Interaction of the solar wind with the moon

1971, Phys. Earth Planet. Interiors 4, 197—198. North-Holland Publishing Company, Amsterdam

INTERACTION OF THE SOLAR WIND WITH THE MOON

NORMAN F. NESS* Laboratory for Extraterrestrial Physics, NASA—Goddard Space Flight Center, Greenbelt, Md., U.S.A.

Abstract During its orbit about the Earth, the Moon is located in the interplanetary medium or the geomagnetosheath—geomagnetotailformed by the solar wind interaction with Earth. In the tail, no evidence is found for a lunar magnetic field limiting the magnetic moment to 3 (< 106 of that of the Earth). In the 1020 G cm interplanetary medium, no evidence exists for a bow shock or a trailing shock although a well definedplasma wake region is observed in the anti-solar wind direction. The Moon absorbs the solar wind plasma which strikes its surface and creates a void region or cavity in the flow. Small perturbations of the interplanetary .

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Presently at C.N.R., University of Rome, Laboratory for Space Plasmas.

magnetic field magnitude (~ 30%) and direction (~ 20~) are observed to be correlated with the location of the solar wind plasma umbra and penumbra. Characteristic perturbations in magnitude are + + + as a satellite traverses the wake region. The magnitude of the anomalies is correlated principally with the diamagnetic properties of the solar wind, as measured by fJ, and less with the direction of the interplanetary magnetic field. The observed lunar Mach cone gives evidence for the anisotropic propagation of waves in the magnetized collisionless warm solar wind plasma. Neither the Gold—Tozer—Wilson mechanism of accretion of field lines or the Sonett—Colburn—Hollweg mechanism of unipolar induction is significant in the interaction. The transmission of microstructural discontinuities in the interplanetary medium past the Moon show little distortion, indicating a low effective —

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INTERPLANETARY MAGNETIC FIELD

FIELD PERTURBAT!ONS~ ~+Ai~j. >1 + FLUCTUATIONS

I—

0


LUNAR MACH CONE _±_ —

SUN 4

—

tO

+__ —

.i

-~—‘~—

—

INCREASE PENUMBRAL DECREASE

—

+

DON SOLAR WIND

--

—

—

—

—

UMBRAL

÷ — —

Fig. 1.

197

P.

INCREASE

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198

NORMAN F. NESS

electrical conductivity (< io~ ~ im_ 1) which implies a relatively cool interior (~ l0~°K)of the lunar body. Fluctuations of the interplanetary magnetic field upstream from the plasma wake are stimulated by the disturbed conditions in that region. The Moon behaves —

like a cold, nonmagnetic fully absorbing dielectric sphere in the solar wind flow (see fig. 1). [The complete text of this paper will be published in the Proc. Leningrad Symp. on Solar—Terrestrial Physics held in May 1970 (Reidel, Dordrecht)].