Role of the solar wind in the structure and dynamics of magnetospheres

Abstract

How the solar wind compresses the magnetic field of a planet to form the dayside magnetosphere is well understood, on a quantitative first-principles level. How the magnetotail on the nightside is formed remains, by contrast, a topic of research and controversy. The configuration of the magnetotail implies an antisunward force on the planet: a continuous supply of antisunward linear momentum, which must be extracted ultimately from the solar wind by some process, most commonly assumed to be the tension of open magnetic field lines extending from the planet into the interplanetary medium. Formation of the magnetotail as the result of magnetic reconnection initially on the dayside and then on the nightside is understood qualitatively; for the basic quantitative parameters of the resulting configuration, however, there are numerous empirical determinations but no generally accepted theoretical estimates. Time variability of reconnection and of magnetotail properties, together with their dependence on solar-wind parameters and their coupling to the ionosphere and atmosphere, are essential ingredients of substorms, magnetic storms, and space weather events at Earth, as well as of analogous (or at least similar) phenomena in the magnetospheres of other planets. At Jupiter and Saturn, the additional effects of rapid rotation and of plasma source from the moons raise the question of how important their roles are in comparison to that of the solar wind.