Fermi Acceleration at Fast Shock in a Solar Flare and Impulsive Loop-top Hard X-ray Source

Fermi acceleration has not been considered to be viable to explain non-thermal electrons (20 100 KeV) produced in solar ares, because of its high injection energy. Here, we propose that non-thermal electrons are e ciently accelerated by rst-order Fermi process at the fast shock, as a natural consequence of the new magnetohydrodynamic picture of the aring region revealed with Yohkoh. An oblique fast shock is naturally formed below the reconnection site, and boosts the acceleration to signi cantly decrease the injection energy. The slow shocks attached to the reconnection X-point heat the plasma up to 10 20 MK, exceeding the injection energy. The combination of the oblique shock con guration and the pre-heating by the slow shock allows bulk electron acceleration from the thermal pool. The accelerated electrons are trapped between the two slow shocks due to the magnetic mirror downstream of the fast shock, thus explaining the impulsive loop-top hard X-ray source discovered with Yohkoh. Acceleration time scale is 0.3{0.6 s, which is consistent with the time scale of impulsive bursts. When these electrons stream away from the region enclosed by the fast shock and the slow shocks, they are released toward the footpoints and may form the simultaneous double-source hard X-ray structure at the footpoints of the reconnected eld lines. Subject headings: MHD { Sun: X-rays, gamma rays { Sun: ares