Abstract
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. HXR RHESSI data on loop dimensions and nonthermal electron parameters and GOES soft X-ray data on hot loop plasma parameters are used to model coronal thick-target physics for two "discovery" 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.
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