Cassini UVIS Update: He 584  Dayglow at Saturn

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

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 (Kh) differ: • 4x107 < Kh < 1.2x108 cm2 s-1 (Sandel et al, 1982; Atreya, 1982) • Kh < 1.2x108 cm2 s-1 (Smith et al, 1983)

A reassessment by Parkinson et al, 1998 show that Kh is likely > 108 cm2 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

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

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

Spectral representation Angstroms

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

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

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

It would appear that the Cassini He 584 brightness is lower now than during the Voyager epoch with the corresponding effects on Kh • 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

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

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.

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

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, Kh would be ~3x107 cm2s-1, and using the EUVT94 model solar flux, ~3x106 cm2s-1 for Cassini measurements.

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

Cassini UVIS Update: He 584  Dayglow at Saturn