Observations of hydrogen in the upper atmosphere.

Observations of hydrogen in the upper atmosphere which were reported since January 1974 are reviewed in the light of four basic questions. The most important results are the following. At the exobase level, the diurnal variation of density in low latitude regions is in agreement with current theories, whereas the latitude variation is not clearly understood. The hydrogen temperature at the exobase level is equal to the temperature of neutral atmosphere as derived by satellite drag data measurements. In the exosphere, the distribution of hydrogen atoms in satellite orbits is largely affected by Lyman-alpha radiation pressure. The absolute value of the thermal escape (Jeans escape flux) is lower than expected from current aeronomic theories, and its variations as a function of solar activity indicates that non-thermal escape mechanisms must exist with a significant contribution to total loss of H from the planet.