18.
The distribution of atomic hydrogen in the thermosphere and exosphere is computed taking into account the upward flow which balances the escape flux. Because of the upward flow the number-density gradient is much steeper than it would be in a static atmosphere. Attention is drawn to the fact that the ratio of the amount of hydrogen above the 100 or 110km levels to the amount of hydrogen above the 200 or 300 km levels is a sensitive measure of the temperature of the exosphere. The evidence on the absolute abundance of atomic hydrogen is examined. It is concluded that the number density at the 120km level is probably about 5 × 10
5/cm
3. The
Ly. absorption line at this level is beyond the linear part of the curve of growth.
Consideration is also given to the steady-state distributions of O+ and H+ ions. In the lower part of the exosphere the number density of O+ ions falls with increase in altitude (the associated scale height being twice that of the O atoms) and the number density of H+ ions rises at the same rate (as was first pointed out by Dungey). The altitude at which the number densities of O+ and H+ ions become equal is calculated on various assumptions regarding the temperature and hydrogen content of the exosphere. It is found to be about 1200 km when the temperature is 1250° K and the hydrogen content corresponds to the number density cited near the end of the preceding paragraph. The gradient of the predicted electrondensity distribution at several Earth radii is much less than that deduced from whistler studies.
The passage from charge transfer to diffusive equilibrium is discussed in an Appendix. 相似文献