High‐resolution spectroscopy of the lunar sodium exosphere

Abstract

We have applied high-resolution Fabry-Perot spectroscopy to the study of the lunar sodium exosphere for the study of exospheric effective temperature and velocity variations. Observing from the National Solar Observatory McMath-Pierce Telescope, we used a dual-etalon Fabry-Perot spectrometer with a resolving power of 180,000 to measure line widths and Doppler shifts of the sodium D2 (5889.95 Å) emission line. Our field of view was 360 km, and measurements were made in equatorial and polar regions from 500 km to 3500 km off the limb. Data were obtained from full moon to 3 days following full moon (waning phase) in March 2009. Measured Doppler line widths within 1100 km of the sunlit east and south lunar limbs for observations between 5 and 40• lunar phase imply effective temperatures ranging between 3260 ± 190 and 1000 ± 135 K. Preliminary line center analysis indicates velocity displacements between different locations off the lunar limb ranging between 100 and 600 m/s from the lunar rest velocity with a precision of ±20 to ±50 m/s depending on brightness. Based on the success of these exploratory observations, an extensive program has been initiated that is expected to constrain lunar atmospheric and surface-process modeling and help quantify source and escape mechanisms.

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

“…Thorium calibration lines were taken nightly to monitor both wavelength and instrumental stability. Instrumental profile (IP) studies indicate an average full width at half maximum of 1.7 km/s, consistent with the earlier work of Mierkiewicz et al (). For these observations, we are able to deconvolve sodium line widths to approximately one half of the IP full width at half maximum.…”

“…The atmospheric transparency is assumed to be similar for purposes of this relative intensity comparison. Consistent with Mierkiewicz et al (), sky background measurements indicate that the terrestrial sodium emissions are negligible for observations taken within one lunar radii of the limb. All observations were obtained with the telescope tracking at the lunar rate.…”

“…Data were collected with the FPS's 3‐arc min field of view (FOV; corresponding to ~336 km at the mean lunar distance of 384,400 km) tangent to the limb and then in a series of defined offsets out to two lunar radii. Only east and west equatorial data off the sunlit limb are presented here, starting from a FOV‐centered 1.5 arc min, 4.9 * cos( δ ) arc min, and 8.5 * cos( δ ) arc min off the limb; where δ is the declination of the Moon (refer to Figure 2 in Mierkiewicz et al, ). For the mean lunar distance, these offsets correspond to 168 km, 550 * cos( δ ) km, and 950 * cos( δ ) km, respectively.…”

“…For these observations, we are able to deconvolve sodium line widths to approximately one half of the IP full width at half maximum. For an explanation on how Doppler line width can be related to effective temperature, refer to Mierkiewicz et al ().…”

High‐Resolution Potassium Observations of the Lunar Exosphere

“…A summary of the observing runs is listed in Table 1. Details about our Fabry-Perot spectrometer are given by Mierkiewicz et al (2014). Details about our Fabry-Perot spectrometer are given by Mierkiewicz et al (2014).…”