Ultraviolet Auroral Pulsations on
Saturn from Cassini UVIS
(focusing on 2009 278-282)
Wayne Pryor, Larry Esposito, Alain
Jouchoux, Greg Holsclaw, Josh
Colwell, Jacques Gustin, Cassini UVIS
team, Frank Crary, Stan Cowley, Don
Mitchell, Ulyana Dyudina
October 2009 ISS observation
Ulyana Dyudina, Andy Ingersoll
Day side
stars
• 472 frames
• 81 hours
• 2 - 3 minute exposures
• Broadband visible wavelengths,
no color information
• Orange aurora is a false color
•UVIS was obtaining
simultaneous observations
UVIS Data during ISS movie..
Dawn
Dawn
Dawn
Dawn Planetary
Rotation
rotation
Dusk
Dusk
Dusk
Dusk
Time->
Stationary UVIS slit aligned E-W on Saturn North
Time marches to right.
Diagonal formed from bright spots
UVIS FUV data overview
2009 days 278 16:50-282 04:04
features with different slopes appear
(different rotation periods)
EUV & FUV data differ somewhat2 mrad and 1.5 mrad wide low-res slits only partially overlap.
EUV 57-75 N mostly shows co-rotational features
FUV 65-84 N shows more periodic bursty features
(seem Mimas, Enceladus related- but TOO POLAR to be “footprints”)
Tested for possible satellite connections
Moon
Period
Orbit radius
Fraction of
co-rotation
speed
Mimas
0.942 d
3.08 Rs
0.47
Enceladus
1.370 d
3.94 Rs
0.32
Tethys
1.889 d
4.4 Rs
0.23
Dione
2.737 d
6.3 Rs
0.16
Rhea
4.518 d
8.74 Rs
0.10
Co-rotation assumed to be 10 hrs 39 minutes here (Voyager epoch)
Drew sub-moon longitudes on these “images”
Found an apparent connection with Mimas
Also found a connection with Enceladus
No obvious link with other 3 moons.
1st 2009 time period
white curve is sub-Enceladus longitude path
black curve is sub-Mimas longitude path
Note longitudinally extended disturbance away from black line
2nd 2009 time period (moon paths don’t show)
white curve is sub-Enceladus longitude path
black curve is sub-Mimas longitude path
3rd 2009 time period
white curve is sub-Enceladus longitude path
black curve is sub-Mimas longitude path
4th (Snake) 2009 time period (best example!)
white curve is sub-Enceladus longitude path
black curve is sub-Mimas path
Snake
UVIS 2009 day 280 data for ISS snakes
281T00:07
ISS “snakes”
280T23:00
UVIS FUV 60 mrad long low-res (1.5 mrad
wide) slit is aligned E-W,
spatial sectors 27-32 (out of 0-59) overlap
ISS NAC and the “Snake” features
As time progresses (upwards in UVIS image)
bursty quasi-periodic features rotate
SLOWLY across Saturn’s nightside
Mimas and Enceladus related?
5th 2009 time period
white curve is Enceladus path
black curve is Mimas path
Different year (2010-177)
white curve is Enceladus path
black curve is Mimas path
Night Side UVIS counts time-series, 4 minute samples: bright bursts are
deep, show up more at long wavelengths (short wavelengths are
absorbed by methane)
Figure from Gustin et al. 2008
bursts
• Large methane
columns above
snakes, other
nightside spots
imply electron
penetration
hundreds of km
below usual ~1100
km auroras found
by Gerard et al.
2009
• Visually, “curtains”
at limb are higher
than “snakes”
• Tests methane
distribution
models
2009_10_6
2009_10_7
2009_10_7
UVIS image sequence
from 2008 day 129
(studied by Mitchell et
al.) shown with subMimas longitude in
white (footprint is the
boxy part)
bursty main oval
disturbance lines up
with sub-Mimas
longitude for 6 hours
next 3 slides enlarge
the material to the left
Conclusions
• UVIS slit aligned E-W and parked monitors auroras in time
and along 1-D.
• “Co-rotational” blobs emit continuously
• “Sub-co-rotational” UVIS features are burstyquasi-periodic bursts separated by ~hour
(such bursts are common in particle data)
some bright high-latitude spots and arcs
appear tied (indirectly, not along field lines) to
the sub-Mimas and sub-Enceladus longitude
• Brightenings like “the snake” are fairly low in the
atmosphere (from both spectra and ISS movie)
Chances to see sub-Mimas effect in 2008 UVIS images (good geometry)
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2008-037
2008-109
2008-129
2008-144
2008-193
2008-194
2008-195
2008-197
2008-200
2008-201
2008-207
2008-204
2008-224
2008-238
2008-238
2008-240
2008-254/255
2008-276
2008-304
2008-334
2008-335
10 hours
4
6
1
2 of 3
2
7.5
7
10
9
8
4
3
3
2
3
4
1.5
2
3
3
no effect
good effect
good
good
good
good
good
no
mimas not available
poor effect
good
good
excellent,close
good
no effect, close
weak/none
weak/none
excellent, close
no
no
weak