Magnetosphere-Ionosphere Coupling and Aurora at
Saturn: Lessons from Cassini
Emma Bunce & Stan Cowley
IonosphericFlowsandCurrents
TakenfromCowley[2001]
TakenfromCowleyetal[2003]
TakenfromCowleyetal[2004]
Cassini’s high-latitude phases
To date there have been
three main periods of
high-laDtude orbits that
have provided (clear) insitu observaDons of the
auroral field aligned
currentsystem(s):
2006/2007 - 5-7 RS, 8
passesSH(dawn-noon),6
NH(~dusk)
2008/9 – 3-6 Rs, ~30
passes N and S, pre- and
post-midnight.
2013–6-8Rs,~15passes
S&N,pre/postmidnight,
somedaysideN
More before the end of
themission!
First direct evidence using coordinated Cassini and HST data that Saturn’s
main auroral oval maps to the boundary between open and closed field lines
(see Bunce et al., JGR, 2008)
Cassini MAG and ELS observations
open
00
closed
18 06
18
06
UT(DOY)
Radial(Rs)
Co-lat(deg)
LT(dechr)
12
Large azimuthal field signature is seen in the MAG
data, as the spacecraft moves from open to closed field
lines (see ELS), near to noon – corresponds to a large
upward field-aligned current – producing the aurora
seen by HST
HST UV image @ 03:21 UT, 17th
January 2007
IMAGE CREDIT: Boston University, HST
Basic Morphology of the High Latitude
Magnetosphere
2008 data – fast passes (2-3 hours)
(a) ω/ΩS
To equator
1
0.8
0.6
0.4
0.2
0
Theoretically, we consider plasma sub-corotation
SH
e.g. Cowley, Bunce & O’Rourke [2004] i.e. PRE-CASSINI
NH
•
(a) Plasma angular velocity profile based on Voyager data
and telescope data in the polar cap
•
(b) Ionospheric meridional current is positive equatorward,
assuming a fixed effective Pedersen conductivity ΣP* of 1 mho
•
(c) Field-aligned current density just above ionosphere,
downward over polar region, upward at OCB (model auroral
oval), and weaker upward again at low latitudes
•
(d) “Lagging” azimuthal field just above ionosphere
associated with the current system, varying along field lines
according to ρBφ ≈ constant (ρ is perpendicular distance from
axis)
Colat/deg
(b) IP/MA rad-1
Ionospheric
meridional current
1
0.8
0.6
0.4
0.2
0
Colat/deg
(c) j||/nA m-2
150
100
50
0
-50
up
Field-aligned current
density
Colat/deg
down
(d) Bϕ/nT
100
80
60
40
20
Saturn’s high-latitude magnetosphere
Plasma angular
velocity
“Lagging” azimuthal
field above
ionosphere
Colat/deg
051015202530
Observationally, we work from the bottom up!
•
We observe the “lagging” Bφ field in the magnetosphere
•
We derive the ionospheric meridional current profile &
hence field-aligned currents
•
Using observed angular velocities we can also infer the (nonconstant) ΣP*
Statistical studies of Saturn’s SH fieldaligned currents Hunt et al [2014]
IM/MArad-1
ω/ΩS
Σ*p/mho
Red=theory
Dots=data
Solid=data
OCB
open
closed
Saturn’s high-latitude magnetosphere
Results derived from ~30 post-midnight highlatitude Cassini passes across southern auroral
zone in 2008
•
(a) Model meridional current profiles based on
Cassini ω/ΩS data (b), and a fixed conductivity
of 0.75 mho
•
Model FAC directed down over polar region,
sharp upward at OCB, and weak upward again
at large co-latitudes (similar to the model)
•
Data profiles (dots/solid line) show distributed
downward current over the polar region
continuing more strongly downward on outer
closed field lines
•
(c) Due to enhanced conductivity derived in
bottom panel (red line)
•
We also find (i) sharp upward (auroral) current
is centred ~2° equatorward of OCB as the
conductivity drops and (ii) lower latitude
upward currents appear not to exist due to low
ionospheric conductivity
Black- Wilson et al. [2009]
Green- Muller et al. [2010]
Blue-Carbary & Mitchell [2014]
* Thomsen et al [2014]
Σ*P =
cos(α i ) I M
ρ i2 Bi Ω S (1 −
ω
ΩS
)
Assumed 0.75 mho
SUMMARY: Variations in ionospheric conductivity
are at least as important as plasma angular
velocity in determining the field-aligned current
profiles due to plasma sub-corotation
IonosphericCola=tude/deg
Major unanticipated effect at Saturn
See Hunt et al [2014]
a
ΨS~90°
b
ΨS~180°
c
ΨS~270°
d
ΨS~360°
The “planetary period oscillation” (PPO)
currents strongly modulate the field-aligned
currents
•
appear to be driven by rotating twinvortex flows in the thermosphere of
uncertain origin, see e.g. Jia and Kivelson
[2012]
•
produces separate current systems driven
from the N and S hemispheres with slightly
differing seasonally-varying periods
Static sub-corotation
current
SH Rotating
PPO current
down
up
a) and c) PPO phase is 90°
and 270°, reducing the
overall current to near zero
in the first case and near
doubling it in the second
No evidence to date of any corresponding effect at Jupiter
b) and d) PPO phase is
180°/360° and the
current is small, so the
sub-corotation system
dominates
Dynamics of the High Latitude Magnetosphere
Approach Phase (solar wind data) + Earth-based
remote sensing support
Dynamic Auroral Storms: First seen during approach campaign!
The “auroral storms” are produced by strong compressions of the
magnetosphere by CIRs/CMEs in the solar wind
HST images: 18.5 hours apart
See Badman et al. [2005]; Meredith et al
[2014]; Nichols et al. [2014]
•
the auroral oval brightens on the dawn side and expands strongly
towards the pole, SKR intensifies and extends to lower
frequencies [see Clarke et al. 2005; Kurth et al., 2005, 2016]
•
suggested by Cowley et al [2005] that this is due to induced bursts
of nightside reconnection that close a significant proportion of the
open tail flux
•
in situ tail data at the expected time of a CIR compression shows
an SKR burst, hot plasma injection, and field dipolarization, see
Bunce et al [2005]
•
occurrence statistics indicate one event every ~6 days, each
lasting ~16 h
What does the in situ data at high-latitudes look like during these storms?
- Unusual events excluded from FAC statistical studies hold the clues!
See Bunce et al., 2010
SKR source region
Due to the approach phase knowledge gained we
were able to interpret the highly unusual fieldaligned current signatures (seen at the same time
as the s/c entered the SKR source region) as being
the in situ consequence of a compression of the
magnetosphere – inducing rapid tail reconnection,
enhanced auroras and radio emission
See Lamy et al., 2010
Lamy et al. 2010
Enhanced field-aligned
currents
Plus dayside field-aligned
currents and hot plasma
at *very* high latitude
Bϕ(nT)
Cowley et al., 2005
See Radioti et al [2013], also Badman et al. [2013]
If compression events close tail flux, dayside
reconnection must open it at low rates (tens kV)
over many-day intervals [Jackman et al, 2004]
-
Evidence in dayside auroras for bursty (FTEstyle) reconnection signatures in post-noon hours
(possibly supressed pre-noon by large flow shear
across boundary)
HST images
See Meredith et al [2013]
Cassini UVIS images
HST images when Cassini was in the upstream solar wind/
IMF show that post-noon auroras are present when IMF Bz is
positive, and not when it is negative
We thus have evidence that the Dungey cycle is active at
Saturn, though in modified form and on longer time scales
than at Earth
It remains to be seen whether there are any corresponding
effects at Jupiter
-
we know the aurora brighten in response to CIR/CMErelated events, but exactly where and when remains to
be determined [see Gurnett et al., 2002; Nichols et al.
2009]
IMFBz+ve
IMFBz-ve
Dynamics of the High Latitude Magnetosphere
2006/7, 2009, and 2013 - slow passes (6-10 hours)
Assuming a static structure...
Ionos Co-lat/deg
Ip / MA rad-1
Energy (eV)
Rev99:January4th2009
NH UVIS images
Cassini UVIS images - January 4th 2009 (Rev 99)
1
2
09:45 UT
10:14 UT
3
10:43 UT
Cassini footprint
06 LT
06 LT
12 LT
4
11:12 UT
06 LT
12 LT
5
11:41 UT
06 LT
06 LT
12 LT
12 LT
12 LT
Animation of images 1-5
Taking into account the oval oscillation...
RPWSauroralhiss
103
Energy (eV)
Frequency (Hz)
Rev99:January4th2009
102
Cos(nh_phase)
Ip / MA rad-1
101
180˚
0˚
1. Oval equatorward
0˚
270˚
2. Oval poleward
90˚
180˚
s/c equatorward
1. Oval motion is equatorward at rate of 2.5° in 2 hours during the UVIS imaging, that is the motion
equatorward >s/c velocity - hence spacecraft is chasing the “typical” downward current region...
2. After peak in cos(NH phase) we assume oval motion is then poleward after the end of the UVIS image
sequence. Cassini now moves rapidly through the down/up current structure
Summary:
CASSINI has taught us a great deal about Saturn’s polar magnetosphere, field-aligned
current systems, and aurora.
It is much more complex than ever anticipated.
The final proximal orbits will bring new insights…
Some interesting “lessons” learned
1)  The approach phase was extremely valuable – an opportunity to investigate how
the aurora respond to CIR/CME related changes in solar wind dynamic pressure,
taught us to use the SKR as a proxy for the solar wind condtions
2)  The timescale for crossing through the auroral field-aligned current systems is
crucial – fast passes allow the assumption that the current system is static
whereas slower crossings (when s/c is further up the field lines) provide an
opportunity to look at dynamics in the polar cap/along the oval
3)  Multi-instrument studies provide an excellent way to understand complex
signatures
Veryfast(few
minutes?)pass
throughthemain
auroralovalFACs
Slower(1.5hour?)
passalong/across
themainauroral
ovalFACs
No storm yet observed from start to finish, but from the overall ensemble the suggested
time sequence is See Badman et al. [2005]; Meredith et al [2014]; Nichols et al. [2014]
Storm 12
Storm 3
Storm 10
Storm 7
1
2
~1-3 h onset post-midnight tail reconnection, expands
~3-8 h bulge expands eastward (~0.6xcorot) to fill whole
rapidly dawnward (~2xcorot), more slowly poleward forming
hot plasma bulge
of dawn sector
Storm 7
Storm 2
3
~8-12 h auroras bifurcate with active poleward edge &
structured bulge, which vents into noon and dusk sector
Storm 1
Storm 11
4
~12-16 h activity at poleward edge dies away from nightside
towards pre-noon, storm auroras dissipate, usual dawn arc
reforms, injected plasma reaches midnight sector
Multi-instrument identification of the open-closed field line boundary
Rev 62: Northern Hemisphere
SeeJinksetal.,2014
RPWS
CAPS
MAG
UpwardFAC
LP
Downwardelectrons
Upwardelectrons
MIMI
Δcola=tude=0.6°
15.7°SH
13.4°NH
Average loca=on of OCB is 1-2°
p o l e w a r d o f t h e m a i n F A C
(polewardedge),andagreeswithin
thedatasetstowithin<1°
ω/ΩS
1
0.8
0.6
0.4
0.2
0
Plasma angular
velocity/ΩJ
2) Polar
cap
IP/MA
50
40
30
20
10
0
j||/nAm-2
300
200
100
0
-100
Bϕ/nT
-100
-200
-300
-400
-500
-600
Applications to Jupiter
The equivalent Jupiter sub-corotation model proposed by
Cowley et al [2001; 2005], comprises
1) Middle
magnetosphere
Ionospheric
meridional current/
MA
1) The description of the upward current associated with
corotation breakdown at ~20 RJ due to outward transport of Io
plasma
- a good description of the location, width, and intensity of the
main auroral oval, and the energy (~50-100 keV) of the electron
primaries determined by UV spectroscopy
2) The description of the polar region is essentially speculative
FAC density/nA m-2
- depends on how much open flux is in the system, and where is
it located?
HST image
“Lagging” azimuthal field
above ionosphere/nT
Main oval
Grodent et al [2003]
View model results for middle magnetosphere field sweep-back due to plasma sub-corotation
Black lines show field lines in the
magnetic meridian mapping from
co-lats of 5°-25° in the ionosphere
Open-closed boundary FAC
-10 nT
Main oval FAC
-20 nT
+20 nT
-5 nT
+5 nT
+10 nT
- black dotted box is the equatorial
current disk
Green lines (also field lines) show
the regions of upward-directed FAC
at the OCB and in the MM, the
latter closing in the equatorial
current sheet
Red and blue lines show contour
maps of Bϕ produced by the FAC
system, -ve in NH and +ve in SH,
from ±2 nT to ± 50 nT
- overall a ‘lagging’ field
configuration is produced in the MM
by the plasma sub-corotation
- angular field deflections are a ~5°
outside the MM current sheet,
reducing to a few tenths of a
degree closer to the magnetic axis
SeeCowleyetal.[2008]
We can project the Juno orbits onto a magnetic meridian
– at start, middle, & end of mission as examples
Start of mission
Middle of mission
End of mission
Inbound
Outbound
- trajectory oscillates at planetary period of 9.9 h due to rotation of magnetic axis around the spin axis
- blue dots plotted every 10 hours relative to the periapsis point
- orbit traverses polar magnetosphere at low altitudes both north & south throughout the mission
- at start of mission the line of apsides is initially near the equatorial plane
- due to non-spherical Jupiter, apoapsis rotates south, line of apsides at ~35° at end of mission
- thus also traverses wide regions of high-lat magnetosphere previously unexplored
Example Juno trajectory is shown in a
magnetic meridian, oscillating at 9.9 hrs
due to rotation of magnetic axis.
Juno traverses the previously unexplored
high-latitude magnetosphere, crossing
directly through MI coupling current
systems (main oval, polar cap)
The blue dots mark 10 hour intervals
relative to periapsis.
-  time-scale for passes over main oval
FACs is ~2 min
-  time scale for passes over polar FACs is
~10 sec
From Bφ we can estimate other model
parameters
Inbound
Lower panel shows model Bφ for north
polar pass from 1.2 to 0.2 hr prior to
periapsis
Middle of mission
z/RJ
The Juno Orbit
ρ/RJ
Bϕ/nT
-1hr
-0.2hr
Polar arc FACs
Outbound
Main oval FACs