Boundary layer dynamics in the description of magnetospheric substorms

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Abstract
Magnetospheric substorm activity has been previously explained primarily as the effects of the episodic unloading of energy stored in the Earth's magnetotail. The physics of the unloading process has been associated with magnetic field line reconnection, with one dominant school of thought contending that reconnection occurs close to the Earth (i.e., inside of ∼15 RE). This near‐Earth neutral line model seeks to explain plasma and field characteristics observed in the magnetotail by the development and ultimate disappearance downtail of a plasmoid involving the population of the plasma sheet antiearthward of the near‐Earth neutral line. In this paper we present a study of a significant number of substorm expansive phase events where the phenomenology at ionospheric levels is clearly defined in terms of onset time and locale of the substorm disturbed region. Through the detailed presentation of a subset of these events it is demonstrated that the magnetotail particle and field observations associated with a substorm expansive phase can equally well be explained through the spatial movement of the boundary layers and central plasma sheet in the magnetotail. The sweeping of these regions past the satellite, even in the absence of temporal variations within the various regions, can lead to a set of plasma flow observations typical of what is observed in the magnetotail during substorm activity. We further demonstrate that filamentary currents in the plasma sheet boundary layer can produce the magnetic field signatures presently attributed to retreating plasmoids in the magnetotail. The major strength of this emphasis on boundary layer dynamics is that it provides a framework which naturally encompasses both the driven system and loading‐unloading system which are recognized to be operative simultaneously during magnetospheric substorm activity.