First performance results of a new field‐widened spatial heterodyne spectrometer for geocoronal H_α research

Abstract

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.

The Unknown Hydrogen Exosphere: Space Weather Implications

“…Ongoing efforts to analyze large data sets should lead to significantly improved models within the next few years. In addition to ongoing studies, some of which are mentioned above (Gardner et al, ; Mierkiewicz et al, ; Nossal et al, ; Qin & Waldrop, ; Waldrop & Paxton, ), upcoming and proposed missions may shed light on this issue. For example, while not directly measuring composition, the Global‐scale Observations of the Limb and Disk satellite (Eastes et al, ) will measure the exobase temperature an important parameter in thermosphere/exosphere models.…”

“…Based on the brief review of the literature (see section 1 above), the immediate need is for improved models of n H in the thermosphere, exosphere, and geocorona that describe the variability of n H with local time, season, and solar cycle. Current data analysis efforts (Gardner et al, ; Kuwabara et al, ; Mierkiewicz et al, ; Nossal et al, ; Qin & Waldrop, ; Waldrop & Paxton, ; Zoennchen et al, ) should provide n H with enough resolution to support development of an improved model. However, we will need ongoing measurements so as to capture climate‐driven changes in n H .…”

Raman measurements using a field‐widened spatial heterodyne Raman spectrometer

“…Gardner et al designed a high‐resolution field‐widened SHS to observe geocoronal Balmer α emission. The results of field‐widened SHS showed excellent agreement with the already established Fabry–Perot interferometer observations . Pannell et al investigated a field‐widened SHS in terms of its fundamental spectral resolution and its sensitivity, which achieved a S/N ration of unity with an input power of tens of femto‐watts.…”

Constraining Balmer Alpha Fine Structure Excitation Measured in Geocoronal Hydrogen Observations

“…The WHAM Balmer β / α line ratio variation with viewing geometry also shows, for the first time, that the cascade contribution to Balmer α is explicitly a function of viewing geometry. This relationship not only is useful in constraining cascade contributions to the total Balmer α emission observed in high‐resolution line profile observations but is also useful in constraining observations obtained by instruments not fully capable of resolving the cascade themselves (e.g., Gardner et al, ). We also show that the unique Balmer β / α line ratio variation with viewing geometry offers a likely forward model constraint toward the retrieval of aeronomical parameters of interest (e.g., [H(z)]).…”

“…Single and multiple Lyman scattering contribute to the observed Balmer emission along the line of sight, in varying proportions dependent on shadow altitude. (Adapted with permission from Gardner et al, ).…”