The solar wind is the extension of the solar corona to very large heliocentric distances. As we shall see later in this chapter the solar wind exists because of the huge pressure difference between the hot plasma at the base of the corona and the interstellar medium.

The existence of a continuous solar wind was first suggested by Ludwig Biermann based on his studies of the acceleration of plasma structures in comet tails. The detailed mathematical theory of the solar wind was put forward by Eugene Parker. The solar wind was first sporadically detected by the Soviet space probes Lunik 2 and 3, but the first continuous observation of the solar wind was made with the Mariner 2 spacecraft.

In this chapter we shall describe the “classic” theory of the solar wind, which is based on the fluid approximation of coronal and interplanetary plasmas.

Hydrostatic Equilibrium — It Does Not Work

The simplest theoretical description of the solar corona is based on the assumption of a spherically symmetric, steady-state hydrostatic corona. Single-fluid equations can be applied since the gas is assumed to be a fully ionized, quasineutral proton—electron plasma. The effects of magnetic field and heat conduction are neglected in this simple approximation.