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

Preamble Type 1Type 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.

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

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

A Optical Data A Low-light CCD cameras developed at MPE Wide-angle optics (86 o x64 o ) Pass band filter, 650nm Exposure time 20ms Digitized images, 768x576x8 Photo: courtesy W. Lieb, MPE Images 4s apart, 8:22 – 8:23. FAST footprint shown as a square. The arc is stable and drifts southward, ~200m/s, equivalent to ~10mV/m westward (if the arc has no proper motion). N E

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

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 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 => E  not const. El. field parallel to arc => E  not 0 FAC – EJ coupling => J  not div free Current Conductance from particle precipitation +

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

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.