Coronal thick-target hard X-ray Sources

A Veronig

John C. Brown

We report a new class of solar flare hard X-ray (HXR) sources in which the emission is mainly in a coronal loop so dense as to be collisionally thick at electron energies up to >50 keV. In most of the events previously reported, most of the emission is at the dense loop footpoints, although sometimes with a faint high-altitude component. For two events (2002 April 14 [23:56 UT] and 2002 April 15 [23:05 UT]), we show that loop column densities N are consistent with (1) a nonthermal coronal thick-target interpretation of the HXR image and spectrum (2) chromospheric evaporation by thermal conduction from the hot loop rather than by electron beam heating and (3) the hot loop temperature being due to a balance of thick-target collisional heating and (mainly) conductive cooling.