Termination Shock as a Source of Unusual Solar Radio Bursts

Abstract

Using centimeter-wave and decimeter-wave solar radio spectral observations of the flares of 2003 November 18 and 2004 September 12, we have discussed two type-II-like bursts at the meter waves. The radio bursts show that the ordinary frequency drift from high to low frequencies slows down and stops, and a frequency drift from low to high frequencies appears. An analysis of all data on the corresponding flares provides evidence of the formation of quasi-standing fast-mode shocks (termination shocks, TSs). TSs are able to generate energetic electrons, responsible for the appearance of new sources of hard X-ray radiation and generation of fast radio bursts (spikes), fibers, and zebra structures. The sources of the radio emission bands with the unusual frequency drift are situated above the top of the post-flare loops (lower TS) or are connected with the erupting prominence or coronal mass ejection (upper TS). Estimations of the critical Mach numbers for the ordinary plasma parameters in the solar flares give the values M cr = 1.1–1.3 easily realized in the flare events. The conditions necessary for the generation of unusual radio bursts are likely to occur in the helmet-shaped magnetic structures in the solar corona.

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.…”