The Lunar Sodium Atmosphere: A Study as Observed Through Four Lunar Eclipses

A.L. Morrill, M Mendillo, J Baumgardner (Boston University)

The Moon's sodium atmosphere has been imaged during four lunar eclipses: November 29, 1993, April 2, 1996, September 27, 1996, and March 24, 1997, using a coronagraph type system at the Boston University four inch telescope located at the McDonald Observatory, TX, and at La Palma, Canary Islands. The Moon is imaged with a 5893A filter with a FWHP of 16A to include the sodium D1 and D2 lines. The eclipse condition provides the opportunity to observe the faint lunar atmosphere when the bright disk of the Moon is within the umbra and penumbra greatly reducing the scattered light in the system.

In all four cases, the sodium atmosphere was imaged out to radial distances of  10 lunar radii. The brightness patterns were essentially uniform in azimuth and exhibited a radial decay far more gradual than seen at sub-solar radial distances at quarter Moon. While some variability appears among the four data sets, the large scale morphology under eclipse conditions was remarkably constant during the 1993 to 1997 events. This implies a steady source of sodium at times of full Moon.