Nitrogen in the upper atmosphere

Abstract

An attempt has been made to find out from theoretical considerations the probable distribution of atomic nitrogen in the upper atmosphere. Two mechanisms of production of atomic nitrogen are considered: 1. (i) predissociation of N 2 molecules by absorption in the Lyman-Birge-Hopfield band within the range 1150–1250 Å as suggested by Herzberg & herzberg, and, 2. (ii) dissociative recombination of N 2 + ions and electrons as suggested by Mitra. It is shown that the N atoms thus produced will disappear largely through two-body radiative recombination process. The method of calculation employed for determining the distribution is a modification of the one adopted by Penndorf in estimating the dissociation of molecular oxygen in the upper atmosphere. The results show that while Herzberg & Herzbergs mechanism is operative within the range of heights 70–170 km, that of Mitra is effective in the much higher range—200–400 km. The dissociation in the former region is found to be less than 1% and the distribution of N atoms is found to have a sharp maximum round the height of 90 km. The dissociation in the latter region, on the other hand, is found to increase from 5% at 170 km to about 25% at 400 km, the distribution not showing any maximum (the rise is at first sharp; this is followed by a very slow but continuous increase with height). Calculations also show that the total number of N atoms in a vertical column of one cm 2 cross-section is about 10 16. Estimate has also been made of the abundance of N 2 + ions in the upper atmosphere. It is shown that although these are largely removed through the dissociative recombination process, a sufficient number is still left to account for the observed twilight flash of negative bands of N 2 + ions. The probable time required for the attainment of equilibrium of the various processes involved in the calculation is also considered. It is found that the concentration of N 2 + ions reaches an equilibrium value very quickly, but it requires years for the concentration of N atoms to reach the same.