Electron Energy Partition in the Above-the-looptop Solar Hard X-ray Sources

Abstract:

Solar flares produce non-thermal electrons with energies up to tens of MeVs. To understand the origin of energetic electrons, coronal hard X-ray sources in particular ‘above-the-looptop’ sources have been studied extensively. However, it still remains unclear how energies are partitioned between thermal and non-thermal electrons within the above-the-looptop source. Here we show that the kappa distribution, when compared to conventional spectral models, can better characterize the above-the- looptop hard X-rays (>~15 keV) observed in four different cases. The combined thermal plus power-law distribution can also fit the data, but it returns unreasonable parameter values due to the artifact of its sharp, lower-energy cutoff E_c. In two cases with extreme-ultraviolet data from SDO/AIA, the analysis of differential emission measure (DEM) did not rule out the kappa distribution model. Based on the kappa distribution model, we found that the 2012 July 19 flare showed the largest non-thermal fraction of electron energies about 50%, suggesting equipartition. Considering results of particle-in-cell (PIC) simulations as well as density estimates of the four cases studied, we propose a scenario in which electron energization is achieved primarily by collisionless magnetic reconnection but the non-thermal tail can be suppressed or thermalized by Coulomb collisions.