Electric Currents through J-shaped and Non-J-shaped Flare Ribbons

Recently solar flares exhibiting a double J-shaped ribbons in the lower solar atmosphere have been paid increasing attention in the context of extending the two-dimensional standard flare model to three dimensions, as motivated by the spatial correlation between photospheric current channels and flare ribbons. Here we study the electric currents through the photospheric area swept by flare ribbons (termed synthesized ribbon area or SRA), with a sample of 71 two-ribbon flares, of which 36 are J-shaped. Electric currents flowing through one ribbon are highly correlated with those through the other, therefore belonging to the same current system. The non-neutrality factor of this current system is independent of the flare magnitude, implying that both direct and return currents participate in flares. J-shaped flares are distinct from non-J-shaped flares in the following aspects: 1) Electric current densities within J-shaped SRA are significantly smaller than those within non-J-shaped SRA, but J-shaped SRA and its associated magnetic flux are also significantly larger. 2) Electric currents through SRA are positively correlated with the flare magnitude, but J-shaped flares show stronger correlation than non-J-shaped flares. 3) The majority (75\%) of J-shaped flares are eruptive, while the majority (86\%) of non-J-shaped flares are confined; accordingly, hosting active regions of J-shaped flares are more likely to be sigmoidal than non-J-shaped flares. Thus, J-shaped flares constitute a distinct subset of two-ribbon flares, probably the representative of eruptive ones. Further, we found that combining SRA and its associated magnetic flux has the potential to differentiate eruptive from confined flares.