ON THE THERMODYNAMICS OF THE PLASMA SHEET

The evidence for nonadiabatic transport in the quiet central plasma sheet is reinvestigated. In this study, we include AMPTE IRM data from the plasma sheet boundary layer and active times selected on the basis of large AE values. We find that as the plasma is transported from the plasma sheet boundary layer into the central plasma sheet, both its temperature (T) and its density (n) increase. The plasma obeys a relation p is-proportional-to n4/3 for quiet times (AE < 100 nT) and p is-proportional-to n5/3 for AE > 300 nT. The remarkable anticorrelation between plasma temperature (T = p/n) and density in the quiet central plasma sheet reported previously is also evident in this data set, but this may not be indicative of nonadiabatic transport. It may simply be related to the fact that the criterion used for deciding whether the spacecraft is in the central plasma sheet depends on the pressure itself or the thermal ion flux which is proportional to nT1/2. Thus low-density and low-temperature observations are assumed to indicate that the spacecraft is in the boundary layer. The data also indicate that the temperature in the quiet plasma sheet is usually less than 6 keV, and high-temperature values are more likely to be observed in what is defined as the active plasma sheet. The observations also suggest that the plasma sheet contains a mixture of high-entropy "bubbles" and low-entropy "blobs." It appears that these either merge or are lost from the tail before they are convected into the near-Earth tail.