(7)
4.3 Plasma sheet morphology
In the run shown in Figures 3 - 7 we saw both an earthward acceleration of plasma and a compression of plasma. This seems consistent with the report of Fairfield et al. [1981] who observed that southward IMF, i.e. convection, enhances accumulation of energy in a magnetotail reservoir and plasma sheet thinning. The thinning was also seen in the Darwin particle code simulations of Pritchett and Coroniti [1995] and the early magnetoinductive particle code simulations of Swift [1983] The thinning went on in spite of the fact that the earthward-directed pressure gradient was not sufficient to balance the field line tension. Swift [1983] suggested a simple model for the thinning, which may be viewed as a process of field line stretching. When a convection electric field is present plasma tends to drift at the E´ B rate. In the lobe this is a slow drift toward the equatorial plane. When the plasma encounters the neutral sheet, the plasma picks up speed and is diverted earthward because the magnetic field becomes weaker and oriented in a north-south direction. The plasma gains momentum that has to be absorbed by the magnetic field. Swift [1983] showed that field line stretching would occur if the convection electric field in the neutral sheet exceeds the critical value
(7)
where n is the plasma density in the lobe, and Bx and Bz are the asymptotic lobe field components, If E is less than Ec the tail field will collapse. In either event there will be an acceleration of plasma earthward.
In the case shown in Figure 13 in which there was no applied convection electric field, the field tended to collapse. The collapsing field gave rise to an inductive electric field which, at least to the plasma earthward of x = -25 RE, appeared to fill the same role as an applied convection electric field and resulted in the acceleration of that plasma earthward. Eventually, the field would collapse to the point where the field line tension can be sustained by the earthward pressure gradient in the remaining plasma. However because of the lower pressures and the diverging flow, we expect the field would collapse to a nearly dipole like configuration, leaving a very extended and very neutral sheet. In the absence of reconnection, this remnant neutral sheet would persist for a long time, because in the absence of a Bz, there would be no field line tension to draw the remaining plasma earthward. Equation (7) also suggests the existence of an equilibrium with both Bz and E = 0. In a full three-dimensional model, however, the situation would likely be different because there would be an exchange of plasma with the magnetosheath on the dawn and dusk flanks.
In the thick plasma sheet run shown in Figures 15 and 16 there was considerably slower earthward flow than in the run shown in Figures 2 – 7, but the field-aligned currents linking to the ionosphere were not much different. This suggests the field-aligned currents are easily generated. It also suggests that if the filamentary currents are generated from the earthward flow, there is a saturation effect whereby increased earthward flow produces only a marginal increase in current intensity to the ionosphere.