IMAGING OBSERVATIONS OF THE EXTENDED SODIUM ATMOSPHERE OF THE MOON

Pilot ground-based optical imaging observations of the extended lunar environment were conducted at McDonald Observatory in February 1991. One set of images taken on February 20 revealed emission at the combined D1 (5896 angstrom) and D2 (5890 angstrom) lines of neutral sodium extending out to about 5 lunar radii (R(M)) from the center of the Moon on the sunward side and much fainter emission extending out to about 15-20 R(M) on the antisunward side. Peak emission (D1 + D2) above the Moon's limb at a distance of 1.8 R(M) along the lunisolar axis was measured to be 900 Rayleighs (R), decreasing with distance as r-4. The morphology of the emitting region is qualitatively that of a comet, i.e., a bright coma centered on the Moon and an extended tail trailing away in the antisunward direction. The tail exhibits a decrease in intensity coincident with the shadow cast by the moon. The sunward extent of the sodium is used to calculate a surface ejection speed of about 2 km s-1, consistent with recent estimates of a meteor impact surface vaporization source. The data agree well with previous observations made within 1 lunar radius above the surface. Modeling yields a surface density (52 atoms cm-3) that decreases with distance as r-5 in the solar direction.