Nocturnal Ionization in the $F_2$ Ionospheric Region

A critical survey was made of the upper atmosphere to an altitude of 300 km, reviewing present knowledge on such factors as composition, winds, temperature, pressuree and diffusion. On the basis of recent information a model atmosphere having a diurnal temperature cycle above 100 km was adopted.

Employing the adopted temperature-altitude function, it was possible to re-examine the rate of recombination occurring in the $F_2$ ionospheric layer. Considering a temperature-dependent recombination coefficient and including the effect of atmospheric contraction arising from the quotidian temperature variation, it was possible to reformulate the equation describing the rate of change of electron density. On this basis, the predicted value of electron density during darkness in the $F_2$ ionospheric layer agreed with the observed values very much better than did those obtained when the simple quadratic recombination law was employed.

In the course of the investigation, all possible sources of nocturnal ionization were examined. For the $F_2$ layer insufficient data are on hand to reveal the magnitude of horizontal transport phenomena, which may be of importance. Vertical transport is quantitatively treated. All other sources of ionization seem to be insufficient to maintain the observed ionization levels.