Termination Shock as a Source of Unusual Solar Radio Bursts

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 valuesM(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.