Interpretation of OGO-5 line shape measurements of Lyman-α emission from terrestrial exospheric hydrogen

The velocity distribution of hydrogen atoms in the terrestrial exosphere was measured as a function of radial distance (up to 7 Earth radii, ER) with the help of a Lyman-α hydrogen absorption cell, flown in 1968 on board the OGO-5 satellite. This paper contains the final analysis of the measurements. As a basis of comparison, the theory for the calculation of projected velocity distribution along a line of sight is established for the theoretical exospheric model of Chamberlain (1963). Self-absorption of Lyman-α photons along a line of sight is included to derive Lyman-α line profiles emerging from the geocorona. The effect of the hydrogen absorption cell, measured by the reduction factor R(p) is predicted as a function of impact parameter p of the line of sight, for various values of the parameters of a Chamberlain's model, nc (density of exobase level), Tc (temperature at the exobase level), and rcs (satellite critical radius). This predicted reduction factor R(p) is compared to the measured Rm(p), with the following findings: the Ly-α line width decreases with radial distance, as expected from the “evaporation and escape” theory of Chamberlain; the measured temperature Tc = 1080 K is in very good agreement with the exospheric temperature prediction from satellite drag data. An upper limit of 8 × 104at. cm−3 is imposed on nc, regardless of photometric absolute calibration. A good fit to data requires the presence of atoms in satellite orbits, distributed in a different fashion than that described by the concept of satellite critical radius. Lyman-alpha radiation pressure is thought to be the cause of this departure from the exospheric theory of Chamberlain (1963), otherwise perfectly confirmed. The same scientific rationale will be applied to exospheric hydrogen of the planets Mars and Venus in subsequent papers.