Interplanetary Slow Shocks

Abstract

Ever since the identification of the first interplanetary shock wave from the Mariner 2 plasma and magnetic field measurements [7.38], shock research has received great attention in solar system plasma physics, and this has resulted in an outstanding collaboration between laboratory and space experimentalists, theorists, and specialists in numerical simulation. This type of research, however, has been devoted, at least from an observational point of view, more or less exclusively to the investigation of fast-mode shock waves, whereas the corresponding study of slow-mode shocks has remained in a fairly primitive state. To some extent this is rather surprising, as MHD per se does not predict any major problems in generating slow-mode waves or shocks, in particular not in the solar wind, where, as we will show, ample “perturbations” are present for their generation. Yet, although long-standing records of solar wind measurements ranging from 0.3 AU out to several tens of AU are at our disposal, the number of observations of interplanetary slow-mode MHD turbulence or slow-mode shocks published over the last few years is remarkably small. In fact, there exists only one published example of low-frequency slow-mode MHD turbulence identified in the solar wind [7.31], and the number of published interplanetary slow shocks, regardless of the level of sophistication with respect to their unique identification, can still be counted on the fingers of one hand [7.9,28].