Observations of the electron concentration Ne and the temperature Te from the electrostatic probes on the Isis 1 satellite were used to examine the location and behavior of the plasmapause at about 3000-km altitude in the vicinity of L=4. At these altitudes the Ne measurements are equivalent to measurements of H+, since the satellite is well into the protonosphere. The plasmapause is evident as a sharp drop in Ne by a factor of 100 as the satellite passes into the polar cap, and a corresponding increase is observed as it enters the plasmasphere on the opposite side of the earth. An enhancement of Te is also observed at the plasmapause, an effect that is most visible at night, when the temperatures at latitudes above and below the plasmapause are usually very low. The position of the plasmapause decreases with magnetic activity but is found to be somewhat less sensitive to Kp than is the equatorial plasmapause. Also, unlike its equatorial behavior, the mid-latitude plasmapause behavior exhibits no detectable late afternoon bulge. These differences imply rather complex coupling of the thermal plasma along the field lines that link these two regions of the plasmasphere. An additional factor may be the recently observed axial asymmetry in the geomagnetic field at high altitudes.
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