Solar WindMagnetosphereIonosphere Interactions at Jupiter

1
Solar Wind-Magnetosphere-Ionosphere Interactions
at Jupiter Saturn - an overview of recent
outer planets work at the University of
Leicester Stan Cowley With thanks to
colleagues at Leicester Sarah Badman, Emma
Bunce, Kay Clarke, Caitriona Jackman, Steve
Milan, Jon Nichols Tim Yeoman And also to
research collaborators at ICL, UCL/MSSL, Liège,
Meudon, Boston, Iowa Lindau
2
Solar Wind-Magnetosphere-Ionosphere Interactions
at Jupiter Saturn Interested in developing
ideas on the large-scale structure and dynamics
of the magnetic fields, plasma flows, current
systems, and auroras at Jupiter Saturn, forming
a background against which new in situ and
auroral data can be tested System dynamics
depends on sources of plasma mass and momentum,
of which there are two main types - solar wind
outflow from the Sun - internal plasma sources
and planetary rotation
3
Earths magnetosphere Cross-section through the
noon-midnight meridian Solid lines field
lines Dotted regions plasma populations
Cowley et al. 2003
Dynamics of the Earths magnetosphere is governed
by the solar wind interaction, mediated by
magnetic reconnection at the magnetopause
boundary Produces a variable cyclic flow, the
Dungey cycle, open field lines are formed at the
dayside magnetopause, transported to the tail by
the solar wind, and then released, usually
explosively The form strength of the flow is
governed by the direction strength of the
IMF Corotation with planet driven by ion-neutral
collisions in ionosphere is generally restricted
to small central core of dipolar flux
tubes Plasma sources are H and He from the
solar wind (green), and H and O from the
ionosphere (blue)
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By contrast, moon/ring plasma sources and
planetary rotation are much more important at
Jupiter and Saturn

Spinning disk of iogenic plasma
For Jupiter, sulphur oxygen plasma is produced
from Ios atmosphere at 6 RJ at a rate 103 kg
s-1 Diffuses outward forming spinning
equatorial plasma disk angular velocity falls
Spin-up torque exerted by ion-neutral
collisions at field line feet Torque is
transferred to equatorial plasma by current
system shown Hill, 1979, involving the bending
of field lines out of magnetic meridian
planes Plasma is lost from the outer edge by
plasmoid ejection down-tail in the
Vasyliunas-cycle
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The Vasyliunas cycle

Vasyliunas 1983
Sub-corotating mass-loaded flux tubes are
confined on the dayside of the magnetosphere by
the solar wind As they flow round into the dusk
sector tail they are no longer strongly confined,
but start to stretch out down-tail A plasmoid
forms within the outer current sheet and
eventually pinches off, carrying plasma mass
down-tail and into the solar wind Mass-reduced
closed flux tubes then rotate around more quickly
in the dawn sector before becoming mass-loaded
once more and stretching out at dusk Conceived
of for simplicity as a steady-state process, but
may not be
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Relative importance of Dungey-cycle and planetary
rotation is usually expressed in terms of the
flux transfer in each process in volts (from
Faraday 1 volt 1 Weber s-1)
For Earth, the Dungey-cycle voltage is measured
to be 50-100 kV, compared with a corotation
voltage of 90 kV gives corotating core of inner
flux tubes
Jupiter D-C voltage Nichols et al.,
2006


Solar Maximum
For Jupiter, we estimate the D-C voltage using an
empirical formula based on SW/IMF parameters,
validated at Earth Plot shows values determined
from Ulysses measurements at 5 AU each panel
is one solar rotation of 26 days Values are
negligible in SW rarefactions, and peak at 2 MV
during compressions/CMEs Corotation voltage is
400 MV thus dominant but most falls across
the inner region our estimates of the voltage
across the middle and outer magnetosphere is
5-10 MV, so peak D-C rates are then comparable
with the V-C rate Average value is 250 kV time
scale for tail replenishment (400 GWb) is 21
days
3 MV
Rising Phase
Solar Minimum
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Saturn D-C voltage Jackman et al.
2004, 2005
Panels show D-C voltage at Saturn estimated in
the same way using 6 solar rotations of Cassini
data obtained during the approach phase in
2003-2004 Values again vary from negligible
values during rarefactions up to 300 kV during
compressions averaged D-C voltage is 50
kV Corotation voltage is 10 MV, and thus again
dominant but again the voltage across the middle
outer magnetosphere is 0.5-1 MV, so peak D-C
rates are competitive At averaged value, time
scale for tail replenishment (35 GWb) is 8-13
days These times for Jupiter Saturn are much
longer than at Earth (3 h), and comparable to
the time scale of CIRs in the heliosphere thus
no substorms like at Earth, tail inflation at J
S takes place over long intervals involving many
solar wind sub-structures Nature of SW
interaction to be discussed later
500 kV



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Combined D-C and corotation/V-C plasma flow in
the equatorial plane - showing that these can
co-exist in a simple steady state
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004


Picture contains three regions of flow
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Combined D-C and corotation/V-C plasma flow in
the equatorial plane - showing that these can
co-exist in a simple steady state
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004


Inner/middle magnetosphere dominated by pick-up
and radial transport of plasma from internal
sources (ionosphere, moons, ring grains) Plasma
near-rigidly corotates in the inner region and
progressively sub-corotates at increasing radial
distances Hill, 1979 Pontius 1997 Saur et
al., 2004
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Combined D-C and corotation/V-C plasma flow in
the equatorial plane - showing that these can
co-exist in a simple steady state
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004


Outer magnetosphere region where planetary
plasma is lost down the dusk-side tail by
plasmoid formation and pinch-off
(Vasyliunas-cycle), leading to faster flows of
the post pinch-off flux tubes in the outer dawn
sector - conceived of here as a steady-state
process Vasyliunas, 1983
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Combined D-C and corotation/V-C plasma flow in
the equatorial plane - showing that these can
co-exist in a simple steady state
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004


Outer magnetosphere region where closed flux
tubes return to the dayside magnetopause via dawn
following reconnection in the tail, corresponding
to the return flow of the solar wind-driven
Dungey-cycle - also conceived of here as a
steady-state process Dungey, 1961
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Combined D-C and corotation/V-C plasma flow in
the equatorial plane - showing that these can
co-exist in a simple steady state
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004


Final point specially for MSSL Cassini CAPS folk
- in principle we can distinguish D-C and V-C
return flows by hot ion composition H,
He, and He for D-C, from solar wind and polar
wind H and S/O for V-C, mainly from
internal moon/ring sources
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Corresponding flow in the polar ionosphere
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004





Inner/middle magnetosphere region at lower
latitudes, where angular velocity of plasma
increasingly sub-corotates with increasing
latitude
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Corresponding flow in the polar ionosphere
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004





Outer magnetosphere ring of closed flux tubes
corresponding to the Vasyliunas-cycle flow
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Corresponding flow in the polar ionosphere
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004





Dungey-cycle flow at high-latitudes, comprising
slow anti-sunward flow (100 m s-1) over the open
field region mapping to the tail, combined with
slow planetary-driven rotation Isbell et al.,
1984, together with fast return flow of closed
field lines via dawn
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Corresponding flow in the polar ionosphere
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004





Pattern of field-aligned currents associated with
the flow (via the divergence of the ionospheric
current) is important for auroras, since upward
current generally implies downward-precipitating
hot magnetospheric electrons bright auroras As
we will see, we get a basic four-zone current
system, down on open field lines and in the V-C
regime, and up at the boundary of open field
lines and in the middle magnetosphere
corotation-breakdown region
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Corresponding flow in the polar ionosphere
Jupiter Cowley, Bunce, Stallard Miller, 2003
Saturn Cowley, Bunce, Prangé, 2004





Now need to quantify these simple qualitative
pictures Set up simple axisymmetric models in
which the flow is purely azimuthal and varies
with co-latitude as determined from a combination
of spacecraft data (e.g. Voyager Galileo),
modelling, and theory calculate currents
auroral parameters Jupiter model Cowley et al.
2005 Saturn model Cowley Bunce 2003,
Cowley, Bunce, ORourke 2004
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Axisymmetric models Angular velocity profiles
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
Rigid corotation
Rigid corotation
?/?J
?/?S
Co-latitude/deg
Co-latitude/deg
Flow regimes are
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Axisymmetric models Angular velocity profiles
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
?/?J
?/?S
Polar cap
Polar cap
Co-latitude/deg
Co-latitude/deg
Flow regimes are Polar cap region of open
field lines, weak sub-corotational flow Isbell
et al., 1984
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Axisymmetric models Angular velocity profiles
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
?/?J
?/?S
Outer
Outer
Polar cap
Polar cap
Co-latitude/deg
Co-latitude/deg
Flow regimes are Polar cap region of open
field lines, weak sub-corotational flow Isbell
et al., 1984 Outer magnetosphere region,
Vasyliunas-cycle and Dungey-cycle return region
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Axisymmetric models Angular velocity profiles
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
?/?J
?/?S
Middle
Middle
Outer
Outer
Polar cap
Polar cap
Co-latitude/deg
Co-latitude/deg
Flow regimes are Polar cap region of open
field lines, weak sub-corotational flow Isbell
et al., 1984 Outer magnetosphere region,
Vasyliunas-cycle and Dungey-cycle return
region Middle magnetosphere region of
sub-corotating flows (Voyager/Galileo models)
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Axisymmetric models Angular velocity profiles
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
Inner
Inner
?/?J
?/?S
Middle
Middle
Outer
Outer
Polar cap
Polar cap
Co-latitude/deg
Co-latitude/deg
Flow regimes are Polar cap region of open
field lines, weak sub-corotational flow Isbell
et al., 1984 Outer magnetosphere region,
Vasyliunas-cycle and Dungey-cycle return
region Middle magnetosphere region of
sub-corotating flows (Voyager/Galileo
models) Inner region of near-rigid corotational
flow
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Axisymmetric models Total equatorward Pedersen
current
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
?P 0.2 mho
?P 1.0 mho
IP/MA
IP/MA
Middle
Outer
Outer
Middle
Polar cap
Polar cap
Inner
Inner
Co-latitude/deg
Co-latitude/deg
Determined from the Pedersen conductivity times
the electric field in the neutral atmosphere rest
frame Field-aligned current density can be
calculated from these profiles from the gradient
of IP - downward current where IP increases,
upward current where IP decreases
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Axisymmetric models Field-aligned current
density
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
ji/?A m-2
?P 0.2 mho
?P 1.0 mho
ji/?A m-2
Middle
Outer
Outer
Polar cap
Polar cap
Middle
Inner
Inner
0
0
-
-
Co-latitude/deg
Co-latitude/deg
FAC profile consists in both cases of
Distributed downward FACs in the polar cap and
outer magnetosphere regions Sheet of upward FAC
at the polar cap boundary 0.2-0.5 ?A m-2 in both
cases Distributed upward FAC in the middle
magnetosphere peaking at 0.5 ?A m-2 for Jupiter
and 0.01 ?A m-2 for Saturn
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Overview of overall current system in
noon-midnight plane FAC flows out at
low-latitudes, then radially outward through the
equatorial plasma the corotation enforcement
current system Closes partly through downward
FAC at higher latitudes, and partly over the
magnetopause Magnetopause current closes through
downward FAC in the tail lobes, which then closes
via ionosphere partly through upward FAC at the
open-closed field boundary (the D-C current
system), and partly via upward FAC at lower
latitudes
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Axisymmetric models Field-aligned current
density
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
ji/?A m-2
?P 0.2 mho
?P 1.0 mho
ji/?A m-2
Middle
Outer
Outer
Polar cap
Polar cap
Middle
Inner
Inner
0
0
-
-
Co-latitude/deg
Co-latitude/deg
Final questions - Do the upward FACs require
downward electron acceleration ? To examine
this we use the kinetic theory of Knight 1973
combined with observed properties of
magnetospheric source electrons in the various
regions
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Axisymmetric models Acceleration of auroral
electrons
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
Middle magnetosphere
Polar cap boundary
Polar cap boundary
?/kV
?/kV
0
0
-
-
Co-latitude/deg
Co-latitude/deg
For Jupiter acceleration voltages required are
5-10 kV at polar cap boundary and 100 kV in
middle magnetosphere For Saturn 10-20 kV at
polar cap boundary, no auroral acceleration
required in middle magnetosphere
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Axisymmetric models Precipitating electron
energy flux
Jupiter Cowley et al., 2005
Saturn Cowley, Bunce, ORourke, 2004
Ef/mW m-2
Ef/mW m-2
Polar cap boundary
Middle magnetosphere
Polar cap boundary
0
0
-
-
Co-latitude/deg
Co-latitude/deg
UV auroral output For Jupiter - at polar cap
boundary luminosity 100 kR, total power 10 GW
- in middle magnetosphere 500 kR, total
power 500 GW For Saturn - at polar cap
boundary luminosity 25 kR, total power 15
GW How do these compare with observations?
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HST Images of Jupiter Saturns UV auroras
Dawn
Dusk
Dawn
Dusk
Noon
Noon
Jupiters auroras consist of - Steady main oval
at 16? mag lat, typically 300 kR intensity
250 GW emitted power corotation breakdown
auroras Variable polar aurora often in a fuzzy
ring, 50 kR intensity 25 GW emitted power
boundary of open field lines (?) Moon auroras
Saturns auroras consist of - Variable main oval
at 15? mag lat, typically 5-100 kR intensity
5-30 GW emitted power boundary of open field
lines (?) Shows persistent dawn-dusk asymmetry
as expected when D-C and V-C are active (new
non-axisymmetric model by Jackman Cowley 2006)
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How do these magnetospheres respond to the solar
wind?
Dawn
Dusk
Dawn
Dusk
Noon
Noon
UV auroras provide a unique global view that can
act as pointers for in situ studies What do we
expect? - not substorms like Earth because tail
replenishment times are very long compared with
solar wind/IMF time scales - so perhaps
trickle-charging of the tail followed by random
or induced collapse? Information for Saturn
provided by the Jan 2004 HST-Cassini campaign
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Cassini-HST Saturn campaign 8-30 January
2004 Selection of HST images (obtained every 2
days) Clarke et al., 2005 Grodent et al., 2005
A
B
C
D
E
F
H
G




These span a range of widely differing solar wind
conditions Crary et al., 2005
Solar wind dynamic pressure
D
A
B
C
E
F
G
H
IMF magnitude
32
Cassini-HST Saturn campaign 8-30 January
2004 Selection of auroral images (which were
obtained every 2 days) Clarke et al., 2005




Badman et al. 2005 have used these images
concurrent interplanetary data to estimate the
magnetopause and tail reconnection
rates Rarefaction regions show slow growth of
open flux in the system depending on IMF
strength - also contain episodic intervals of
tail reconnection net flux closure Compression
regions - initiate episodes of strong tail
reconnection and closure of open flux - net
accumulation of open flux can occur in the strong
IMF region that follows Infer that episodic tail
activity should be a significant feature at
Saturn particularly compression-induced
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Equatorial plane Ionosphere
Magnetospheric auroral effects of tail
reconnection - results in hot plasma injection
into the post-midnight magnetosphere related
auroral bulge in the ionosphere - hot plasma
rotates about the planet due to M-I coupling, so
the bulge rotates first into the dawn sector,
then around towards noon - after 20-30 h,
the hot plasma rotates back into the nightside
forming an auroral spiral structure, as
observed We have found evidence that such an
event occurred during the SOI pass when Cassini
was outbound in the tail, resulting in a hot
plasma injection
Cowley et al. 2005

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Nichols et al. 2006
What about Jupiter? In fact a similar
Cassini-HST campaign was conducted during the
Jupiter fly-by in Dec 2000 Jan 2001, but the
two data sets have only recently been put
together Because of irregularly-spaced HST
observations, by chance 6 out of 7 HST intervals
lie in rarefaction regions these all show
auroras similar to the example shown previously
(from 16 Dec 2000) Only one case (13 Jan 2001)
shows moderate compression-region conditions (and
not under very favourable viewing geometry) -
what does it show?
35
Compression-region image of Jupiters auroras 13
Jan 2001
Dawn
Dusk
Noon
Nichols et al. 2006
Main oval located on the usual reference oval,
but now strongly enhanced in intensity to 500 kR
1 MR, and widened in the dusk sector by bright
active sector auroras - opposite to
theoretical prediction (main oval should dim on
compression !) Mottled 100 kR auroras now
almost fill the whole polar cap expanded open
flux? To address these open questions a new HST
campaign of 128 orbits (J S) will take place in
Jan-June 2007 we expect this to form a
definitive data set
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Summary (1) Simple models of M-I coupling at
Jupiter Saturn have been developed, which
provide insight into the nature of the overall
current systems and the origins of the planetary
auroras (2) Flows associated with the D-C and
V-C on closed field lines can be distinguished by
ion composition (3) Quantitative estimates of
the Dungey-cycle solar wind driving at Jupiter
Saturn have also been made, and show that peak
flux transfer rates are competitive with
planetary-rotation driven transport in the outer
and middle magnetospheres (4) Tail
replenishment times are long , however,
comparable with the time scale for CIRs in the
solar wind (1-2 weeks for Saturn, 3 weeks for
Jupiter) (5) Intervals of rapid tail
reconnection associated with CIR compression
regions appear to play a major role in governing
the overall open flux budget at Saturn, though
(substorm-related?) tail reconnection can occur
at other times as well (6) Response at Jupiter
to major interplanetary compression events has
not yet been observed in detail, and is an
important topic for future study