Deformation of a magnetic dipole field by trapped particles

Abstract

A model for the stormtime ring current, based on a self-consistent steady state solution of Vlasov’s and Maxwell’s equations for a finite pressure plasma immersed in the earth's dipole field, is described. The particular choice of the particle distribution function is justified by the good fit to OGO 3 particle energy density and the Explorer 26 magnetic field measurements made during moderate geomagnetic storms. The model is then used to calculate a set of self-consistent equilibrium configurations that correspond to increasing dipole moments of the ring current. These calculations prove the existence of equilibrium solutions that contain a neutral point and regions in which the particle energy density exceeds the undisturbed magnetic field energy density for a factor of up to 27. For a given shape of the particle distribution function in field line space, we show that the particle energy density, the total confined particle energy, and the disturbance field on the earth's surface are limited. For a particle distribution centered around the field line with the shell coordinate L = 3.3 in the undisturbed state, these limiting values were found to be 1.55×10−6 ergs/cm³, 5.2×1022 ergs, and −230 γ, respectively, where the dependence of these results on the shell coordinate of the undisturbed center field line follows simple scaling laws.