Doppler line profiles in a planetary corona: An extended approach

Abstract

Measurements of the line profile of the geocoronal Lyman alpha emission by the hydrogen cell aboard Ogo 5 and a marked day‐night asymmetry in the measured intensity distribution have led Bertaux (1974, 1978) to conclude that the population of satellite particles in the exosphere cannot be described by any single value of the satellite critical radius (Chamberlain, 1963). Rather, he concludes (on the basis of a computer simulation of the effect) that Lyman alpha radiation pressure from the sun destroys satellites in a complicated manner which depends upon the initial orbital parameters of each particle and that the satellite critical radius formalism may no longer be a viable way to distinguish a satellite population. He does not, however, consider the alteration of the profile due to proton‐neutral charge exchange when the ionic temperature differs from that of the neutrals (Chamberlain, 1977; Prisco and Chamberlain, 1978), nor does he take into account the fact that the satellite temperature may differ from that of the remaining neutral population. This investigation takes the point of view that Bertaux's interpretation of his data may not be unique. In particular, the influence of these last two effects on the inferred profile (characterized by a ‘reduction factor’) is examined, and several conclusions are reached. First, we find that the perturbation to the reduction factor by charge exchange among the ballistic and escaping populations is negligible. Second, we are able to reproduce the Ogo 5 data satisfactorily by introducing a satellite population characterized by λcs=1.5 and a temperature which equals the neutral temperature beyond about 6 RE but increases inward to about 1.2 times the neutral temperature at 2 RE. We also argue that radiation pressure effects are symmetrical and hence are unlikely to be the cause of the observed asymmetry. Thus the question of how best to describe a satellite population appears still to be open.