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

UVIS long-slit spectroscopy EUV channel nm FUV channel 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 R s Saturn’s emissions: H Lyman-  and H 2 bands from auroras and dayglow. Reflected sunlight spectrum: Rayleigh scattering in H 2 and acetylene absorption bands

June 21 (Day 172), :30-14:30 “EUVFUV” from 35 R s N-S-N UVIS scan Slit E-W S Auroral oval imaged twice Images deconvolved Blue H 2, H emission Orange reflected sunlight Aurora changes over ~1 hour Oval 70-75S

Enceladus footprint search Wannawichian et al 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

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

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

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

FUV Spot Spectrum: enhanced H Lyman-alpha 1216 A and H 2 band emissions

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

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

2007 Day 145 N Auroral Movie Black “Clock Hand” is at 330 longitude Features generally co-rotational Double arc on nightside

2008 Day 002: N. Polar Cap Transient Spots FUV EUV UVIS_055SA_NAURMOV001_PRIME T15:18:00 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

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

2008 Day 197 S aurora shows spiral forms

2008 Day 201 Flare in N Ends in a “Q” shape flare

2008 Day 208 Flare in N and Stable Transpolar Arc flare

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

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

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

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 B y -effect) Two brief N auroral flares (2008 Day 201, 208) in the polar cap: spectra show particles penetrate below methane homopause (methane vertical column ~3x10 16 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!