Model calculation of N₂ Vegard-Kaplan band emissions in Martian dayglow

Abstract

A model for N₂ Vegard-Kaplan (VK) band (A³Σ_u⁺−X¹Σ_g⁺) emissions in Martian dayglow has been developed to explain the recent observations made by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) ultraviolet spectrograph aboard Mars Express. Steady state photoelectron fluxes and volume excitation rates have been calculated using the analytical yield spectra technique. Since interstate cascading is important for triplet states of N₂, the population of any given level of N₂ triplet states is calculated under statistical equilibrium considering direct excitation, cascading, and quenching effects. Relative population of all vibrational levels of each triplet state is calculated in the model. Line of sight intensities and height-integrated overhead intensities have been calculated for VK, first positive (B³Π_g−A³Σ_u⁺), second positive (C³Π_u-B³Π_g), and Wu-Benesch (W³Δ_u−B³Π_g) bands of N₂. A reduction in the N₂ density by a factor of 3 in the Mars thermospheric general circulation model is required to obtain agreement between calculated limb profiles of VK (0-6) and SPICAM observation. Calculations are carried out to asses the impact of model parameters, namely, electron impact cross sections, solar EUV flux, and model atmosphere, on the emission intensities. Constraining the N₂/CO₂ ratio by SPICAM observations, we suggest the N₂/CO₂ ratios to be in the range 1.1-1.4% at 120 km, 1.8-3.2% at 140 km, and 4-7% at 170 km. During high solar activity the overhead intensity of N₂ VK band emissions would be ~2.5 times higher than that during low solar activity.