Magnetosphere of Jupiter

Abstract

THE rotation of Jupiter is very rapid (36°/h). If its magnetospheric plasma corotates with the planet, the effect of centrifugal force on the plasma distribution will be considerable. At 6 equatorial radii from the centre (which corresponds roughly to the orbit of the satellite Io) the centrifugal force on a corotating body is eighteen times that of gravity; the two forces are equal at 2.3 radii. Thus, we may expect that the corotating plasma trapped in the magnetic field will be thrown out to those parts of the lines of force which are the most remote from the axis of rotation. There is, however, a maximum plasma density which can corotate at a given distance from the axis. The magnetic field must be able to exert enough force on the plasma to provide the required centripetal acceleration. If the plasma density is too great, it will break away, carrying the magnetic field with it. We may estimate the maximum number density of the plasma in the equatorial plane by equating the magnetic and rotational kinetic energy densities B is the magnetic induction at radius r = Lr₀, where r₀ is the equatorial planetary radius and L will later be used to identify the magnetic field line on which this plasma lies. N_max is the maximum electron or ion number density which can corotate, m the mass of an ion, Ω the angular velocity and μ₀ the permeability of free space. Assuming the magnetic field to be that of a dipole aligned along the rotational axis, of induction B₀ at the equatorial planetary surface, we find This equation has been derived by a different method by Hines¹.