Transcript MPE Science Highlight

Auroral Arc Electrodynamics from Satellite Data
Abstract The ALADYN (AuroraL Arc electroDYNamics) method enables a realistic description of an
auroral arc (Marghitu, 2003; Marghitu et al., 2004). The method is 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 one has to take into account, as a minimum, the ionospheric polarization, the contribution
of the Hall current to the meridional closure of the field-aligned current (FAC), and the coupling between
the FAC and the electrojet (EJ) flowing along the arc.
ALADYN is illustrated with a wide, stable, winter evening arc, as seen in the ‘Data’ panels. 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 terms are considered, and YPYHX, where
the FAC–EJ coupling is added. Outside of the ion beams the potential drops at FAST and 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 Pedersen and Hall components of
Jx compensate each other, and the ionospheric connection between the downward and upward FACs is vanishingly
small (panel e). This quite atypical configuration is caused by the close vicinity of the FAC and convection reversal.
Data
Results
(a)
(b)
(c)
(d)
(e)
(f)
Left (a) Ground optical data. (b–e) FAST data: electron and ion energy spectrograms, high altitude electric potential,
perturbation magnetic field. FR and CR indicate the FAC and convection reversal, respectively.
Right: Results obtained by ALADYN: (a, b) IEF for polarization length scales of 4km (red), 8km (green), and 20km
(blue); (c, d) potential drop at FAST (black) and ionospheric level (red); (e) field-aligned (black) and ionospheric
current (Jx red, Jy green, together with their respective Pedersen and Hall components); (f) schematic view of the arc.
References:
• Marghitu, O., Ph.D. Thesis, TU Braunschweig, MPE Report 284, 2003.
• Marghitu, O., et al., J. Geophys. Res., in press, 2004.
O. Marghitu, B. Klecker, G. Haerendel, J.P. McFadden