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

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.

太阳耀斑终止激波被认为是太阳耀斑中粒子加速的有希望的驱动因素之一，但观测证据仍然很少。通过对一次爆发耀斑中分米波段随机尖峰暴的射电动态光谱成像，陈等人发现这些暴形成了一个位于环顶上方快速等离子体向下流动端点的动态表面状特征，被解释为耀斑终止激波。一个有力支持终止激波解释的观测证据是发射带偶尔在频率上分裂成两个更细的条带，类似于快速日冕激波驱动的II型射电暴中看到的分裂带特征。在这里，我们对耀斑终止激波事件的分裂带特征进行空间、光谱和时间分辨分析。我们发现来自两个分裂带条带的射电质心集合各自勾勒出一个几乎共空间的表面。高频条带比低频条带略低约0.8 Mm，这有力地支持了激波上下游的解释。在这种情况下，激波前沿的密度压缩比可以从频率分裂中推断出来，这意味着激波的马赫数高达2.0。此外，沿激波前沿的密度压缩的时空演化与磁流体动力学模拟的结果吻合良好。我们得出结论，激波压缩比的详细变化可能是由于重联流出中动态等离子体结构的影响，这导致了激波前沿的扭曲。