Study of Particle Acceleration and Magnetic Field Structure in Solar Flares

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By studying temporal evolution of solar flare hard X-ray footpoint sources with the Hard X-ray Telescope (HXT) aboard Yohkoh satellite, we have investigated magnetic field structure associated with flares and particle acceleration process(es). Amongst flares observed, a large flare which commenced on 14 July, 2000 has provided a unique opportunity to study hard X-ray footpoint sources in great detail. Analysis of this flare revealed a striking result that non-thermal (above 30 keV) footpoint sources show a pair of thin elongated ribbons which reminds one the so-called "two-ribbon structure" in H-alpha observations. This suggests that magnetic arcade structure, not just a single loop, should take part in solar flare energy release. Detailed analysis on the footpoint sources has indicated that magnetic shear decreases as the flare progresses, and that the sources tend to show harder spectra towards outer edges. These observational findings are expected to provide important clues for investigating particle acceleration mechanisms in flares. In addition to analyses of hard X-ray footpoint sources, theoretical studies, stimulated with the analyses, have been in progress with an attempt to understand solar flare energy release in the framework of 3-dimensional magnetic reconnection.