X-ray plasma ejections associated with coronal type II shocks

Abstract

Context. Recent observations suggest that X-ray plasma ejections can drive coronal shocks and metric type IIs that are also generated in close association with the X-ray erupting features. Aims. The physical relationship between the plasma ejections and metric type II radio bursts are studied by analyzing the characteristics of ejecta and type IIs. Methods. We present the first comprehensive analysis of a set of 18 events of X-ray plasma ejections associated with coronal shocks inferred from metric type II radio bursts. For this study, we have utilized a list of 137 limb X-ray plasma ejection events and multi-wavelength observations from GOES X-ray, Yohkoh SXT, SOHO/LASCO, and SOHO/EIT. Results. (i) type IIs are reported only for about 15% of the 137 limb ejections; (ii) there exists a close temporal relationship among the starting time of type IIs, the hard X-ray flare peak, and the ejecta time; (iii) there exist negative correlations between X-ray loop length and starting frequency, and between the ejecta height and starting frequency of type IIs; (iv) the type II formation height computed using the starting frequency and 1 $\times$ Newkirk electron density model is in close association with or above the height of X-ray ejecta; and (v) while there is no correlation between the speeds of type II and ejecta, there seems to be a weak correlation between the speeds of type II and CME. Conclusions. Though the results suggest that some type IIs are generated in close association with the X-ray erupting features, it is not likely that X-ray plasma ejections are the main drivers of all coronal shocks and metric type II radio bursts due to the absence of correlation between both speeds and mostly sub-Alfenic speeds of the ejections.