We present the first comprehensive set of lunar exospheric line width and line width derived effective temperatures as a function of lunar phase (66° waxing phase to 79° 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 ~ 180,000) to observe emission from exospheric sodium (5,889.9509 Å, D2 line). The 3‐arc min field of view of the instrument, corresponding to ~336 km at the mean lunar distance (384,400 km), was positioned at several locations off the lunar limb; only equatorial observations taken out to 950 km 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 2500–9000 K. For phase angles greater than 40° we find that temperatures flatten out to ~1700 K.

A new, high‐resolution field‐widened spatial heterodyne spectrometer (FW‐SHS) designed to observe geocoronal Balmer α (Hα, 6563 Å) emission was installed at Pine Bluff Observatory (PBO) near Madison, Wisconsin. FW‐SHS observations were compared with an already well‐characterized dual‐etalon Fabry‐Perot Interferometer (PBO FPI) optimized for Hα, also at PBO. The FW‐SHS is a robust Fourier transform instrument that combines a large throughput advantage with high spectral resolution and a relatively long spectral baseline (~10 times that of the PBO FPI) in a compact, versatile instrument with no moving parts. Coincident Hα observations by FW‐SHS and PBO FPI were obtained over similar integration times, resolving powers (~67,000 and 80,000 at Hα) and fields of view (1.8° and 1.4°, respectively). First light FW‐SHS observations of Hα intensity and temperature (Doppler width) versus viewing geometry (shadow altitude) show excellent relative agreement with the geocoronal observations previously obtained at PBO by FPI. The FW‐SHS has a 640 km/s (14 Å) spectral band pass and is capable of determining geocoronal Hα Doppler shifts on the order of 100 m/s with a temporal resolution on the order of minutes. These characteristics make the FW‐SHS well suited for spectroscopic studies of relatively faint (~12–2 R), diffuse‐source geocoronal Hα emission from Earth's upper thermosphere and exosphere and the interstellar medium in our Galaxy. Current and future FW‐SHS observations extend long‐term geocoronal hydrogen observation data sets already spanning three solar minima. This paper describes the FW‐SHS first light performance and Hα observational results collected from observing nights across 2013 and 2014.

Cascade contributions to geocoronal Balmer α airglow line profiles are directly proportional to the Balmer β/α line ratio and can therefore be determined with near simultaneous Balmer β observations. Due to scattering differences for solar Lyman β and Lyman γ (responsible for the terrestrial Balmer α and Balmer β fluorescence, respectively), there is an expected trend for the cascade emission to become a smaller fraction of the Balmer α intensity at larger shadow altitudes. Near‐coincident Balmer α and Balmer β data sets, obtained from the Wisconsin H alpha Mapper Fabry‐Perot, are used to determine the cascade contribution to the Balmer α line profile and to show, for the first time, the Balmer β/α line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer α line profile fits obtained independently by high‐resolution Fabry‐Perot at Pine Bluff, WI. We also demonstrate with radiative transport forward modeling that a solar cycle influence on cascade is expected, and that the Balmer β/α line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen's evaporative escape rate and exobase density).

Ground-based hydrogen Balmer-α observations from Northern midlatitudes span multiple solar cycles, facilitating investigation of decadal scale variations, including natural variability in the hydrogen response to solar geophysical changes. Here we present a reanalysis of ground-based hydrogen emission observations from the early 1990s and their comparison with observations obtained in [2000][2001] in the context of the extended Northern Hemisphere midlatitude geocoronal hydrogen emission data set. This work suggests an increase in hydrogen emission intensity between the solar-maximum period of 1990-1991 (Solar Cycle 22) and the near-solar-maximum period of 2000-2001 (Solar Cycle 23), with the caveat that this is a limited data set and that there are calibration uncertainties discussed in this paper. Solar activity was higher during the earlier solar maximum period. Thus, the apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. This increase was also not seen in comparison of intensities from three solar minima periods. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide. We will discuss the reanalysis and recalibration of the 1990-1991 observations using current analysis approaches and the interpretation of these observations in the context of observations and modeling of hydrogen variation over different time scales. The detailed review of the calibration procedures has also provided insights to guide design of future observations. Plain Language Summary Atomic hydrogen is a key constituent of the thermosphere and exosphere, the uppermost region of the Earth's atmosphere. Hydrogen in this region is a by-product of molecules at lower altitudes that contain hydrogen such as water vapor and methane, two of the greenhouse gases most important to the energy balance of the Earth. The 11-year solar cycle is a major source of natural variability in the upper atmosphere. Here we present observations of geocoronal hydrogen emissions (~400 km and above) taken by ground-based Fabry-Perot instruments at Wisconsin and at the Kitt Peak, Observatory in Arizona. These observations span two solar cycles and suggest an increase in the geocoronal hydrogen emission intensity between the 1990-1991 solar maximum (Solar Cycle 22) and the 2000-2001 near-solar-maximum (Solar Cycle 23) periods, with the caveat that this is a limited data set and there are calibration uncertainties. Solar activity was higher during the earlier solar maximum period. The apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide.