbulence and ion heating have all been proposed, but observations have not yet conclusively distinguished between these mechanisms. Here we perform an unprecedented geometric test for flare footpoints, using 4,593 Hinode/EIS spectra from 407 C- to M-class flares. Line widths decrease systematically from disk centre to limb in all coronal emission lines, showing that the dominant broadening component is magnetic field aligned rather than isotropic or transverse. Cooler lines retain substantial broadening into the early decay phase, consistent with persistent unresolved field-aligned flows or line-of-sight velocity gradients. Hotter lines show an impulsive component that decays rapidly after the soft X-ray peak, consistent with preferential ion heating and ion temperature anisotropy. These findings resolve the long-standing question of the nature of line broadening at flare footpoints, place direct limits on flare energetics, and motivate a new direction in flare physics incorporating distinct field-aligned and perpendicular ion temperatures that exceed the electron temperature.

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astro-ph.SR 2026-06-05

Collisional rates for He I solar line polarization computed

by Moncef Derouich, Saleh Qutub

Depolarization and Polarization-Transfer Rates for Solar He I Lines due to Collisions with Neutral Hydrogen

Multi-level and multi-term rates for 10830 Å and D3 lines supply input for statistical equilibrium equations

abstract click to expand

Context. Neutral helium (He I) produces several spectral lines that are widely used for solar diagnostics. The role of collisions between He I atoms and neutral hydrogen (H I) in the modeling of solar He I lines remains insufficiently quantified. Accurate determination of collisional rates affecting atomic polarization is needed for solar spectropolarimetry. Aims. Our aim is to provide a set of multi-level and multi-term collisional depolarization, polarization-transfer, and population-transfer rates, due to isotropic collisions with neutral hydrogen, for He I levels and terms involved in the main solar He I diagnostic lines. Methods. The calculations are performed within the frozen-core approximation, in which the inner 1s electron is treated as a core with Lc = 0, Sc = 1/2, and Jc = 1/2, while the outer electron is treated as the active valence electron. Results. We compute both multi-level rates, describing depolarization and polarization transfer between fine-structure J-levels, and multi-term rates, which additionally account for coherences between different J-levels belonging to the same term. Conclusions. Our results provide the collisional input needed for the statistical equilibrium equations (SEE) of the polarization of the main He I solar lines, including the 10830 {\AA}, D3 5876 {\AA}, and related triplet transitions, and allow a quantitative reassessment of the role of neutral-hydrogen collisions in He I spectropolarimetry.

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astro-ph.SR 2026-06-05

Imbalanced turbulence inverts minor-ion heating scaling

by Michael F. Zhang, Evan L. Yerger +2 more

Minor Ions as a Diagnostic of Solar Wind Heating: Inverted Mass-to-Charge Scaling in Imbalanced Turbulence

Flat spectra from parallel proton-cyclotron waves cause heating to decrease with rising mass-to-charge ratio

abstract click to expand

Alfv\'enic turbulence is vital to powering the solar wind and corona, yet eludes a comprehensive understanding of the kinetic processes by which it dissipates. Minor ions are sensitive tracers of these processes, showing extreme perpendicular temperatures and mass-weighted temperature trends that can either correlate or anticorrelate with mass-to-charge ratio, $A_i/Z_i$. We use a combination of quasilinear theory and 3D hybrid-kinetic simulations to explain these features and their correlations with properties of turbulence in the fast solar wind. When Alfv\'enic turbulence is imbalanced, its cascade to ion-Larmor scales is throttled by the helicity barrier. This barrier ultimately leads to high-frequency proton-cyclotron waves (PCWs), both oblique and parallel, the latter of which produce very flat electric-energy spectra ($\mathcal{E}_{E_{\perp}}\sim k_\parallel^{-\eta}$ with $\eta<2$) over the range of scales that are cyclotron resonant with minor ions. While steeper spectra lead to a positive correlation of heating with $A_i/Z_i$, the shallower spectra cause the dependence to invert, with $Q_i\propto Q_{\mathrm{p}}A_i(A_i/Z_i)^{\eta-2}$. Six simulations of balanced and imbalanced turbulence spanning $\beta_{\rm p0}=\{1,0.3,1/16\}$ corroborate this prediction, showing minor-ion heating rates that follow $(A_i/Z_i)^a$. Minor-ion heating is strongest and most perpendicular in our lowest $\beta_{\rm p0}=1/16$ simulation of imbalanced turbulence, reaching $T_{\perp{\rm O}^{5+}}/T_{\perp{\rm p}}\approx40$ and $T_{\perp{\rm O}^{5+}}/T_{\parallel{\rm O}^{5+}}\approx10$, consistent with low-coronal observations. Future minor-ion measurements should test whether intervals in which minor-ion thermal speeds decrease with increasing mass-to-charge ratio are associated with a history of large cross helicity, enhanced power in parallel PCWs, and a steep transition-range spectrum.

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astro-ph.EP 2026-06-03

Entry angle controls bolide infrasound detection

by Miro Ronac Giannone, Elizabeth A. Silber

The Role of Source Geometry and Atmospheric Propagation in Global Bolide Infrasound Detectability

Steeper trajectories with lower energy deposition reach global arrays more reliably than shallow high-altitude ones across the 623-event sam

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Global infrasound monitoring provides a persistent means of detecting energetic bolide atmospheric entries, complementing optical observations and extending coverage over remote regions. We present a global assessment of the physical factors governing bolide infrasound detectability by correlating 623 bolide events reported by the Center for Near-Earth Object Studies between 2007 and 2025 with waveform data from the International Monitoring System. We identify 311 events with confirmed infrasound detections, corresponding to a detection rate of approximately 50%, substantially higher than inferred from earlier surveys, reflecting both the maturation of the global infrasound network and advances in automated, multi-frequency array processing. Analysis of flight parameters shows that infrasound detectability is selective rather than uniform across the bolide population. Detected events are preferentially associated with steeper entry angles and lower-altitude energy deposition, while shallow, high-altitude trajectories are less consistently observed. Very high-energy events remain detectable regardless of geometry, but for the more common lower-energy regime, observability depends on specific combinations of entry parameters and propagation conditions. This geometric dependence persists across comparable energy ranges and atmospheric conditions, indicating that entry angle exerts a primary control on detectability, with energy and propagation acting as secondary modulating factors. These results provide new physical constraints on bolide-atmosphere interactions and improve interpretation of global infrasound observations for planetary defense and atmospheric-entry studies.

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physics.plasm-ph 2026-06-03

Phase-space tools separate population roles in pressure-strain during reconnection

by M. Hasan Barbhuiya, Paul A.Cassak +8 more

Velocity space origins of pressure-strain interaction in multi-population distributions and its application to magnetic reconnection

Velocity-space diagnostics applied to PIC simulations isolate distinct electron contributions near the diffusion region.

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A forefront research question is how energy evolves in weakly collisional plasmas for which departures from local thermodynamic equilibrium (LTE) are significant. The standard approach is studying the terms in the non-LTE energy evolution equation derived by taking the second moment of the Boltzmann equation, but the resultant fluid metrics do not retain information about which particles at which velocities drive energy evolution. A widely studied channel for internal energy density evolution is the pressure-strain interaction. Here we employ the kinetic pressure-strain [S. A. Conley et al., ${\it Phys. Plasmas,} {\bf 31}$, 122117 (2024)], a phase space diagnostic whose velocity-space integral recovers the pressure-strain interaction to disambiguate the contributions to pressure-strain interaction from disparate particle populations in composite phase-space densities. We develop phase-space analogs of the pressure-strain interaction decompositions to provide the phase-space origins of normal vs. sheared flow. We introduce the "kinetic strain-rate" tensor, the phase-space analog of strain-rate tensor, which we argue is needed to interpret phase-space origins of pressure-strain interaction. To demonstrate the utility of these quantities, we investigate them for composite electron distributions near the electron diffusion region in two-dimensional particle-in-cell simulations of antiparallel symmetric magnetic reconnection. We find that the phase space-based diagnostics isolate the roles of distinct populations. These results contribute to a growing body of work providing new methods for quantifying phase space energy evolution for a broad array of processes, from magnetic reconnection to collisionless shocks and turbulence, opening new pathways for answering longstanding problems of particle energization in weakly collisional plasmas.

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physics.space-ph 2026-06-02

Shock nose connection explains higher SEP fluxes at one spacecraft

by Weihao Liu, Xianyu Liu +5 more

Counterintuitive Magnetic Connectivity and Energetic Particle Flux Differences among Nearby Spacecraft During the 2023 February 24 Solar Energetic Particle Event

In the 2023 event, Solar Orbiter linked to stronger shock region while nearby Earth and STA connected to weaker flanks due to footpoint shif

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For solar energetic particles (SEPs), it is generally expected that observers magnetically closer to the eruption source region exhibit higher particle intensities than those poorly connected to the eruption site. However, the 2023 February 24 SEP event departs from this simple picture: Earth and STA, near 1 au, are nominally better connected to the source region, whereas Solar Orbiter (SolO), at 0.77 au but less favorably connected, observed SEP fluxes more than an order of magnitude higher. This difference cannot be simply explained by nominal magnetic connectivity or radial scaling of SEP fluxes alone. To investigate this behavior, we perform a global magnetohydrodynamic simulation of the associated coronal mass ejection (CME) using the Alfv\'{e}n Wave Solar-atmosphere Model-Realtime (AWSoM-R). The simulation reveals that the CME flux rope originates close to a coronal streamer and as it propagates and expands, the CME-driven shock is effectively distorted, developing into two distinct flanks with different strengths. Although the three spacecraft are separated by only $\lesssim$30$^{\circ}$ in heliolongitude, their magnetic footpoints differ by $\gtrsim$50$^{\circ}$ in longitude because of a nearby stream interaction region. Specifically, Earth and STA connect to a weaker shock region, while SolO connects to the shock nose with a higher compression ratio and more efficient particle acceleration. We further simulate SEPs using the Multiple-Field-Line Advection Model for Particle Acceleration (M-FLAMPA) coupled with AWSoM-R, obtaining results that reproduce the observed flux differences among the three spacecraft, demonstrating that this counterintuitive behavior results from their connections to different regions of the inhomogeneous CME-driven shock.

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astro-ph.SR 2026-06-02

Ephemeral regions occupy low-flux end of BMR spectrum

by Rambahadur Gupta, Anu Sreedevi +2 more

Large ephemeral regions and their tilt angles

Short-lived ones show turbulent tilts; longer-lived ones hint at Coriolis effect, adding to the Sun's flux budget.

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The ephemeral regions (ERs), which are short-lived bipolar magnetic regions that emerge across the solar cycle but do not appear as sunspots, play a crucial role in the Sun's magnetic flux budget. However, their properties, particularly the tilt distribution, are poorly constrained by observations. In this study, we isolate ERs from the Automatic Tracking Algorithm for Bipolar Magnetic Regions (AutoTAB) catalog during Solar Cycles 24 and 25 by applying flux and footpoint-separation thresholds. Although AutoTAB was designed to track high-flux regions, it also records ephemeral regions with fluxes of 10^19 to 10^20 Mx, placing them at the upper end of the ER spectrum. The isolated ERs have an average lifetime of 1.2 days. Footpoint separation begins at supergranular scales (about 20 Mm), grows during the first half of the lifetime, and then saturates. ERs occur most frequently near solar minima, consistent with earlier studies and likely reflecting AutoTAB's greater sensitivity to weaker regions when strong BMRs are scarce. Tilt properties reveal a more complex picture. For lifetimes shorter than two days, ERs show a broad, noisy distribution with no systematic latitude dependence. Including longer-lived ERs produces a weak, though statistically insignificant, increasing trend with latitude, suggesting that short-lived ERs are shaped by turbulent convection, while stronger, longer-lived ERs may retain Coriolis-imparted tilts. Overall, these results support the view that ERs occupy the low-flux end of the BMR spectrum and contribute meaningfully to the solar dynamo.

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astro-ph.IM 2026-06-01

Coherence framework flags biosignatures in LSST data

by Andjelka B Kovacevic, Nigel J. Mason +5 more

Prospects for Astrobiology and Technosignature Searches with the Vera C. Rubin Observatory Legacy Survey of Space and Time

Candidates treated as structured departures from natural manifolds yield measurable thresholds like D≈5.1 and f_crit≈0.13 in simulations.

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The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will map sources in multiband colour--variability space. We present a prototype coherence-based framework for astrobiology and technosignature searches, in which candidates are treated as structured departures from natural astrophysical manifolds rather than isolated photometric outliers. We illustrate the framework with three simulated cases: five Kuiper Belt Object (KBO) surface/activity states, a grid of 649 synthetic exoplanet spectra with vegetation-red-edge-like (VRE) perturbations, and 500 synthetic multiband light curves, each projected into LSST-like observable space and analysed through colour geometry, chromatic variability, and cross-band coherence. Key results include a full-colour Mahalanobis distance $D\approx5.1$ for the weak-coma KBO state (${\sim}5\sigma$ in the five-dimensional colour vector), an indicative VRE coherence threshold at $f_{\rm crit}\approx0.13$, and an idealised stacking forecast reaching $5\sigma$ under optimistic assumptions. We show, using a small Gaia~DR3 stellar sample, that stellar colour and photometric stability may inform the prioritisation of Galactic regions for applying such coherence diagnostics.

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math.DS 2026-06-01

Cycler orbits emerge as hubs in three-body orbital networks

by Abdullah Braik, Shane D. Ross

Orbital Networks in the Three-Body Problem

Reachable-set overlaps reveal which periodic orbit families connect easily and which stay isolated under limited budgets.

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Orbital transfers in multi-body systems are often studied as isolated trajectory design problems, making it difficult to identify the larger transport structure connecting families of periodic orbits, including which families act as hubs, gateways, relays, or persistently difficult-to-access regions. This work introduces a reachable-set-based framework for constructing orbital networks in the circular restricted three-body problem. Finite-$\Delta V$ and finite-time-of-flight reachable-set overlaps are used to infer accessibility relationships between representative periodic orbit families on a common Jacobi energy manifold and to assemble these relationships into a weighted orbital network. Applied to the Earth-Moon system, the resulting network reveals distinct accessibility regimes in which direct reachability, graph connectedness, and feasible multileg closure emerge separately. The analysis identifies multi-orbiter cycler orbits as the dominant hub, gateway, and relay families, with the (3,2)-cycler dominating across much of the sampled budget plane and the short-period (1,1)-cycler dominating in the low-time-of-flight regime, while the stable 2:1 resonant orbit remains persistently difficult to access. Although the maximum-budget network is nearly complete in a binary sense, its weighted accessibility remains strongly non-uniform. Selected proxy-supported connections are refined into concrete trajectories through differential correction, with corrected transfer costs remaining below the proxy estimates in all tested cases. Together, the results demonstrate how reachable-set overlap geometry can expose large-scale transport structure in nonlinear gravitational systems without requiring exhaustive pairwise trajectory optimization.

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physics.space-ph 2026-06-01

Scattering kernels extended across independent roughness scales

by Sabin-Viorel Anton, Bernardo Sousa Alves +3 more

An extended scattering kernel formalism for multi-scale gas-surface dynamics

Local kernels lifted by single- and multi-reflection operators preserve reciprocity and normalization at every larger scale.

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Gas-particle interactions with non-absorbing surfaces are commonly described using the scattering-kernel formalism. In this framework, an operator $\mathbf{K}$ maps incident velocity distributions to reflected velocity distributions. The operator is self-adjoint and has norm $\lVert \mathbf{K} \rVert = 1$ in an $L^2$ space weighted by the three-dimensional Maxwell-Boltzmann distribution, and must satisfy non-negativity, normalisation, and reciprocity. In standard formulations, $\mathbf{K}$ represents the aggregate effect of all gas-surface interaction mechanisms through a single operator, without distinguishing the physical scales at which these mechanisms occur. For gas scattering from a rough surface, however, it is advantageous to separate geometric effects associated with distinct roughness scales from the underlying thermochemical processes occurring at the atomic scale. We therefore introduce a roughness-based extension of the scattering-kernel formalism, in which a local kernel is successively lifted to larger scales via single- and multi-reflection operators associated with statistically defined surface morphologies. We derive sufficient conditions under which the resulting global kernels preserve reciprocity, normalisation, and non-negativity whenever these properties hold for the smallest-scale kernel. We further show that these constructions define operators on the space of scattering kernels, and establish the associated multi-scale composition laws that allow independent roughness contributions to be combined recursively. The resulting framework provides a general basis for modelling gas-surface scattering on rough surfaces with arbitrary scale decompositions.

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physics.space-ph 2026-06-01

Radar tracks 11 km/s plasma burst in ionosphere during storm

by Magnus F Ivarsen, Saif Marei +3 more

Extreme, transient bursts of energy in the auroral ionosphere. I. Predictive radar tracking

New method recovers 11240 m/s motion implying 560 mV/m electric fields as five-second clusters on closed field lines

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The bulk motion of E-region radar aurora provides a sparsely distributed, direct measurement of the ionospheric electric field in intermittent bursts. We present a tracking procedure for \textsc{icebear} VHF measurements of Farley-Buneman waves. Each cluster is represented as an $\alpha$-shape; frame-to-frame association is a Hungarian linear-assignment problem with a cost combining centroid distance and shape Intersection-over-Union; kinematic prediction amounts to a degenerate Kalman filter. Births, deaths, splits, and mergers are monitored; each tracked trajectory is reduced to per-segment velocities by piecewise-linear regression. We validate against \textit{in-situ} observations. During the G5 storm of 10 May 2024, on closed dayside field-lines, our method recovers a five-second cluster moving at $11{,}240\pm660$~m/s, implying an electric field strength of $\approx 560$~mV/m, a value that exceeds documented sub-auroral thermal emission speeds and the most extreme reported sub-auroral drifts. The detection is consistent with extreme E-field structures appearing as short-lived bursts, representing field variability, and we provide parameterizations of this variability for space weather modeling.

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physics.space-ph 2026-05-29

Cusp precipitation tracks IMF reconnection geometry during storms

by Shirsh Lata Soni, David M. Miles +13 more

Storm-Time Cusp Precipitation: Insights from TRACERS Multi-Crossing Observations

TRACERS data show storm-time changes in cusp width and latitude follow solar-wind conditions, not intrinsic storm-phase effects.

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The dayside cusp provides a direct pathway for solar wind plasma entry into the magnetosphere ionosphere system through magnetic reconnection. Using low altitude ion and electron measurements from TRACERS, together with upstream solar wind and geomagnetic conditions, we investigate the evolution of the cusp during a geomagnetic storm on 30 September 2025, spanning its rising, main, and recovery phases, and compare these with a quiet-time reference. Storm-time observations show broader and more poleward precipitation regions and enhanced electron energy flux, indicating intensified dayside coupling. To interpret these variations, we combine solar wind and IMF measurements with the maximum magnetic shear reconnection model to determine X-line locations and use a Tsyganenko field model to compute event-specific field-line transit distances between the X-line and TRACERS. The results demonstrate that cusp morphology and latitude track IMF-driven reconnection geometry, and that realistic path lengths are essential for quantitative reconnection-rate estimates, highlighting the capability of TRACERS to resolve storm-time cusp evolution. Enhanced cusp precipitation during the recovery phase is consistent with IMF conditions, indicating sustained solar wind driving rather than intrinsic storm-phase effects.

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physics.space-ph 2026-05-29

Perpendicular electrons dominate dissipation in turbulent reconnection

by Rachel Wang, Hantao Ji +8 more

Statistical study of energy dissipation in magnetic structures during turbulent reconnection in the Earth's magnetotail

MMS statistical study finds bidirectional energy exchange in magnetotail structures with only small net transfer to particles.

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Magnetic reconnection is a ubiquitous plasma phenomenon that plays a critical role in particle heating and energization. During reconnection, the topology of magnetic field rearranges, depositing energy into the surrounding plasma through bulk flow, thermal heating, or non-thermal particle acceleration. While the pathways of this transformation from magnetic energy into kinetic have been studied extensively in recent years through theoretical or case-by-case observations, comprehensive statistical studies remain limited. In this paper, we present a statistical investigation using data from the Magnetospheric Multiscale (MMS) mission, and detail the particle energization mechanisms in magnetic structures found near reconnecting regions in turbulent Earth's magnetotail. We find that electrons with motion perpendicular to the magnetic field dominate $\vec{j}\cdot\vec{E}$ dissipation. In contrast to the conventional picture of unidirectional energy transfer to particles by laminar two-dimensional (2D) reconnection, we find that energy exchange within magnetic structures during turbulent reconnection tends to be bidirectional with only a small positive bias from electromagnetic fields to particles. Specific electron energization mechanisms are quantified, including those due to parallel electric field, Fermi energization from curvature drift, betatron heating from magnetic field inhomogeneity, and polarization drift.

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astro-ph.EP 2026-05-28

Volatiles let tiny meteoroid form 30 m shock at 92 km

by Elizabeth A. Silber, Denis Vida +8 more

Shock wave formation in the thermosphere by an earthgrazing fireball: Empirical evidence for volatile-enhanced hydrodynamic shielding

Multi-station data show a 45 g object produced an acoustic source far larger than itself, requiring density boost from volatiles in thin air

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Hydrodynamic shielding is a theoretically well-established but observationally elusive and experimentally difficult-to-replicate phenomenon with implications that extend far beyond meteor physics. Rare earthgrazing meteoroids with infrasound signatures that penetrate to the ground can be used to probe hydrodynamic shielding that leads to strong shock formation at high altitude. Here, we report the first coordinated optical and multi-station infrasound observations of a centimeter-scale earthgrazing fireball that generated sustained cylindrical line shock at thermospheric altitudes near 92 km. The event was recorded by numerous optical stations and three infrasound arrays, allowing trajectory reconstruction, ablation behavior, acoustic source localization, and shock characteristics. Optical observations indicate early mechanical erosion and ablation/evaporation at exceptionally low dynamic pressure, consistent with a cometary or a porous, volatile-bearing CM chondritic object. Independent infrasound detections localize shock generation to multiple points along a 164 km trajectory segment near perigee. Weak-shock modeling yields a consistent blast radius of ~30 m, implying an acoustic-equivalent source size far exceeding the physical dimensions of the ~45 g nucleus. We demonstrate that classical gas dynamics and ablation-driven hydrodynamic shielding alone cannot account for these observations under ambient thermospheric conditions. We show that volatile release provides the additional flow-field density enhancement required to amplify hydrodynamic shielding, reduce the effective local Knudsen number, and sustain a shock envelope capable of radiating detectable infrasound. These results demonstrate that small, volatile-rich meteoroids can transiently establish continuum-like flow in rarefied environments.

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physics.space-ph 2026-05-28

Meteor radar detects Falcon 9 re-entry at 55-75 km

by Juha Vierinen, Dabrowka Knach +10 more

Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry

Plasma echoes match optical fragment paths, indicating the same systems could track smaller satellites like Starlink on re-entry.

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We investigate the February 19, 2025, re-entry of a Falcon 9 upper stage using optical observations from 43 meteor cameras across central Europe together with radar detections of re-entry plasma obtained with the 32.55 MHz SIMONe Germany multistatic radar system. Optical observations of fragment emissions between 85 and 36 km altitude were used to reconstruct 30 fragment trajectories, identify two main fragment families, and fit ballistic trajectories to estimate kinetic energy loss per unit mass. The optical detection-height distribution peaks near 60 km with a standard deviation of 10 km, and both optical and radar signatures occur in the same broad altitude region as the maximum kinetic-energy loss. Radar echoes were detected at altitudes between 55 and 75 km, and the radar-derived positions are consistent with those obtained from optical observations. Two distinct radar echo types associated with the re-entry plasma were identified: (1) specular trail echoes from overdense wake plasma, with radar cross-sections (RCS) of up to 60 dBsm, and (2) short-lived non-specular trail echoes with RCS values of 20--30 dBsm, exhibiting a delay of 1--2 s compared to optical signatures. The characteristic decay time of both echo types is approximately 1 s. In the radar-echo altitude range, the estimated Knudsen numbers for meter-scale fragments are well below unity, consistent with continuum-flow conditions and shock-driven plasma production rather than ordinary meteor-like impact ionization. These serendipitous radar observations demonstrate that the atmospheric re-entry of other spacecraft, including objects smaller than the Falcon 9 upper stage such as Starlink satellites, may likewise be detectable using comparable multistatic meteor radar systems deployed globally.

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physics.space-ph 2026-05-28

Cross-spectral method recovers camera delays to under 50 microseconds

by Juha Vierinen, Pavithiran Sivasothy +1 more

Estimating sub-frame time differences in camera image sequences

Phase-slope analysis of intensity signals extracts sub-frame timing from ordinary video, validated on smartphone recordings of a pulsed cali

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Some optical measurements require relative timing of intensity variations with accuracy much finer than the camera frame period. One motivating example is dynamic aurora, where different prompt emissions are expected to originate from different altitude regions and can therefore have millisecond-scale relative delays caused by finite energetic-electron velocities and other electron-transport effects. These delays are predicted to be a small fraction of the frame duration of typical auroral video cameras. We present a cross-spectral technique for estimating the relative delay between two time-varying optical intensity signals recorded by one or more image sensors. The method is validated with a calibration device that generates two pseudorandomly pulsed optical emissions with a known relative delay, recorded using a consumer smartphone camera. For the tested recordings, the method estimates relative delays between image-sensor regions with better than $50$~$\mu$s accuracy. Although developed for high-frame-rate auroral imaging, the technique has numerous other imaging applications, including camera timing calibration and measurements of time-varying optical signals. The single-camera tests demonstrate that the method can characterize sub-frame timing differences across an image sensor, such as those produced by rolling-shutter readout. The same analysis applies to separate cameras when they observe the same time-varying signal and are synchronized to a shared clock.

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physics.space-ph 2026-05-28

Polarization method images interplanetary magnetic fields

by Chuanpeng Hou, Huirong Yan +1 more

A Method for Imaging Interplanetary Magnetic Field Strength and Orientation

Ground-state alignment and Hanle effect polarization allow remote mapping of weak heliospheric magnetic fields.

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Measurements of interplanetary magnetic fields have long relied on spacecraft measurements, which provide only in-situ sampling and therefore cannot capture the global magnetic structure. Faraday rotation of radio signals extends in-situ measurements to line-of-sight measurements, but it still depends on the number and spatial distribution of available radio sources. The Zeeman effect offers another route to remote sensing of magnetic fields, but it is generally too weak to diagnose the weak interplanetary magnetic fields. Here, we present a remote-sensing method to constrain weak magnetic field strength and orientation using spectral-line polarization induced by ground-state alignment (GSA) and Hanle effect, with collisional effects taken into account. This method is sensitive to weak magnetic fields in environments ranging from the high solar atmosphere and solar wind to the outer heliosphere, and we identify suitable spectral lines for different targets. We further perform forward modeling of Mercury's magnetosphere to demonstrate the feasibility of this imaging method. Spectral-polarization imaging therefore provides a new way toward remote imaging of dynamic heliospheric magnetic structures.

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cs.LG 2026-05-27

Neural nets predict low-thrust fuel costs and times

by Zhong Zhang, Giacomo Acciarini +3 more

Pretrained Approximators for Low-Thrust Trajectory Cost and Reachability

Scaled data plus a self-similar transform lets one model generalize across orbit sizes, tilts, and central bodies.

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Low-thrust trajectory design relies heavily on repeated evaluations of fuel consumption and transfer feasibility, which require expensive optimal control solutions. In this work, we show these quantities can be accurately approximated by machine learning surrogates, enabling fast and scalable evaluation across a wide range of scenarios. By increasing both dataset size and model capacity, we observe that low-thrust trajectory optimization follows a scaling law, with performance improving linearly with the logarithm of training data and network parameters, and no evidence of saturation within the explored regime. Guided by this observation, we construct a large-scale dataset using the proposed homotopy-ray strategy tailored to mission design requirements. A key is the introduction of a self-similar transformation, which allows generalization across semi-major axes, inclinations, and central bodies avoiding retraining. As a result, the same neural approximator can be applied to diverse orbital environments and mission classes. The proposed models accurately predict optimal fuel consumption and minimum transfer time for single- and multi-revolution transfers. Their performance and generalization are demonstrated on a public dataset, a multi-asteroid flyby problem from the Global Trajectory Optimization Competition, and an asteroid rendezvous mission design. The models and datasets are released as open-source to support the space community.

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physics.space-ph 2026-05-26

Active-phase CMEs carry stronger fields than quiet-phase ones

by Yakub Olufadi, Nada Al-Haddad +7 more

Evolution of Coronal Mass Ejection Properties through Superposed Epoch Analysis from 0.2 to 2.2 au

Speed-matched analysis of 1600 events shows intrinsic differences in magnetic strength and density by solar cycle phase.

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Coronal mass ejections (CMEs) are explosive and energetic events consisting of strong magnetic structures erupting from the solar corona. We use superposed epoch analysis to investigate the general properties of CMEs as measured {\it in situ} from 0.2 to 2.2 au based on over 1600 events obtained from the HELIO4CAST catalog. We examine the dependence of the CME global properties on solar cycle phase, and compare the CME parameters derived in the active phase (AP) with the quiet phase (QP). Our findings show that during the AP of the solar cycle, the occurring CMEs are faster and have stronger magnetic field strength than during the QP, which has denser but weaker magnetic strength. These differences in magnetic field strength and density remain even when controlling for the speed. This may indicate that the enhanced profiles observed during the AP are not only a consequence of the CME propagation speed but may also reflect intrinsic differences in the eruption mechanism during different solar cycle phases. We also study how the magnetic field strength and components of the CME magnetic ejecta (ME) structure evolve with heliocentric distance. We find that the toroidal and poloidal ME magnetic field components have a similar power law decrease with distance, indicating a comparable expansion behavior of CMEs in these dimensions. We further quantify the CME magnetic field asymmetry %(often associated with CME aging) using the front-to-rear ratio of the toroidal component across heliocentric distance and find evidence of an increase of this ratio with heliocentric distance.

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astro-ph.SR 2026-05-25

Parallel waves damp to heat the fast solar wind

by Evan L. Yerger, Benjamin D. G. Chandran +3 more

Cyclotron breaking: a mechanism for parallel ion cyclotron waves to heat the fast solar wind

PICWs driven unstable near the sun increase their normalized wavenumber while propagating outward and damp when exceeding the turbulent thre

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The $\textit{Parker Solar Probe}$ ($\textit{PSP}$) mission has observed near-continuous power in parallel ion cyclotron waves (PICWs) in the young, fast solar wind. These waves are unlikely to be directly produced by the turbulent cascade and are likely born of a local instability; yet, they are observed to both cool -- and heat -- the plasma. We propose that these observations can be self-consistently explained as the natural consequence of PICWs propagating in the inhomogeneous solar wind after they have been driven unstable. In this work, we argue that strong proton heating by a turbulent cascade of oblique ICWs will result in PICWs being driven unstable in a process known as quasi-linear focusing. Because the power in the turbulent cascade is concentrated at scales above the turbulent transition region, PICWs will be driven unstable within a range of wave numbers parallel to the background magnetic field, $k_\parallel$, that is bounded from above by $k_{\parallel\rm P}^*$, corresponding to the start of the transition region. As unstable PICWs propagate away from the sun to regions of lower proton density, their $k_\parallel$, multiplied by the proton inertial length $d_{\rm p}$, increases. Eventually, the $k_\parallel d_{\rm p}$ of the PICWs becomes larger than $k_{\parallel\rm P}^*d_{\rm p}$ and the waves damp, heating the solar wind. We call this effect `cyclotron breaking', in analogy with ocean waves breaking on the shore. We then discuss the testable predictions of the theory, including a distinct heating signature in which PICWs cool fast protons and heat slow protons at any given heliocentric distance $r$. Finally, we conjecture that cyclotron breaking can lead to net heating by PICWs if the power emitted as PICWs decreases sufficiently rapidly with $r$ that local emission of PICWs is overwhelmed by the local damping of PICWs generated closer to the sun.

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eess.SY 2026-05-25

Maximum initial mass decides low-thrust reachability

by Giacomo Acciarini, Dario Izzo +1 more

Reachability for Low-Thrust Trajectories via Maximum Initial Mass

One scalar threshold per target replaces grid-based reachable-set construction

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Reachability analysis plays a central role in low-thrust spacecraft trajectory optimization by identifying which target states can be achieved under constraints on time, thrust, and propellant. Classical approaches construct reachable sets by solving many optimal control problems over grids of terminal states, requiring extensive forward simulations with fixed initial conditions. While effective, this approach is computationally expensive and becomes impractical for high-dimensional systems or strongly nonlinear dynamics, such as those encountered in cislunar environments or solar sail missions. This work introduces a dual formulation of the reachability problem. Instead of computing reachable sets directly, we determine, for fixed transfer time and boundary conditions, the maximum allowable initial mass (or, for solar sails, a scalar sail-strength parameter) that permits a successful transfer. A target is reachable if the spacecraft's initial mass does not exceed this threshold. This reformulation reduces reachability assessment to a scalar optimization problem for each target, producing a smooth scalar field that encodes equivalent feasibility information to classical reachable sets. We develop indirect maximum-initial-mass (MIM) formulations for both electric low-thrust and solar-sail dynamics and show how they can serve as efficient reachability oracles. Building on this formulation, we construct data-driven surrogate models to approximate the MIM-based reachability indicator. We investigate fully connected neural networks and demonstrate that residual networks provide the best trade-off between accuracy, training stability, and model complexity. The resulting surrogates enable rapid reachability evaluation while preserving the numerical advantages of the dual formulation, offering a practical tool for preliminary mission design and feasibility assessment.

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physics.space-ph 2026-05-25 Recognition

Circular space economy receives first structured definition

by Jonas Bahlmann, Michael Saidani +2 more

Conceptualizing and Defining the Circular Space Economy

10R Space Framework and three environments separate in-orbit, terrestrial, and celestial operations to close resource loops.

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Space faces significant sustainability issues including orbital congestion and debris accumulation. The continued growth of space operations, accelerated by advancements such as reusable launch systems, further intensifies these pressures. Current mitigation strategies, such as deorbiting spacecraft or transferring them to graveyard orbits, remain inherently linear. This "take-make-waste" approach is environmentally unsustainable and economically inefficient. On Earth, similar challenges have driven the development of the circular economy (CE), which aims to eliminate waste and pollution, circulate resources at their highest value, and decouple economic growth from finite resource consumption. While these objectives have been extensively studied across terrestrial sectors, their application to the space domain remains largely unexplored. In particular, the concept of a circular space economy (CSE) remains constrained by narratives centered on reuse, recycling, and in-orbit servicing, lacking a structured definition, consistent terminology, and a clearly defined, comprehensive scope. This lack complicates the systematic integration of circularity into mission design, policy frameworks, and space system architectures. After a detailed analysis of established CE definitions and CSE definition proposals, this work conceptualizes the CSE and introduces a structured definition for the first time. It analyzes Earth-space distinctions, clarifies the relationship between space sustainability and the CSE, establishes the 10R Space Framework to narrow, slow, and close resource loops, and distinguishes three operational environments: (I) the CE in space, (II) the CE of the terrestrial (space) sector, and (III) the CE of celestial bodies beyond Earth. Ultimately, this work enables a shared understanding and aims to strengthen the concept's recognition in the space sustainability debate.

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astro-ph.SR 2026-05-25

Solar radio burst pairs show delayed turbulent echoes

by Suli Ma, Eduard P. Kontar +3 more

Imaging spectroscopy reveals spike-like repeating radio burst pairs in the solar corona

Imaging of 613 pairs at 30-50 MHz finds displaced sources with reduced drift rates matching simulations of anisotropic plasma scattering.

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Solar radio bursts exhibit complex fine structures that reveal intricate coronal plasma dynamics. Here, we report detection of spike-like repeating burst pairs, characterized by two short-lived (0.1-2 s), narrowband components separated by about 4 s at frequencies 30-50 MHz. Using high-resolution dynamic spectra and spectroscopic imaging, we analyzed 613 burst pairs, measuring their durations, bandwidths, drift rates, flux densities, and spatial characteristics. Imaging links sources to an active region, with earlier components spatially concentrated above the region while delayed components are displaced and exhibit reduced drift rates. Radio-wave propagation simulations support the delayed bursts as turbulent echoes of harmonic emission in anisotropic coronal plasma. The location of the burst sources high in the corona suggests ongoing magnetic reconnection and electron acceleration well above typical flare heights. Our findings offer new insights into coronal turbulence effects while advancing diagnostics of coronal plasma and the elusive nature of solar radio echoes from ground-based transmitters.

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physics.space-ph 2026-05-22 Recognition

1-in-100-year solar event could cost $5.2B in U.S

by D. Bor, E. J. Oughton +5 more

C-SWIM: A Coupled Space Weather Impact Model for Satellite Fleet Vulnerability and Economic Loss Under a 1-in-100-Year Solar Energetic Particle Event

Model finds 1 percent of fleet at critical risk with daily economic impacts up to $1.3 billion

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Modern economies depend critically on satellite infrastructure, yet the aggregate economic consequences of extreme solar energetic particle (SEP) events have not been rigorously assessed. This study develops an integrated framework linking SEP hazard characterization, dynamic geomagnetic cutoff rigidity modeling, radiation dose transport, and fleet-wide failure probability estimation to macroeconomic impact analysis. Using extreme-value analysis of 160 SEP events over 27.4 years (1996-2025), failure probability is estimated for ~10,650 US operational satellites under orbital regime-dependent shielding assumptions. The assessment reveals that ~100 satellites (1.0%) are at Critical risk, concentrated in high-altitude low Earth orbit and highly elliptical orbit, while medium Earth orbit and geosynchronous orbit satellites fall in the Negligible class (P_fail < 10^-9) under the assumed radiation-hardened components and shielding. The expected capital loss across the ~$254B fleet totals ~$5.2B. Three failure scenarios, expanding from Critical satellites only (P_fail > 10^-2), to Critical and Elevated (P_fail > 10^-3), and to all satellites with non-negligible risk (P_fail > 10^-6), yield daily economic impacts of ~$70M, ~$270M, and ~$1.3B, respectively. Earth observation suffers up to 95.6% capacity loss in the worst case, while military services experience 16.1-20.4% disruption across scenarios. Results are first-order estimates: hardware failure counts are conservative because only total ionizing dose is modeled, and daily economic impacts represent upper bounds because operator response and recovery are not included.

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physics.space-ph 2026-05-22

Two-stage cascade forecasts aurora visibility at 0.937 ROC-AUC

by Zongyuan Ge, Chenwaner Zhang +5 more

Aurora Hunter: A Two-Stage Framework for Probabilistic Visibility Forecasting

Separating occurrence from cloud and moonlight effects improves single-stage baseline by 0.087 on test and independent sites.

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Forecasting aurora borealis visibility matters for space weather research and aurora tourism. Visibility at a site and night depends on two distinct factors: (1) whether aurora is physically occurring, driven by solar wind-magnetosphere coupling, and (2) whether observing conditions allow naked-eye detection, mainly cloud cover and lunar illumination. We present Aurora Hunter, a two-stage cascade that decouples these factors. Stage 1 predicts P(occurring) with XGBoost using 51 physics-driven features trained on joint Tromso+Kiruna data (about 16,600 hourly samples, 2015-2023) with labels from the Tromso AI all-sky image classifier. Stage 2 predicts P(clear observation given occurring) with logistic regression using 21 cloud-cover and lunar-illumination features trained only on aurora-occurring hours. The cascade P(visible)=P(occurring)*P(clear|occurring) reaches ROC-AUC 0.937 (Tromso test, 2019-2020) and 0.905 (independent Kiruna, 2024), improving a single-stage baseline by +0.087. Held-out Skibotn data (2022-2025) confirm cross-site generalization. SHAP identifies the Kp x nightside interaction, MLT position, and auroral oval distance as dominant predictors (39% combined). Prototype: https://aurora-hunter.onrender.com.

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physics.plasm-ph 2026-05-22 2 theorems

Turbulence boosts non-thermal electrons at oblique shocks

by Karol Fulat, Eloise Moore +5 more

PIC simulations of nonrelativistic high-Mach-number oblique shocks propagating in a turbulent medium

Simulations show pre-existing compressive turbulence shortens the foreshock and raises both the number and maximum energy of accelerated non

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Collisionless shocks are common in astrophysical systems and stand as sites of particle acceleration. While particles at perpendicular shocks may not return to the upstream region, at oblique shocks a fraction of energetic electrons manage to escape the shock and travel upstream. An extended region known as the electron foreshock is formed, where these reflected particles drive various instabilities that may promote electron acceleration. Here we present the first 2D3V particle-in-cell (PIC) simulations of electron-ion non-relativistic oblique shocks that explore the interaction of the foreshock with pre-existing compressive turbulence with relative amplitude of 15% based on interstellar medium estimates. We find that pre-existing turbulence influences the emergence and behavior of the whistler-wave instability, as it enhances the amplitudes of the magnetic-field fluctuations and leads to larger nonlinear structures. This impacts the dynamics of the reflected electrons, resulting in a shorter and hotter electron foreshock. At the end of our simulations, with pre-existing upstream turbulence we observe non-thermal electrons that are more numerous, reach higher energies, and carry a larger portion of the total energy.

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astro-ph.SR 2026-05-21 Recognition

Plasma diagnostics agree because they measure one effective temperature

by Victor Edmonds

Multi-diagnostic convergence: a single measurement in weakly collisional plasmas

In weakly collisional plasmas the convergence of electron-temperature methods reflects a shared ionization limit rather than independent sam

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When multiple electron temperature diagnostics converge on the same value, the standard inference is that the measurement is robust. We show that this convergence is a structural consequence of the shared ionization bottleneck in any plasma where the electron Knudsen number exceeds $\sim 0.01$: all diagnostics downstream of collisional ionization report the effective temperature $T_{\rm eff}$, not the core temperature $T_{\rm core}$. Their agreement is a single measurement reported $N$ times. We introduce a taxonomy: Type A (ionization-gated, $T_{\rm eff}$), Type B (bulk-sampling, $T_{\rm core}$), Type C (distribution-resolving). The ratio $R = T_A/T_B$ yields $\kappa = 3R/[2(R-1)]$ directly. Applied to the solar corona ($R = 2.4$, $\kappa \approx 2.5$) and the tokamak scrape-off layer, single kappa distributions ($\kappa \approx 2$--$10$) reproduce published bi-Maxwellian EEDF decompositions to 3--8\% RMS with one fewer parameter, and Thomson scattering confirms the predicted Type B temperature. We test applicability in planetary nebulae (the 80-year CEL--ORL abundance discrepancy). Knudsen calculations with the Shoub $v^4$ mean-free-path scaling show ionizing electrons are collisionless in the corona even when the bulk is fluid; in PNe, both ionizing ($\sim 55$ eV) and excitation ($\sim 5$ eV) electrons are collisional over nebular scales, identifying PNe as the falsification boundary; in the SOL, non-local parallel transport maintains tails even where local collisionality is high. For $\kappa \approx 3$--$5$, the raw Spitzer--H\"arm formula with spectroscopic $T_e$ overestimates parallel heat flux by factors of 3--25$\times$; flux-limited models inherit the bias through their boundary conditions, relevant to ITER divertor predictions. Every diagnostic campaign on a weakly collisional plasma should include at least one Type B measurement.

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physics.ao-ph 2026-05-20 Recognition

Sakurai formula matches reentry infrasound to 9 percent

by Elizabeth A. Silber

Benchmarking Cylindrical Blast Wave Theory Against the OSIRIS-REx Sample Return Capsule Reentry

Benchmark against known non-ablating capsule shows common meteor approximation overestimates blast radius by more than factor of three.

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Weak shock theory based on cylindrical blast waves has been used to interpret meteor infrasound, but it has not been systematically benchmarked against a non-ablating hypersonic source with independently known parameters. The objective of this study is not to propose a new theoretical framework, but to evaluate the operational validity of the existing suite of blast radius formulations against a high-fidelity ground truth dataset. The OSIRIS-REx Sample Return Capsule reentry on 24 September 2023 provides such a benchmark because the capsule geometry, trajectory, and infrasound emission points are constrained from mission data and ray tracing, reducing source-side uncertainty associated with ablation. Using observations from 39 infrasound stations, this benchmarking study evaluates six published blast radius (R_0) formulations and three weak-shock transition coefficients (C) within a stratified atmospheric propagation model to predict signal period and peak overpressure. The benchmarking identifies the Sakurai formulation as the best-performing formulation for non-ablating bodies, with the Jones/Plooster formulation performing comparably when a physically appropriate C is adopted. Sakurai and Jones/Plooster yield linear-period median absolute percentage residuals of 9% and 11%, respectively. The period predictions show only weak sensitivity to C at these propagation distances. The Mach-diameter approximation commonly used in meteor studies overestimates R_0 by more than a factor of 3 in the absence of ablation. These results establish a performance baseline for applying cylindrical blast wave theory to non-ablating hypersonic bodies and demonstrate that the signal period is a robust observable for constraining R_0.

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gr-qc 2026-05-20 2 theorems

Pre-inflationary phase suppresses large CMB correlations

by M. Montes, José Edgar Madriz Aguilar +2 more

A novel pre-inflationary model in view of the lack of angular correlation of CMB

Decelerated expansion before inflation raises the Hubble horizon and imposes a cutoff that reduces power at the largest scales.

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In this paper we propose a novel unified cosmological model that connects a pre-inflationary epoch, starting at the Planckian time, with the onset of inflation within a single scalar-field framework. The pre-inflationary phase is characterized by a decelerated expansion with an increasing comoving Hubble horizon, followed by a gradually transition to an accelerated inflationary regime. This early dynamics leads to a modified causal structure that naturally accounts for the suppression of large-angle $(\theta \gtrsim 60^\circ)$ correlations in the cosmic microwave background (CMB) reported by the satellite PLANCK. We study the quantum fluctuations of the scalar field using the Mukhanov-Sasaki formalism and a canonical quantization procedure based on energy minimization. We find that the vacuum state is well-defined only for sub-horizon modes at the onset of inflation, which induces a natural cutoff in the primordial power spectrum. The resulting spectrum exhibits a suppression at large scales while remaining nearly scale-invariant at small scales. In the appropriate limit, the model recovers the standard de Sitter result, in agreement with current observational constraints. These results highlight the relevance of pre-inflationary dynamics for addressing large-scale anomalies within a consistent inflationary framework.

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astro-ph.EP 2026-05-20 Recognition

SGP4 beats high-fidelity models for most Starlink TLE forecasts

by Dimitrije Jankovic

How long can you trust a Starlink TLE? An empirical comparison of SGP4 and high-fidelity propagation against operator-updated truth across a megaconstellation

Across 24,000+ pairs, the standard propagator wins 65-75 percent of comparisons even at seven-day horizons.

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We characterise position-error behaviour of Two-Line Element (TLE) propagation against operator-updated truth on Starlink, sweeping 24,641 next-TLE-truth pairs across 501 satellites stratified by altitude shell (540, 550, 560 km) and platform generation (v1.0, v1.5, v2-mini) over April 2026. Each pair is propagated with SGP4 and GMAT at high fidelity (EGM2008 $70\times70$, NRLMSISE-00 drag, Sun and Moon third-body gravity, conical-shadow SRP), then compared against the operator's next TLE as proxy truth. Three findings: First, position error follows a per-cell power law $\lVert\Delta\mathbf{r}(\Delta t)\rVert \approx A\,\Delta t^{k}$ with fitted exponents in $(1,2)$ on every v2-mini cell and on the high-fidelity v1.x cells at 540 and 560 km, while SGP4 v1.x and high-fidelity v1.x at 550 km are sub-linear ($k \lesssim 1$); the cohort-specific mix of mean-motion bias and unmodelled in-track acceleration sets the per-cell exponent. Pooled $L_{2}$ medians grow from $\sim 1$ km at 6 h to $\sim 38$ km (SGP4) / $\sim 76$ km (high-fid) at 7 d. Second, high-fidelity propagation from public-TLE inputs does not improve over SGP4 at any of the four staleness horizons; SGP4 wins on $\sim 65$--$75\%$ of pairs, with v2-mini at long $\Delta t$ the one regime where high-fidelity wins on a majority of pairs at both populated shells. The negative result reflects operator-OD residual dominance at epoch, SGP4-vs-SGP4 truth-construction kernel alignment, and spacecraft-property bias amplification on the high-fidelity arm. Third, the per-satellite SGP4 staleness coefficient regressed against F10.7 returns a positive slope clearing conventional significance at one shell (560 km) on the 30-day, $\sim 17$ sfu window -- direction-consistent with the LEO density-gradient expectation, not a calibrated F10.7-modulation measurement.

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physics.space-ph 2026-05-20 1 theorem

Reassessment finds no GRB ionospheric effects after all

by Maosheng He, Quanhan Li +6 more

Reassessment of Ionospheric Responses to GRB~221009A: Disentangling Instrumental, Illumination and Geophysical Effects

Reported signals match recurring orbital illumination patterns and solar wind changes instead of the gamma-ray burst arrival.

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Gamma-ray bursts (GRBs) have long been proposed to perturb Earth's ionosphere, with occasional reports of disruptions in ultra- and extremely-low-frequency radio signals. The exceptionally bright GRB~221009A was recently claimed to induce multi-altitude ionospheric responses, including perturbations in satellite electric fields, regional total electron content (TEC), and the equatorial electrojet (EEJ). These claims have renewed interest in the potential near-Earth impacts of astrophysical transients. Here we perform an independent reassessment using expanded datasets spanning multiple altitudes. We find no coherent, burst-like TEC enhancement, show that the reported electric-field anomalies recur under specific illumination conditions each orbit, and demonstrate that the EEJ fluctuations preceded the burst and coincide with solar-wind variability. Together, these results indicate that the reported GRB-induced ionospheric responses are fully attributable to other natural geophysical processes and instrumental artefacts, thereby resolving a high-profile controversy and clarifying the true limits of GRBs'ionospheric effects.

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astro-ph.SR 2026-05-13 1 theorem

Helicity ratio reaches 0.38 at solar eruption onset

by Xinkai Bian, Chaowei Jiang +6 more

An investigation of magnetic energy and helicity thresholds at the onset of solar eruptions based on numerical simulations

Simulations across topologies show only 10 percent variation, outperforming other energy and helicity metrics

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Identifying universal, topology-independent thresholds in the coronal magnetic fields at onset of solar eruptions is crucial for physics-based prediction of eruptions. To this end, we systematically analyze the evolution of magnetic energy and helicity in twelve high-fidelity 3D magnetohydrodynamic simulations where eruptions are triggered by magnetic reconnection. The simulations encompass a comprehensive parameter space, including bipolar and quadrupolar configurations, sheared arcades and pre-existing flux ropes, and various photospheric driving motions. We find that the ratio of current-carrying helicity to total relative helicity $(H_j/H_r)$ exhibits a remarkably consistent threshold of $0.38 \pm 0.04$ at eruption onset across all cases, with a coefficient of variation of only $\sim 10$\%. This threshold specifically characterizes the critical conditions at eruption onset and is largely independent of the subsequent temporal evolution, making it the most robust eruptivity indicator identified. In contrast, other normalized helicity and energy metrics show greater scatter. Crucially, we further find that $H_j/H_r$ does not necessarily achieve its peak at the eruption onset time and its post-eruption evolution diverges based on magnetic topology: it continues to increase in bipolar configurations due to tether-cutting reconnection, which transforms sheared arcade into the erupting current-carrying magnetic flux, but decreases in quadrupolar configurations as breakout reconnection peels off the erupting flux. These results highlight the helicity ratio as a promising and consistent eruptivity indicator and provide new insights into its dynamic evolution due to different reconnections.

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physics.plasm-ph 2026-05-12

Dynamic alignment reflects survival of intense fluctuations

by Amir Jafari

Dynamic Alignment as a Statistical Survival Effect

Amplitude-weighted diagnostics show smaller angles because high-amplitude misaligned states deplete faster, leaving typical angles nearly sc

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Dynamic alignment in magnetohydrodynamic turbulence is often interpreted as scale-dependent alignment of counterpropagating Els"asser increments (\delta_r z^\pm), with consequences for inertial-range spectra. We show that standard amplitude-weighted measurements do not establish progressive alignment of typical fluctuations. We separate angular statistics from Els"asser-amplitude weighting and interpret the signal as finite-time retention of amplitude--angle states, tested with Johns Hopkins Turbulence Database simulations and NASA Wind measurements. In the simulations, the unweighted folded angle (\theta_r) between (\delta_r z^+) and (\delta_r z^-), with alignment and anti-alignment folded together, remains only moderately below the random 3D baseline and shows no monotonic decrease across inertial-range separations. Smaller angles in weighted diagnostics are produced mainly by large (A_r=|\delta_r z^+||\delta_r z^-|) events, giving a negative covariance between (A_r) and (\sin\theta_r) that is removed by shuffled controls. Transition measurements show that high-amplitude large-angle states deplete faster than high-amplitude small-angle states. The source--depletion balance reconstructs second-order Els"asser amplitudes and gives an effective rms increment scaling close to (\ell_\perp^{1/4}), although the typical folded angle is nearly scale independent. Mean-log increment-amplitude checks give larger slopes than second-order-amplitude fits in both simulation and Wind data, consistent with stronger intermittent-event weighting of second-order statistics. Wind measurements reproduce the same amplitude--angle hierarchy and negative covariance under Taylor sampling. Conventional dynamic-alignment diagnostics therefore measure selective retention of intense Els"asser fluctuations, not progressive alignment of typical fluctuations.

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physics.space-ph 2026-05-12 2 theorems

Electron-only reconnection emerges in solar wind turbulence simulations

by Joaquín Espinoza-Troni, Giuseppe Arrò +3 more

Secondary Electron-Only Reconnection Driven by Large Scale Ion-Coupled Reconnection and Electron Kelvin-Helmholtz Instabilities in Hybrid Simulations of Solar Wind Turbulence

It arises from plasmoid interactions and electron instabilities, suggesting a role in kinetic-scale energy dissipation even in large systems

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Electron-only reconnection (EREC) is a magnetic reconnection regime occurring within subion-scale current sheets (CSs), exhibiting only electron jets, without any ion outflows. EREC has been first observed in the Earth's magnetosheath, where its occurrence is linked to the small correlation length of magnetic fluctuations, limiting the growth of CSs to very large scales. On the other hand, the development of EREC in open systems with large magnetic correlation lengths, such as the solar wind (SW), remains an open question. To address this problem, we employ a large-scale 2D hybrid simulation with finite electron inertia, investigating the development of EREC driven by turbulence. By injecting energy at very large scales, we allow EREC to develop spontaneously due to the turbulent cascade, without any external small-scale forcing or imposed constraints on the turbulence correlation length. We find that EREC develops in our simulation via two distinct turbulence-driven mechanisms: (1) secondary EREC induced by the interaction of plasmoids in the outflows of large-scale ion-coupled reconnection; (2) EREC directly driven at subion scales by the electron Kelvin-Helmholtz instability in small-scale velocity shears. Furthermore, we perform a statistical analysis of CSs using the machine-learning clustering algorithm HDBSCAN, showing that subion-scale CSs capable of hosting EREC are dominant in our simulation. Our results suggest that EREC could occur even in large-scale space and astrophysical systems, like the SW, driven by secondary turbulent processes, potentially playing a key role in dissipating energy at kinetic scales.

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astro-ph.EP 2026-05-11 2 theorems

Ice shell variations on Enceladus produce magnetic signals tied to ocean conductivity

by Alexander Grayver, Joachim Saur

Exploring Enceladus's Interior Structure Using Electromagnetic Induction

If measured from low orbit, these signals would indicate a conductive ocean and set lower limits on its salinity and volatile content.

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Electromagnetic (EM) sounding can constrain the electrical structure of Enceladus and, in turn, the salinity of its ocean and the porosity, fluid content, and thermal state of its hydrothermally active core. Here, we assess the feasibility of EM sounding at Enceladus using both global (orbiter) and local (lander) EM induction transfer functions. We provide a physical framework for modeling EM induction for 1-D and 3-D subsurface conductivity models and discuss how transfer functions can be estimated from global or local measurements of the magnetic and electric fields. We simulate 3-D induction effects arising from variations in ice-shell thickness. The magnitude of these effects in the magnetic field correlates with the ice-shell thickness at the surface and is strongly dependent on the ocean's conductivity. These magnetic variations, if observed, would favor a moderately to highly conductive ocean, providing lower bounds on salinity and volatile content. The absence of these effects indicates a thicker, more homogeneous ice shell and/or a lower-conductivity ocean. Given plausible magnitudes, a polar-orbiting mission with low-altitude measurements will be required to detect these effects. In summary, an orbiter will constrain global ocean conductivity using long-period induction and possibly map the ice thickness variations. The detailed EM sounding of both the hydrosphere and the core can be achieved by a lander-based broadband EM sounding at periods $\approx 10^1-10^5$ s to probe ocean salinity and thickness, as well as core properties including porosity, fluid content, and temperature.

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astro-ph.SR 2026-05-11 2 theorems

Pre-flare oscillations detected hours ahead of X9 solar flare

by Louis Seyfritz, Maria Kazachenko +1 more

Investigating Pre-flare Signatures in Spectroscopic Observations of an X9-class Solar Flare

IRIS data show 7-21 minute periods and steady parameter increases consistent with slow magnetic destabilization before rapid reconnection.

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On October 3rd, 2024, the Sun emitted an X9.0-class flare from active region NOAA 13842. The event was recorded by multiple space-based instruments, beginning hours before the eruption, granting a unique opportunity to provide insight into the flare's pre-flare phase. In this study, we employ analysis of Interface Region Imaging Spectrograph (IRIS) spectroscopic data to investigate pre-flaring phenomena associated with this flare. We present time-series and wavelet analysis of non-thermal velocity, Doppler velocity, and line intensity quantities of the IRIS Si IV 1403 angstrom line. We find two ranges of periodic oscillations during the pre-flare phase: ~7-10 min and ~18-21 min oscillations, with local enhancements occurring near the polarity inversion line. We also find a steady rise in Si IV line parameters beginning 3 hours before the flare in the same region, transitioning into strong non-thermal velocities and blueshifts ~15 minutes before onset. These findings are consistent with a slow destabilization of the coronal magnetic field, possibly driven by the gradual activation of a flux rope, followed by a rapid shift to intense reconnection activity leading to flare onset.

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astro-ph.CO 2026-05-08

Cluster data back constant speed of light except with Planck calibration

by R. F. L. Holanda, Marcelo Ferreira +2 more

Revisiting the Constancy of the Speed of Light: Galaxy Cluster Mass Bias Implications

X-ray gas fractions and supernova distances show no deviation from constant c under CLASH or CCCP mass biases but mild 2-sigma tension under

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In recent years, improvements in galaxy cluster observations have enabled a variety of tests of fundamental physics using these systems. In this work, we test the constancy of the speed of light, $c$, by combining X-ray gas mass fraction measurements from galaxy clusters with SNe Ia luminosity distance measurements from Pantheon+. We adopt the SH0ES prior on $H_0$ and the $\Omega_b/\Omega_m$ ratio from galaxy clustering observations, thereby minimizing the dependence of our analysis on any specific cosmological model. We explore different assumptions for the cluster mass calibration (mass bias), including \textsc{CLASH}, \textsc{CCCP}, and Planck-based estimates. We find no deviation from a constant $c$ when adopting \textsc{CLASH} or \textsc{CCCP} priors, while Planck-based calibration yields a mild tension, with the hypothesis of constant $c$ being only marginally consistent at the $2\sigma$ level, indicating a non-negligible sensitivity of the results to the adopted calibration scheme.

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physics.acc-ph 2026-05-08 2 theorems

Orbital colliders need 1,000-100,000 km radii for PeV-EeV energies

by Viktor Danchev, Alex Dyer +2 more

The Case for Space-Based Particle Colliders: Orbital Infrastructure as a Path to Grand Unification Energy Scales

Terrestrial accelerators fall short of the scales where grand unification is expected, but space orbits following the standard energy-radius

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The Standard Model of particle Physics has been validated to extraordinarily high precision by the Large Hadron Collider (LHC). Yet it leaves some of the most fundamental questions in Physics unresolved: the nature of dark matter, the hierarchy problem, and the unification of forces. Multiple next-generation terrestrial colliders have been proposed such as the Future Circular Collider (FCC) which will reach centre-of-mass energies of $\approx$100 TeV, yet the energy scales at which hints of Grand Unified Theories (GUTs) and string theory are expected to be observed ($10^{11}-10^{13}$ TeV) remain orders of magnitude beyond the reach of any terrestrial facility. We argue that the path to these energy frontiers inevitably leads to Space. By examining the fundamental scaling law for circular proton colliders, we establish that colliders of radius $10^3-10^5$ km are required to enter the PeV-EeV regime. In addition, Space-based colliders benefit from virtually free ultra-high vacuum ($< 10^{10}$ particles/m$^3$ above 1000 km altitude), passive cryogenic cooling, reduction of geological and political constraints, and perhaps most importantly -- the substantial reduction of the thermodynamic penalty that dominates terrestrial cryogenic power budgets. We survey existing proposals for beyond-Earth colliders, derive order-of-magnitude requirements for an orbital collider constellation, and assess feasibility against current and near-term spacecraft capabilities in formation flying, power generation, and precision attitude control. We conclude that recent developments in orbital infrastructure -- particularly gigawatt-scale orbital power architectures being developed for Space-based data centers -- are converging with the needs of a Space-based mega collider, making serious feasibility studies warranted and promising a more certain path towards the core questions of modern Physics.

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gr-qc 2026-05-07

Wormhole ringdown damping tracks galactic compactness

by Shauvik Biswas, Sayan Chakrabarti

Perturbations in the parametrized wormhole spacetime and their related quasinormal modes

Shadow bounds on Sgr A* yield viable parameters where decay rates vary with compactness while oscillation frequencies stay stable.

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We study electromagnetic perturbations and the associated quasinormal modes (QNMs) of parametrized static, spherically symmetric wormhole spacetimes, focusing on Damour-Solodukhin and braneworld geometries as well as their galactic extensions. Using the Bronnikov-Konoplya-Pappas parametrization, we express the metric functions in terms of a compactified radial coordinate and characterize the spacetime through far-field and near-throat parameters. The far-field coefficients govern the asymptotic structure and post-Newtonian behaviour, while the near-throat continued-fraction expansion captures the strong-field geometry near the throat. We first apply the parametrization to isolated wormholes and identify its range of validity, showing that non-polynomial metric functions can limit the convergence of the near-throat expansion and hence the accuracy of a truncated representation. We then extend the framework to a galactic Damour-Solodukhin wormhole embedded in a Hernquist dark matter halo. Imposing observational bounds from the shadow of Sgr A$^*$, we constrain the galactic compactness and deformation parameters and obtain an observationally viable parametrized metric. Within the allowed parameter space, we compute the fundamental QNM frequencies using the transfer matrix method and analyze the corresponding time-domain ringdown signals. We find that the damping rate is more sensitive to galactic compactness, whereas the oscillation frequency remains comparatively stable. Although the spectral shifts are small within the shadow-allowed region, the framework provides a systematic link between geometric parametrization, shadow constraints, and dynamical response. Our results establish an observationally consistent parametrized description of wormhole perturbations for strong-field tests of horizonless compact objects.

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physics.space-ph 2026-05-06

Electrons follow tangled field lines but leak across them via drifts and waves

by Daniel Verscharen, Natasha Jeffrey +7 more

Transport of electrons in tangled magnetic fields

Review outlines how inhomogeneities and instabilities let electrons deviate from strict field-line confinement in cosmic plasmas.

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Cosmic magnetic fields are typically inhomogeneous and often highly tangled due to large-scale plasma flows, turbulence, and instabilities. If the variations in the magnetic field occur on scales that are large compared to the gyro-radius of the plasma electrons, the electrons are primarily confined to gyro-centre trajectories along the field lines. Therefore, in-situ electron measurements help us map out the connectivity of the magnetic field in space plasmas. Gyro-centre drifts, wave-particle interactions, trapping, and cross-field diffusion are processes related to field inhomogeneities and fluctuations; they have the potential to modify or even disrupt the transport of electrons along field lines. We introduce the basic principles of electron transport in tangled magnetic fields and review the creation of tangled fields through turbulence and instabilities as well as the modulation of parallel electron transport through kinetic instabilities. We then describe trapping and de-trapping effects in inhomogeneous magnetic fields, as well as electron diffusion and energisation across the magnetic field. The transport of electrons in tangled fields results from a complex interplay of plasma processes that occur on a broad range of scales. A combination of in-situ plasma measurements, remote-sensing plasma observations, and plasma theory and simulations is required to resolve this contemporary challenge to the fields of heliophysics and astrophysics.

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physics.space-ph 2026-05-05

Technique restores whistler magnetic spectra from electric data

by Declan Frawley, Dmitri L. Vainchtein +2 more

Whistler-mode waves in near-equatorial THEMIS measurements: reconstruction of magnetic field spectra from electric field and plasma measurements

Reconstruction using cold plasma relation recovers THEMIS E and D wave amplitudes to within factor of 1.5 of full measurements.

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Electromagnetic whistler-mode waves are a natural emission in the outer radiation belt and the Earth's magnetotail. The resonant interaction of these waves and energetic electrons are responsible for electron acceleration and losses, thus coupling the magnetosphere and ionosphere. Near-equatorial spacecraft use search-coil magnetometers for whistler-mode wave measurements, and one of the largest (covering the longest period of time) dataset of such waves has been collected by the THEMIS mission operating in the near-Earth magnetosphere within 2008-2025. However, after 2017, the search-coil magnetometers on two THEMIS spacecraft, THEMIS E and D, experienced problems with their signal along the spacecraft spin axis and were only able to detect the spin plane components of the wave vector. This significantly reduces our ability to detect the total wave amplitude wave magnitudes and limits our ability to incorporate the THEMIS E, D datasets into investigation of whistler-mode waves. In this technical report, we propose and validate a technique for reconstruction of magnetic field spectral density for Fast Fourier transform data product collected during Fast-Survey mode hereafter referred to as the fff dataset collected by THEMIS E and D. We use measurements of the electric field instrument and cold plasma dispersion relation to evaluate the whistler-mode magnetic field spectral density. Verification of this technique by comparison with THEMIS A measurements (which retained their 3D measurement capability intact) confirms that restored magnetic field spectral density is within a factor of ~1.5 of the actually measured magnitudes.

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astro-ph.SR 2026-05-04

Matched solar wind streams show 45% speed gain per radial decade

by Jean-Baptiste Dakeyo, Tamar Ervin +11 more

On the Radial Evolution of the Solar Wind : The Source Alignment Method Applied to Parker Solar Probe and Solar Orbiter Observations

Statistics from 548 intervals demonstrate ongoing acceleration beyond the corona.

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The properties of the solar wind, as measured in situ throughout the heliosphere, depend both on the characteristics of its coronal source and on the intrinsic processes governing its interplanetary evolution. Recently, radial and Parker spiral alignment techniques have been applied to Parker Solar Probe (PSP) and Solar Orbiter (SO) observations to investigate the radial evolution of the same solar wind parcel. These studies have shown that the solar wind can undergo significant acceleration even beyond its primary acceleration region (i.e., above 15 solar radii). However, such radial and Parker spiral alignments are rare in practice, which limits the statistical significance and general applicability of the results. We introduce a new source alignment technique designed to overcome these limitations. Using magnetic backmapping, we associate similar solar wind streams observed by the two spacecraft based on the proximity of their photospheric footpoints, combined with additional in-situ stream similarity criteria. Applying the source alignment method to PSP and SO observations, we identify a total of 548 alignment intervals, each lasting 30 minutes. By constructing statistics over all alignments, we find that the solar wind speed increases by an average of 45% per radial decade (approximately 147 km/s) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 solar radii. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density, show the strongest anti-correlation with the increase in bulk velocity.

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physics.space-ph 2026-05-04

SEP rise time power law flattens from Earth to Mars

by Yihang Cao, Jingnan Guo +4 more

Statistical analysis of solar energetic particle rise times using Earth and Mars observations and constraints on particle transport parameters

Multi-planet observations show weaker energy dependence of rise times at larger distances, implying rigidity-independent scattering.

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The propagation of solar energetic particles (SEPs) in interplanetary space is modulated by solar wind turbulence, which significantly influences particle diffusion and energy evolution through scattering processes. Traditional analyses based on absolute flux measurements face inherent difficulties in disentangling source acceleration from subsequent transport, while temporal features such as onset and peak times are less affected and better suited for studying SEP transport. This study establishes a statistical relationship between the rise time of SEP events at different energies using multi-satellite observations at Earth and Mars. We use data from SOHO/ERNE and Tianwen-1/MEPA between November 2020 and March 2025, selecting 75 SEP events at 1 AU and 58 near Mars. For each energy range, onset times are determined by linear fitting, and peak times are extracted via a sliding median filter combined with Savitzky-Golay smoothing; the difference gives the SEP rise time. Comparing with the pure diffusion equation prediction, we examine the statistical behavior of rise time at Earth and Mars. Despite event selection uncertainties, SEP rise time follows a clear power-law relation with energy. The flatter power-law at Mars indicates weaker energy dependence with increasing solar distance. Using these empirical relations, we constrain the rigidity dependence of the parallel mean free path within the parallel diffusion model. Our results show that turbulence scattering at Mars approaches a rigidity-independent regime, reflecting turbulence evolution toward a dissipation-dominated state from Earth to Mars.

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