Electromagnetic induction in the Moon


  • C. P. Sonett


Except for a boundary layer of uncertain properties the moon constitutes a rigid (nonhydromagnetic) but electrically conducting target to the solar wind when unscreened by the earth's magnetosphere. At frequencies below 3 mHz, observed in the rest frame of the moon, fluctuations of the interplanetary and the geomagnetic tail fields excite eddy currents which penetrate deeper than half the lunar radius. At long wavelength (λ ≫ rM, where rM is the lunar radius) a time-varying magnetic dipole is induced. Shorter wavelengths excite correspondingly higher-order magnetic multipoles, but primarily in the outer parts of the moon because of skin depth limitations. The lunar response reaches a peak as frequency increases, diminishing with further increase in frequency. This is interpreted as evidence for the presence of the magnetic quadrupole moment. In the solar wind and magnetosheath the transverse electric mode in the moon is not accompanied by the corresponding transverse magnetic mode, since a bow shock wave is not detected. The latter is probably damped by the nonconducting lunar crust, though the moon is still polarized electrically. It is marginally possible that transient bow wave phenomena appear at times, but because of the high dynamic pressure of the solar wind and magnetosheath, most induced fields are confined to the lunar interior and perhaps the boundary layer. Furthermore, a theorem by Herbert et al. (1976) seems to preclude time-variable induced magnetospheres. Magnetometer measurements of induction using Explorer and Apollo instruments are reviewed, from both the harmonic and the transient standpoint, and the resulting determination of internal bulk electrical conductivity is discussed. Estimates of both internal temperature and metallized core radius maxima are examined. The closeness of the estimated temperature to the Ringwood-Essene solidus at 150- to 250-km depth may be indicative of a layer of enhanced conductivity in lieu of high temperature. If so, this suggests a reduction in temperature, bringing the temperature into coincidence with other selenophysical parameters. A reduced core radius estimate with a one-sigma upper limit of 360 km is reported. Application of inverse theory resolution tests has not been made pending completion of current work involving autoregressive models in attempting to isolate the angular spectrum of real or apparent incident wave normals, upon which the quadrupole response depends. This is also aimed at bypassing current limitations imposed by data gap convolution, data digitization noise, and noise thought to be generated by the interaction of the solar wind with local and regional permanent fields on the lunar surface. Because of length limitations, discussion of lunar electrodynamics has been restricted to the problem of induction, with accompanying subjects such as the flow field and regional electric fields on the moon mentioned only as required. A fuller treatment should include these matters. The review closes prior to ongoing work on high-frequency induction involving the full lunar scattering problem.