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Cambridge 2004. Wolfgang Baumjohann IWF/ÖAW Graz, Austria With help from: R. Nakamura, A. Runov, Y. Asano & V.A. Sergeev. Magnetotail Transport and Substorms. Sun-Earth Connection. Standard Model. Magnetospheric convection is driven by solar wind.

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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 Transport and Substorms

standard model
Standard 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
inner magnetosphere
Inner Magnetosphere

In inner magnetosphere quasi-static convection:

  • 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
lobe convection 1
Lobe Convection - 1
  • Cluster/EDI gives first direct measurements of convection in lobe (measuring electric field in extremely thin plasma over polar cap)
  • Dependence of convection velocity toward plasma sheet on polarity of IMF BZ clearly visible
  • Cluster Electron Drift Instrument (EDI) uses same principle as GEOS-2 Electron Beam Experiment
lobe convection 2
Lobe Convection - 2
  • EDI data also show IMF BY effect
  • Shear flow in Y-Z plane
magnetotail
Magnetotail

Pressure Catastrophe:

  • Adiabatic convection: d/dt PV g = 0, V = ò 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
near earth neutral line
Near-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
flow curv b reversal
Flow & 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
reconnection hall effect
Reconnection & 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
electron hall current
Electron 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
currents at psbl and x line
Currents 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

braking dipolarization
Braking & 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
aurora electrojet
Aurora & Electrojet

Flow braking and flow shear generate:

  • Substorm electrojet (adds to convection electrojet)
  • Aurora (in upward field-aligned current region electrons are accelerated downward)
flow channel width
Flow 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
aurora and bursty bulk flow
Aurora and Bursty Bulk Flow
  • Isolated flow burstswith E>2mV/m (Geotail)alwayscorrespond toauroral activations(Polar).
  • Auroral activationsnear foot pointof satellite startwithin 1 minofflow burst onset.
structure of flow bursts
Structure of Flow Bursts

Spatial scale of flows

Small 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
flow bursts and fac
Flow Bursts and FAC
  • Aurora corresponds to upward FAC in bubble model
  • Scale size consistent with ionospheric observations
substorm recovery
Substorm 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:

summary scenario
Summary 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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