Abstract
Spatial variations of the atomic hydrogen Lyman-alpha airglow were observed at 550 km from the OSO-4 spacecraft. Near the subsolar position, the zenith, horizon and nadir emission rates were 27,50 and 35 kR, respectively, while near the antisolar position, the respective emission rates were 2·0, 2·7 and 1·2 kR on 27 October 1967. Analyses of these results shows that the mean vertical optical depth above satellite altitude is in the range 0·87–3·5 with 1·3 as the preferred value. Atomic hyrdogen undergoes a diurnal density variation of the order of a factor of 1·7 with a maximum in the early morning and a minimum in the afternoon. A hydrogen altitude profile corresponding to 1100 K exospheric temperature is adequate to explain the data. Temporal variations of the airglow observed from OGO-4 show that the solar line-center Lyman-alpha flux should typically increase by 50 per cent for an increase in Zurich sunspot count from 30 to 210.
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