Non-Thermal processes and Superhot Plasma Creation in Solar Flares

Abstract

We review new aspects of solar flare energy release and related non-thermal/thermal processes revealed by the Hard X-ray Telescope (HXT) aboard Yohkoh. Hard X-ray imagery of impulsive solar flares with HXT has shown that the bulk of hard X-rays above 30 keV is emitted near the footpoints of a flaring magnetic loop by electrons accelerated near the top of the loop, manifesting itself most frequently as “double sources” located on both sides of a magnetic neutral line. Besides these “double footpoint sources”, a third hard X-ray source, located above the top of the flaring loop visible in soft X-rays, was discovered in some impulsive flares near the solar limb. This discovery provides us with clear evidence that solar flare energy release takes place outside the closed loop, most likely via mignetic reconnection in an open magnetic field configuration. Observations of superhot plasma creation with HXT are also briefly reviewed and their implication in the energy release and transfer in solar flares is discussed.