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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. Sun-Earth Connection. Standard Model. Magnetospheric convection is driven by solar wind.

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Cambridge 2004

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  1. Cambridge 2004 Wolfgang Baumjohann IWF/ÖAW Graz, Austria With help from: R. Nakamura, A. Runov, Y. Asano & V.A. Sergeev Magnetotail Transport and Substorms

  2. Sun-Earth Connection

  3. 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

  4. 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

  5. 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

  6. Lobe Convection - 2 • EDI data also show IMF BY effect • Shear flow in Y-Z plane

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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)

  15. 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

  16. 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.

  17. 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

  18. Flow Bursts and FAC • Aurora corresponds to upward FAC in bubble model • Scale size consistent with ionospheric observations

  19. 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:

  20. 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

  21. Thanks…

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