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Volume 29, issue 4
Ann. Geophys., 29, 679–685, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 29, 679–685, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

ANGEO Communicates 14 Apr 2011

ANGEO Communicates | 14 Apr 2011

Downward auroral currents from the reconnection Hall-region

R. A. Treumann1,2,*, R. Nakamura3, and W. Baumjohann3 R. A. Treumann et al.
  • 1Department of Geophysics and Environmental Sciences, Munich University, Munich, Germany
  • 2Department of Physics and Astronomy, Dartmouth College, Hanover NH 03755, USA
  • 3Space Research Institute, Austrian Academy of Sciences, Graz, Austria
  • *currently at: International Space Science Institute, Bern, Switzerland

Abstract. We present a simple (stationary) mechanism capable of generating the auroral downward field-aligned electric field that is needed for accelerating the ionospheric electron component up into the magnetosphere and confining the ionospheric ions at low latitudes (as is required by observation of an ionospheric cavity in the downward auroral current region). The lifted ionospheric electrons carry the downward auroral current. Our model is based on the assumption of collisionless reconnection in the tail current sheet. It makes use of the dynamical difference between electrons and ions in the ion inertial region surrounding the reconnection X-line which causes Hall currents to flow. We show that the spatial confinement of the Hall magnetic field and flux to the ion inertial region centred on the X-point generates a spatially variable electromotive force which is positive near the outer inflow boundaries of the ion inertial region and negative in the central inflow region. Looked at from the ionosphere it functions like a localised meso-scale electric potential. The positive electromotive force gives rise to upward electron flow from the ionosphere during substorms (causing "black aurorae"). A similar positive potential is identified on the earthward side of the fast reconnection outflow region which has the same effect, explaining the observation that auroral upward currents are flanked from both sides by narrow downward currents.

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