Determination of Atomic Hydrogen Densities in the MLT and Thermospheric Regions From Coincident Ground-Based and Satellite Airglow Data

Coincident ground- and satellite-based geocoronal hydrogen emission data are presented, along with forward-modeling analysis results. Atomic hydrogen plays several unique roles in the terrestrial atmosphere. For example, as a daughter of the important mesospheric minor species H2O and CH4, knowledge of the MLT atomic hydrogen density distribution and associated vertical flux may prove to be valuable in understanding the chemistry in that region. The interactions of the geocorona, plasmasphere, and ring current via atomic hydrogen transport provide another example of increasing interest, given the use of energetic neutral atom imaging in recent (e.g., NASA/IMAGE) and upcoming magnetospheric missions. Recent advances in Fabry-Perot instrumentation (e.g., annular summing spectroscopy) have greatly increased the quality and quantity of ground-based geocoronal Balmer α emissions data, and have enabled the first detailed ground-based measurements of the extremely faint (sub-Rayleigh) Balmer β airglow. Extensive Balmer α and β data sets have been obtained with two large aperture (15 cm), double-etalon, Fabry-Perot spectrometers located at Pine Bluff, WI (PBO) and at Kitt Peak, AZ (WHαM). Absolute intensity calibration has been made through comparisons with well-established nebular emission sources. Complementing these measurements, satellite missions have provided large data sets of FUV and EUV emission measurements, including Lyman line intensities. The EURD instrument for measurements of the diffuse interstellar radiation field, on the Spanish satellite MiniSAT-01, provided numerous data sets coincident with our ground-based measurements. Data and analysis results for early March 2000 are being presented. The Balmer α and β intensity measurements, through the intensity variations with solar depression angle and viewing geometry, provide tight constraints on thermospheric and exospheric atomic hydrogen abundances. The coincident EURD measurements of Lyman β and γ nightside intensities provide independent constraints given the different viewing schemes and optically thick character of the resonance transitions. Forward modeling using the lyao_rt radiative transport code is currently being applied to seek self-consistent fits to the diverse data sets and to assess the feasibility of extracting vertical fluxes. The PBO Fabry-Perot operates at high resolving power (˜ 85,000), allowing Balmer α line profile observations to be carried out. A significant decrease in Balmer α Doppler width with increasing shadow altitude is detected. Preliminary applications of lyao_rt indicate good agreement with Doppler width trends in the nonisothermal geocorona.