fran bagenal university of colorado
TRANSCRIPT
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Fran Bagenal University of
Colorado
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Magnetosphere Dynamics
Internal
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A B A
B
In rotating magnetosphere If fluxtube A contains more mass than B – they interchange
Radial Transport
If β << 1, interchange of A and B does not change field strength.
Rayleigh-Taylor instability where centrifugal potential replaces gravity
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A B A
B
In rotating magnetosphere If fluxtube A contains more mass than B – they interchange
Radial Transport
You can think of centrifugally-driven fluxtube interchange as a kind of diffusion. - How will density vary with distance from the source? - How will diffusion rate depend on gradient of
density?
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A B A
B
If β << 1, interchange of A and B does not change field strength.
1035
1034 1035
1036
RJ RS
Jupiter Saturn
NL2 NL2
FAST outward Inward SLOW
Radial Transport Rayleigh-Taylor instability
where centrifugal potential replaces gravity
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Plasmaspheres / Plasma disks
Earth's Plasmasphere
Image observations 30.4 nm He+
Jupiter's plasmadisk Heats up as it moves outwards 10s keV plasma – β~10s
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Plasma Heating - Flux tube expands faster than bounce time - violation of 2nd adiabatic invarient
Vogt et al. (2014)
bounce-averaged
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Interchange transports material from inner m’sphere, but plasma β increases outward and
ultimately ballooning replaces interchange
If β << 1, interchange of A and B does not change field strength. - This is the ‘ideal’ interchange
However for finite β, if A has more plasma, pressure will increase relative to surroundings at A’s new location once it is displaced.
- Now the field pressure has to change also to maintain quasi-equilibrium - The motion is no longer an ‘ideal’ interchange - Once field strength changes, field must bend as well
A B A
B small β A
B
large β
Radial Transport – high β
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Magnetosphere Dynamics
Solar-Wind-Ionosphere-Magnetosphere Coupling
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Dungey Cycle
Dynamics at Earth driven by the solar wind coupling the Sun's magnetic field to
the Earth's field
Opening reconnection
Closing reconnection
Rotating Plasmasphere
• Variable opening & closing rates
• Must be equal over time to conserve magnetic flux
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Magnetic Reconnection
NASA Magnetospheric Multi-Scale mission – Launch spring 2015
Ion Scale
Electron Scale
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Polar view Connected to solar wind
Closed magnetic field
Variable opening & closing rates Must be equal over time to conserve magnetic flux
Solar Wind
note: ionosphere is incompressible
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Ionosphere - Sets boundary conditions for magnetospheric dynamics
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Solar Wind
Econvection = - ζ VSW X BSW ζ ~ efficiency of reconnection ~10-20%
The Dungey Cycle Solar wind driven
magnetospheric convection*
B V
E
E V
B
Vconvection
~ ζVSW(R/RMP)3
(where 3 power assumes a dipole - in reality, the flow is not uniform and the power somewhat less)
crude approximation!! Econv~ constant in m'sphere
(*strictly speaking not convection but advection or circulation)
This is the conventional E-J approach. See Parker 1996; Vasyliunas 2005,11 for B-V approach
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growth phase
substorm onset
Substorm Energy Storage
solar wind kinetic energy converted to magnetic energy
SW kinetic energy
magnetic energy
kinetic heat
magnetic energy
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Evolutionary Phases for Substorm Plasmoid
1. Energy storage:
2. Onset:
3. Recovery:
Aurora: • Open-closed boundary • Stronger on nightside • Highly variable
DNL=Distant Neutral Line NENL = Near Earth Neutral Line
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Reality = Messy & 3D
Needs sophisticated, complicated numerical models – and color
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Dynamics Dayside magnetopause • Response to BSW direction • Solar wind ram pressure
Tail Reconnection • Depends on recent history of dayside reconnection and state of plasmasheet
Space Weather!
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Magnetosphere Dynamics
Solar-Wind-Ionosphere-Magnetosphere Coupling
vs.
Ionosphere-Magnetosphere Coupling
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Rplasmapause/RMP
~ [ rpRMP Ω / ζVSW]1/2
∝ Ω1/2µ1/6 /(ρSW)1/12 VSW2/3
Vco ~ Ω X R Vconvection
~ ζVSW(R/RMP)3
rp = planetary radius µ = magnetic moment of planet Bo Rp
3
Fraction of planetary magnetosphere that is rotation dominated is…
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What if... How would location of plasmapause change? 1. Reconnection more/less
efficient at harnessing the solar wind momentum
2. Planet’s spin slows down
Rplasmapause/RMagnetoPause
~ [ rpRMP Ω / ζVSW]1/2
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Rplasmapause / RPlanet =
6.7 350 95
Solar-wind vs. Rotation-dominated magnetospheres
Assumptions: 1. Planet’s rotation coupled to magnetosphere 2. Reconnection drives solar wind interaction
RMP~10Rp RMP~100Rp RMP~25Rp
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Jupiter's 3 Types of Aurora
Aurora associated with moons
Steady Main Auroral Oval
Variable Polar Aurora
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Clarke et al. Grodent et al.
HST
Jupiter’s Aurora -
The Movie
Fixed magnetic
co-ordinates rotating
with Jupiter
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Main Aurora
• Shape constant, fixed in magnetic co-ordinates
• Magnetic anomaly in north
• Steady intensity • ~1° Narrow Clarke et al., Grodent et al. HST
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• As plasma from Io moves outwards its rotation decreases (conservation of angular momentum)
• Sub-corotating plasma pulls back the magnetic field
• Curl B -> radial current Jr
• Jr x B force enforces rotation
Coupling the Plasma to the Flywheel
Field-aligned currents couple magnetosphere
to Jupiter’s rotation
Khurana 2001
Cowley & Bunce 2001
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The aurora is the signature of Jupiter’s attempt to spin up its magnetosphere
Clarke et al. Hill 1979
• Outward transport of Iogenic plasma • J⎟⎟ transfers load to ionosphere • Transfer of angular momentum limited by ionospheric conductivity
20-30Rj 1 ton/s ? 0.3 S ? Cowley et al. 2002
• Empirical field+plasma model • Sub-corotating plasmasheet 20-60 Rj • Knight relation
• E|| ~ 100 kV • Upward current ~1 µA/cm2 • 1° narrow auroral oval
?
?
Upward ~ OK
Downward ???
Parallel electric fields: potential layers, φ||, “double layers”
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Where is the clutch slipping?
Mass loading
A - Between deep and upper atmosphere? B - Between upper atmosphere and ionosphere? C - Lack of current-carriers in magnetosphere-> E||?
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What if there are strong thermospheric winds that drive circulation in the ionosphere – What might be the manifestation in the magnetosphere? Hint: Think of a very symmetric magnetosphere – e.g. Saturn
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What if there are strong thermospheric winds that drive circulation in the ionosphere – What might be the manifestation in the magnetosphere?
Jia, Kivelson, Gombosi (2012)
Vortex in ionosphere
Produces longitudinal asymmetries in otherwise symmetric magnetosphere
SATURN
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Ionosphere - Sets boundary conditions for magnetospheric dynamics
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Scale, rotation-dominated, Io source
Main Aurora 20 RJ
RMP 60-100 RJ
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Radial Current
Azimuthal Current
Configuration 𝜵xBobserved ➔J
∇•J=0 ➔J||
Side View
Expands, stretches field
Looking Down
Bends field back
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Coupling - 1 (De-)Coupling - 1
Magnetospheric Factors: Ṁ, φ||, Alfven wave travel time Ionosphere/Thermosphere factors: ∑p, winds, chemistry, heating, radiation, etc;
φ||
φ||
Communica)on breaks down ~25RJ -‐> aurora Magnetosphere & atmosphere stop talking > 60 RJ
Ṁ=mass flux
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Vasyliunas Cycle
Outward in afternoon
Inward in morning
Reconnection sending plasmoids down the tail
Vasyliunas Cowley et al. Southwood & Kivelson
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Vogt et al. 2010
X-Line Observations of plasmoid events in Galileo data
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Ionosphere
Magnetosphere
Solar Wind
B – Rota)ng Plasmasphere
C -‐ Viscous Interac)on
A -‐ Dungey Circula)on
Which Form of Coupling Dominates -‐> Controls Dynamics
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Shear-driven Kelvin-Helmholtz instability
Otto & Fairfield 2001
Can small-scale boundary-layer processes act like viscosity?
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Upstream IMF
Upstream IMF wrapped around
flattened magnetospause
Mass & momentum transport – boundary layers
Small-scale, intermittant, turbulent, non-linear – reconnection
A bit like viscosity?
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Delamere & Bagenal 2010, 13
#1 Rota)ng plasmadisk Non-‐uniform for flux conserva)on
#2 tail reconnec)on line is variable in space and )me
Combining Rota)onal and Solar Wind stresses
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Delamere & Bagenal 2010, 13
#3 Large region on dusk flank with few observa)ons
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Closed flux
Open, polar cap flux
Tail-theading open flux
MHD model shows storage of flux and mass in wings on the sides of the magnetosphere - caused by SW interaction in equatorial boundary layer?
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Delamere & Bagenal 2010
Equatorial Plane
#4 Magnetosphere is flattened
Noon-Midnight Plane
#5 Polar cap is small
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Delamere & Bagenal (2013)
“wings” – closed field?
Drizzle?
Reconnection?
Small polar cap?
Cross-magnetopause Transport of mass & momentum
Cross-magnetopause Transport of mass & momentum Draped
IMF
Plasmoids
Jupiter
Tin-Tin tuft?
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Could Jupiter be a Colossal Comet? Plasma-plasma interaction with magnetic field playing less of a role than at Earth Venus- or comet-like rather than field-controlled terrestrial tail.
Solar wind hung up on the boundary layers
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Arrives at Jupiter 2016!
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Uranus - Highly asymmetric, - Highly non-dipolar - Complex transport (SW + rotation) - Multiple plasma sources (ionosphere + solar wind + satellites)
Toth et al.
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Zieger et al.
Neptune Similarly complex as Uranus
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Mercury - Magnetic field detected by Mariner 10 in 1974
Mercury & Ganymede
Diameter of Earth
Solar Wind
Ganymede - Magnetic field detected by Galileo in 1996
Bsurface ~ 1/100 Earth
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Testing our understanding of Sun-Earth connections through application to other planetary systems
Planetary Magnetospheres See vol. III ch. 7 & vol. I ch. 13
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Ionosphere
Magnetosphere
Solar Wind
B – Rota)ng Plasmasphere
C -‐ Viscous Interac)on
A -‐ Dungey Circula)on
Which Form of Coupling Dominates -‐> Controls Dynamics
How do INTERNAL properties control a magnetosphere? How do EXTERNAL properties control a magnetosphere?
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Magnetosphere
Solar Wind
A -‐ Dungey Circula)on
Ionosphere Solar wind drives flow over poles
Return circula)on required by ionosphere
incompressibility
Alfven wing
Ridley & Kivelson 2007
Reconnec)on-‐driven circula)on
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Ionosphere
Solar Wind
E∥
Magnetosphere
B – Rota)ng Plasmasphere Radial Transport
• Fluxtube interchange • Mass flux out • Empty magne)c flux in • Decoupling p, E∥ and/or Vr~VA
Mass Flux
Alfven Radius
p
p
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Ionosphere
Solar Wind
Sheath
Magnetoshere
Upstream IMF
Upstream IMF wrapped around flaZened -‐ magnetospause Magnetosphere
• Small-‐scale, intermiZant reconnec)on
• Mixing of sheath and m’sphere plasmas
• How much IMF penetrates inside m’sphere?
C–Viscous Interac)on
Delamere, Desroche Kelvin-‐Helmholtz Instability
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Summary • Diverse planetary magnetic fields &
magnetospheres • Earth, Mercury, Ganymede
magnetospheres driven by reconnection • Jupiter & Saturn driven by rotation &
internal sources of plasma • Uranus & Neptune are complex – need to
be explored! Stay tuned… Juno, JUICE missions to Jupiter Mission(s) to Uranus?! Neptune?!