Three time intervals have been selected during which the Active Magnetospheric Particle Tracer Explorers (AMPTE) Irm satellite traversed the plasmapause around 2130, 1900, and 1730 LT. The cold plasma density is monitored with the wave experiment whose frequency range includes the upper hybrid and the plasma frequencies; in this manner, the plasmapause is easily observed as a reduction in the plasma density from 10--100 cm-3 to 0--5 cm-3. The AMPTE IRM three-dimensional plasma instrument detects ions in the energy range 20 eV to 40 keV and allows the determination of the inner edge of the ring current ions, which penetrate the duskside plasmapause by 0.5--2 RE.

This boundary is characterized by termination of the plasmaspheric hiss and an outward pointing gradient in the energetic ions. Another signature of this boundary is an eastward current which is detectable by the plasma instrument as well as the magnetometer; this current is consistent in direction and magnitude with the diamagnetic current associted with the observed gradient in the ring current ions. In two of the three cases, current-driven lower hybrid waves occur. On all three days, the ring current overlap with the plasmasphere is marked by sunward convection (opposite to corotation) and by a plateau formation in the density profile, both of which indicate that the ring current has a significant effect on the electric field. On one of the three days (June 6, 1985), the region where the ring current overlaps the plasmasphere is marked by electromagnetic waves with frequencies below the local helium gyrofrequency. These waves appear to be about an equal mix of the left-hand-polarized L mode and the right-hand-polarized R mode ion cyclotron waves and seem to support the general explanation of such low-frequency waves based on GEOS and ATS observations.

On all three days, the overlap region is populated by magnetic pulsations with periods of the order of minutes. These are accompanied by fluctuations in the cold plasma density which are in phase with the compressional part of the pulsations and seem roughly consistent with adiabatic variation in the cold plasma density in response to the changes in the field. ¿ American Geophysical Union 1988