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Thin Electron Current Sheets and Auroral Arcs

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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

upward field-aligned

current

J||

Shear flow,

for small scales carried by electron Hall current

U shaped potential

Electron precipitation,

Acceleration by E||

jHall

x = -4

x = 0

- current in thin sheet carried
by ExB drifting electrons

- pressure dominated by ions
- el. stat. potential

- 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

- 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

x = -4

x = 0

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.)

Field-aligned currents at x = -1

twisted or sheared

magnetic field

field-aligned current

flow

Obs.: Nakamura et al.

velocity vx

2 min

2 min

4 min

4 min

6 min

6 min

8 min

8 min

pressure change

2 min

2 min

4 min

4 min

6 min

6 min

8 min

8 min

Earthward moving bubble

Current layers

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 (EB drift)

field-aligned currents E E||

Small scales:

- perp. currents from enhanced p, reduced B

- Hall current sheets electron EB drift E

- closure of converging E E|| U-shaped potentials

- electron shear flow, vorticity field-aligned currents