Termination shock as a source of unusual solar radio bursts

TL;DR

This paper investigates unusual solar radio bursts linked to termination shocks in the solar corona, showing how these shocks generate energetic electrons and diverse radio emissions during solar flares.

Contribution

It provides evidence for the formation of quasi standing fast mode shocks (termination shocks) as sources of unusual radio bursts and related energetic phenomena in solar flares.

Findings

Termination shocks can generate energetic electrons responsible for hard X-ray radiation.

Unusual radio bursts are associated with shocks above post flare loops or erupting prominences.

Critical Mach numbers around 1.3 are sufficient for these shocks to occur in solar flares.

Abstract

Using centimeter wave and decimeter wave solar radio spectral observations of the flares of November 18, 2003 and September 12, 2004, 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 formation of quasi standing fast mode shocks (termination shocks, TS). TS 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 (CME, upper TS). Estimations of the critical Mach numbers for the ordinary plasma parameters in the solar flares give the values 1.3 easily realized in the flare events. The conditions necessary for generation of unusual radio bursts are likely to occur in the helmet shaped magnetic structures in the solar corona.