Evidence of solar-cycle related structural changes in the solar convection zone

While it has been relatively easy to determine solar-cycle related changes in solar dynamics, determining changes in structure in the deeper layers of the Sun has proved to be difficult. By using helioseismic data obtained over two solar cycles, and sacrificing resolution in favour of lower uncertainties, we show that there are significant changes in the solar convection zone, and perhaps even below it. Using MDI data, we find a relative squared sound-speed difference of $(2.56\pm 0.71)\times 10^{-5}$ at the convection-zone base between the maximum of solar Cycle~23 and the minimum between Cycles~23 and 24. The squared sound-speed difference for the maximum of Cycle~24 obtained with HMI data is $(1.95\pm 0.69)\times 10^{-5}$. GONG data support these results. We also find that the sound speed in the solar convection zone decreases compared to the sound speed below it as the Sun becomes more active. We find evidence of changes in the radial derivative of the sound-speed difference between the solar minimum and other epochs at the base of the convection zone implying possible small changes in the position of the convection-zone base, however, the results are too noisy to make any definitive estimates of the change.