Authors: Jeongbhin Seo (Los Alamos National Laboratory), Fan Guo (Los Alamos National Laboratory), Xiaocan Li (Los Alamos National Laboratory), Bin Chen (New Jersey Institute of Technology), ChengcaiShen (Harvard-Smithsonian Center for Astrophysics), and Hui Li (Los Alamos National Laboratory)

Solar flares are among the most dynamic phenomena in the solar system, releasing substantial magnetic energy and accelerating electrons to energies ranging from a few keV to several tens of MeV. Observations in hard X-rays and microwaves clearly indicate emissions from nonthermal electrons. Notably, in some events, the above-the-looptop region has been reported to host a significant population of nonthermal electrons in both number and energy. For the first time, we employ a novel numerical approach that couples magnetohydrodynamics with energetic particles, incorporating feedback from nonthermal electrons, to study their energization and transport in solar flares. Our findings reveal that nonthermal electrons are accelerated along the current sheet and even more efficiently at the termination shock. As a result, electrons in the above-the-looptop region can reach energies exceeding ∼100 keV. In our fiducial model, the energy distribution of nonthermal electrons in the above-the-looptop region follows a power-law spectrum which becomes steeper closer to the flare arcade. Interestingly, we find that increasing the injection rate of nonthermal electrons results in a steeper energy spectrum. Moreover, the energy density of nonthermal electrons exhibits oscillations due to the periodic collision of magnetic islands with the above-the-looptop region, which may help explain the quasi-periodic pulsations observed in flare emissions. Our simulations offer new insights into the generation of nonthermal electrons and the associated emissions originating from the above-the-looptop region.