Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-Band Feature as Evidence for Shock Compression

TL;DR

This study provides observational evidence of a solar flare termination shock through radio spectroscopic imaging, identifying a split-band feature that reveals shock compression and Mach number, advancing understanding of particle acceleration in solar flares.

Contribution

First spatially, spectrally, and temporally resolved analysis of split-band features in radio emissions from a solar flare termination shock, linking observations to shock physics and MHD simulations.

Findings

Split-band radio features are nearly co-spatial, supporting shock interpretation.

The high-frequency lane is ~0.8 Mm below the low-frequency lane, indicating upstream-downstream regions.

Shock Mach number is inferred to be up to 2.0 from the density compression ratio.

Abstract

Solar flare termination shocks have been suggested as one of the promising drivers for particle acceleration in solar flares, yet observational evidence remains rare. By utilizing radio dynamic spectroscopic imaging of decimetric stochastic spike bursts in an eruptive flare, Chen et al. found that the bursts form a dynamic surface-like feature located at the ending points of fast plasma downflows above the looptop, interpreted as a flare termination shock. One piece of observational evidence that strongly supports the termination shock interpretation is the occasional split of the emission band into two finer lanes in frequency, similar to the split-band feature seen in fast-coronal-shock-driven type II radio bursts. Here we perform spatially, spectrally, and temporally resolved analysis of the split-band feature of the flare termination shock event. We find that the ensemble of the radio centroids from the two split-band lanes each outlines a nearly co-spatial surface. The high-frequency lane is located slightly below its low frequency counterpart by ~0.8 Mm, which strongly supports the shock upstream-downstream interpretation. Under this scenario, the density compression ratio across the shock front can be inferred from the frequency split, which implies a shock with a Mach number of up to 2.0. Further, the spatiotemporal evolution of the density compression along the shock front agrees favorably with results from magnetohydrodynamics simulations. We conclude that the detailed variations of the shock compression ratio may be due to the impact of dynamic plasma structures in the reconnection outflows, which results in distortion of the shock front.