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Saturn’s Aurora from Cassini UVIS

Saturn’s Aurora from Cassini UVIS. Wayne Pryor (Central Arizona College) for the UVIS team. UVIS long-slit spectroscopy. EUV channel 56.3-118. 2 nm FUV channel 111.5-191.3 nm 64 spatial x 1024 spectral pixels Spectral imaging is done by spacecraft slews

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Saturn’s Aurora from Cassini UVIS

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  1. Saturn’s Aurora from Cassini UVIS • Wayne Pryor (Central Arizona College) for the UVIS team

  2. UVIS long-slit spectroscopy EUV channel 56.3-118. 2 nm FUV channel 111.5-191.3 nm 64 spatial x 1024 spectral pixels Spectral imaging is done by spacecraft slews FUV imaging pixel (1 mrad tall) using low-res slit (1.5 mrad wide) = 600 km x 900 km at 10 Rs Saturn’s emissions: H Lyman-a and H2 bands from auroras and dayglow. Reflected sunlight spectrum: Rayleigh scattering in H2 and acetylene absorption bands

  3. June 21 (Day 172), 2005 03:30-14:30 “EUVFUV” from 35 Rs • N-S-N UVIS scan • Slit E-W • S Auroral oval imaged twice • Images deconvolved • Blue H2, H emission • Orange reflected sunlight • Aurora changes over ~1 hour • Oval 70-75S

  4. Enceladus footprint search • Wannawichian et al. 2008 set an upper limit on the Enceladus footprint in HST data < few kR • Rymer et al. (2009 MOP meeting) presented evidence for episodic field aligned “beams” just downstream of Enceladus in CAPS and INCA data • Cowley et al. computed footprint spot location based on magnetic field models • Footprint boxes were added to several hundred UVIS images

  5. The Enceladus Auroral Footprint From Frank Crary… The idea is that field lines from Enceladus (500 km diameter) converge to a smaller (60 km) wide glowing spot on Saturn

  6. 3-frame EUV movie of 2008 Day 239 images Noon to the left Sub-Enceladus footprint in white box EUV Spot Peak Brightness: 400 R, 450 R, 200 R

  7. Cut-throughs of the FUV spotsSpot is spatially extended in longitude: interaction with extended cloud? Enceladus, wake to right

  8. FUV Spot Spectrum: enhanced H Lyman-alpha 1216 A and H2 band emissions

  9. Search Summary • 5 images (out of hundreds) show an obvious spot (several hundred R) in the box • An image pair (84 min. apart) from 2008-239 from close range (6-7 Rs altitude) shows a good dayside spot near 65 N that moves with Enceladus, looks better with red end out. • A 3rd EUV image 3 hours 40 minutes later on 2008-239 shows a faint spot in box • Enceladus orbital period of 1.37 days-> 11 degrees/hr • Saturn rotation period of 10.66 hrs-> 34 degrees/hr • Spot is usually absent or below our detection threshhold

  10. UVIS Movies • Selected movies will now be shown • Mostly of the North pole • Reflected sunlight on left or bottom part of the images indicates dayside • Terminator is marked in white • Cross-bar on terminator is on dawn side

  11. 2007 Day 145 N Auroral MovieBlack “Clock Hand” is at 330 longitudeFeatures generally co-rotationalDouble arc on nightside

  12. 2008 Day 002: N. Polar Cap Transient Spots • UVIS_055SA_NAURMOV001_PRIME • 2008-002T15:18:00 to T21:48:00 • Notice one frame in looped 6 frame movie has polar outburst in semicircle of bright beads • Range 16.4 Rs, spacecraft at 37N FUV EUV

  13. 2008 Day 129 N UVIS Aurora and MIMI INCA 50-80 keV protons (Mitchell et al. 2009)

  14. 2008 Day 197 S aurora shows spiral forms

  15. 2008 Day 201 Flare in NEnds in a “Q” shape flare

  16. 2008 Day 208 Flare in N and Stable Transpolar Arc flare

  17. 2008 Day 201, 208 Flares (Cuts Across Image):Flares are much brighter than oval

  18. 2008 UVIS Flare Spectra are unusual: large methane column above emissions (~30keV electrons)Day 201 05:39Day 208 05:55

  19. 2009 Day 023 N. Nightside “Horseshoe” with 3 resolved arcs 5 frames looped Note “jet” leaving main oval Near noon forming a “Q”

  20. Conclusions • Enceladus auroral footprint exists! Usually NOT Seen, but ~5 images show it • Multiple auroral arcs are common- suggests auroras are not solely on the open-closed field line boundary • Strong correlations with INCA magnetospheric images • Spiral forms are common • Persistent “transpolar arcs” inside oval seen on a large fraction of the observations near local noon (Expected for southward IMF, location in oval may be a By-effect) • Two brief N auroral flares (2008 Day 201, 208) in the polar cap: spectra show particles penetrate below methane homopause (methane vertical column ~3x1016 cm-2). • Auroral electrons near ~10 keV, but flares near ~30keV, • RPWS saw one flare at low-frequencies (1-2 min. duration) • 2008 Day 201 storm: arcs form near noon at right angles to the oval, lengthen, split in the middle and separate: “Q” auroras • Several other “Q” auroras shown near noon: disturbance crosses L-shells!

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