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Auroral Physics
  Auroral arc electrodynamics
  Possible relationship between visible auroral arcs and ion beams
  The altitude of the auroral acceleration region
  Experimental investigation of auroral generator regions by Cluster/FAST conjunctions
  Catalog of geomagnetic events recorded on-board the MAGION-2 satellite during 1990
  Investigation of field-aligned currents in double-oval configuration
  Small-scale structure of field aligned currents and discrete auroral arcs
Auroral Arc Electrodynamics

In a simple model the auroral arc is represented as a homogeneous stripe of increased conductance; field-aligned current (FAC) sheets that flow in and out of the ionosphere at the arc boundaries are connected across the arc through Pedersen current, while the electrojet (EJ), that flows along the arc as Hall current, is divergence free. To evaluate the deviation from this ideal configuration we developed the ALADYN (AuroraL Arc electroDYNamics) method (Marghitu, 2003; Marghitu et al., 2004), based on a parametric arc model, that allows the derivation of the parameters by numerical fit to the experimental data. In order to obtain consistent results it turns out that one has to take into account, as a minimum, the ionospheric polarization, the contribution of the Hall current to the meridional closure of the FAC, and the coupling between the FAC and the EJ.

The method is illustrated with a wide, stable, winter evening arc, as seen in the "Data" panels below. The ionospheric electric field (IEF), potential, and current obtained by ALADYN are presented in the "Results" panels. The IEF and potential are given for two arc models: YPYH, where only the polarization and Hall contribution are considered, and YPYHX, where the FAC?EJ coupling is also parametrized. Outside of the ion beams the potential drops at FAST altitude and at ionospheric level match each other (as expected, because the magnetic field line is equipotential) for model YPYHX (panel d), which is not the case for model YPYH (panel c). This is a key feature, pointing to the importance of the variations along the arc. The negative excursions of Ex at the arc boundaries indicate polarization charge double layers, as sketched in panel f. Once the IEF is derived, one can also find the ionospheric current. In our case the ionospheric connection between the downward and upward FACs is vanishingly small: Jx≈0 at the arc leading edge (panel e), a quite atypical configuration caused by the close vicinity of the FAC and convection reversal.

Left: Ground optical data (a), as well as particle and field data collected by FAST in the vicinity of the arc: electron (b) and ion (c) energy spectrograms; high altitude electric potential (d); perturbation magnetic field (e). The FAC reversal (FR) and convection reversal (CR) are indicated.
Right: Results obtained by ALADYN: IEF along FAST footprint (a, b) for polarization length scales of 4km (red), 8km (green), and 20km (blue); potential drop (c, d) at FAST altitude (black) and ionospheric level (red); field-aligned (black) and ionospheric (Jx red, Jy green, together with their respective Pedersen and Hall components) current (e); schematic view of the arc (f).


Marghitu, O., Ph.D. Thesis, TU Braunschweig, MPE Report 284, 2003.
Marghitu, O., et al., J. Geophys. Res., 109, A11305, doi:10.1029/2004JA010474, 2004.

Contact: Dr. Octav Marghitu
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