He 584 A Dayglow at Saturn: A Reassessment

Physics

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Scientific paper

New calculations of the saturnian He 584 A airglow intensity, using radiative transfer models with partial frequency redistribution and inhomogeneous atmospheric models, are presented. Our reference conditions assume a helium mixing ratio, f_He, of 0.033 and a He 584 A solar flux of 2 x 10^9 photons cm^-2 s^-1 at 1 AU, typical of the EUVAC model, with a solar line FWHM of 120 mA and an atmosphere consistent with the Voyager UVS occultation results. For these conditions we require the eddy diffusion coefficients at the homopause, K_h, to be greater than 10^9 cm^2 s^-1 in order to fit the Ultraviolet Spectrometer measurements of Voyager 1 and 2 He 584 A airglow measurements. These values of K_h are even higher than obtained in the earlier work of Sandel et 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 that K_h is likely to be greater than 10^8 cm^2 s^-1 during 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 A is it possible to begin to reconcile K_h from the He 584 A airglow and the work of Smith et al. (1983, J. Geophys. Res. 88, 8667-8678).

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