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Thin Electron Current Sheets and Auroral Arcs

Thin Electron Current Sheets and Auroral Arcs. Joachim Birn LANL Karl Schindler Ruhr-Univ. Bochum Michael Hesse NASA/GSFC. Relationship between magnetospheric features and auroral electric fields and currents: role of flows, currents, electric potentials. Auroral arc brightening.

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Thin Electron Current Sheets and Auroral Arcs

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  1. Thin Electron Current Sheets and Auroral Arcs Joachim Birn LANL Karl Schindler Ruhr-Univ. Bochum Michael Hesse NASA/GSFC Relationship between magnetospheric features and auroral electric fields and currents: role of flows, currents, electric potentials

  2. Auroral arc brightening upward field-aligned current J|| Shear flow, for small scales carried by electron Hall current U shaped potential Electron precipitation, Acceleration by E||

  3. Relation between auroral arcs and shear flow jHall

  4. Vlasov model for thin embedded current sheet x = -4 x = 0 • current in thin sheet carried by ExB drifting electrons • pressure dominated by ions •  el. stat. potential

  5. Relation between thin current sheet and auroral arc • current in thin sheet carried by eastward ExB drifting electrons • rapid thinning provides Alfvenic pulse with field-aligned currents •  may lead to quasi-steady potential pattern

  6. Summary: electron current sheets & arcs •  thin current sheets dominated by electron flow (eastward ExB drift) •  but pressure dominated by ions •  potentials up to fraction of kTi/e •  bifurcation of current earthward of thin sheet •  double current sheet possible  onset from rapid thinning in late growth phase or initial reconnection •  Alfvenic pulse with firl-aligned currents •  may lead to persisting potential •  precedes fast flow and dipolarization

  7. Vlasov equilibrium approach

  8. Vlasov model for thin embedded current sheet x = -4 x = 0

  9. Double electron current sheet

  10. Bubble propagation 2 min 4 min velocity vx (300 km/s) 6 min (-200 km/s) depleted flux tube (reduced pressure or flux tube volume, entropy: pVg) 8 min Earthward motion from buoyancy, interchange (Pontius & Wolf, Chen & Wolf) Association with bursty bulk flows? (Sergeev et al.)

  11. Macro-scale field-aligned current generation Field-aligned currents at x = -1 twisted or sheared magnetic field field-aligned current flow Obs.: Nakamura et al.

  12. pressure change velocity vx 2 min 2 min 4 min 4 min 6 min 6 min 8 min 8 min

  13. cross-tail current density pressure change 2 min 2 min 4 min 4 min 6 min 6 min 8 min 8 min

  14. Relation between tail flows (“bubbles”), thin current sheets, and auroral arcs Earthward moving bubble Current layers

  15. Summary: Bubble/BBF effects Summary: electron current sheets - current dominated by electrons - pressure dominated by ions - potentials up to fraction of kTi/e - bifurcation of current possible - double current sheet possible, cannot extend to z=0 inside plasma sheet Large scales: - ion & electron vorticity, shear flow (EB drift)  field-aligned currents  E  E|| Small scales: - perp. currents from enhanced p, reduced B - Hall current sheets  electron EB drift  E - closure of converging E  E||  U-shaped potentials - electron shear flow, vorticity  field-aligned currents

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