1. 3D Current Topology in the Vicinity of an Evening Arc O. Marghitu (1,3), G. Haerendel (2), B.Klecker (3), and J.P. McFadden (4) (1)Institute for Space Sciences, Bucharest, Romania (2)International University of Bremen, Germany (3)Max-Planck-Inst. f. extraterr. Physik, Garching, Germany (4)Space Sciences Lab., Univ. of California at Berkeley, US EGS-AGU-EUG, Nice, April 9, 2003 Credit: Jan Curtis, http://climate.gi.alaska.edu/Curtis

2. Acknowledgement C. Carlson – FAST R. Ergun – FAST Electric field R. Strangeway – FAST Magnetic field FAST Team J. Vogt, H. Frey – Optical data World Data Center for Geomagnetism, Kyoto – AE Index

3. Contents Experimental setup: FAST and ground based optics Data: Optical and FASTmeasurements Current topology Summary Prospects

4. Setup: Ground Optics Low-light CCD cameras developed at MPE, Garching Wide-angle optics (86 o x64 o ) Pass band filter Exposure time 40 ms multiplied with powers of 2 Digitized images, 768x576x8 Location: Deadhorse, Alaska, 70.22 o LAT, 211.61 o LON Time: Feb. 9, 1997, 8:22UT Photo: courtesy W. Lieb, MPE

5. Setup: FAST 2nd NASA SMEX Mission PI Institution UCB/SSL Launch: August 21, 1996 Lifetime: 1 year nominal; still operational Orbit: 351 x 4175km, 83 o Spin axis perpendicular to the orbit plane Electric field: three orthogonal boom pairs equipped with spherical probes Magnetic field: a DC fluxgate and an AC search coil Mass spectrometry: TEAMS – measures full H+ and O+ distributions in ½ spin and He+ in one spin Plasma analyzers: IESA, EESA, SESA – high time resolution electron and ion data, with uninterrupted 360 o coverage http://www-ssc.igpp.ucla.edu/fast

6. Data: AE index, Feb. 9, 1997 http://swdcdb.kugi.kyoto-u.ac.jp

7. Optical Data: 8 min Selection of images, 1 min apart, taken at UT 8:18 – 8:26. FAST crosses the camera´s FoV in the frames 4, 5, 6; the satellite´s ionospheric footprint is shown as a square. The limits of the ion beams detected by FAST are overlaid in all the frames, to provide a reference. N E

8. Optical Data: 1 min Images 4 s apart during 8:22-8:23. FAST footprint is shown as a square. ´11´ and ´22´ are the limits of the first two ion beams. The arc is stable and drifts with ~200m/s, equivalent to ~10mV/m (assuming the arc has no proper motion).

9. FAST Data: Trajectory Magnetic noon at the top N=magnetic pole X=arc FAST Path Auroral Oval Terminator at 110km

10. FAST Data: Large Scale Top: Potential. Middle: Magnetic field in the Satellite Associated System (SAS). Bottom: Magnetic field in the Arc Associated System (AAS).

11. FAST Data: Medium Scale Panel 1: Magnetic field. Panels 2-4: Electrons, energy. Panel 5: Electrons, p.-a. Panels 6: Ions, energy Panels 7: Ions, p.-a. Panel 8: Electric potential The arc is north of the convection reversal

12. Current Configuration: FR vs. CR The relative positions of the FAC reversal (FR), the convection reversal (CR), and the arc. The CR is very close to the FR and just a negligible fraction of the downward FAC returns to the magnetosphere as upward FAC.

13. Current Configuration: Flow Topology Type 1 Type 2 From Bostrom (1964) Current Electric field Plasma convection

14. Current config: quantitative evaluation Electric field High-altitude data cannot be mapped to ionosphere when FAST crosses the AAR FAST does not measure the E–W electric field Method based on a parametric model of the arc, prezented in the poster session. In order to obtain consistent results one has to take into account, as a minimum: Ionospheric polarization => E  not const. =>a 1,....., a n Hall current perpendicular to the arc => E  not 0 => b 0 Coupling FAC – electrojet => J  not divergence free => c 1 Current Conductance from particle precipitation +

15. Current config: quantitative evaluation J , J , J ||

16. Summary 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 data suggests the standard Bostrom Type 2 (1B2) configuration, the current topology looks like 2 times Bostrom Type 1 (2B1)

17. Prospects Check the current topology for other orbits. First step: the relative positions of FR and CR. Investigate the conditions under which the 1B2 topology develops. Check the results with ground observations, when conjugated data exists.