X-ray Ejecta, White-Light CMEs and a Coronal Shock Wave

Abstract

We report on a coronal shock wave inferred from the metric type II burst of 13 January 1996. To identify the shock driver, we examined mass motions in the form of X-ray ejecta and white-light coronal mass ejections (CMEs). None of the ejections could be considered fast (> 400 km s−1) events. In white light, two CMEs occurred in quick succession, with the first one associated with X-ray ejecta near the solar surface. The second CME started at an unusually large height in the corona and carried a dark void in it. The first CME decelerated and stalled while the second one accelerated, both in the coronagraph field of view. We identify the X-ray ejecta to be the driver of the coronal shock inferred from metric type II burst. The shock speed reported in the Solar Geophysical Data (1000–2000 km s−1) seems to be extremely large compared to the speeds inferred from X-ray and white-light observations. We suggest that the MHD fast-mode speed in the inner corona could be low enough that the X-ray ejecta is supermagnetosonic and hence can drive a shock to produce the type II burst.