1. Saturn’s Auroras from the Cassini Ultraviolet Imaging Spectrograph
Wayne Pryor
Robert West
Ian Stewart
Don Shemansky
Joseph Ajello
Larry Esposito
Joshua Colwell
William McClintock
Alain Jouchoux
Candice Hansen
Frank Crary
William Kurth
John Clarke
Kevin Baines
Denis Grodent
Emma Bunce
Presented at MOP, August 2005

2. Abstract
Cassini's Ultraviolet Imaging Spectrograph (UVIS) is making detailed studies of Saturn's auroras. Two long slit spectral channels obtain EUV data from nm and FUV data from nm. 64 spatial pixels along each slit are combined with slit motion to build spectral images of Saturn, with sufficient spatial resolution to reveal Saturn's auroral oval. Observed emissions include H Lyman-alpha and H2 bands from Saturn's auroras and dayglow. The auroral spectrum is similar to that of Jupiter, showing short-wavelength FUV absorption due to methane, CH4. Saturn's auroral and dayglow spectra show significant differences. Saturn's aurora is observed to vary in brightness by at least a factor of four. The brightest auroral emissions seen so far occurred after 2004 day :30 when Cassini CAPS recorded passage of a solar wind shock. The enhanced auroral brightness persisted for days, and is seen at both poles of Saturn. Cassini RPWS observed enhanced auroral kilometric emissions during several auroral brightening events seen by UVIS. A campaign of Hubble Space Telescope UV imaging with ACS (Advanced Camera for Surveys) of Saturn's dayside southern auroral zone took place on 2005 February 17. Cassini UVIS and VIMS observed Saturn's nightside northern aurora during this period. The UVIS long slit was aligned with lines of latitude on Saturn, providing information about intensity and spectral variations along the auroral oval.
Two recent auroral images show a full oval with a variable polar cap.

3. UVIS is on the Cassini Orbiter

4. UVIS long-slit spectroscopy
EUV channel nm
FUV channel nm
With 64 spatial x 1024 spectral pixels
Spectral imaging is possible during spacecraft slews
Saturn’s emissions:
H Lyman-a and H2 bands from auroras and dayglow.
Saturn’s reflected sunlight spectrum:
Rayleigh scattering in H2 modulated by acetylene absorption bands

5. UVIS H2 band data on Saturn

6. Sample Saturn Spectral Images EUV FUV

7. UV Saturn image July 13, 2004
H Lyman-a
H2 bands

8. Saturn H Lyman-a

9. Saturn UV Oxygen 130 nm image

10. Saturn Day 172, 2005 N-S-N scan Slit E-W Auroral oval imaged twice
Images deconvolved
Blue H2, H emission
Orange reflected sunlight
Aurora changes over ~1 hour

11. Polar Dark Spot (2005 day 172)
ratio 1st/2nd
wavelengths (A)
Spot in 1st image is gone at longer wavelengths (2nd image)
Localized small hydrocarbons? (spectrum is noisy)

12. ISS south pole methane, uv3 images

13. 50 day time-series moving away from Saturn near phase angle 90...

14. UVIS auroral time-series as Cassini recedes from Saturn…

15. UVIS, RPWS and MAG trends http://saturn. jpl. nasa

16. EUV, SKR, CAPS solar wind

17. EUV, SKR, CAPS solar wind

18. Saturn Auroral Spectrum

19. Color Ratio Trend

20. Hubble Auroral Campaign
John Clarke, J.-C. Gerard program: 5 orbits HST ACS UV dayside auroral Feb 17, 2005 tied to Cassini night side observations
Cassini VIMS_003SA_THRCYLMAP001_UVIS_FOV was 8 hours 22 minutes long night side observation
Start time T23:08:00 GMT
~800,000 km range
Aurora was relatively weak that day

21. HST Campaign ACS Images: Feb 17, 2005
16:20-16:58
17:53-18:33
19:29-20:09
21:04-21:45
22:40-23:21

22. VIMS_003SA_THRCYLMAP001HST Campaign UVIS Geometry

23. HST Campaign UVIS spatial images
Time increases upwards
VIMS mosaic leads to repeated UVIS passes across oval
<-East-West->
EUV FUV

24. UVIS dark-side spectrum

25. Conclusions
Saturn has extended H, O clouds Polar auroras vary over ~1 hour time scale Small polar dark spot appears near 1750 ± 75 Angstroms (hydrocarbons?) Auroral brightness varies by a factor of ~3 Daily-averaged auroral emissions vary with SKR (RPWS) Auroras respond to solar wind shocks seen in MAG + CAPS Auroras brightest during (and after) solar wind shock 2004 d :30 Auroral spectrum like Jupiter’s, with CH4 absorption Auroral color ratio ( A/ A) steady, near ~3±0.3 (2.0 for HST campaign period) : ~20 keV electrons

26. References
Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation. Space Sci. Reviews, 115, , 2004.
Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, , 2005.
Ajello, J. M., et al., The Cassini Campaign Observations of the Jupiter Aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph, Icarus (in press), 2005.
Pryor, W. R., et al., Cassini UVIS Observations of Jupiter’s Auroral Variability, Icarus (in press), 2005.

27. Future Work Auroral studies from higher inclinations
Auroral time-dependence, longitude-dependence
High-latitude absorber studies (acetylene, haze, ?)
H cloud variability (source(s): rings + moons + hot H from sunlit Saturn due to photoelectron impact on H2)
Comparison with Saturn atmosphere models

28. Upcoming UVIS Auroras 2004-196T06:26:00..UVIS_000SA_AURORAB001_PRIME
T02:21:00..UVIS_000SA_AURORAA002_PRIME
T16:16:00...UVIS_000SA_AURORAA003_PRIME
T07:41:00...UVIS_000SA_AURORAB002_PRIME
T00:33:00..UVIS_042SA_NAURMOV001_PRIME
T09:10:00..UVIS_045SA_NAURMOV001_PRIME
T23:31:00..UVIS_054SA_EUVFUV001_PRIME
T15:18:00..UVIS_055SA_NAURMOV001_PRIME
T05:36:00..UVIS_058SA_NAURMOV001_PRIME
T08:18:00..UVIS_065SA_NAURMOV001_PRIME
T06:30:00..UVIS_067SA_NAURMOV001_PRIME

29. Saturn EUVFUV’S 2005-087T07:46:00..UVIS_005SA_EUVFUV001_PRIME
T03:30:00..UVIS_010SA_EUVFUV003_PRIME (the oval pictures)
T01:00:00..UVIS_010SA_EUVFUV002_PRIME
T21:15:00..UVIS_011SA_EUVFUV002_PRIME
T05:50:00..UVIS_012SA_EUVFUV001_PRIME
T20:27:00..UVIS_016SA_EUVFUV001_PRIME
T17:48:00..UVIS_016SA_EUVFUV003_PRIME
T16:57:00..UVIS_019SA_EUVFUV004_PRIME
T13:13:00..UVIS_020SA_EUVFUV003_PRIME
T15:28:00..UVIS_020SA_EUVFUV004_PRIME
T10:36:00..UVIS_021SA_EUVFUV005_PRIME
T16:05:00..UVIS_022SA_EUVFUV003_PRIME
T07:49:00..UVIS_022SA_EUVFUV005_PRIME
T06:11:00..UVIS_024SA_EUVFUV005_PRIME
T20:41:00..UVIS_024SA_EUVFUV002_PRIME
T02:37:00..UVIS_025SA_EUVFUV002_PRIME
T01:06:00..UVIS_025SA_EUVFUV005_PRIME
T00:21:00..UVIS_026SA_EUVFUV007_PRIME
T23:56:00..UVIS_026SA_EUVFUV006_PRIME
T01:51:00..UVIS_027SA_EUVFUV003_PRIME
T01:36:00..UVIS_027SA_EUVFUV002_PRIME
T00:16:00..UVIS_027SA_EUVFUV006_PRIME
T18:27:00..UVIS_031SA_EUVFUV003_PRIME
T21:03:00..UVIS_032SA_EUVFUV003_PRIME

30. Saturn EUVFUV’s (2) 2007-034T12:10:00..UVIS_038SA_EUVFUV001_PRIME