Lyman α airglow emission: Implications for atomic hydrogen geocorona variability with solar cycle

Satellite-based measurements of geocoronal Lyman α (Ly _α) emission at 121.6 nm, created through multiple scattering of solar Ly_α photons by atomic hydrogen, offer a valuable means of inferring the hydrogen abundance, [H], in the terrestrial thermosphere and exosphere on a global, long-term basis. We present initial results from an analysis of Ly_α radiance measurements acquired across the Earth's limb from 2002 to 2007 by the Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. This data spans nearly half of a solar cycle, and both the absolute Ly_α radiance as well as its relative variation across the limb are shown to exhibit a significant dependence on solar activity. We describe sensitivities of a forward radiative transport (RT) model to key parameters governing the [H] distribution in order to assess implications for [H] estimation from the GUVI limb scan data throughout the solar cycle. Based on data-model comparisons, we conclude that the observed solar cycle variability is indicative of a decrease in dayside H density at the exobase with increasing solar activity. These results, along with additional forward RT modeling based on NRLMSISE-00 model specification of [H], are also used to assess contemporary semiempirical model accuracy. Key PointsDayside H density decreases by a factor of ~4 from solar min to max.The MSIS model overestimates dayside H density.The GUVI data support the existence of an extended geotail on the nightside.