Neutral hydrogen density profiles derived from geocoronal imaging

N. Ostgaard, S. B. Mende, H. U. Frey, T. J. Immel, G. R. Gladstone

Space Sciences Laboratory, University of California, Berkeley 
(nikost@ssl.berkeley.edu)


The geocorona is produced when solar Lyman alpha radiation is resonance scattered 
by exospheric neutral hydrogen. In this paper the Lyman alpha column brightness 
measurements from the Geocorononal Imager (GEO), part of the FUV imaging system 
on-board the IMAGE satellite, have been used to present an empirical model of the 
neutral hydrogen density distribution at high altitudes. The variable solar Lyman 
alpha flux is obtained from the UARS/SOLSTICE measurements and the scattered 
solar Lyman alpha emissions from interplanetary hydrogen is obtained from a model. 
Assuming that the exosphere at high altitudes (>3 RE) 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 alpha column brightness can easily be 
converted to hydrogen density profiles. The hydrogen density distribution in the 
nightside of the Earth is found to be cylindrical symmetric around the Sun-Earth 
line with a ~40% increase at 180 deg solar zenith angle (compared to 90 deg) above 
6-7RE. The hydrogen density shows temporal variation which is not controlled by 
any single solar quantity alone. Our hydrogen density profiles show densities in 
the upper range of what has been modeled by others. Our results are valid above 
3 RE.

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Presented at the 34th COSPAR Scientific Assembly, Houston, Texas, U.S.A.  
10-19 October 2002