Modeling the Propagation of Flare-Accelerated Electrons and Coupled Return Currents

In flares, electrons are accelerated out of the ambient atmosphere to high energies. These electrons heat the ambient plasma as they propagate through it. The location and intensity of radiation produced by the heated plasma depend upon the electron energy distribution, so this distribution is key to understanding the flare acceleration process as well as the thermal response of the atmosphere. Details of flare-accelerated electron spectra can be inferred from the X-ray bremsstrahlung produced as the electrons interact with the ambient plasma. Electrons moving in a loop produce an electric current that must be neutralized by a counter-streaming return current (RC). The RC produces a potential drop acting to decelerate beam electrons. This can be detected by a flattening in the X-ray spectrum at low energies. Therefore, techniques which invert X-ray spectra to obtain electron distributions must account for RCs. Here we present simulations of the transport of electrons through and corresponding heating of the ambient solar atmosphere specifically accounting for RCs. We demonstrate how the energy distribution of flare-accelerated electrons changes in its transport from the acceleration region to the footpoint. We use the state-of-the-art coupled codes, RADYN+FP, to model the electron beam transport and the radiative hydrodynamic response of the solar atmosphere to electron beam heating.