He 584 Angstroms Dayglow at Saturn: a Reassessment

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New calculations of the Saturnian He 584 Angstroms airglow intensity, using radiative transfer models with partial frequency redistribution and inhomogeneous atmospheric models, are presented. For our standard conditions the results indicate that the eddy diffusion coefficient at the homopause, K_h, lies in the range 4.3x 10(7) to 1.2x 10(8) cm(2) s(-1) for Voyager 1 and 2x 10(8) to 5x 10(8) cm(2) s(-1) for Voyager 2 brightness measurements. Our standard conditions assume a helium mixing ratio, fHe, of 0.033, a He 584 Angstroms solar flux of 4x 10(9) photons cm(-2) s(-1) at 1 AU with a solar line 1/e half-width of 73 m Angstroms, and an atmosphere consistent with the Voyager UVS occultation results. The results of the earlier work of Sandel et al. (1982) and Atreya (1982) are in reasonable accord with this work. Sandel et al. (1982) used an isothermal atmosphere and a simpler scattering model with a limited analysis of stellar occultation data. We have also considered variations in the model atmosphere about the standard one obtained from the occultation data and find that the result is robust. This result underscores the original difference found for values of K_h obtained from the He 584 Angstroms airglow and the occultation data (Smith et al., 1983), which yielded a much smaller value of 5x 10(6) cm(2) s(-1) . However, we note that there is a possibility that the He 584 Angstroms line center solar flux could be lower by a factor of two than our standard value. In this case, K_h would have to be ~ 10(9) cm(2) s(-1) or greater. Hence, it would be very difficult to fit the Voyager He 584 Angstroms airglow data with reasonable values of K_h and the standard value of fHe. Increasing fHe to 0.06, the upper limit of the IRIS estimate, helps to reduce the descrepancy between measurement and model results, but is far from ameliorating the problem. In any case, the inference of high values of K_h remains, and also suggests that future work modelling the Saturnian thermosphere should consider the higher K_h values as more appropriate.

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