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J. K. Wilson, M. Mendillo, H. E. Spence (Center for Space Physics/B.U.)
The lunar sodium exosphere owes its existence to the impact of solar photons, solar wind plasma, and meteorites on the lunar surface. Near full Moon, the Moon lies in the Earth’s magnetotail where it is shielded from the solar wind. It has therefore been assumed that the sodium exosphere observed at full Moon is indicative of purely non-plasma sources. However, the Moon passes through the thin magnetotail current sheet a few times during each magnetotail passage, and the plasma in the current sheet is roughly similar to solar wind plasma in energy and density. It is therefore possible that the plasma-impact source of the exosphere is temporarily revived for small intervals of time near each full Moon.
We have observed the lunar sodium exosphere during five full-Moon phases, (when the Moon was in eclipse) and we have used the current sheet model of Tsyganenko et al. (1998) to estimate when the Moon passed through the current sheet in each case. We find that the amount of sodium in the full-moon exosphere depends on the length of time since the Moon’s most recent current sheet passage, consistent with the idea that plasma-impact in the current sheet provides a temporary but significant source of sodium.
Tsyganenko, N.A., S.B.P. Karlsson, S. Kokubun, T. Yamamoto, A.J. Lazarus, K.W. Ogilvie, C.T. Russell, and J.A. Slavin, J. Geophys. Res., 103, 6827-6841, 1998.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.