He 584 Å Dayglow at Saturn: a Reassessment

C. D. Parkinson, E. Griffioen, J. C. McConnell (York University), G. R. Gladstone (Southwest Research Institute), B. R. Sandel (Lunar and Planetary Laboratory)

New calculations of the Saturnian He 584 Å 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 tex2html_wrap_inline22 , lies in the range 4.3 tex2html_wrap_inline24 to 1.2 tex2html_wrap_inline26 cm tex2html_wrap_inline28 s tex2html_wrap_inline30 for Voyager 1 and 2 tex2html_wrap_inline26 to 5 tex2html_wrap_inline26 cm tex2html_wrap_inline28 s tex2html_wrap_inline30 for Voyager 2 brightness measurements. Our standard conditions assume a helium mixing ratio, f tex2html_wrap_inline40 , of 0.033, a He 584 Å solar flux of 4 tex2html_wrap_inline42 photons cm tex2html_wrap_inline44 s tex2html_wrap_inline30 at 1 AU with a solar line 1/e half-width of 73 mÅ 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 tex2html_wrap_inline22 obtained from the He 584 Å airglow and the occultation data (Smith et al., 1983), which yielded a much smaller value of 5 tex2html_wrap_inline50 cm tex2html_wrap_inline28 s tex2html_wrap_inline30 . However, we note that there is a possibility that the He 584 Å line center solar flux could be lower by a factor of two than our standard value. In this case, K tex2html_wrap_inline22 would have to be tex2html_wrap_inline58 cm tex2html_wrap_inline28 s tex2html_wrap_inline30 or greater. Hence, it would be very difficult to fit the Voyager He 584 Å\ airglow data with reasonable values of K tex2html_wrap_inline22 and the standard value of f tex2html_wrap_inline40 . Increasing f tex2html_wrap_inline40 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 tex2html_wrap_inline22 remains, and also suggests that future work modelling the Saturnian thermosphere should consider the higher K tex2html_wrap_inline22 values as more appropriate.