Introduction

Abstract

Abstract This book is concerned with the magnetohydrodynamic processes in the interplanetary medium, the region near the solar equatorial plane extending from approximately 0.5 AU to 40 AU. The interplanetary medium lies between the sun and the interstellar medium. The observations that are discussed in this book were made by instruments on spacecraft moving through the interplanetary medium. Thus, our view of the interplanetary medium based on these observations is that of one who is immersed in the medium that he seeks to understand. There is another view of the interplanetary medium, the view that would be seen by an observer standing far beyond the interplanetary medium, say two or three hundred astronomical units from the sun. This section provides a brief overview of the interplanetary medium from the latter point of view. The sun and the interstellar medium compete for control of the interplanetary medium. The sun that we see during the day has a sharp boundary, the photosphere, having a temperature of 4000 K. However, anyone who has seen a solar eclipse knows that the sun reaches out to at least 10 solar radii. The solar “corona” is hot, of the order of a million degrees (van de Hulst, 1953; Billings, 1959). One of the great unanswered questions is how the sun produces such a hot corona (Parker, 1961; Scudder, 1992). The interstellar medium is relatively cool, the temperature being only of 10,000 K. The matter in the corona is fully ionized, consisting primarily of protons and electrons. The local interstellar medium is only weakly ionized, consisting primarily of neutral hydrogen. The density of the solar corona is high, of the order of 108 cm-3• The density of the interstellar medium is low, of the order of 0.1 cm-3• The magnetic field in the corona is approximately 105 nT, while that in the interstellar medium is perhaps 0.5 nT.