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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
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
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||



Vlasov model for thin embedded current sheet
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
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
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





Bubble propagation
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
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
pressure change

velocity vx

2 min

2 min

4 min

4 min

6 min

6 min

8 min

8 min


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

Earthward moving bubble

Current layers


Summary bubble bbf effects
Summary: Bubble/BBF effects sheets, and auroral arcs

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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