Phenomenology and physics of magnetospheric substorms

Abstract

This paper provides a phenomenological and theoretical framework for understanding magnetospheric substorms based on the boundary layer dynamics model developed by Rostoker and Eastman [1987] updated to be consistent with modern observations. The model is designed to account for both the directly driven and storage‐release aspects of substorm activity. The essence of the model is that enhanced frontside reconnection leads to both growth of the directly driven electrojets and storage of energy in the magnetic field and particle drift in the tail. The growth of particularly intense cross‐tail current near the inner edge of the plasma sheet is terminated by an abrupt collapse marking the onset of the expansive phase of the substorm. This collapse normally takes place in the midnight sector inside ∼ −12 RE. The collapse of this cross‐tail current is consistent with the breakdown of shielding associated with region 2 field‐aligned currents. The expansive phase proceeds through antiearthward propagation in the tail of a wave triggered by the collapse. When the wave reaches the neutral line in the tail, it enhances the reconnection rate leading to stronger velocity shear along the interface between the low latitude boundary layer and central plasma sheet. This velocity shear is responsible for activity at the high latitude edge of the expanded auroral oval. The physics of the processes characterizing the model requires a reexamination of the Harang discontinuity as it would appear that there are actually two regions of electric field reversal in the nightside magnetosphere both of which play important roles in the substorm process.