Energetic ion beams at the edge of the plasma sheet: ISEE 1 observations plus a simple explanatory model

The ISEE 1 satellite experienced numerous encounters with the plasma sheet on May 3, 1978, during the period 0400–2000 hours UT. ISEE 1 was in the dusk‐midnight quadrant of the tail and ranged in geocentric distance from 22 to 16 R E . We have examined the energetic ion (≥24 keV) three‐dimensional distributions during this period and find a multitude of energetic ion beams, generally observed within two gyroradii of the plasma sheet edge. Beam characteristics include E −γ spectra, single‐peaked energy spectra with peak energies dependent on time and pitch angle and ranging upwards of 100 keV, double‐peaked energy spectra having peaks at tens and hundreds of keV, streaming directions parallel and antiparallel to B , and streaming directions at arbitrary angles to B . The time evolution of these energetic ion energy and angular distributions indicates that the plasma sheet edge is dynamic and not an equilibrium situation. From a close study of the data we conclude that all the observed energetic ion beam characteristics are explained by single‐particle motion along B under conservation of the first adiabatic invariant µ = E ⊥ / B . Thus, for example, tailward streaming beams are simply the result of mirror point reflection of an earthward moving ion population. Round‐trip distances of 35–40 R E are observed, implying unimpeded travel to low‐altitude mirror points and back. Energetic ion beams at arbitrary angles to B are simply azimuthal asymmetries around B due to the sharp energetic ion density gradient at the plasma sheet edge. A sporadic source of the general form of a Maxwellian plus an E −γ high‐energy tail is required on the ISEE 1 flux tube. The variable nature of the source can be due to either an intrinsic source property or intermittent contact of the satellite flux tube with the source. The observations and analysis indicate that the existence of energetic beams propagating at apparently arbitrary directions through lower‐energy plasmas is a natural result of single‐particle dynamics. Large, positive phase space density velocity gradients, ∂ f /∂υ ≫0, naturally develop at all energies and pitch angles, dependent on spatial location with respect to the source. These results can unify a variety of previous energetic ion streaming observations in the tail and relate them to a common source.