Magnetosheath electrons at lunar distance

Abstract

Properties of the electron population in the magnetosheath at lunar distance have been analyzed using measurements from the Apollo 14 Charged Particle Lunar Environment Experiment (CPLEE). Average magnetopause and bow shock locations are computed, and it is shown that the axis of symmetry of each is aligned with the aberrated solar-wind direction within the standard deviations of the measurements. Measured electron differential-flux spectra are fitted to appropriate distribution functions, and the distribution functions are integrated to yield number densities, most probable thermal energies, and flow velocities based on assumed flow directions. The lowest-energy electrons (4 eV < E < 1 eV) are well characterized by a nearly isotropic Maxwellian distribution, with number densities ranging from about 4-8 (cm-) at the bow shock to 1-3(cm-3) near the magnetopause. Most probable thermal energies fall in the range 15-25 eV. The spectra exhibit non-Maxwellian high-energy "tails", with flux enhancements at higher energies (1 eV < E < 2 eV). The high-energy flux is anisotropic, and in general is more intense and more energetic in the dawn sheath, often taking the form of a secondary peak. This secondary peak is similar both in number density and mean energy to the plasma-sheet electron population at lunar distance