Lunar electromagnetic scattering 3. Propagation at arbitrary angles to the cavity axis

Horning, B. L.; Schubert, G.

doi:10.1029/ja080i031p04215

Cited by 3 publications

(2 citation statements)

References 20 publications

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“…In a broad discussion of induction at arbitrary angles of incidence, Homing and Schubert [1975] considered the case of the two-layer moon with core of finite conductivity. However, their primary attention was directed to the cavity fields.…”

Section: Synthesis Of Axial and Transverse Propagationmentioning

confidence: 99%

Electromagnetic induction in the Moon

Sonett¹

1982

Reviews of Geophysics

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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 ...

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Section: Synthesis Of Axial and Transverse Propagationmentioning

confidence: 99%

Electromagnetic induction in the Moon

Sonett¹

1982

Reviews of Geophysics

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“…Theories of asymmetric confinement of the induced magnetic field are now being devised (e.g. Schwartz Horning & Schubert 1975) and these newer theories will hopefully give a more accurate description of the day-side inductive response, together with a consistent interpretation of a further mass of data arising from studies of transient phenomena on the night-side of the Moon. The above Backus-Gilbert technique, although a linearized approximation, seems an appropriate way to analyse such data, and it is to be hoped that this application will encourage the technique to be applied to the newer theories, and to an extended data set.…”

Section: Resultsmentioning

confidence: 99%

The electrical conductivity of the Moon: an application of inverse theory

Hobbs

1977

Geophysical Journal International

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Inverse theory of Backus & Gilbert is used to analyse the day‐side electromagnetic response of the moon to magnetic fluctuations in the solar wind. The data consist of two transfer functions, both tangential to the lunar surface, and in the theoretical development, the required Fréchet kernels corresponding to these transfer functions are determined. The ensuing calculations show that the data are sufficiently good to determine the conductivity down to a depth of about 600 km. The results are very encouraging and it is suggested that as newer theories of lunar induction are developed to consistently interpret the day‐and night‐side measurements, they should be used in the above enlightened manner.

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The deep lunar electrical conductivity profile: Structural and thermal inferences

Hood

Herbert²,

Sonett

1982

J. Geophys. Res.

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Simultaneous Apollo 12 surface and Explorer 35 orbital magnetometer records have been reexamined for intervals suitable for measuring the lunar inductive response in the solar wind and terrestrial magnetosheath. Improvements in the selected time series are obtained by (1) editing of intervals containing extraneous surface field fluctuations caused by plasiha interaction with the local crustal magnetic field, and (2) minimization of magnetometer data errors in the Apollo 12 record. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the frequency range 10−5 to 10−3 Hz using methods introduced earlier by Wiskerchen and Sonett (1977). Maximum consistency of estimates from different time intervals is found when the analysis is limited to the tangential direction of maximum incident power as measured at Explorer 35 for each interval, i.e., the direction in which the ratio of signal to background noise is greatest. A simplified version of spherically symmetric plasma (SSP) confinement theory is used to interpret the transfer function data via forward model calculations under the assumption that the electrical conductivity increases continuously with depth. The inferred bounds on the internal electrical conductivity profile are narrowest in the depth range 450–1350 km and are more restrictive at these depths than those obtained previously from plasma‐confined response data. An upper limit of ≤360 km for the radius of a metallic core with assumed conductivity ≥102 mhos/m is estimated from the one standard deviation error limits of the measurements. Using the bulk conductivity versus temperature model proposed by Huebner et al. (1979), rough limits on the selenotherm are calculated which appear to be in agreement with independent geophysical constraints at depths less than about 800 km.

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Lunar electromagnetic scattering 3. Propagation at arbitrary angles to the cavity axis

Cited by 3 publications

References 20 publications

Electromagnetic induction in the Moon

Electromagnetic induction in the Moon

The electrical conductivity of the Moon: an application of inverse theory

The deep lunar electrical conductivity profile: Structural and thermal inferences

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