Interaction of fast steady flow with slow transient flow: A new cause of shock pair and interplanetary Bz event

Abstract

The occurrence of the nonspiral magnetic field, high helium density, “cold magnetic enhancement” and counterstreaming suprathermal electron flux in the slow flow around a forward shock indicates that the slow flow is the interplanetary counterpart of the coronal mass ejection (ICME). The characteristics of the field and plasma in the fast flow around the reverse shock are typical for a fast steady flow. Thus the shock pair here appears to be caused by interaction of a fast steady flow with a slow ICME. The fact that the slow ICME possesses a planar magnetic structure and a large −Bz component suggests that the slow ICME may be disconnected from the Sun. It is shown that compression alone appears to be adequate to explain the large southward interplanetary magnetic field component within the shocked slow plasma because of the large southward field component present in the ICME ahead of the forward shock. In addition, a new method to infer the shock angle and Mach number from the observed upstream plasma β and the jump ratios of proton density and total magnetic flux density across a shock is suggested.