Coronal Trapping of Energetic Flare Particles:Yohkoh/HXT Observations

Abstract

We examine spectroscopic data from the Yohkoh Hard X-Ray Telescope in a search for spectral evidence of the coronal trapping of energetic particles during solar flares. Two distinct particle populations with significantly different spectral properties are found to be present in three of the six flares studied; the first population is trapped in the corona, where it encounters a "thick-thin" target, while the second population precipitates directly to the footpoints. In the remaining three events, a single population of energetic particles appear to be responsible for the observed hard X-ray emission, either via thermal bremsstrahlung (one case) or nonthermal thin-target emission (two cases). For the three events in which a trapped population is observed, the spectroscopic observations imply first that there is likely to be a single acceleration mechanism for both the trapped and the precipitating populations and second that the magnetic field geometry in these flares is conducive to trapping in a confined region high in the corona, above the soft X-ray loops. Both conditions are consistent with magnetic reconnection models of flares in which energetic particles are trapped between MHD slow-mode shocks attached to the reconnection region and a fast-mode shock formed by the reconnection outflow jet.