HXT Observations of Solar Flares — A Review and Perspective —

We present hard X-ray imaging observations of impulsive solar flares made with the Hard X-ray Telescope (HXT) aboard the Yohkoh satellite, and emphasise the magnetic field structure and its evolution during flares.The HXT has revealed that the bulk of hard X-rays above 30 keV is emitted from the footpoints of a flaring magnetic loop by electrons accelerated near the loop top.This implies that the evolution of hard X-ray footpoints reflects that of magnetic field structure responsible for the energy release, through which we can infer the magnetic field structure itself. We review progress so far made along these lines, including the relationship between magnetic field structure and thermal/non-thermal aspects of impulsive flares, followed by discussion of the (three-dimensional) magnetic field structure inferred from motions of individual footpoints.We also discuss, from a viewpoint of magnetic field structure, near-future perspective of hard X-ray solar physics.