Electron observations between the inner edge of the plasma sheet and the plasmasphere

Observations of electron intensities over the energy range ∼100 ev to 50 kev are presented for L values between 3 and 10 RE (earth radii) near local midnight at low magnetic latitudes. These measurements were obtained over the period June 11 through July 23, 1966, with an array of electrostatic analyzers borne on the earth satellite Ogo 3. The earthward edge of the plasma sheet is characterized by severe decreases in electron energy densities with decreasing geocentric radial distance. These decreases have a distinct structure with fluxes of higher energy electrons decreasing further from the earth for the electron energy range ∼700 ev to 20 kev. A trough of ∼100-ev electrons with number densities of ∼1(cm)−3 is observed to fill the region between the earthward edge of the plasma sheet and the plasmasphere. The densities of electrons with energies greater than 700 ev are often less than 0.1 (cm)−3 in this region. The observed separation between the inner edge of the plasma sheet and the plasmapause was ∼1 to 3 RE in June and ∼3 to 5 RE in July. Analysis of subsequent observations should determine whether this is a latitude or local time effect. The plasmapause is usually observed to be located at the minimum of the energy density profile for electrons with energies above 700 ev. At electron energies of ∼200 ev a plasmapause structure comprising a sharp radially outward boundary and a broader inward boundary is observed. The observed lifetimes of ∼10 days for lower energy electron (E∼1 kev) intensities in the plasmasphere that were enhanced during a moderate geomagnetic storm are similar to previously reported lifetimes for more energetic electron (E≥500 kev) intensities in the same region. Kilovolt electron intensities between the plasmasphere and the earthward edge of the plasma sheet decreased more rapidly and fell to typically quiescent values within the satellite’s orbital period of 48 hours. Simultaneous observations of the angular distributions of electron intensities at two pitch angles in the plasma sheet revealed that these angular distributions approached isotropy for the several plasma sheet crossings that were examined. Differential energy spectrums of electron intensities for the plasma sheet, the earthward edge, the electron ‘trough,’ and the plasmasphere are also presented.