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Cambridge 2004 Cambridge 2004
Wolfgang Baumjohann
IWF/ÖAW Graz, Austria
With help from: R. Nakamura, A. Runov, Y. Asano & V.A. Sergeev
Magnetotail Magnetotail Transport and Transport and
SubstormsSubstorms
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Sun-Earth ConnectionSun-Earth Connection
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Standard ModelStandard Model
Magnetospheric convection is driven by solar wind.
Merging of dipolar field with southward IMF at MP
Open field lines move tailward over polar cap
Reconnection to dipolar & SW field lines in tail
Closed field lines move sunward in equatorial plane
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Inner MagnetosphereInner Magnetosphere
GEOS-2 Electron Beam Experiment measures "shift" of gyration circle of 1 keV electrons by electrical drift
Southward IMF: convection towards magnetopause Northward IMF: only weak plasma flow
Mean values for southward IMF correspond to standard model
In inner magnetosphere quasi-static convection:
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Dependence of convection velocity toward plasma sheet on polarity of IMF BZ clearly
visible
Lobe Convection - 1Lobe Convection - 1
Cluster Electron Drift Instrument (EDI) uses same principle as GEOS-2 Electron Beam Experiment
Cluster/EDI gives first direct measurements of convection in lobe (measuring electric field in extremely thin plasma over polar cap)
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Lobe Convection - 2Lobe Convection - 2
EDI data also show IMF BY
effect
Shear flow in Y-Z plane
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MagnetotailMagnetotail
Pressure Catastrophe:
Adiabatic convection:
d/dt PV = 0, V = B -1ds
Flux tube volume strongly decreases toward Earth
Convection stops to avoid pressure catastrophe
Tail observations with IMP show:
Fast Earthward flow for 25-40 RE
Closer in, convection severely slows down
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Near-Earth Neutral LineNear-Earth Neutral Line Tail-like field geometry
weakens pressure gradient
Reconnection leads to smaller flux tube volume
Earthward convection by bursty bulk flows
Reversal of fast flow direction observed by Geotail
Near-Earth neutral line located ~25 RE
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Flow & Curv B ReversalFlow & Curv B Reversal
Magnetic field components in Cluster barycentre: 4 current sheet traversals
Field line curvature:
curv B = (b.grad)b
Flow and field line curvature reversal
X-line moves tailward over Cluster
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Reconnection & Hall EffectReconnection & Hall Effect
Ion Flow Reversal during 4 neutral sheet crossings
X-line moves tailward over Cluster
500 km thin CS around X-line
bifurcated current sheet on both sides
Hall effect (By) during ‘outer’ crossings shows ion decoupling
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Electron Hall CurrentElectron Hall Current Cluster 2003 tail passes can resolve fine structure of currents
JY shows very thin current sheet (triple peaks?)
JX consistent with electron Hall current in ion diffusion region
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Currents at PSBL and X-lineCurrents at PSBL and X-line
Strong flow shear (N-S electric field) and thin field aligned current layer (1500 km) during a substorm
Consequence of Hall-effects in reconnection region and closure of the Hall-electric current
Ion diffusion region
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Braking & DipolarizationBraking & Dipolarization
Intermittent high-speed Earthward flow bursts (~500 km/s)
Bursty fast flows accompanied by dipolarization
Fast flow braked near 10 RE by dipolar field
More dipolar flux added by flow
Pressure gradients lead to current wedge and aurora
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Aurora & ElectrojetAurora & Electrojet
Aurora (in upward field-aligned current region electrons are accelerated downward)
Substorm electrojet (adds to convection electrojet)
Flow braking and flow shear generate:
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Flow Channel WidthFlow Channel Width Cluster gives direct estimate of spatial scale of bursty bulk
flows
Vertical: 1.5-2 RE, Azimuthal: 2-3 RE
Sharper gradient on duskside flank
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Aurora and Bursty Bulk FlowAurora and Bursty Bulk Flow Isolated flow bursts with
E>2mV/m (Geotail) always correspond to auroral activations (Polar).
Auroral activations near foot point of satellite start within 1 min of flow burst onset.
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Structure of Flow BurstsStructure of Flow Bursts
Spatial scale of flowsSmall expansion, pseudo-breakup 1.4 MLT (4-5 RE)
Auroral streamer (N-S aurora) 0.7 MLT (3-4 RE)
Flow bursts are centered 0.4 MLT east of aurora
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Flow Bursts and FACFlow Bursts and FAC
Scale size consistent with ionospheric observations
Aurora corresponds to upward FAC in bubble model
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Substorm RecoverySubstorm Recovery
45 min after onset dipolarization front meets neutral line
No reconnection in dipolar field; recovery phase begins
Neutral line retreating tailward
Fast flows transport dipolar field inward:
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Summary ScenarioSummary Scenario Plasma energy dominant outside of 25 RE
Flow uninhibited
Magnetic energy increases near 20-25 RE
Pending pressure catastrophe leads to NENL
Magnetic field dominant inside 15 RE
Dipolar field brakes BBF: current wedge & aurora are generated
Dipolarization front travels downtail and meets NENL
near-Earth reconnection stops
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Thanks…Thanks…