Lunar surface solar wind observations at the Apollo 12 and Apollo 15 sites

Eleven months of simultaneous data were obtained by the Alsep solar wind spectrometers at the Apollo 12 and 15 sites. There were no observed differences between the properties of the upstream solar wind and the plasma observed at Apollo 15, where the local magnetic field is 3plus-or-minus3 γ. However, the solar wind flow is often strongly perturbed at the Apollo 12 site, where the field is approx.38 γ. These perturbations include (1) the acceleration of solar wind electrons to energies of less than or equal to120 eV, (2) the deceleration of solar wind protons by as much as 70 km/s, (3) the deflection of solar wind protons by less than or equal to10degree, (4) the focusing or defocusing of the ion flux by factors as large as 3, (5) the heating of the protons by a factor of less than or equal to2 in excess of any adiabatic heating associated with compression, and (6) an increased level of fluctuations of plasma parameters with frequencies in the range 3times10-5 to >2times10-2 Hz. The deceleration and focusing are both functions of the angle of incidence of the plasma and of the component of solar wind dynamic pressure normal to the surface. These effects require that there be a charge-separation electric field above the lunar surface and that the magnetic field at the Apollo 12 site have a scale size of less than approx.5 km. The plasma perturbations observed at this site suggest that remanent lunar magnetic fields are the cause of lunar limb compression waves and that these waves should be more noticeable for low solar wind dynamic pressures. Those physical and chemical properties of the surface layers of the moon which depend on the chemical composition, the energy, or the flux of the bombarding plasma may depend strongly on the strength and scale size of the local magnetic field