Solar Lyman alpha changes and related hydrogen density distribution at the Earth's exobase (1969-1970)

International audienceThe University of Paris experiment, operating on board the Oso 5 spacecraft since January 22, 1969, observes the solar Lyman α line integrated over the whole solar disk. The hydrogen and deuterium resonance cells yield information about the solar flux at the center of the line and on the blue wing. The center of the solar line as observed from the spacecraft is strongly absorbed by the earth's geocorona. Therefore the hydrogen resonance data contain solar, as well as geocoronal, information. Comparison of the data with sophisticated exospheric models gives the following principal results: (1) A high correlation (0.87 coefficient) exists between the total Lyman α line flux and the center line flux. A 30% variation in the total flux corresponds to a 47% variation in the central flux. The average fluxes over 1969 and 1970 are 3 × 1011 ph cm−2 sec−1 total flux and 5 × 1011 ph cm−2 sec−1 A−1 central flux. (2) The absolute value of the hydrogen density at 500-km altitude (independent of the experiment's calibration) is of the order of 5 × 104 atoms cm−3 on the day side and 9 × 104 atoms cm−3 on the night side. (3) The earth's hydrogen exobase is controlled by the zero net ballistic flux condition (i.e., the outgoing ballistic flux is everywhere equal to the incoming ballistic flux). This is why an average ratio of 1.7 was found between the maximum and minimum densities at the exobase. A discrepancy appeared between the absolute values of the hydrogen density measured by this experiment and the corresponding absolute values predicted by the model of G. Kockarts and M. Nicolet (1963). In order to fit our data, the values of the model should be increased to the following hydrogen densities at an altitude of 100 km: 2.3 × 107 atoms cm−3 for low solar activity (Zurich number Rz ∼ 60); 2.5 × 107 atoms cm−3 for mean solar activity (Rz ∼ 120); and 4 × 107 atoms cm−3 for high solar activity (Rz ∼ 180)