Plasma sheet at lunar distance - Structure and solar-wind dependence.

Analysis of Explorer 35 observation of low-energy (0.1 to 3 keV) electrons in the distant geomagnetic tail at 60 earth radii, and comparison with the solar-wind conditions monitored simultaneously by Explorer 33. The plasma sheet extends to the lunar distance with almost the same characteristics as those observed in the near tail region; the thickness is several earth radii, and the flux, density, and energy are of the order of 10 to the 9th per sq cm per sec, 1 per cu cm, and 0.6 keV, respectively. The latitudinal boundaries of the plasma sheet are essentially parallel to magnetic-field lines. Indication is seen of the thinning of the plasma sheet during polar substorms. The average energy of electrons is correlated with the solar-wind velocity in agreement with the view that plasma-sheet electrons originate from the magnetosheath. The flux is correlated with the solar-wind dynamic pressure, reflecting the dynamic balance between the solar wind and the geomagnetic tail.