Solar radio bursts associated with standing shock waves (termination shocks)
Gennady Chernov, IZMIRAN (Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio wave propagation of Russian Academy of Sciences)
The standard model of the solar flare in the region of magnetic reconnection contains fast shock fronts outgoing upward and downward from the current sheet. This conventional scheme is used to interpret many flare phenomena observed in extreme ultraviolet, hard X-ray and radio emission. To explain the numerous data SDO and RHESSI, as well as radio bursts (including various fine structures of radio emission), the appearance of accelerated particles is often associated with acceleration on the shock front. In recent years, this acceleration has been attributed to a standing shock wave (termination shock), and in a number of works (Aurass, Mann, Warmuth (2009)), the slowing down of frequency drift of bursts similar to type II bursts (and even their reverse drift) is associated with this phenomenon. An additional confirmation of this mechanism is given in Chen, Bastian et al. (2015). At the same time, classical type II bursts always observed in large flares (with initial frequencies in the meter range) are usually not associated with these shock fronts. It is believed that they are caused by coronal matter emissions (CME), which excite piston shock waves. But in the decimeter and microwave ranges, there are sometimes unusual bands in the radiation (slowly drifting to both low and high frequencies), similar to short-term bursts of type II. The numerous examples of spectra of presented here such bursts give grounds for assuming their connection with termination shocks, moreover with both the lower and the upper outgoing fronts. The lower termination shock is explained by the collision of the shock front with a closed flare loop, and the upper termination shock can be caused by a collision of the upper front with the global loop located above (or with a magnetic cloud, a potential CME). Drift acceleration of particles in a termination shock should lead to the generation of fast radio bursts such as spikes and bursts in soft and hard X-rays. In all considered here events these bursts are revealed. We considered four events with radio bursts in the decimeter and microwave ranges, similar to meter type II bursts. Estimates of the critical Mach number Mcr for ordinary parameters of a flare plasma showed that the obtained values (Mcr = 1.2–1.4) can be easily realized in the events under consideration, and the radiation can be associated with the Buneman instability. Thus, we actually observed miniature type II bursts.