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Ann. Geophys., 26, 201-210, 2008
www.ann-geophys.net/26/201/2008/
© European Geosciences Union 2008
Multi-instrumentation observations of a transpolar arc in the northern hemisphere
A. Goudarzi1, M. Lester1, S. E. Milan1, and H. U. Frey2
1Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
2Space Science Laboratory, University of California, Berkeley, CA 94720-7450, USA
Abstract. A transpolar arc was imaged by the FUV instrument on the IMAGE spacecraft
during a 3-h interval on 5 February 2002. Observations indicate that a
burst of reconnection in the geomagnetic tail, which was not associated with
a substorm, was responsible for the formation of the arc. The arc initially
formed across the central polar cap, extending from near midnight to noon
such that the polar cap was approximately divided in half. The subsequent
motion of the arc was controlled by the amount of open flux being added to
the dawn sector cap from a magnetopause reconnection site on the post-noon
side of the magnetosphere. The dayside reconnection happened during a period
when the IMF By component was dominant, although the Bz component initially
remained positive, and resulted in strong westward azimuthal flows in the
noon sector. The arc continued to move towards the duskside auroral oval
after the IMF Bz turned southward. A keogram of the FUV/WIC auroral
observations along the dawn-dusk meridian provides further evidence of the
expansion and contraction of the polar cap during the period in which
different IMF orientations occurred. Furthermore, comparing images from
IMAGE and ionospheric convection flow from SuperDARN measurements, vortical
convection flows occurred exactly at the time and location of the formation
of the transpolar arc and subsequently followed the head of the transpolar
arc as it moved across the polar cap. The observations are consistent with
the prediction of a recent model for the formation of the transpolar cap by
the closure of open flux in the geomagnetic tail, and its subsequent motion
through changes in the open flux distribution within the polar cap.
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