Observations of Moon‐plasma interactions by orbital and surface experiments

Abstract

Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity‐associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. The axis of the lunar wake is aberrated from the moon‐sun line by the relative velocity of the solar wind with respect to the moon, and the cross section of the wake is elliptical, reflecting the anisotropic propagation of magnetoacoustic waves in the solar wind. Enhancements in B are often observed just external, i.e., on the solar wind side, of the rarefaction‐associated dips in the field magnitude. These are of perturbation magnitude at the altitude of Explorer 35, whereas at the low height of the subsatellites they are often large field increases or limb compressions that occur either just upstream of or directly above lunar limbs. Particular surface regions of the moon seem more effective than others as a source of the limb compressions; the pertinent regional property is likely to be remanent magnetization. Propagation of the limb disturbances downstream from the source and the efficacy with which a regional property can create a disturbance in the solar wind flow past the moon's limb depend on properties of the plasma and interplanetary magnetic field. The main enhancement of B in the shadow region behind the moon is a more conspicuous feature of the magnetosheath interaction, whereas the field strength dips and the limb compressions become more difficult to identify in the highly variable state of the magnetic field. In the lobes of the geomagnetic tail, remanent lunar magnetic fields are readily detected at the subsatellite altitude. There is a distinct structure associated with the average field magnitude measured by the Apollo 15 subsatellite in the plasma sheet of the geomagnetic tail; B is enhanced over the day side quadrant of the subsatellite orbit. This variation in the average magnitude of the magnetic field may be associated with the drift of plasma sheet particles toward the earth, creating a wake region centered approximately on the day side orbital quadrant of the subsatellite. Alternatively, flux tube depletion earthward of the moon may explain the observed day side enhancement of average magnetic field magnitude. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous Ogo 5, Vela 5, and Apollo 12 spectrometer data of ion density and velocity taken when the Ogo and Vela satellites were in the solar wind and the Apollo spectrometer was on the lunar day side also exposed to solar wind plasma show that the proton velocity is smaller and the proton density is larger at the Apollo site than in the free stream solar wind, a result of proton deceleration at the site by an electric field established via the plasma‐remanent magnetic field interaction. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.