The Venus nitric oxide night airglow: Model calculations based on the Venus thermospheric general circulation model

Abstract

Pioneer Venus (PV) orbiter ultraviolet spectrometer (OUVS) images of the nightside airglow in the (0, 1) δ band of nitric oxide showed a maximum whose average location was at 0200 local solar time just south of the equator. The average airglow brightness calculated over a portion of the nightside for 35 early orbits during the Pioneer Venus mission was a factor of 4 lower than this maximum. Recent recalibration of the PV OUVS instrument and reanalysis of the data yield new values for this statistical maximum (1.9±0.6 kR) and the nightside average (400–460± 20 R) nightglow. This emission is produced by radiative recombination of N and O atoms transported from their source on the dayside to the nightside by the Venus thermospheric circulation. The Venus Thermospheric General Circulation Model (VTGCM) has been extended to incorporate odd nitrogen chemistry in order to examine the dynamical and chemical processes required to give rise to this emission. Its predictions of dayside N atom densities are also compared with empirical models based on Pioneer Venus measurements. Calculations are presented corresponding to OUVS data taken during solar maximum (F10.7 = 180–200). The average production of nitrogen atoms on the dayside is about 9.0×109 atoms cm−2 s−1. Approximately 30% of this dayside source is required for transport to the nightside to yield the observed dark‐disk nightglow features. The statistical location and intensity of the bright spot are well reproduced, as well as the altitude of the airglow layer. The importance of the large‐scale transport and eddy diffusion on the global N(4S) distribution is also evaluated. Uncertainties in measured rate coefficients and branching ratios and their effects on the calculated airglow are discussed. It is found that model parameters can be adjusted to reproduce either the OUVS nightglow or the observed dayside N densities; a best match to both implies a VTGCM global wind system that is a factor of 1.5–2.0 slower than at present. Predictions are also made for comparison with OUVS solar minimum data.