1. Cassini UVIS Update: He 584  Dayglow at Saturn Christopher Parkinson Ian Stewart and Yuk Yung January 05, 2006

2. Before Voyager 1 and 2 epoch UVS He 584 A measurements showed brightness at disk center to be 3.1 +/- 0.4 and 4.2 +/- 0.5 R, respectively (Sandel et al., 1982) Previous Voyager determinations of eddy diffusion at the homopause (K h ) differ: –4x10 7 < K h < 1.2x10 8 cm 2 s -1 (Sandel et al, 1982; Atreya, 1982) –K h < 1.2x10 8 cm 2 s -1 (Smith et al, 1983)

3. A reassessment by Parkinson et al, 1998 show that K h is likely > 10 8 cm 2 s -1 during Voyager encounters Main uncertainties are the –integrated He 584 solar line flux –planetary mixing ratio of He in the deep atmosphere –unknown uncertainties in the airglow measurement due to calibration

4. Now More precise, recent Cassini measurements give us the ability to use helium as a effective diagnostic tool to say something more conclusive about the dynamics in the Saturnian atmosphere, ameliorating previous difficulties

5. UVIS Data collection XUV summary structure file data 1024x64 Rows are spatial, columns spectral

6. Spectral representation Angstroms

7. Spatial representation: edge of planet clearly at row 16 and 46 for both EUV and FUV S N Ring shadow effects In the northern hemisphere?

8. Total count rate over 1 “movie” segment (450 min)

9. Data analysis A series of Saturnian observations totalling 60 hours has been looked at He 584 A line IS present with sunlit disk averaged brightness of 0.77 +/- 0.13 R obtained (5-sigma detection) Hard work needed to get more than a long term disk average out of data

10. It would appear that the Cassini He 584 brightness is lower now than during the Voyager epoch with the corresponding effects on K h Possible causes could be –shadowing of the northern hemisphere of the planet by the rings, which wasn't the case during the Voyager measurements, –seasonal variations, viz., the data examined from the Cassini mission falls approximately 24 and 23 years following the Voyager encounters. –differences in the solar flux

11. Shadowing by the rings As seen from Earth, the rings change orientation with a period of 29.5 years after being twice edge on at 15.5 and 13.5 years. Voyager epoch measurements the rings we edge on, whereas the northern polar region is shadowed now

12. Seasonal variation A Saturnian year is 29.5 earth years and Voyager measurements were 24.5 and 23.5 years ago indicating seasonal differences Comparing to the earth, if the Cassini measurements were corresponding to Jan 01, the Voyager encounters would have been occurring sometime in March, which is an appreciable difference.

13. Solar flux Voyager encounter occurred during a period of solar maximum Cassini measurements were taken approximately 2 years following solar maximum, so the solar flux is somewhat reduced from solar max

14. Preliminary results Compare ~ 1 R for Cassini to the Voyager values shown in Figures 4 and 5 in Parkinson et al., 1998 For their standard parameters, K h would be ~3x10 7 cm 2 s -1, and using the EUVT94 model solar flux, ~3x10 6 cm 2 s -1 for Cassini measurements.

15. C C

16. Issues and Ongoing work Current "best estimate" for the He mixing ratio needs to be quantified, but Figure 5 in that paper explores a parameter space of mixing ratios, from which we can obtain a preliminary estimate if it deviates from the standard value quoted there. Need to properly quantify solar flux and effects of ring shadowing in calculations