Energy release and particle acceleration in solar flares with respect to flaring magnetic loops

We study the spatial evolution of hard X-ray double footpoint sources in 14 intense impulsive flares observed with Hard X-ray Telescope (HXT) aboard Yohkoh. In 7 out of the 14 events, the separation between the double footpoints increased (at the 3 sigma confidence level) during the impulsive phase, with a typical separation velocity, v(sep) similar to 50 km/s (corrected for projection effects on the solar surface). The remaining 7 flares, however, do not show significant increase in separation; some of them even showed decreasing footpoint separation (v(sep) < 0). The magnetic field configuration inferred from the former class of events (i.e., with v(sep) > 0) suggests a "loop-with-a-cusp" picture where magnetic reconnection takes place at a magnetic X- (or Y-) point above a closed loop. The latter class of events (with v(sep) less than or equal to 0) may imply an "emerging-flux-type" field configuration. We also find a significant difference between these two classes of flares. Events with separating footpoints show a hard X-ray source in the L-band of HXT (14 - 23 keV) located in between the footpoint sources. This source most likely originates from a super-hot plasma at the top of the flaring loop, producing spectral enhancement below 20 - 30 keV. In contrast to this, events with non-separating footpoints show double sources in the L-band, without the loop-top source. We conjecture that (1) two kinds of magnetic field configuration, one is "loop-with-a-cusp" and the other is "emerging-flux-type", are responsible for solar flare energy release via magnetic reconnection, and that (2) superhot plasma is produced at the loop top during the impulsive phase only in flares with. the loop-with-a-cusp field configuration. Little such plasma is created in flares with the emerging-flux-type configuration, resulting in a (spatially averaged) hard X-ray spectrum which breaks down towards higher energies at around 20 - 30 keV, so that (3) this spectrum may reflect purely non-thermal emission from the footpoints of flaring magnetic loop(s).