Magnetospheres

Abstract

A planetary magnetic dipole acts as an obstacle to the supersonic flow of the solar wind. Upstream, on the bow of this obstacle, a shock wave develops; on the stern, a wake arises, which extends to a very large distance downstream. Solar wind particles can penetrate the magnetic dipole cavity and reach the ionosphere through the polar regions. This complex interaction is highly variable: there is a seasonal effect due to the changing angle between the magnetic axis and the solar wind; the interplanetary magnetic field may invert its direction, triggering reconnection of lines of force, which results in major magnetospheric perturbations; solar activity produces gusts in the wind. The study of planetary magnetospheres is mainly based on spacecraft measurements that use in situ instrumentation, radio propagation and active experiments which affect the state of the plasma. In this chapter, three topics are selected for more detailed discussion, with particular reference to the Earth: the motion of particles in an intense magnetic field in the guiding centre approximation and their trapping in the magnetosphere; the upstream bow shock, which deflects the solar wind on the sides; and Alfvén waves, a generalization of acoustic waves. The magnetospheres of other planetary bodies are reviewed in the last Section.