Plasma flow around the magnetosphere

A review is provided of the salient features of the flow of solar plasma past the magnetosphere, as they are revealed by observations in space and by theory. Discussed first are the properties of the solar wind, including the velocity, density, temperature and temperature anisotropies, and magnetic field, and their variations in space and time as observed beyond the disturbing influence of the earth and its magnetic field. Theoretical aspects of the steady‐state interaction of the solar wind and the geomagnetic field are taken up next, using the continuum description provided by the equations of magnetohydrodynamics of a perfect dissipationless gas as a basis. An outline is given of the series of approximations, and their observational and theoretical justifications, used to reduce the complex free‐boundary magnetohydrodynamic flow problem to a series of tractable problems; and a set of results is provided for representative conditions in the solar wind. The results are shown to be in good accord, even to numerous details, with observations made in space during quiet times. The results are also shown to be useful in interpreting many features of observations during disturbed times. Attention is drawn to other features of the flow of solar plasma past the magnetosphere for which quantitative theoretical descriptions must still be devised. These include the irregular or fluctuating character of the flow downstream of the bow wave, the nature of the magnetosphere tail and surrounding flow at great distances from the earth, numerous transient aspects of the flow, and the mechanisms underlying the many statistical correlations being discovered between variations of the surface geomagnetic field and various properties of the solar wind.