Propagating slow-mode shocks discovered in dynamical solar flare loops

Solar flares are the largest energy releasing events in the solar system, where the open magnetic field lines reconnect and form the closed flare loops. During this process, rapid magnetic reconnection, the associated shock waves, and chromospheric evaporation are expected but not yet well understood. These processes are crucial for understanding similar features in stellar flares and other astrophysical jets. Here, we report the characteristics of propagating slow-mode shocks in the flare loop system, by combining a 3D high-resolution magnetohydrodynamics modeling and spectral analysis of Extreme Ultra-Violet observations. It is found that normal slow shocks are recurrently formed after the collision between the post-reconnection downflows and evaporation flows in the flare loops, which subsequently propagate toward the chromosphere at speeds comparable to the evaporation flows. In particular, the Doppler analysis of the Fe XXI 1354 Å line normally shows a sharp change in blueshifted velocity and an asymmetrical line broadening once the line-of-sight passes through the shock front. This study highlights that propagation of slow shocks can facilitate energy release in flares and affect energy transport, suggesting an advancement in the standard flare model framework. Slow shocks have been invoked to explain flare observations, but simulations suggest that they result in relatively small energy release in the post-loops. Here, the authors show evidence for the existence of propagating slow shocks, due to the downflow-evaporation collision, which contributes to efficient energy dissipation in the shrinking post-flare loops.