Saturn’s Aurora from Cassini UVIS

• Wayne Pryor (Central Arizona College) for the UVIS team

UVIS long-slit spectroscopyEUV 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

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

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

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 spotsSpot is spatially extended in longitude: interaction with extended

cloud?

Enceladus, wake to right

FUV Spot Spectrum: enhanced H Lyman-alpha 1216 A and H2 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 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

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 MovieBlack “Clock Hand” is at 330 longitude

Features generally co-rotationalDouble arc on nightside

2008 Day 002: N. Polar Cap Transient Spots

FUV

EUV

• 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

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

2008 Day 197 S aurora shows spiral

forms

2008 Day 201 Flare in NEnds 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 201 05:39

Day 208 05:55

2009 Day 023 N. Nightside “Horseshoe”

with 3 resolved arcs

5 frames loopedNote “jet” leaving main ovalNear 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 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!