Neutral hydrogen density profiles derived from geocoronal imaging

Measurements of the Lyman α column brightness by the Geocoronal Imager (GEO), part of the FUV imaging system on board the IMAGE satellite, have been used to derive an empirical model of the neutral hydrogen density distribution at high altitudes (>3.5 RE geocentric distance) on the night‐side of the Earth. The model presented is an effort to provide the density profiles needed to analyze the energetic neutral atom imaging data at ring current altitudes and above. The variable solar Lyman α flux is obtained from the UARS/SOLSTICE measurements and the scattered solar Lyman α emissions from interplanetary hydrogen are obtained from a model. Assuming that the exosphere at high altitudes (>3.5 RE geocentric distance) can be considered as an optical thin medium and that the hydrogen density profile can be expressed as a double exponential we show that the Lyman α column brightness can be converted to hydrogen density profiles. The hydrogen density above 5 RE is found to be slightly higher for large solar zenith angles than for 90° solar zenith angle. The hydrogen density shows temporal variations which are not controlled by any solar quantity or geomagnetic parameter alone. Our Lyman α profiles and derived hydrogen density profiles are close to what was observed by Dynamics Explorer 1 [Rairden et al., 1986]. Above 8 RE we find higher densities than they did for all solar zenith angles >90°. We do not find any evidence of depletion due to charge exchange with solar wind protons outside the magnetopause. Our results are only valid above 3.5 RE.