Distributions of Nighttime F‐Region Molecular Ion Concentrations and 6300‐Å Nightglow Morphology

Abstract

A numerical technique is employed in solving the coupled, nonlinear system of equations for the O+, NO+, and O2+ number densities in the nighttime F region, including the effects of diffusion, E×B drift, and neutral air motions. The best fits to the ‘sunset’ [Johnson, 1967] and ‘midnight’ [Holmes et al., 1965] rocket observations of the ion profiles obtained at White Sands, New Mexico, are achieved by iteratively varying the neutral wind, the electrodynamic drift, the neutral atomic nitrogen and nitric oxide concentrations, and the ion‐atom interchange and dissociative recombination rate coefficients. The best fit results for both sets of profiles with a Jacchia model atmosphere are achieved with the rate coefficients (cm3 sec−1) at 300°K given by γN2 = 1.1×10−12, γO2 = 2.0×10−11, αNO+ = 3.5×10−7, and αO2+ = 3.1×10−7, each with an assumed inverse temperature dependence. In addition, the ‘sunset’ best fit solutions require that the chemical time constant τ for reaction of O2+ with N and NO be 500 seconds at 200 km and that the net vertical plasma transport velocity wz due to E×B drift and a neutral wind be downward with a speed of 14 m sec−1. The midnight best fit solutions are achieved with τ = 2.5×103 at 200 km and wz = 10 m sec−1 upward. The 6300‐Å nightglow morphology in the inter‐tropical region is computed directly from the ion concentrations calculated for assumed drift and neutral wind models representative of equinoctial, sunspot‐minimum conditions. Isophote contours of zenith intensity are presented to illustrate the nightglow morphology associated with the decay of the Appleton anomaly. Calculated results are compared with the observations of Barbier [1964] to demonstrate the influence of electrodynamic drift on the morphology of nightglow enhancements. The results show that the electrodynamic drifts required to produce the observed enhancements are consistent with the drifts observed at Jicamarca.