High-Spectral Resolution, May 2013 Ground-Based Observations of the Lunar Sodium and Potassium Exosphere

We apply high resolution spectroscopy to investigate the lunar exosphere by measuring sodium and potassium spectral line profiles to determine the variations in exospheric effective temperatures and velocities as a function of time, solar conditions and geometries. Observations are made with a dual-etalon Fabry-Perot spectrometer from the National Solar Observatory McMath-Pierce Telescope. The spectrometer has a Field of View (FOV) of 3 arcmin (~336 km at semimajor axis = 384,400 km) and a resolving power of 180,000 and 150,000 to measure the line widths and radial velocity Doppler shifts of the sodium D2 (5889.951 Å) and potassium D1 (7698.965 Å) emission lines, respectively. We first measured the sodium profile in March 2009 [1] followed by the first potassium line profile measurements in June 2012 (Fig. 1) during the moon’s waning gibbous phase. As previously reported [2], [3] potassium has a smaller scale height than sodium (Fig. 1). We only detected the potassium emission within ~0.25 Rmoon of the lunar limb while we measure sodium out to ~1 Rmoon. A lunar scattered light gradient underlying the FabryPerot circular interference fringes is the dominant continuum source and limiting factors in the precision of these measurements.