Acceleration Processes in the Plasma Sheet
The environment of the plasma sheet is described and the possible importance of noise and parallel electric fields near the outer boundary noted, but attention is concentrated on the neutral sheet, A significant feature in the plasma sheet is that the parallel pressure must exceed the perpendicular pressure.
The motion of particles in the neutral sheet is reviewed for models of increasing complexity. Cowley’s modification of Alfvén’s model raises the question of space charge in the middle of the sheet and this provides a natural explanation for field aligned currents, so it is fortunate that observations of these yield information about the neutral sheet. Recent observations suggest that polar wind plasma entering the neutral sheet from the lobes contributes only a small proportion of the plasma sheet population and recent work emphasises plasma entering from the magnetosheath. Acceleration in the neutral sheet leading to double streaming of electrons is likely to cause instability and hence anomalous resistivity, so that the system is explosive and may explain the sudden onset of substorms.
Away from the neutral line the normal component of magnetic field is important and drifts are discussed, but it is hard to decide whether a typical particle has come through the neutral sheet. Systematic variations in the particle population with position in the plasma sheet should be studied.
KeywordsAnisotropy Helium Explosive Resis Geophysics
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