Fermi acceleration has not been considered to be viable to explain non-thermal electrons ( 20 \sim 100 KeV ) produced in solar flares , because of its high injection energy .
Here , we propose that non-thermal electrons are efficiently accelerated by first-order Fermi process at the fast shock , as a natural consequence of the new magnetohydrodynamic picture of the flaring region revealed with Yohkoh .
An oblique fast shock is naturally formed below the reconnection site , and boosts the acceleration to significantly decrease the injection energy .
The slow shocks attached to the reconnection X-point heat the plasma up to 10 \sim 20 MK , exceeding the injection energy .
The combination of the oblique shock configuration 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 \sim 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 field lines .