S. M. Nossal

Solar cycle variations of geocoronal Balmer α emission

Observations of the geocoronal Balmer α nightglow have been made from Wisconsin for more than a solar cycle with an internally consistent intensity reference to standard astronomical nebulae. These measurements were made with a double-etalon, pressure-scanned, 15-cm aperture Fabry-Perot interferometer. The resulting long time line data provides an opportunity to examine solar cycle influence on the mid-latitude exosphere and to address accompanying questions concerning the degree to which the exosphere is locally static or changing. Our exospheric Balmer α absolute intensity measurements show no statistically significant variations throughout the solar cycle when the variation with viewing geometry is removed by normalizing the data to reference exospheric model predictions by Anderson et al. However, the relative intensity dependence on solar depression angle does show a solar cycle variation. This variation suggests a possible related variation in the exospheric hydrogen density profile, although other interpretations are also possible. The results suggest that additional well-calibrated data taken over a longer time span could probe low-amplitude variations over the solar cycle and test predictions of a slow monotonic increase in exospheric hydrogen arising from greenhouse gases.

Cascade excitation in the geocoronal hydrogen Balmer α line

This paper reports high-accuracy measurements of geocoronal Balmer α line profiles and demonstrates that the profiles are well fit with a model which includes cascade excitation by solar Lyman series radiation from n > 3 in addition to the direct excitation of n = 3 by solar Lyman β. The increase in the signal-to-noise of our data is made possible by the use of the Fabry-Perot annular summing technique implemented at our Fabry-Perot facility at the University of Wisconsins Pine Bluff Observatory. The new sensitivity has allowed us to make a detailed examination of line profile asymmetries and to conclude that they are compatible with predictions that of the order of 10% of the geocoronal Balmer α emission is caused by the cascade process. Cascade excitation alters the observed profile because it produces Balmer α emission along fine structure paths yielding slightly shifted wavelengths not present in direct Lyman β excitation, which is the predominant excitation mechanism for geocoronal Balmer a. We discuss how fine structure excitation affects studies of non-Maxwellian exospheric hydrogen velocity distributions and effective temperatures through Balmer α line profile measurements. In a broader context, we consider how inclusion of the cascade excited emission in future radiation models can enhance their accuracy and their potential for assisting in the isolation in the data of shorter-term solar geophysical effects and longer timescale changes in exospheric hydrogen densities.

Analysis of Balmer α intensity measurements near solar minimum

Abstract Balmer α intensity measurements made with a dual etalon Fabry–Perot spectrometer at Haleakala during two campaigns in 1988 are presented. The data from each campaign demonstrate night-to-night stability, despite variations in geophysical conditions. Analysis of these data using a nonisothermal Lyman β radiative transport code, updated solar Lyman β line-center flux estimates, and corrected thermospheric atomic hydrogen density profiles points to the resolution of the “factor of 2” problem. A careful reassessment of other mechanisms for upper atmospheric Balmer α excitation has also been carried out.

Geocoronal hydrogen Balmer-α line profiles obtained using Fabry-Perot annular summing spectroscopy: Effective temperature results

A Fabry-Perot annular summing spectroscopy technique has been used at the University of Wisconsins Pine Bluff Observatory to acquire geocoronal Balmer-α line profile data with significantly improved precision and height resolution. The double-etalon Fabry-Perot interference pattern is imaged onto a Photometries PM512 charge-coupled device (CCD) chip, thus enabling light to be gathered in multiple spectral bins simultaneously. In comparison with scanning systems we used earlier, the high quantum efficiency of the CCD and the multichannel detection associated with the Fabry-Perot annular summing technique have enabled us to save a factor of about 10 in the integration time required for studies of the line profile. As a result, we are now able to both more precisely observe the line shape of the very faint (1–10 R) Balmer-α emission and obtain data using shorter integration times. Our data illustrate the scientific potential for using this technique for the study of very faint extended emission line sources. We present exospheric effective temperatures obtained from line profile data acquired during the period of 1992–1993. When these data are compared with predictions from the Anderson et al. [1987] and MSIS90 models, there are points of agreement as well as some discrepancies. Included in this paper are discussions of both technical issues associated with applying annular summing spectroscopy for geocoronal Balmer-α observations and results of data obtained using this technique.

The geocoronal H α cascade component determined from geocoronal H β intensity measurements

Geocoronal H α and H β intensity measurements using the Wisconsin H α Mapper Fabry-Perot are used to determine the intensity of the H α cascade component. From basic atomic physics and the work of Meier (1995), we show that the total cascade in geocoronal H α emission is 0.52 ± 0.03 times the geocoronal H β intensity, I(H β), for solar Lyman series excitation of geocoronal hydrogen. The results are consistent with the H α cascade measurements of Mierkiewicz et al. (2012), which were determined directly from the analysis of H α line profile measurements, and significantly narrow the range of uncertainty in the cascade measurement. Accounting for cascade is essential in determining exospheric effective temperatures and dynamics from the shape of the geocoronal H α line.

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

A new, high-resolution field-widened spatial heterodyne spectrometer (FW-SHS) designed to observe geocoronal Balmer α (Hα, 6563 A) 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 A) 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 Earths 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.

Constraining Balmer Alpha Fine Structure Excitation Measured in Geocoronal Hydrogen Observations

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 Hα Mapper (WHAM) 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 hydrogens evaporative escape rate and exobase density). Index Terms: Hydrogen Geocorona, Balmer-alpha Line Profile, Fabry–Perot Interferometry