Night-time ionization in the lower ionosphere-I: recombination processes

Abstract

Both height and time variations of the effective recombination coefficient at night for the lower ionosphere are obtained from ionospheric data involving the use of radio waves over a wide range of frequencies. The experimental data used include measurements on ionospheric absorption at 18.3 Mc/s and 150 kc/s; hourly values of the night-time E-region critical frequencies for Watheroo, phase height observation at 16 kc/s, and virtual height measurements at 60 kc/s and 150 kc/s. It is found that although at levels near 80 km the effective recombination coefficient increases appreciably immediately after sunset, at higher levels the coefficient decreases. There are both height and time variations of the coefficient. Theoretical interpretation of these results is based on the idea, recently put forward by Nicolet, that an appreciable number of atomic ions of low ionization potential exist in the lower ionosphere. While the positive molecular ions disappear rapidly through dissociative recombination with electrons, the positive atomic ions disappear very slowly.