High-Resolution, Ground-Based Observations of the Lunar Sodium Exosphere During the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission

Abstract

Plain Language Summary We present the first comprehensive set of lunar exospheric line width and line width derived effective temperatures as a function of lunar phase (66 degrees waxing phase to 79 degrees waning phase). Data were collected between November 2013 and May 2014 during six observing runs at the National Solar Observatory McMath-Pierce Solar Telescope by applying high-resolution Fabry-Perot spectroscopy (R similar to 180,000) to observe emission from exospheric sodium (5,889.9509 angstrom, D2 line). The 3-arcmin field of view of the instrument, corresponding to similar to 336km at the mean lunar distance (384,400km), was positioned at several locations off the lunar limb; only equatorial observations taken out to 950km are presented here. We find the sodium effective temperature distribution to be approximately a symmetric function of lunar phase with respect to full Moon. Within magnetotail passage we find temperatures in the range of 2500-9000K. For phase angles greater than 40 degrees we find that temperatures flatten out to similar to 1700K. High spectral resolution observations of optical line emissions are used to investigate the morphology and dynamics of the lunar sodium exosphere. These observations were obtained from the National Solar Observatory McMath-Pierce Solar Telescope, coincident with the Lunar Atmosphere and Dust Environment Explorer mission. The equatorial data presented here are the first comprehensive set of direct sodium emission line profile observations of the lunar exosphere. These observations will help constrain atmospheric and surface process modeling, and help quantify lunar exospheric source and escape mechanisms. Studying the morphology of the lunar exosphere with altitude and local time provides a useful laboratory for testing space weather effects at Earth and theoretical models of other bodies with similar exospheres (e.g., Mercury).