Relative Importance of Thermosphere/Ionosphere in Magnetospheric Eletrodynamics at Earth, Jupiter, and Saturn

Abstract:

A planet's thermosphere affects the electrodynamics of its magnetosphere through plasma-neutral collisions in the ionosphere, which tend to make ionospheric plasma move with the thermosphere while magnetic forces tend to make it move with the magnetosphere. The resulting deformations of the magnetic field as well as modifications of plasma flow in magnetospheres of different planets can be compared in terms of dimensionless parameters. One of the parameters (not often considered) is the ratio of electron-neutral collision frequency to electron gyrofrequency at the bottom of the ionosphere: if its value is >>1, thermospheric flow could be directly transmitted to the magnetosphere, and the fundamental decoupling of magnetosphere motions from planet/atmosphere motions first proposed by Gold (1959) would no longer apply. The increasingly popular interpretation of Saturn's puzzling magnetospheric periodicities as the result of some type of neutral-wind dynamo raises the question: is it a phenomenon unique to Saturn, or could something similar be occurring (albeit masked by a much more obvious rotational modulation of a tilted dipole) also at other planets? A first look at the relevant dimensionless parameters suggests that, for the three planets considered, neutral-wind dynamo effects in the magnetosphere should be most significant at Earth, less so at Saturn, and least of all at Jupiter.