Numerical Study on Excitation of Turbulence and Oscillation in Above-the-loop-top Region of a Solar Flare

Extreme ultraviolet imaging spectroscopic observations often show an increase in line width around the loop-top or above-loop-top (ALT) region of solar flares, suggestive of turbulence. In addition, recent spectroscopic observations found the oscillation in the Doppler velocity around the ALT region. We performed three-dimensional magnetohydrodynamic (MHD) simulations to investigate the dynamics in the ALT region, with a particular focus on the generation of turbulence and the excitation of the oscillatory motion. We found a rapid growth of MHD instabilities around the upper parts of the ALT region (arms of the magnetic tuning fork). The instabilities grow more rapidly than the magnetic Rayleigh-Taylor-type instabilities at the density interface beneath the reconnecting current sheet. Eventually, the ALT region is filled with turbulent flows. The arms of the magnetic tuning fork have bad-curvature and transonic flows. Therefore, we consider that the rapidly growing instabilities are combinations of pressure-driven and centrifugally driven Rayleigh-Taylor-type instabilities. Despite the presence of turbulent flows, the ALT region shows a coherent oscillation driven by the backflow of the reconnection jet. We examine the numerical results by re-analyzing the solar flare presented in Reeves et al. 2020. We find that the highest non-thermal velocity is always at the uppermost visible edge of the ALT region, where oscillations are present. This result is consistent with our models. We also argue that the turbulent magnetic field has a significant impact on the confinement of non-thermal electrons in the ALT region.
Comment: 21 pages, 20 figures, This article has been accepted for Astrophysical Journal