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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…