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
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Auroral arc brightening
J
upward field-aligned current
Shear flow, for small scales carried by electron
Hall current
U shaped potential
Electron precipitation, Acceleration by E
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Relation between auroral arcs and shear flow
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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

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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

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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

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Vlasov equilibrium approach
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Vlasov model for thin embedded current sheet
x -4
x 0
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Double electron current sheet
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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.)
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Macro-scale field-aligned current generation
Obs. Nakamura et al.
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pressure change
velocity vx
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cross-tail current density
pressure change
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Relation between tail flows (bubbles), thin
current sheets, and auroral arcs
Earthward moving bubble
Current layers
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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 z0
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