Photoelectric Ionization in the Ionosphere

Abstract

The ionization equilibrium in the various atmospheric gases is calculated from the Saha theory on the assumption that the spectral energy curve of sunlight for ultraviolet frequencies above the limits of the principal series is that of a black body at 6000°K. A calculated region of ionization due mainly to oxygen and partly to nitrogen is found at about 200 km with maximum electron density 1.6×${10}^6$, in agreement with $F_2$ observation in winter temperate latitudes. The assumption of an expansion of the high atmosphere under the sun, with resulting winds and diffusion, leads to a partly quantitative explanation of the observed seasonal changes in $F_2$, and to a qualitative explanation of $F_1$. In the case of $E$ region the equations do not indicate what radiation causes the ionization nor what gases are ionized. This may be due to the omission in the theory of the possibility of ionization by frequencies below the series limits. Calculation suggests that $E$ ionization is mainly ionic resulting from two processes, the first being the formation by the ionizing radiation of positive ions and free electrons, and the second being the formation of negative ions by the attachment of the electrons to oxygen molecules.