Planetary Magnetospheres

Abstract

This is a brief, equation-free introduction to the physics of planetary magnetospheres, emphasizing their relationship to the heliosphere at large, over the life of our solar system. It will focus on unifying principles, rather than providing a taxonomy of the many different magnetospheres in our own solar system. A planetary magnetosphere is a closed cell of plasma bound together by a permeating magnetic field, which may originate from any combination of internal or external currents. The essential features are thus an ionized atmosphere (ionosphere or plasmasphere), created by intrinsic and/or extrinsic sources of energy, and a magnetic field. At planetary scales, parcels of plasma sharing a common magnetic flux tube must move together as a unit distributed along that flux tube. The magnetic field thus plays the role of a connective tissue threading plasmas and binding them together via Maxwell stresses analogous to those of surface tension, but distributed throughout the volume. More powerfully than surface tension acts to confine water in droplets, magnetic fields confine plasmas in magnetic cells, or magnetospheres. Rotation and relative motion are important factors in the character of magnetospheres, while magnetic linkages within and between them exert strong control over their interactions via the process known as reconnexion, which acts as a plasma pump and energizer.