Suppression of energetic electron transport by double layers in flares

The transport of electrons from a coronal acceleration site to the chromosphere is a key issue in understanding the dynamics of flares. Recent observations from RHESSI revealed coronal hard X-ray emissions came from an above-the-loop-top source, which was first discovered in the Masuda flare in 1994. The above-the-loop-top source is suggested to be the acceleration site of the X-ray-producing electrons. The physics of how these energetic electrons transport from high corona to the chromosphere remains poorly understood. We are exploring the problem of electron transport in this context using particle-in-cell simulations. We set up an initial system of very hot electrons in contact with cold electrons along the local magnetic field, and let it evolve over time. After a short phase of diffusion, a large-amplitude, localized electrostatic electric field (in the form of a classic double layer) emerges from ion-electron streaming instability at the the boundary between the hot and cold electrons. The double layer then suppresses the hot electron transport into the cold region. It grows over time and therefore enhances the suppression. The barrier supports a significant drop in temperature and hence heat flux between the two regions that is sustained for the duration of the simulation. The dynamics of the double layer and the associated transport suppression are being investigated.