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

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-upstreamdownstream 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.

A model of double coronal hard X-ray sources in solar flares

“…The flare TS was often invoked in the standard flare model to explain the looptop HXR source (Masuda et al 1994;Shibata et al 1995;Magara et al 1996), and both theoretical and numerical models showed that the TS can be a promising candidate of acceleration mechanism (Tsuneta & Naito 1998;Mann et al 2009;Warmuth et al 2009;Guo & Giacalone 2012;Kong et al 2013;Li et al 2013). The flare TS has long been predicted in numerical magnetohydrodynamic (MHD) simulations (e.g., Forbes 1986;Magara et al 1996;Yokoyama & Shibata 1998;Takasao et al 2015;Takasao & Shibata 2016;Shen et al 2018;Cai et al 2019;Ruan et al 2020;Wang et al 2021), and observational evidence for the existence of a TS in multiple wavelengths has also been reported (e.g., Masuda et al 1994;Aurass et al 2002;Aurass & Mann 2004;Mann et al 2009;Warmuth et al 2009;Chen et al 2015Chen et al , 2019Polito et al 2018;Cai et al 2019;Luo et al 2021).…”

Radio Spectral Imaging of an M8.4 Eruptive Solar Flare: Possible Evidence of a Termination Shock

“…Among others, shocks are believed to be an important cal experiments (Forbes & Malherbe 1986;Forbes 1986Forbes , 1988Seaton & Forbes 2009;Workman et al 2011;Chen et al 2015;Takasao et al 2015;Takasao & Shibata 2016;Takahashi et al 2017;Shen et al 2018;Cai et al 2019), and have been frequently depicted in schematics of the standard solar flare model (Masuda et al 1994;Shibata et al 1995;Magara et al 1996;Forbes & Acton 1996;Lin & Forbes 2000), observational evidence for these shocks remains relatively rare. There have been only a handful of reports on possible signatures of flare termination shocks at radio, ultraviolet, and X-ray wavelengths (Masuda et al 1994;Aurass et al 2002;Aurass & Mann 2004;Mann et al 2009;Warmuth et al 2009;Chen et al 2015Chen et al , 2019Guo et al 2017;Polito et al 2018;Cai et al 2019;Tan et al 2019;Fomichev & Chernov 2020). Here we refer interested readers to Chen et al (2019) and references therein for more in-depth discussions regarding observational signatures of flare termination shocks, as well as possible reasons for the scarcity of these observations.…”

“…There have been only a handful of reports on possible signatures of flare termination shocks at radio, ultraviolet, and X-ray wavelengths (Masuda et al 1994;Aurass et al 2002;Aurass & Mann 2004;Mann et al 2009;Warmuth et al 2009;Chen et al 2015Chen et al , 2019Guo et al 2017;Polito et al 2018;Cai et al 2019;Tan et al 2019;Fomichev & Chernov 2020). Here we refer interested readers to Chen et al (2019) and references therein for more in-depth discussions regarding observational signatures of flare termination shocks, as well as possible reasons for the scarcity of these observations.…”

Multiple electron acceleration instances during a series of solar microflares observed simultaneously at X-rays and microwaves

“…However, a number of authors found such emissions to originate from a different, secondary acceleration site (e.g. Battaglia & Benz 2009;Benz et al 2011;Carley et al 2016;Chen et al 2015Chen et al , 2019. Large solar flares usually have a long duration (up to several hours) and often display a complex morphology with multiple loop systems, hence it is not surprising that multiple acceleration sites are found.…”