ISEE 1 and 2 observations of ion distributions at the plasma sheet‐tail lobe boundary

Observations of plasma ions (0.075–29 keV) at or near the plasma sheet‐tail lobe boundary made with the ISEE 1 and 2 spacecraft are the principal topic of this paper. We describe crossings of this boundary that occurred during various levels of geomagnetic activity at distances X SM = −7 to −13 R E , near local midnight, and 2 R E or more above the estimated location of the tail current sheet. Fluxes of ions directed along the magnetic field lines (referred to as ion beams) with kinetic energies of 1‐ > 29 keV are commonly observed during crossings of this boundary. The ion beams nearest the edge of the plasma sheet are directed earthward, while deeper within the plasma sheet, additional ion beams, directed tailward, are encountered, usually resulting in bidirectional or counterstreaming ion distributions. The ion distribution functions for these clearly indicate that the tailward directed beams are reflected versions of the earthward directed beams, returning after mirroring in the stronger magnetic field near the Earth. From an intersatellite timing analysis the thickness d B of the region of solely earthward streaming ions is estimated to be a few tenths of an Earth radius. The fastest particles (plasma electrons and the highest‐energy ions) are the first to be seen as a satellite enters the plasma sheet from the lobe and last to be seen as it leaves the plasma sheet to enter the lobe. This suggests that the ion beams are components of a layer of plasma that is projected earthward along field lines from a more or less steady source tailward of the satellite and that is E × B drifting equatorward. The observations are consistent with theoretical predictions for an open model of the magnetosphere having a neutral line tailward of the satellite and having the plasma sheet‐lobe interface coincident with the separatrix between closed and open field lines. We therefore identify the beam‐containing plasma layer contiguous to, and extending equatorward from, this interface as the separatrix layer. The velocity, temperature, and kinetic energy flux of the earthward beams in the separatrix layer are found to correlate positively with the AE index. We rarely observe isolated tailward streaming ion beams near the boundary, although slow (<300 km/s) tailward flows there sometimes result from an imbalance of particle fluxes carried by the oppositely streaming beams. We show that such net tailward ion anisotropies, occasionally observed in our data set, differ distinctly from the tailward bulk flow event reported by Hones et al. (1986). That event had all ion components moving tailward with a net velocity of ∼500 km/s; it was interpreted to be the result of magnetic reconnection at a near‐Earth neutral line that formed at the onset of a substorm and tailward departure of the resulting plasmoid.