Intensities of the Venusian N 2 electron‐impact excited dayglow emissions

Dayglow emissions are signatures of both the energy deposition into an atmosphere and the abundances of the species from which they arise. The first N 2 dayglow emissions from Mars, the (0,5) and (0,6) bands of the N 2 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 N 2 triplet state ( A 3Σ u + ; v ′ ) to the electronic ground state ( X 1Σ g + ; v ″ ) . 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 N 2 emissions have been detected. Because the N 2 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 N 2 emissions to be significantly more intense than those of Mars. A prediction of the intensities of various N 2 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 N 2 band systems and limb profiles of the Vegard‐Kaplan bands. The ratios of the predicted intensities of the various N 2 bands at Venus to those at Mars are in the range 5.5–9.5.