Direct measurement of hydrogen density at exobase level and exospheric temperatures from Lyman α line shape and polarization, 1. Physical background and first results on day side

Measurements of the polarization and line shape of the Lyman α hydrogen geocoronal emission obtained on board the satellite D2A Tournesol are compared with theoretical predictions using Kockarts-Nicolet (K-N) profiles. From these data the value of the hydrogen density at the exobase ρ500 can be inferred without a large error, whatever the model; the value of the exospheric temperature Tex, however, depends on the value of the gradient δρ/δz at the exobase level. Data obtained over one orbit (April 27, 1971) provide some values and features in agreement with other experimenters' measurements as follows: (1) ρ500 density at noon near the equator (11.5 ± 2.5)104 atoms cm−3, (2) ρ500 density variation consistent with a diurnal variation of 2 between 6 and 17 h LT, and (3) maximum exospheric temperature Tex late in the evening (1140° ± 100°K). However, the present data show two new features: (1) a density increase of 2 between southern latitudes (22°S–46°S) and northern latitudes (46°N to the equator) and (2) an altitude gradient of hydrogen density different from the gradient predicted by K-N theory in a region between 46°N to the equator.