Local perturbations of the atomic hydrogen density distribution near the exobase, inferred from D2A Lyman α airglow measurements

The atomic hydrogen density variation near the exobase has been inferred from measurements of both polarization and resonance‐cell absorption of the hydrogen Lyman‐alpha geocoronal emission, performed onboard the D2A satellite. Comparison of simultaneous measurements made in three different lines‐of‐sight, on April 27, 1971, have shown firstly, a steep North pole depletion at 0600 L.T., with a density ratio of a factor of 2.4 between midlatitude and North pole densities, and secondly, a non‐sinusoidal diurnal variation with two steep, local density depletions occuring around 0400 L.T. (+ 40° latitude), and around 1600 L.T. (− 20°). These observations, compared to earlier measurements of hydrogen density and correlated parameters, suggest that the non‐thermal loss processes are dominant in determining the density distribution, and subject to large and local perturbations.