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

Acknowledgement

C. Carlson – FASTR. Ergun – FAST Electric fieldR. Strangeway – FAST Magnetic fieldFAST Team

J. Vogt, H. Frey – Optical data

World Data Center for Geomagnetism, Kyoto – AE Index

Contents

• Experimental setup: FAST and ground based optics

• Data: Optical and FASTmeasurements

• Current topology

• Summary

• Prospects

Setup: Ground Optics

• Low-light CCD cameras developed at MPE, Garching

• Wide-angle optics (86ox64o)

• Pass band filter

• Exposure time 40 ms multiplied with powers of 2

• Digitized images, 768x576x8

• Location: Deadhorse, Alaska, 70.22o LAT, 211.61o LON

• Time: Feb. 9, 1997, 8:22UTPhoto: courtesy W. Lieb, MPE

Setup: FAST

• 2nd NASA SMEX Mission

• PI Institution UCB/SSL

• Launch: August 21, 1996

• Lifetime: 1 year nominal; still operational

• Orbit: 351 x 4175km, 83o

• 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 360o coverage

http://www-ssc.igpp.ucla.edu/fast

Data: AE index, Feb. 9, 1997

http://swdcdb.kugi.kyoto-u.ac.jp

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

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).

FAST Data: Trajectory

Magnetic noon at the top N=magnetic pole X=arcFAST PathAuroral Oval Terminator at 110km

FAST Data: Large Scale

Top: Potential. Middle: Magnetic field in the Satellite Associated System (SAS). Bottom: Magnetic field in the Arc Associated System (AAS).

FAST Data: Medium Scale

Panel 1: Magnetic field. Panels 2-4: Electrons, energy. Panel 5: Electrons, p.-a.Panels 6: Ions, energyPanels 7: Ions, p.-a. Panel 8: Electric potential

The arc is north of the convection reversal

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.

Current Configuration: Flow Topology

Type 1

Type 2

From Bostrom (1964)CurrentElectric fieldPlasma convection

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. => a1 , ..... , an

• Hall current perpendicular to the arc => E not 0 => b0

• Coupling FAC – electrojet => J not divergence free => c1

Current

Conductance from particle precipitation

+

Current config: quantitative evaluation

J , J , J||

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)

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.