COLLISIONLESS 3-DIMENSIONAL RECONNECTION IN IMPULSIVE SOLAR-FLARES

Two subclasses of impulsive solar flares, observed with the Hard X-Ray Telescope (HXT) onboard Yohkoh, have been discovered by Sakao et al. The two subclasses can be characterized as more impulsive (MI) and les s impulsive (LT) flares, the former having a shorter total duration of the impulsive phase in the hard X-ray emission than the latter. We as sume that in both subclasses, the collisionless three-dimensional reco nnection process occurs at the separator with a longitudinal magnetic field. The high-temperature turbulent-current sheet (HTTCS), located a long the separator, generates accelerated particles and fast outflows of ''superhot'' (T greater than or equal to 30 MK) plasma. Powerful an omalous heat-conductive fluxes along the reconnected field lines maint ain a high temperature in the superhot plasma. The difference between the LI and MI flares presumably appears because the footpoint separati on (the distance between two brightest hard X-ray sources) increases i n time in the LI flares, but decreases in the MI hares. According to o ur model, in the LI hares the three-dimensional reconnection process a ccompanies an increase in the longitudinal magnetic field at the separ ator. In contrast, in the MI hares the reconnection proceeds with a de crease of the longitudinal field; hence, the reconnection rate is high er in the MI flares. Since reconnection in the MI flares proceeds with a decrease of the longitudinal field, the reconnected field lines bec ome shorter in this process, As the reconnected lines become shorter, accelerated electron beams arrive at the upper chromosphere faster. So , in the MI hares chromospheric evaporation begins earlier than in the LI flares. The evaporation process driven by accelerated electron bea ms generates upflows of ''warm'' (T less than or equal to 10 MK) plasm a that interacts with downflows of superhot plasma and can switch off the accumulation of superhot plasma in the MI flares during the impuls ive phase. In the LI flares, however, an observable amount of superhot plasma is accumulated even during the impulsive phase. Moreover, sinc e the cooling timescales increase with the length of the reconnected f ield lines, our argument for the association of superhot plasma with l onger lines may remain valid to a reasonable extent even if the chromo spheric evaporated plasma mixes with the reconnected outflow and super hot temperatures are reached in this mixture. Further analysis of the Yohkoh data obtained simultaneously with the Hard and Soft X-Ray Teles copes and the bent crystal spectrometer (BCS) is necessary to distingu ish the superhot components of chromospheric and coronal origins in di fferent classes of flares as well as at different phases of their deve lopment.