Collisionless Reconnection and High‐Energy Particle Acceleration in Solar Flares

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Abstract
Observations with the Hard X-Ray Telescope (HXT) and the Soft X-Ray Telescope (SXT) on board Yohkoh show that the reconnection process is common to impulsive and gradual flares. We apply the collisionless reconnection theory—more exactly, the model of a high-temperature turbulent-current sheet (HTTCS)—to the coronal conditions derived from the Yohkoh data on the site and mechanism of magnetic energy transformation into kinetic and thermal energies of "superhot" plasma and accelerated particles. We consider the reconnecting current sheet as the source of flare energy and the first-step mechanism in a two-step acceleration of electrons and ions to high energies. According to our model, reconnected field lines rapidly move out of the HTTCS, being frozen into superhot plasma, and form magnetic loops on the upstream side of a fast oblique collisionless shock (FOCS) situated above the soft X-ray-emitting loops of a strong magnetic field. The electrons and ions energized and preaccelerated by the HTTCS are trapped in magnetic loops. The top of each loop moves with a high speed toward the FOCS, while its feet penetrate through the shock front. For these reasons, two mechanisms—the adiabatic heating inside the collapsing trap and acceleration by the shock front at the two feet of the trap—efficiently increase the particle energy. The lifetime of an individual collapsing trap can be identified with the observed few-second delay to higher energies of hard X-ray and gamma-ray emission. The trap of accelerated electrons can be seen as the coronal hard X-ray "above-the-loop-top source." Precipitation of accelerated electrons from the trap through the FOCS into the chromosphere is responsible for the hard X-ray "footpoint sources." The model explains timing, location, and motion of the hard X-ray sources in solar flares as well as the observed relative intensity of the coronal and chromospheric hard X-ray sources and other physical properties.