Evolution of Coronal and Interplanetary Shock Waves Inferred from a Radio Burst

Abstract

Studying the evolution of the source of a radio burst, which is recognized as a shock wave, is important for understanding its generation mechanism and predicting its hazards. Estimating the kinematics of radio-burst sources using electron-density models is not easy. In this article, the kinematics of the Type-II radio-burst source is estimated without using electron-density models by studying the density variation along the leading surface of the coronal mass ejections (CMEs) (hereafter ejecta) during Type-II radio-burst emission. This technique is valid for analyzing the Type-II radio-burst spectrum in metric and DH ranges, from which we can infer ejecta propagation from the corona into interplanetary space. It is found that the Type-II radio burst can be described by the Sedov–Taylor blast-wave equation by matching the calculated theoretical frequencies with that observed by the RAD1 and RAD2 receivers. The theoretical model showed a good fit with the observed spectra of Type-II radio bursts of different Type-II events. The analysis was consistent with the previous work regarding the conditions of the Sedov–Taylor equation and statistical studies of the density variation on the surface area of an interplanetary CME. The kinematics of a Type-II radio-burst source and the temporal variation of its energy are estimated during the Type-II radio-burst emission. The results of the two cases studied show that the energy of ejecta degraded by ≈ 14 % of its initial energy at the beginning of metric Type-II radio emission on 16 March 2016, while the energy of ejecta degraded by ≈ 86 % and ≈ 20 % for DH Type-II radio burst as recorded by RAD1 and RAD2 on 7 November 2004, respectively. The analysis shows that the radial speed of the blast wave is lower than its transversal speed along the surface of ejecta and extends to a small fraction of R⊙ from its source point on the ejecta. The magnetic-field strength of the ejecta and the ambient medium are estimated during the Type-II radio-burst emission. This study emphasizes that the emission of a blast wave from the reconnection sites within the ejecta is one of the processes that degrades the energy of ejecta during their propagation.