Neutral hydrogen density profiles derived from geocoronal imaging 

N. Ostgaard, S.B. Mende, H.U. Frey, G.R. Gladstone, and H. Lauche


Measurements of the Lyman a 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 a flux is obtained from the UARS/SOLSTICE measurements and
the scattered solar Lyman a 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
a 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 a 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. 

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Journal of Geophysical Research, 108(A7), 1300, doi:10.1029/2002JA009749, 2003.