Date of Award

12-2020

Document Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

Physical Sciences

Committee Chair

Dr. Edwin Mierkiewicz

First Committee Member

Dr. Ashley Kehoe

Second Committee Member

Dr. Ronald J. Oliversen

Third Committee Member

Dr. Menelaos Sarantos

Abstract

This dissertation outlines and presents the most comprehensive set of velocity-resolved measurements of sodium D2 (5889.9509 Å) lines taken over multiple lunations spanning seven years (2011 – 2017). These data are used to study the morphology and dynamics of the lunar exosphere. Additionally, potassium D1 (7698.9646 Å) is used as a complement to sodium. The two species approach, with sodium being the main attraction, provides constraints on the critical drivers of the lunar exosphere. Observations were made at the National Solar Observatory McMath – Pierce Telescope, Kitt Peak, Arizona where I personally collected both sodium and potassium data over about a dozen observing runs. This work builds on the initial velocity resolved measurement analysis of sodium by Mierkiewicz et al. (2014) and includes the sodium analysis of Kuruppuaratchi et al. (2018), and potassium analysis of Rosborough et al. (2019). This work is divided into an analysis of exospheric responses as a function of phase angle (temporal) and an analysis of exospheric responses as a function of latitude and altitude (spatial). Sodium results suggest that photon stimulated desorption (PSD) is the major source mechanism for the lunar sodium exosphere near subsolar points while contributions from charged particle sputtering and micrometeoroid impact vaporization are more important near the dawn and dusk terminators. For potassium, the exosphere exhibits dawn-dusk asymmetry because of impact vaporization and a connection to the surface abundance of potassium. A two species study investigating the response of the exosphere to micrometeoroid impacts shows that sodium appears to have a smooth, gradual response to meteor activity, while potassium appears to have a short-term but significant response. Potassium is found to track closely with surface abundance tied to the regions on the Moon where there are known abundances of potassium (KREEP regions). The spatial study is performed keeping in mind prior works that speculated the possible contribution of plasma sheet ions in the response of the exosphere during magnetotail passage. A large part of this work is focused on the analysis of sodium and potassium based on the near lunar plasma environment: (i.e., solar wind, magnetosheath, and magnetotail) as a function of latitude. Sodium exospheric temperatures and relative intensities indicate that the relative importance of source processes vary depending on the near lunar plasma environment. Potassium, on the other hand, hints at being modulated by micrometeorite impact vaporization throughout all plasma regions. Potassium also shows enhancements in brightness around latitudes where KREEP regions are. Sodium shows some indication of surface enhancement since relative intensity data tend to peak at low-mid latitudes in both North and South regions. Sodium temperatures also show an increase from low to high latitudes. A closer look at the amount of time the Moon spent interacting with the plasma sheet revealed that sodium temperatures showed a positive correlation with time spent in the plasma sheet. Sodium intensities however did not seem to respond as well. The radial dependence of the sodium exosphere indicated that linewidths narrow as a function of altitude. Analysis of these velocity resolved observations from this unique dataset, taken over multiple lunations and years as a function of latitude, altitude, and phase, has helped define baseline and variable conditions of the lunar exosphere - thereby making important contributions towards the study of the near space environment of the Moon which is not only important for testing theoretical models of Surface Bounded Exospheres (SBEs) but also for understanding the atmosphere – surface interactions that will affect human exploration.

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