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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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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

upward field-aligned

current

J||

Shear flow,

for small scales carried by electron Hall current

U shaped potential

Electron precipitation,

Acceleration by E||


Relation between auroral arcs and shear flow

jHall


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


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


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


Vlasov equilibrium approach


Vlasov model for thin embedded current sheet

x = -4

x = 0


Double electron current sheet


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.)


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.


pressure change

velocity vx

2 min

2 min

4 min

4 min

6 min

6 min

8 min

8 min


cross-tail current density

pressure change

2 min

2 min

4 min

4 min

6 min

6 min

8 min

8 min


Relation between tail flows (“bubbles”), thin current sheets, and auroral arcs

Earthward moving bubble

Current layers


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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