He 584 Å Dayglow at Saturn: A Reassessment

Abstract

New calculations of the saturnian He 584 Å airglow intensity, using radiative transfer models with partial frequency redistribution and inhomogeneous atmospheric models, are presented. Our reference conditions assume a helium mixing ratio,fHe, of 0.033 and a He 584 Å solar flux of 2 × 109photons cm−2s−1at 1 AU, typical of the EUVAC model, with a solar line FWHM of 120 mÅ and an atmosphere consistent with the Voyager UVS occultation results. For these conditions we require the eddy diffusion coefficients at the homopause,Kh, to be greater than 109cm2s−1in order to fit the Ultraviolet Spectrometer measurements of Voyager 1 and 2 He 584 Å airglow measurements. These values ofKhare even higher than obtained in the earlier work of Sandelet al.(1982,Geophys. Res. Lett.9,1077–1080) and Atreya (1982,Planet. Space Sci.30,849–854) and seem unreasonably high. This suggests that either the values of one or more of the parameters of our model are not correct or the measured UVS airglow is too bright and that there is a problem with calibration. Even so, we suggest thatKhis likely to be greater than 108cm2s−1during the period of the Voyager encounters. Only if we increase the helium mixing ratio to the solar value and use the EUVT94 solar flux model for He 584 Å is it possible to begin to reconcileKhfrom the He 584 Å airglow and the work of Smithet al.(1983,J. Geophys. Res.88,8667–8678).