Energetic (>35 keV) ion bursts in the deep geomagnetic tail associated with the passage of 37 plasmoids are examined using data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3. These bursts can usually be divided into four distinct phases: (1) strongly tailward streaming ions observed in the lobe for a few minutes prior to plasmoid entry, commencing ~25 min after geomagnetic substorm onset; (2) the plasmoid interval, when the energetic ions have a broader tailward angular distribution arising from convection with the plasmoid; (3) the ''post-plasmoid'' plasma sheet, where more strongly tailward streaming ions are observed in the plasma sheet on field lines disconnected from the earth at the substorm neutral line; and (4) a strongly tailward streaming ion population extendin into the lobe for a few minutes after exit from the plasma sheet.

We concentrate here on the streaming ion ''boundary layers'' observed in the lobe at the leading and trailing edges of these bursts. In a majority of these layers, a clear dawn-dusk gradient anisotropy and energy dispersion are evident at the leading edge, and a similar gradient anisotropy with ''reverse'' dispersion is evident at the trailing edge. It is shown however that the dispersion at onset is not consistent with simple time of flight from a near-earth neutral line or from a neutral line retreating tailward during substorm recovery. Instead, observations of 90¿ pitch angle ions with a time resolution of 16 s are used to infer that the ion onset is due to a layer of energetic ions expanding outward from the tail center plane and engulfing the spacecraft. At the trailing edge of the burst, this layer contracts back across the spacecraft toward the center plane.

Mean expansion and contraction speeds are 94¿74 km s-1 and 99¿100 km s-1 respectively, with boundary layer thicknesses of ~3 RE. From these observations, it is concluded that the expansion of the ion layer is caused predominantly by the ion layer being swept across the spacecraft by the arrival of the plasmoid in the deep tail, contributing ~60 km s-1 to the expansion speed, rather than by a thickening of the region of lobe field lines disconnected at the substorm neutral line which expands at ~35 km s-1. The energy dispersion at the leading edge can be reconciled with a near-earth neutral line in this case. Using this dispersion and the measured expansion speed of the layer, the electric field along the near-earth substorm neutral line is deduced. Values derived from layer expansions and contractions are both ~0.4 mV m-1, equivalent to ~110 kV across a ~4-RE tail width.