He 584 Å dayglow at Neptune

The Voyager 2 Ultraviolet Spectrometer measured the emission intensity of the He resonance line at 584 Å to be 0.34‐0.15+0.2 R on the day side of Neptune. Our calculations of the He 584 Å intensity at Neptune using partial frequency redistribution and inhomogeneous atmospheric models show that the product of the volume mixing ratio (mole fraction), f_He, and the eddy diffusion coefficient at the homopause, K_h, is f_HeK_h = 10⁷ cm² s⁻¹ with upper and lower bounds to the uncertainty of about a factor of 3 and 9, respectively. If f_He is taken as the current Infrared Interferometer Spectrometer working value, f_He = 0.19, then K_h = 5×10⁷ cm² s⁻¹ with a similar uncertainty. This range of K overlaps that obtained from analysis of the hydrocarbon distributions on Neptune (Broadfoot et al., 1989).