Some characteristics of propagation of flare- or CME-associated interplanetary shock waves

Abstract

Many interplanetary shock waves have a fast mode MHD wave Mach number between one and two and the ambient solar wind plasma and magnetic field are known to fluctuate. Therefore a weak, fast, MHD interplanetary shock wave propagating into a fluctuating solar wind region or into a solar wind stream will be expected to vary its strength. It is possible that an interplanetary shock wave, upon entering such a region will weaken its strength and degenerate into a fast-mode MHD wave. It is even possible that the shock may dissipate and disappear. A model for the propagation of a solar flare - or CME (Coronal Mass Ejections) - associated interplanetary shock wave is given. A physical mechanism is described to calculate the probability that a weak shock which enters a turbulent solar wind region will degenerate into a MHD wave. That is, the shock would disappear as an entropy-generate entity. This model also suggests that most interplanetary shock waves cannot propagate continuously with a smooth shock surface. It is suggested that the surface of an interplanetary shock will be highly distorted and that parts of the shock surface can degenerate into MHD waves or even disappear during its global propagation through interplanetary space. A few observations to support this model will be briefly described. Finally, this model of shock propagation also applies to corotating shocks. As corotating shocks propagate into fluctuating ambient solar wind regions, shocks may degenerate into waves or disappear.