An ion‐molecule mechanism for the formation of neutral sporadic Na layers

Abstract

This paper describes a laboratory study into the chemical pathways by which Na+ is converted to Na in the upper atmosphere. The termolecular clustering reactions of Na+ with N2, O2, and CO2 were studied in a low‐temperature fast flow reactor coupled to a quadrupole mass spectrometer. This yielded k(Na+ + N2 + He, 93–255 K) = (1.20±0.13) × 10−30 (T/ 200 K)−(2.20±0.09), k(Na+ + O2 + He, 93–130 K) = (5.20±2.62) × 10−31 (T/ 200 K)−(2.64±0.74), k(Na+ + CO2 + He, 158–300 K) = (9.05±1.38) × 10−30 (T/ 200 K)−(2.84±0.48), where are cm6 molecule−2 s−1 and the stated errors are a combination of the 2σ standard errors in the kinetic data and the systematic errors in the temperature, pressure, and flow rates. It was then shown that atomic O will ligand switch with Na. N2+ but not with Na.CO2+, and that the former reaction proceeds essentially at the Langevin collision frequency. The neutralization of Na+ in the upper atmosphere is therefore rather complex. The first step is formation of the Na.N2+ ion from the recombination of Na+ with N2. This cluster ion can then either switch with O2, which leads to a stable cluster ion that will undergo dissociative electron recombination to form Na; or switch with atomic O, which reforms Na+. The result of this is that the lifetime of Na+ changes very rapidly from more than a day above 100 km to just a few minutes at 90 km. Furthermore, the rate of neutralization is largely independent of the electron concentration. A simple model describing the conversion of Na+ to atomic Na in a descending sporadic E layer demonstrates that this ion‐molecule mechanism appears to fulfil many of the major criteria for producing sporadic sodium layers.