Simultaneous Cassini, HST, and
ground-based observations of
Saturn’s aurorae during the 2013
auroral campaign
Henrik Melin
!
S. V. Badman, T. S. Stallard, K. H. Baines, J. Nichols, W. R. Pryor, U. Dyudina,
S. Cowley, G. Provan, J. O’Donoghue, J. S.D. Blake, C. Meredith, S. Miller, J.
Gustin, A. Radioti, C. Tao
UVIS Team Meeting 17/06/14
Space Environment Technologies
University of Leicester
Motivation
•
Observe the same phenomenon in multiple
wavelengths (i.e. what is the relationship between
ultraviolet H, H2 and infrared H3+ emissions?)
•
Observe aurora from multiple vantage points
simultaneously (dayside/nightside).
•
Observe auroral emission at many spatial scales (i.e.
on what spatial and temporal scales do we observe
variability?).
•
Icarus special issue!
60 00 00
Lots of auroral
observations in March and
April 2013
50 00 00
•
Mimas
Declination (d m s)
-11 47 50
40 00 00
-11 48 20 -11 48 10 -11 48 00
20 00 00
30 00 00
-11 48 30
Cassini observing
nightside, Earth-based
platforms observing the
day-side.
-11 48 40
•
Cassini VIMS, UVIS, ISS
with Hubble Space
Telescope, NASA IRTF,
and Keck simultaneous
observations on 21 April
2013 (2013-111)
Declination (d m s)
•
Saturn Viewer Results
Enceladus
Dione
Tethys
Mimas
Rhea
3
00 00
Enceladus
Time (UTC):
Ephemeris:
Viewpoint:
Moon selection:
Ring selection:
2 00 00
1 00 00
0 00 00
Right Ascension (h m s)
2013-111T11:07:04.68600
SAT351 + SAT353 + DE421
Cassini
Mimas-Phoebe
A,B,C
Dione
2013-111 Timeline
•
Consider an interval of 8 hours during which 6
instruments observed, covering ~3/4 of Saturn rotation.
•
At most 5 instruments were observing at the same time.
•
Shift observing times to be time of emission at Saturn (i.e.
shift Earth-based observations back by 4,403 seconds.
HST - ACS
• 2.4 m telescope in low-Earth
orbit, launched in 1990
• ACS was installed 2002, by
Shuttle service mission 3B.
• Wide field images of UV
wavelengths, including H2
Lyman & Werner bands
• Science products: - Brightness
Hubble Space Telescope
•
ACS, 2 HST orbits, observing auroral H2 Lyman & Werner bands
•
Midnight oval not visible
•
Relatively quiet aurora
•
Dawn brightened, but variable over the interval, although total emitted power
does not vary significantly.
NASA Infrared Telescope
Facility - CSHELL
• 3 m telescope on Mauna Kea,
Hawaii.
• CHSELL is a high resolution
spectrograph (R ~ 35,000). 30” slit
aligned East-West on the northern
pole. Short wavelength coverage.
• Science products: - H3+ intensity
- H3+ ion wind velocities across the
polar cap
-11 48 20 -11 48 10
20 00 00
30
-11 48 30
-11 48 40
-11 48 50
Declination (d m s)
Enceladus
Dione
Tethys
Mimas
NASA IRTF
CSHELL
Rhea
3
00 00
Enceladus
2 00 00
1 00 00
0 00 00
23 00 00
Right Ascension (h m s)
Time (UTC):
Ephemeris:
Viewpoint:
Moon selection:
Ring selection:
2013-111T11:07:04.68600
SAT351 + SAT353 + DE421
Cassini
Mimas-Phoebe
A,B,C
Dione
Rhea
Generated by the Saturn Viewer Tool, PDS Rings Node, Sun Jan 12 00:54:49 2014
•
14 28 12
14 28 11
Telescope ‘wobble’ introduces
significant errors on H3+ intensity
and derived ionospheric velocities.
14 28 10
14 28 09
Right Ascension (h m s)
Time (UTC):
Ephemeris:
Viewpoint:
Moon selection:
Ring selection:
2013-111T11:07:04.68600
SAT351 + SAT353 + DE421
Mauna Kea Observatory (19.827, -155.472, 4215.)
Mimas-Phoebe
A,B,C
Generated by the Saturn Viewer Tool, PDS Rings Node, Sun Jan 12 00:54:06 2014
14 28 08
Keck II - NIRSPEC
• 10 m telescope on Mauna Kea,
Hawaii.
• NIRSPEC is a high resolution
spectrograph (R ~ 25,000).
Cross dispersed - long
wavelength coverage. Slit
aligned North-South, traversing
both poles.
• Science products: - H3+ Intensity (high S/N)
- H3+ temperatures
0
TC):
meris:
point:
ction:
ction:
Keck NIRSPEC
Rhea
2 00 00
1 00 00
0 00 00
23 00 00
Right Ascension (h m s)
Mimas
2013-111T11:07:04.68600
SAT351 + SAT353 + DE421
Cassini
Mimas-Phoebe
A,B,C
dus
wer Tool, PDS Rings Node, Sun Jan 12 00:54:49 2014
Dione
•
+
Measure H3 intensity along the north-south aligned slit, seeing both northern
and southern aurora
Rhea
•
Initially, both midnight and noon oval visible on the north
•14 Southern
aurora
28 11
14 28 10
brightens
around 14
12:00
14 28 09
28 08 UT
Right Ascension (h m s)
Low latitude structures
UTC): •
2013-111T11:07:04.68600
meris: SAT351 + SAT353 + DE421
- ring rain? (O’Donoghue et al., 2013)
Comparing hemispheres
•
Keck sees northern aurora brightest when largest max field aligned
current (FAC) passes over northern noon.
•
Keck sees southern aurora brightest about when southern max FAC is
at noon.
2013-111 Cassini Geometry
•
Cassini pointing ~fixed in local-time
Cassini UVIS
•
Ultraviolet H, H2 auroral
emissions
•
Main oval, varies slightly in
latitude, at a minimum when
the southern PPO phase ~0o
•
Discontinuity at 09:00 UT
•
Diffuse equatorward emission
below -70o latitude
Cassini VIMS
+
•
Infrared H3 emissions at 3.5 micron
•
‘Well behaved’ auroral oval oval
•
Discontinuity at noon at 09:14 UT - associated with the rotating electric field?
•
NO diffuse equator ward emission as seen in the ultraviolet by UIVS. Is this
regions cooler than the rest of the oval?
Multi-spectral
comparison I
•
General
agreement with
the main oval
•
Differences in
morphology
mostly due to
instrumental
effects
Cassini
ISS
•
H Balmer-alpha emissions in the visible
•
High spatial resolution at high cadence
•
Fine structure within the main auroral oval
Ultraviolet
Infrared
Visible
Multi-spectral
comparison II
•
Good correspondence in
the three wavelengths of
the main oval
•
Emission visible in IR and
UV not present in visible
•
Fine structure seen in
individual ISS images not
present in this average
view
Summary
•
Good agreement on the location of the main oval in the
infrared, visible, and ultraviolet
•
Significant differences are present:
✤
Differences in emission mechanism
✤
Differences in how the instruments operate
✤
Differences in temperature?
•
There is spatial variability on every resolution observed (tens
of km upward)
•
There is temporal variability on minutes, but also longer term
over hours
IAU Longitude vs. PPO Longitude
•
Are IAU longitudes (defined in the Voyager era) useful
for showing features fixed relative to rotation?
S
N
PPO phases
sampled every IAU
rotation
Provan et al., (today)
60 00 00
40 00 00
50 00 00
Mimas
Keck
Enceladus
Dione
-11 48 30
-11 48 20
-11 48 10
2048
0000
00
-11
30 00 00
-11 47 50
Saturn Viewer Results
Rhea
3 00 00
2 00 00
1 00 00
0 00 00
23 00 00
Right Ascension (h m s)
Tethys
VIMS
Mimas
Time (UTC):
Ephemeris:
Viewpoint:
Moon selection:
Ring selection:
2013-111T11:07:04.68600
SAT351 + SAT353 + DE421
Cassini
Mimas-Phoebe
A,B,C
Enceladus
Generated by the Saturn Viewer Tool, PDS Rings Node, Sun Jan 12 00:54:49 2014
48 40
Declination (d m s)
Declination (d m s)
Next: Nightside/dayside Temperatures
Dione