A Coronal Thick-Target Interpretation of Two Hard X-Ray Loop Events

Abstract

We report a new class of solar flare hard X-ray (HXR) sources in whichthe emission is mainly in a coronal loop so dense as to be collisionallythick at electron energies up to >~50 keV. In most of the eventspreviously reported, most of the emission is at the dense loopfootpoints, although sometimes with a faint high-altitude component. HXRRHESSI data on loop dimensions and nonthermal electron parameters andGOES soft X-ray data on hot loop plasma parameters are used to modelcoronal thick-target physics for two ``discovery'' events (2002 April 14[23:56 UT] and 2002 April 15 [23:05 UT]). We show that loop columndensities N are consistent with (1) a nonthermal coronal thick-targetinterpretation of the HXR image and spectrum; (2) chromosphericevaporation by thermal conduction from the hot loop rather than byelectron beam heating; and (3) the hot loop temperature being due to abalance of thick-target collisional heating and (mainly) conductivecooling.