Plasma boundaries in the inner magnetosphere

Abstract

Plasma boundaries in the inner magnetosphere are investigated with measurements chiefly by the DE 1 and 2 spacecraft. The boundaries studied here are: (1) the low‐energy (or thermal) ion transition (LEIT) frequently observed at or near the conventional plasmapause density gradient, defined here as the outer boundary of observable, cold, isotropic light ions and transition to warm field‐aligned ions; (2) the earthward or inner edges of the plasma sheet electrons having pitch angles near 90° and the three energies 100 eV, 1 keV, and 10 keV; and (3) the equatorward boundaries of precipitating auroral oval electrons at those same energies. Some simultaneous observations of the aurora are also examined. Observations reported here have been obtained in the evening and the morning local time sectors; however, it is found that distinct equatorward boundaries of the auroral oval and the plasma sheet are infrequently observed in the morning sector, and the report focuses on boundary measurements in the evening sector (mainly 1800–2300 MLT). These measurements show that in the evening sector, the LEIT and the 100‐eV inner edge of the electron plasma sheet are normally coincident to within identification uncertainties (∼0.3L), and these are reasonably coincident with the field lines threading the 100‐eV equatorward edge of the auroral electron precipitation. These boundaries respond similarly to variations in magnetic activity. A characteristic electron energy dispersion is typically observed in the plasma sheet inner edges at the different energies, with the lower‐energy boundaries located earthward of the boundaries at higher energies. The dispersion appears to increase from the midnight sector toward dusk, and also to decrease with increasing magnetic activity. It is suggested that these boundaries are reasonably sensitive and accurate signatures, in the evening sector, of the boundary between closed and open convection trajectories as determined by the convection electric field, and that the evening sector characteristic electron plasma sheet energy dispersion is similarly governed by the convection pattern so that the inner edges may be regarded as the Alfvén layers at those energies.