Measurements of the helium 584 Å airglow during the Cassini flyby of Venus

Abstract

The helium resonance line at 584Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320R, reaching ∼700R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×106cm−3 at the level of where the CO2 is 2×1010cm−3.