The lunar sodium atmosphere: April–May 1998 1This article is part of a Special Issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

Abstract

Atmospheric sodium at the Moon was observed for altitudes from the ground up to 1800 km from 18 April to 13 May 1998, a period including an entire lunar month. Because of bad weather, coverage was incomplete but we present the analysis of 36 spectra largely from near first and third quarter phases and passage through full moon. The apparatus used was the specially built clear aperture 16.5 cm, 203 cm focal length, Mt. Lemmon Lunar Coronagraph on Mt. Lemmon in the Catalina Mountains of Tucson, Arizona. Data are interpreted in terms of Chamberlain fits to the observed emission rate using both one equivalent temperature and two source components (with both cold and hot density distributions). The extent and equivalent temperatures of the exosphere during this period trended towards an extended coma with very little fraction in the thermally accommodated regime, even near the surface. Preliminary data–model comparisons suggest that these measurements are consistent with rates and spatial distributions expected from a photon-stimulated desorption source, although evidence for one, or possibly two, periods of notable meteoroid impact source might be present. A compilation of new and previously published observations illustrates an increased emission by sodium at third quarter over that of first quarter, a result that suggests that the dayside exospheric content increases, at least occasionally, following transit of the Moon through the Earth’s magnetosphere.