Intensities of the Venusian N2 electron‐impact excited dayglow emissions

Abstract

Dayglow emissions are signatures of both the energy deposition into an atmosphere and the abundances of the species from which they arise. The first N2 dayglow emissions from Mars, the (0,5) and (0,6) bands of the N2 Vegard‐Kaplan band system, were detected by the Spectroscopy for Investigations of the Characteristics of the Atmosphere of Mars (SPICAM) UV spectrometer on board the Mars Express spacecraft. The Vegard‐Kaplan band system arises from the transition from the lowest N2 triplet state to the electronic ground state . It is populated by direct electron‐impact excitation and by cascading from higher triplet states. The Venus UV dayglow is currently being probed by an instrument similar to SPICAM, the Spectroscopy for the Investigations of the Characteristics of the Atmosphere of Venus (SPICAV) UV spectrometer on Venus Express, but no N2 emissions have been detected. Because the N2 mixing ratios in the Venus thermosphere are larger than those in the thermosphere of Mars and the solar flux is greater at the orbit of Venus than that at Mars, we expect the Venus N2 emissions to be significantly more intense than those of Mars. A prediction of the intensities of various N2 emissions from Venus could be used to guide observations by the SPICAV and other instruments that are used to measure the Venus dayglow. Employing updated data, we here construct models of the low and high solar activity thermospheres of Venus, and we compute the integrated overhead intensities of 17 N2 band systems and limb profiles of the Vegard‐Kaplan bands. The ratios of the predicted intensities of the various N2 bands at Venus to those at Mars are in the range 5.5–9.5.