Calculations of N2 triplet states vibrational populations and band emissions in Venusian dayglow

TL;DR

This paper models N2 triplet state emissions in Venus's dayglow, calculating vibrational populations and band intensities under different solar conditions, and compares them with observations and other planetary data.

Contribution

The study introduces a detailed model for N2 triplet state emissions in Venus's dayglow, including cascading effects and vibrational populations, with predictions for intensities under various solar activities.

Findings

Calculated intensities of N2 triplet bands for solar minimum and maximum.

Model results agree with Pioneer Venus observations.

Venus's N2 emissions are about ten times stronger than on Mars.

Abstract

A model for N2 triplet states band emissions in the Venusian dayglow has been developed for low and high solar activity conditions. Steady state photoelectron fluxes and volume excitation rates for N2 triplet states have been calculated using the Analytical Yield Spectra (AYS) technique. Model calculated photoelectron flux is in good agreement with Pioneer Venus Orbiter-observed electron flux. Since inter-state cascading is important for the triplet states of N2, populations of different levels of N2 triplet states are calculated under statistical equilibrium considering direct electron impact excitation, and cascading and quenching effects. Densities of all vibrational levels of each triplet state are calculated in the model. Height-integrated overhead intensities of N2 triplet band emissions are calculated, the values for Vegard-Kaplan (A^3Sigma_u^+ - X^1Pi_g^+), First Positive (B^3Pi_g - A^3Sigma_u^+), Second Positive (C^3Pi_u - B^3Pi_g), and Wu-Benesch (W^3Delta_u - B^3Pi_g) bands of N2, are 1.9 (3.2), 3 (6), 0.4 (0.8), and 0.5 (1.1) kR, respectively, for solar minimum (maximum) conditions. The intensities of the three strong Vegard-Kaplan bands (0, 5), (0, 6), and (0, 7) are 94 (160), 120 (204), and 114 (194) R, respectively, for solar minimum (maximum) conditions. Limb profiles are calculated for VK (0, 4), (0, 5), (0, 6) and (0, 7) bands. The calculated intensities on Venus are about a factor 10 higher than those on Mars. The present study provides a motivation for a search of N2 triplet band emissions in the dayglow of Venus.