Sources for energetic ions at the plasma sheet boundary: Time varying or steady state?

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Abstract
Analyses of energetic ions observed at the plasma sheet boundary usually have assumed or inferred the existence of either a time‐varying or a steady state source. Because these assumptions are both important and conflicting, we have investigated the uniqueness with which they can be applied to the energetic ion observations. We have applied the results of assuming either a time‐varying or a steady state source to analytic distribution function mappings and to a computer simulation of the ISEE 1 medium energy particles instrument (MEPI). The predicted MEPI ion signatures then are compared with observations. We find that in general it is not possible to determine the time‐varying or steady state nature of the source with single‐point observations of the ion distribution function. In specific instances, however, the time sequence of the ion distribution may allow such a determination. The likelihood of time variations in various parameters of the models (electric field, magnetic field, boundary velocity, etc.), plus the existence of complex boundary features (e.g., transverse and parallel waves), further complicates the issue, making it very difficult for even multipoint observations to determine the time‐varying or steady state nature of the source. Nevertheless, independent of these assumptions, energetic particle events at the plasma sheet boundary analyzed to date show that the observations are consistent with single‐particle motion from a source several tens of earth radii down the tail which is characterized by an E−γ spectrum. Whether the source is time varying or steady state cannot be uniquely inferred from the data.