Coronagraphic Observations of the Lunar Sodium Exosphere 2018–2019

Introduction: In order to observe the lunar sodium exosphere out to one-half degree around the Moon, we designed, built and installed a small robotically controlled coronagraph at the Winer Observatory in Sonoita, Arizona. Details of the instrument and data reduction procedure are given in Killen et al., 2019 [1]. A 1.5 Å bandpass filter centered at 588.995 nm measures the sodium D₂ line and a 1.5 Å bandpass offband filter centered 3 Å blueward of the onband filter measures the continuum. Our observations were obtained from approximately 143 km off the lunar surface to about one lunar radius above the surface, 1738 km.

Results and Discussion: We report on observations taken during fall and spring, 2018, and spring, 2019. The data were reduced following procedures given in [1]. We observed on 44 nights in 2018, 33 of which were successful, and 11 nights in 2019, 9 of which were successful. Our observations show a more uniform latitudinal distribution than previously proposed. We also see that the column abundances in the extended corona peak at dawn and dusk. Scale heights are consistent with temperatures averaging about 4500 K. This shows that the data is not strictly symmetric about the equator nor do they follow any cosine distribution with a peak at noon as expected from theory or as found by Potter and Morgan. The highly N/S asymmetric sodium profile seen at many lunar phases is consistent with observations published by Potter and Morgan on Feb. 21, 1989. When reduced to surface number density or column abundance the values are consistent with previous work. We show for the first time that the column abundance is greatest when the ion flux to the surface is enhanced, as measured by the ARTEMIS ElectroStatic Analyzer. The largest column abundance was obtained on March 3, 2018, when the Moon entered the Earth’s magnetosheath and the ion fluxes measured by ARTEMIS suddenly increased.