The magnetosheath electron population at lunar distance: General features

The spatial extent and energy distribution function of the magnetosheath electron population at lunar distance have been analyzed by using measurements from the Apollo 14 charged particle lunar environment experiment (CPLEE). The magnetotail is shown to be approximately 52 R/sub E/ in diameter. The mean bow shock surface has a cross section of about 91 R/sub E/. The average aberration angle measured is about 3degree. The electron distribution function reveals two distinct particle populations. The first, at low energies (40 eVless than or equal toEless than or equal to200 eV), is well characterized by a nearly isotropic Maxwellian distribution, with temperatures in the range 15-25 eV. Densities calculated from fitted functions range from 4-8 cm-3 at the bow shock to 1-2 cm-3 near the magnetopause. The high-energy portion of the spectrum (200 eVless than or equal toEless than or equal to2000 eV), however, is generally anisotropic and is generally denser and more energetic in the dawn magnetosheath than in the dusk magnetosheath. Separate Maxwellian fits to the high-energy population yield densities near 0.02(0.10) cm-3 and temperatures near 200(250) eV for the dusk (dawn) magnetosheath. It is argued that these particles originate at the bow shock rather than in the plasma sheet