MHD shocks and simple waves in CMES

Abstract

Previous studies have shown how the various types of MHD shocks (slow, intermediate, and fast may form near the leading edge of coronal mass ejections in relatively simple geometric (2-D, Cartesian) models. These models also neglect the important contribution of the outflow velocity and the current sheet since they do not incorporate a coronal streamer configuration in the initial state. It is shown that some of these previous results remain true for a model with a more realistic spherical geometry (2-D and 3-D) and when the CME propagates through a streamer. However, there are several important differences. Some are of a more detailed character such as the slow shock becoming a perpendicular shock at the center of the current sheet rather than a gas dynamic shock and being less noticeably concave upward. One very important difference is that (except for possibly the very fastest disturbances), the results indicate that fast MHD shocks probably do not form within the field of view of most coronagraphs. This is true despite the fact that the disturbance speed greatly exceeds the fast mode speed. This apparent anomaly occurs due to the formation of fast simple expansion waves ahead of the central part of the disturbance which modify the initial corona such that either a slow or intermediate shock forms. The leading fast simple waves become compressive at the flanks of the disturbance but have not steepened into shocks. The three-dimensional simulations also provide information on the global CME structure as well as the anticipated reduction in observed brightness as the CME occurs further from the limb. For the results given here the CME is shaped more like a radially expanding arcade than a bubble. The CME brightness is reduced by 30% as the CME moves from the limb to 45° away from it. The peak brightness in a polar view is about 65% less than the maximum in a meridional projection. These results are discussed in more detail in: R. S. Steinolfson, “The three-dimensional structure of coronal mass ejections,” J. Geophys. Res. (in press) 1992.