Stromkonfiguration in der Nhe eines Polarlichtbogens O Marghitu

Stromkonfiguration in der Nähe eines Polarlichtbogens O. Marghitu (1, 3), G. Haerendel (2, 3), B. Klecker (3), and J. P. Mc. Fadden (4) (1) (2) (3) Institute for Space Sciences, Bucharest, Romania International University of Bremen, Germany Max-Planck-Institut für extraterrestrische Physik, Garching, Germany (4) Space Sciences Lab. , Univ. of California at Berkeley, USA (5) AEF Tagung, Kiel, März 11, 2004 Photo: Jan Curtis, http: //climate. gi. alaska. edu/Curtis

Preamble Type 1 Type 2 From Bostrom (1964) Type 1: Substorm current wedge, convection electrojets Type 2: Auroral arcs, large scale Birkeland currents Our case: The current circuit resembles Type 1 in the vicinity of a wide, stable, winter evening arc.

Outline A. Experimental setup and data B. Current configuration C. Summary and prospects

A Conjunction Map and Geophysical Data A http: //swdcdb. kugi. kyoto-u. ac. jp Magnetic noon at top; N=Magnetic pole X=Arc: Deadhorse, AK, 70. 22 o x 211. 61 o Time: Feb. 9, 1997, 8: 22 UT FAST; Aur. Oval ; Terminator at 110 km Growth phase of a small substorm Kp = 2 Dst = -27

E N A Optical Data A Photo: courtesy W. Lieb, MPE • Low-light CCD cameras developed at MPE • Wide-angle optics (86 ox 64 o) • Pass band filter, 650 nm • Exposure time 20 ms • Digitized images, 768 x 576 x 8 Images 4 s apart, 8: 22 – 8: 23. FAST footprint shown as a square. The arc is stable and drifts southward, ~200 m/s, equivalent to ~10 m. V/m westward (if the arc has no proper motion).

A FAST Data A (a) Electrons (b) Ions http: //www-ssc. igpp. ucla. edu/fast • 2 nd NASA SMEX Mission • PI Institution UCB/SSL • Launch: August 21, 1996 • Lifetime: 1 year nominal; still alive • Orbit: 351 x 4175 km, 83 o • Full set of plasma and field sensors CR very close to FR. Just a small bit of the dwd. FAC returns to magnetosphere as upwd. FAC. (c) Potential (d) Sheet current (e) Mag. Perturb.

B Current and Plasma Flow Topology B Type 1 Type 2 Current; Electric field; Plasma convection FR=FAC reversal; CR=Convection reversal AS, AN=Southern and northern arc edges

B Quantitative Evaluation B Conductance from particle precipitation + Electric field • Data cannot be mapped to ionosphere when FAST crosses the AAR • FAST does not measure the DC E–W electric field The new ALADYN method, based on a parametric arc model, can be used north of the CR: • Polarization => Ex not const. • El. field parallel to arc => Eh not 0 • FAC – EJ coupling => Jh not div free Current

B Tentative Equatorial Mapping B From Heelis and Hanson, 1980 Convection studies based on Atmospheric Explorer C data From Heelis et al. , 1980

C Summary C • Because of the close proximity of the CR and FR the downward and upward FACs appear to be electrically separated in the ionosphere. • The current continuity is achieved at the expense of the electrojets. • Although the magnetic field signature suggests the standard ’Type 2’ configuration, the current topology resembles the ’Type 1’, in a modifed realisation, with the FAC distributed along the arc.

C Prospects C • Current topology for other FAST orbits. First step: FR vs. CR. • Check the results with conjugated ground data, when available. • Is there any association with the substorm growth phase? • Model the complete current circuit, including the magnetospheric closure.