Exobase hydrogen density temperature from Ly α Absorption and polarization measurements, 2. Dayside and nightside results during April 1971

Measurements of polarization and absorption by a hydrogen cell of the Lyman alpha geocoronal emission, performed on board the French spacecraft D2-A Tournesol over the working period of the tape recorder (April 19 to May 2, 1971), are compared with theoretical predictions given by spherical models of transfer using Kockarts-Nicolet and Chamberlain profiles. From polarization measurements the value of the hydrogen density at the exobase ρ500 and its latitudinal-diurnal variation are inferred. From absorption measurements, by using the exospheric temperature Tex of Jacchia, another independent way of deducing ρ500 variations is presented. Owing to calibration problems, an absolute measurement of Tex derived from absorption measurements is no longer available. However, Tex departures from the Jacchia temperatures are deduced from departures between the two values of ρ500 deduced from absorption and polarization measurements. Data obtained over the considered period of time provided ρ500 measurements independent of the solar flux. For example, the ρ500 density at noon near the equator is equal to (10 ± 2) 104 atoms cm−3. Intensity measurements at Ly α and 1304 A are also reported and are compared with the Oso 4 and 5 results. Measurements of the extrageocoronal Ly α emission are first made from the core of the geocorona. The values obtained during a satellite spin vary between 430 and 780 R. These measurements permit a ρ500 midnight density near the equator equal to (11 ± 3)104 atoms cm−3 to be deduced. The measured latitudinal-diurnal variation of ρ500 over 24 hours LT is compared with the previous experimental and theoretical values. The agreement is roughly good over 16 hours between 0900 LT and 0100 LT: the maximum occurs around 0900 LT, the minimum occurs near 1800 LT, and the midnight to midday ratio is around 1.1. However, the maximum to minimum density ratio ρmax/ρmin is higher than the expected one: 2.7 ± 0.8 as deduced from absorption measurements and 3.8 ± 1.2 from polarization measurements. The discrepancy between these two values can be interpreted by a ∼50°K departure from the Jacchia 71 temperature at southern latitudes in the afternoon. In the second part of the night, between 0100 LT and 0900 LT, a steep and large depression is observed; it can be interpreted as a local one around 35°N or a remote one, above the northern geomagnetic pole. This northern depression, the weak values of densities measured at southern latitudes, and the high density ratio ρmax/ρmin suggest the existence of a hydrogen bulge whose axis seems to be located around the sun-earth axis and to be linked to depressions over the poles.