The effect of thermal escape on the neutral hydrogen density above 120 kilometers

Recent studies of the escape problem of hydrogen in the terrestrial atmosphere indicate that its effusion velocity at the base of the exosphere is noticeably reduced compared to the loss rate deduced from Jean's simple theory of escape and that this minor constituent is effectively cooled by evaporation. Consequently, the hydrogen density profile is considerably modified both in the thermosphere below the critical level, as well as in the exosphere above it. While the density of hydrogen in the thermosphere is enhanced because of the retarded loss, its cooling leads to a steeper gradient of its abundance in the exosphere. The over-all effect is, however, an increase in the total hydrogen abundance above its production region near the turbopause. Numerical results, presented here, show that the hydrogen content of the atmosphere above 120 km may be enhanced by a factor of ∼1.1 (at 2500°K) to ∼2.0 (at 1000°K) and suggest an even larger relative increase for the range of temperatures prevailing near solar minimum (600–800°K). Though the spherical symmetry of the atmospheric model underlying these results is subject to serious objections, both the predicted over-all enhancement of hydrogen density and its greater swing between temperature extrema appear to point in the right direction.