Efficiency of electron acceleration by shock waves in the solar corona according to observational data on the fine structure of type II radio bursts

Abstract

Herringbone bursts (HB-bursts) are the type III-like fine structure in type II bursts of solar radio emission and are usually interpreted as plasma radiation arising from fast electrons accelerated by shock waves in the solar corona. In general outline, the radiation mechanism of HB-bursts is similar to that of type III bursts. However, HB-bursts have brightness temperatures that are about an order of magnitude higher than those of type III bursts. The frequency drift of BH-bursts is about two or three times lower than that for type III bursts. All this shows that the fast-electron beams responsible for HB-bursts and type III bursts differ markedly in their parameters. We calculated expected brightness temperatures of HB-bursts at the fundamental and second harmonic and compared our results with Culgoora radiometer and radioheliograph data and Tremsdorf spectrograph data in order to estimate parameters of fast-electron beams generating HB-bursts. We found that the observed brightness temperatures of HB-bursts give velocities of fast electrons accelerated by shock waves within the limits (0.02–0.17)c. These velocities are several times lower than those for type III bursts (0.15–0.5)c. The density of the fast electrons responsible for HB-bursts is in the interval 3·10−6 <nb/n<6·10−5, which exceeds by 1–2 orders of magnitude the relative densities in type III sources. This gives a clue to understanding the markedly higher brightness temperatures of HB-bursts compared to those of type III bursts. We concluded that the parameters obtained for the agent exciting HB-bursts favor of turbulence mechanisms of electron acceleration by shock waves in the solar corona.