TRANSPORT TOWARD EARTH OF IONS SPUTTERED FROM THE MOONS SURFACE BY THE SOLAR-WIND

The transport of typical ions from the surface of the Moon to the vicinity of Earth was calculated using a test particle approach. It was assumed that the ions were sputtered from the surface by the solar wind, with fluxes in the range determined experimentally by Elphic et al. (1991), and were accelerated initially to 10 eV by the potential of the Moon on its sunlit side. Si+ and Ca+ ions were selected for this transport analysis because their masses are within two prominent ion mass groups that have high sputtering yields. In the solar wind the ion trajectories were traced in the following superimposed fields: (1) a steady magnetic field B-0 at an angle of 45 degrees to the solar wind velocity V-SW, (2) the motional electric field E(0) = -V-SW x B-0, and (3) turbulent magnetic and electric fields generated by hydromagnetic waves with a k-space power spectrum of \k\ (-5/3) propagating along both directions of the magnetic field B-0. Interactions with Earth's bow shock and magnetosphere were included. Case histories of the ions were recorded in the X(GSM), Y-GSM plane and in various planes perpendicular to the E(0) x B-0 drift direction of the ions between the Moon and Earth. The number density, energy and angular distributions, and directional and omnidirectional fluxes of the ions were constructed from the case histories. It was found that the diffusion of the ions increases rapidly as the amplitude of the turbulence delta B-rms increases beyond the value 0.04B(0). The directional fluxes of Si+ and Ca+ in the solar wind, upstream from the bow shock, were found to be of the order of tens of ions cm(-2) s(-1) sr(-1) keV(-1), with energies 35-70 keV, for delta B-rms = 0.04B(0), B-0 = 5 nT, and V-SW = 400 km/s. The fluxes of Si+ and Ca+ ions that originate in the Moon's atmosphere were estimated to be smaller than the corresponding sputtered-ion fluxes by about an order of magnitude. Recent measurements of lunar ions upstream of the bow shock by Hilchenbach et al. (1992) generally confirm the predicted behavior of the ions.