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

Increasing attention has recently been paid to solar flares exhibiting double-J-shaped ribbons in the lower solar atmosphere, 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 the synthesized ribbon area (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 flowing through the other, and they therefore belong to the same current system. The nonneutrality 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 ways: (1) electric-current densities within the J-shaped SRA are significantly smaller than those within the non-J-shaped SRA, but the J-shaped SRA and its associated magnetic flux is also significantly larger. (2) Electric currents through the SRA are positively correlated with the flare magnitude, but J-shaped flares show a 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 representative of eruptive ones. Further, we found that combining the SRA and its associated magnetic flux has the potential to differentiate eruptive from confined flares.