Plasma Void Research Articles

Satellite measurements of the moon's magnetic field: A preliminary report

A preliminary analysis of the data from the UCLA magnetometer on board the Apollo 15 subsatellite indicates that remnant magnetization is a characteristic property of the Moon, that its distribution is such as to produce a rather complex pattern or fine structure, and that a detailed mapping of its distribution is feasible with the present experiment. The analysis also shows that lunar induction fields produced by transients in the interplanetary magnetic field are detectable at the satellite orbit so that in principle the magnetometer data can be used to determine the latitudinal and longitudinal as well as radial dependences of the distribution of electrical conductivity within the Moon. Finally, the analysis indicates that the plasma void or diamagnetic cavity which forms behind the Moon when the Moon is in the solar wind, is detectable at the satellite's orbit and that the flow of the solar wind near the limbs is usually rather strongly disturbed.

Magnetic field structure behind the Moon

Details concerning the magnetic field structure expected on the basis of the author's model of solar wind interaction with the moon are discussed. A derivation is presented that supports the finding of Colburn et al. that the magnetic field in the plasma void region downstream of the moon (relative to the solar wind flow) is more intense than the ambient interplanetary magnetic field. In the expansion region surrounding the void, the field is less than ambient. These effects should be smallest when the magnetic field is perpendicular to the flow vector. A standing shock wave trailing several lunar radii behind the moon is expected regardless of the magnetic field orientation.

Diamagnetic Solar-Wind Cavity Discovered behind Moon

Preliminary Ames-magnetometer data from Explorer 35, the lunar orbiter, show no evidence of a lunar bow shock. However, an increase of the magnetic field by about 1.5 gamma (over the interplanetary value) is evident on Moon's dark side, as well as dips in field strength at the limbs. Interpretation of these spatial variations in the field as deriving from plasma diamagnetism is consistent with a plasma void on the dark side, and steady-state (B = 0) magnetic transparency of Moon.