Helium, an important constituent in the lower exosphere

The atmospheric content of He4, of the order of 1.13×1020 cm−2 [Nicolet, 1957], produced by the radioactive decay of uranium and thorium, is obtained in less than 5×106 years. A continuous escape of the order of 106 He4 atoms cm−2 sec−1 can be supported by vertical diffusion in the thermosphere, and the vertical distribution of neutral helium atoms can, therefore, be determined by an application of the diffusive state equilibrium. In the homosphere, where the mixing state is prevalent, the ratio of helium and molecular concentrations n(He)/n(N2) is equal to 6.7×10−6. Such a ratio leads to a concentration n(He) = 7.4 × 107 cm−3 at 100 km when the partial pressure of molecular nitrogen is not less than 2.2×10−4 mm Hg.