Examination of type II origin with SOHO/LASCO observations

Abstract

We examine a possibility that metric type II solar radio bursts are all caused by coronal mass ejection (CME) generated shocks. For this we consider 129 type II flare events from February 1997 to October 2000 and examine their associations with SOHO/LASCO CMEs according to their temporal and spatial closeness using SOHO/EIT and GOES data. We then carefully inspected 26 CME‐less events to examine if there are CME‐related features in LASCO and EIT images. In addition we examined 28 limb type II CME events to compare the kinematics of coronal shocks with those of the CME fronts. Under the assumption that the observed type IIs are all generated by CME‐related shocks, we determine the formation heights of the CME‐associated type IIs using LASCO CME speeds and type II onset times. From these studies, we have found (1) a large fraction (81%) of the type II bursts have temporal and spatial association with CMEs, and the association increases as their source position approaches to the limb; (2) most of the events without the association are related with weak flares and/or disk events; (3) most of the events are super‐Alfvenic with a mean speed of 900 km s −1 ; (4) the front heights of all CMEs except for a few events are in the range of 1 to 3 solar radii, which are consistent with the type II formation heights; (4) the onset time difference (CME‐type II) of all events are within about ±1 hour, mostly −30 min to 10 min; (5) the CME speeds have a possible correlation ( r = 0.6) with coronal shock speeds, when two outliers are excluded. Considering a possibility that some outliers could result from some effects such as the coronal shock generation at CME flanks and CME accelerations, our results show that most of the type II bursts can be explained by the CME origin.