A Dynamical Model for the Onset of Magnetospheric Substorms

Abstract

A model for the onset of magnetospheric substorms is developed based on the observations of Pytte et a1 (1976). North-South harmonic motion across the plasma sheet becomes instrumental in coupling the plasma sheet to the tail lobes when the plasma sheet is sufficiently thinned. The number density times the square of the plasma sheet thickness is found to be the critical parameter (n ℓ 2≤ 8.1 x 1015 protons/cm). A merging theory is developed based on magnetic instabilities. The nightside merging rate is controlled by the value of the magnetic field in the tail lobes which, in turn, is dependent on the dayside merging rate. Merging and collisional damping of the J x B force contribute to the heating of the plasma sheet. The 10–15 minute interval between onsets is interpreted in terms of a weak coupling between the tail lobes and the plasma sheet. A straightforward application of small- amplitude oscillation theory shows that the period between onsets is a direct measure of the mean collision frequency in the plasma sheet. The obtained value is in good agreement with that calculated from the resistivity relation. An individual particle treatmenty in terms of analytic functions leads to similar conclusions regarding harmonic motion in the plasma sheet.KeywordsField LineCollision FrequencyPlasma SheetHarmonic ComponentHarmonic MotionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.