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 P21B-01 Tuesday 0830 h Spring AGU 2005

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 H 2 bands from Saturn's auroras and dayglow. The auroral spectrum is similar to that of Jupiter, showing short- wavelength FUV absorption due to methane, CH 4. 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.

UVIS is on the Cassini Orbiter

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-  and H 2 bands from auroras and dayglow. Saturn’s reflected sunlight spectrum: Rayleigh scattering in H 2 modulated by acetylene absorption bands

Sample Spectral Images EUV FUV

UV Saturn July 13, 2004 H Lyman-  H 2 bands

Saturn H Lyman- 

Saturn UV Oxygen 130 nm image

Saturn Auroral Spectrum

Saturn’s H 2 Dayside S. Aurora & UVIS lab spectrum: (differ due to CH 4, etc.)

UVIS Aurora

UVIS H 2 band data on Saturn

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

UVIS auroral time-series as Cassini recedes from Saturn…

UVIS auroral trend & Cassini RPWS SKR signal

EUV, SKR, CAPS solar wind

Acetylene (C 2 H 2 ) absorption is seen in FUV solid=Saturn reflected sunlight spectrum dashed=scaled C 2 H 2 cross-section

Rayleigh scattering model with C 2 H 2

Hubble Auroral Campaign John Clarke (Boston U.) PI program: 5 orbits HST ACS UV dayside auroral imaging time for Feb 17, 2005 tied to Cassini night side observations John Clarke (Boston U.) PI program: 5 orbits HST ACS UV dayside auroral imaging time for Feb 17, 2005 tied to Cassini night side observations Cassini VIMS_003SA_THRCYLMAP001_UVIS_FOV is 8 hours 22 minutes long night side observation Cassini VIMS_003SA_THRCYLMAP001_UVIS_FOV is 8 hours 22 minutes long night side observation Start time T23:08:00 GMT Start time T23:08:00 GMT ~800,000 km range ~800,000 km range Aurora was relatively weak that day Aurora was relatively weak that day

HST Campaign ACS Images: Feb 17, :20-16:5817:53-18:33 19:29-20:0921:04-21:45 22:40-23:21

VIMS_003SA_THRCYLMAP001 HST Campaign UVIS Geometry

Summed UVIS spectrum

Conclusions Auroral spectrum similar to Jupiter’s, with CH 4 absorption Auroral brightness varies by a factor of 3 or more Auroras brightened the most during (and after) solar wind shock 2004 day :30 Daily-averaged auroral emissions vary with SKR and solar wind ion fluxes Auroral color ratio ( A/ A)=2.0 for HST campaign period Saturn has an extended H cloud (and a ring-related O cloud) Reflected sunlight spectrum maps acetylene distribution. Acetylene bands are stronger at Saturn than Jupiter.

References Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation. Space Sci. Reviews, 115, , Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation. Space Sci. Reviews, 115, , Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, , Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, , 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), 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), Pryor, W. R., et al., Cassini UVIS Observations of Jupiter’s Auroral Variability, Icarus (submitted), Pryor, W. R., et al., Cassini UVIS Observations of Jupiter’s Auroral Variability, Icarus (submitted), 2005.

Future Work EUVFUV image cubes for dayglow, and acetylene latitude- dependence (30-60 elements across Saturn radius) EUVFUV image cubes for dayglow, and acetylene latitude- dependence (30-60 elements across Saturn radius) Auroral studies from higher inclinations Auroral studies from higher inclinations Auroral time-dependence, longitude-dependence Auroral time-dependence, longitude-dependence H cloud variability H cloud variability