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

Abstract Cassini's Ultraviolet Imaging Spectrograph (UVIS) is making detailed studies of Saturn's auroras. Two long slit spectral channels obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 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 207 19:30 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.

UVIS is on the Cassini Orbiter

UVIS long-slit spectroscopy EUV channel 56.3-118. 2 nm FUV channel 111.5-191.3 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

UVIS H2 band data on Saturn

Sample Saturn Spectral Images EUV FUV

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

Saturn H Lyman-a

Saturn UV Oxygen 130 nm image

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

Polar Dark Spot (2005 day 172) 1738-1816 1816-1894 ratio 1st/2nd 1328-1426 wavelengths (A) Spot in 1st image is gone at longer wavelengths (2nd image) Localized small hydrocarbons? (spectrum is noisy)

ISS south pole methane, uv3 images

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

UVIS auroral time-series as Cassini recedes from Saturn…

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

EUV, SKR, CAPS solar wind

EUV, SKR, CAPS solar wind

Saturn Auroral Spectrum

Color Ratio Trend

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 2005-048 T23:08:00 GMT ~800,000 km range Aurora was relatively weak that day

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

VIMS_003SA_THRCYLMAP001HST Campaign UVIS Geometry

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

UVIS dark-side spectrum

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 207 19:30 Auroral spectrum like Jupiter’s, with CH4 absorption Auroral color ratio (1550-1620 A/1230-1300 A) steady, near ~3±0.3 (2.0 for HST campaign period) : ~20 keV electrons

References Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation. Space Sci. Reviews, 115, 299-361, 2004. Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, 1251-1255, 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.

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

Upcoming UVIS Auroras 2004-196T06:26:00..UVIS_000SA_AURORAB001_PRIME 2004-197T02:21:00..UVIS_000SA_AURORAA002_PRIME 2004-198T16:16:00...UVIS_000SA_AURORAA003_PRIME 2004-199T07:41:00...UVIS_000SA_AURORAB002_PRIME 2007-096T00:33:00..UVIS_042SA_NAURMOV001_PRIME 2007-145T09:10:00..UVIS_045SA_NAURMOV001_PRIME 2007-350T23:31:00..UVIS_054SA_EUVFUV001_PRIME 2008-002T15:18:00..UVIS_055SA_NAURMOV001_PRIME 2008-037T05:36:00..UVIS_058SA_NAURMOV001_PRIME 2008-109T08:18:00..UVIS_065SA_NAURMOV001_PRIME 2008-129T06:30:00..UVIS_067SA_NAURMOV001_PRIME

Saturn EUVFUV’S 2005-087T07:46:00..UVIS_005SA_EUVFUV001_PRIME 2005-172T03:30:00..UVIS_010SA_EUVFUV003_PRIME (the oval pictures) 2005-179T01:00:00..UVIS_010SA_EUVFUV002_PRIME 2005-197T21:15:00..UVIS_011SA_EUVFUV002_PRIME 2005-212T05:50:00..UVIS_012SA_EUVFUV001_PRIME 2005-283T20:27:00..UVIS_016SA_EUVFUV001_PRIME 2005-289T17:48:00..UVIS_016SA_EUVFUV003_PRIME 2005-362T16:57:00..UVIS_019SA_EUVFUV004_PRIME 2006-012T13:13:00..UVIS_020SA_EUVFUV003_PRIME 2006-020T15:28:00..UVIS_020SA_EUVFUV004_PRIME 2006-063T10:36:00..UVIS_021SA_EUVFUV005_PRIME 2006-083T16:05:00..UVIS_022SA_EUVFUV003_PRIME 2006-084T07:49:00..UVIS_022SA_EUVFUV005_PRIME 2006-138T06:11:00..UVIS_024SA_EUVFUV005_PRIME 2006-143T20:41:00..UVIS_024SA_EUVFUV002_PRIME 2006-178T02:37:00..UVIS_025SA_EUVFUV002_PRIME 2006-187T01:06:00..UVIS_025SA_EUVFUV005_PRIME 2006-200T00:21:00..UVIS_026SA_EUVFUV007_PRIME 2006-208T23:56:00..UVIS_026SA_EUVFUV006_PRIME 2006-227T01:51:00..UVIS_027SA_EUVFUV003_PRIME 2006-231T01:36:00..UVIS_027SA_EUVFUV002_PRIME 2006-233T00:16:00..UVIS_027SA_EUVFUV006_PRIME 2006-303T18:27:00..UVIS_031SA_EUVFUV003_PRIME 2006-314T21:03:00..UVIS_032SA_EUVFUV003_PRIME

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