Spectroscopy of the N₂ Vegard-Kaplan bands in the dayglow

Abstract

A synthetic spectrum of the N₂ Vegard-Kaplan (VK) bands is developed for dayglow conditions at thermospheric altitudes. Due to the change in electron spin, the A³Eu state is metastable (lifetime 2-3 s.) and excited from the X¹Eg⁺ ground state primarily by photo-electron impact. Cascade from higher-energy triplets contributes to the A³Eu⁺ population and, due to its long lifetime, losses due to quenching are significant. Because of quenching, the VK bands were the last of the major N₂ emissions to be observed, as is explained in an historical review of the work of Vegard and Kaplan. Taking as inputs the solar soft x-ray measurements of the Student Nitric Oxide Explorer (SNOE), the model computes steady-state, ro-vibrational A³Eu⁺ populations and generates the synthetic VK spectrum in Rayleighs. Comparison with observations is hampered by a lack of VK data for days within the SNOE mission (1998-2003). As such, model results for a day in 1999 are compared with VK observations from 1992. The error is within 50% for altitudes below 150 km, above which it steadily decreases to 8% at 280 km. These errors are reasonable considering the unknown solar conditions of the 1992 observations.