Impulsive energization of ions in the near-earth magnetotail during substorms

Abstract

At the onset of a magnetospheric substorm, the stretched, tail-like field lines in the near-Earth plasma sheet collapse rapidly in towards the Earth, and assume a more dipolar configuration. Low energy ions, whose initial speeds are comparable with, or less than the field collapse speed (hundreds of kilometres per second), are accelerated parallel to the magnetic field in the equatorial region during the collapse to ∼ keV energies, giving rise to the field-aligned “bouncing ion clusters” observed near geostationary orbit. It is shown that the acceleration of these particles can be readily understood, and estimated quantitatively for given model fields, in terms of particle motion in a one-dimensional field configuration, representing the local equatorial field. Estimates based on this simple theory are compared with the results of guiding-centre integrations of particle trajectories, and show good agreement. The theory shows that the low energy ions which cross the equatorial plane during the collapse are accelerated along the field to a speed which is comparable with the equatorial collapse speed of the field, a result which is independent of ion mass. A simple explanation is thus provided of the major features of recent numerical models of low energy ion acceleration during the substorm collapse.