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

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

3. 3 Relation between auroral arcs and shear flow j Hall

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

5. 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. 6  thin current sheets dominated by electron flow (eastward ExB drift)  but pressure dominated by ions  potentials up to fraction of kT i /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 Summary: electron current sheets & arcs

7. 7

8. 8 Vlasov equilibrium approach

9. 9 x = -4x = 0 Vlasov model for thin embedded current sheet

10. 10 Double electron current sheet

11. 11 2 min 4 min 6 min 8 min (300 km/s) (-200 km/s) Bubble propagation velocity v x depleted flux tube (reduced pressure or flux tube volume, entropy: pV  ) Earthward motion from buoyancy, interchange (Pontius & Wolf, Chen & Wolf) Association with bursty bulk flows? (Sergeev et al.)

12. 12 twisted or sheared magnetic field field-aligned current flow Macro-scale field-aligned current generation Field-aligned currents at x = -1 Obs.: Nakamura et al.

13. 13 2 min 4 min 6 min 8 min 2 min 4 min 6 min 8 min pressure changevelocity v x

14. 14 2 min 4 min 6 min 8 min 2 min 4 min 6 min 8 min cross-tail current densitypressure change

15. 15 Relation between tail flows (“bubbles”), thin current sheets, and auroral arcs Earthward moving bubble Current layers

16. 16 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 Summary: Bubble/BBF effects Summary: electron current sheets - current dominated by electrons - pressure dominated by ions - potentials up to fraction of kT i /e - bifurcation of current possible - double current sheet possible, cannot extend to z=0 inside plasma sheet