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Ann. Geophys., 23, 2657-2670, 2005
www.ann-geophys.net/23/2657/2005/
© European Geosciences Union 2005
Multi-instrument mapping of the small-scale flow dynamics related to a cusp auroral transient
K. Oksavik1,2, J. Moen1,3, H. C. Carlson4, R. A. Greenwald2, S. E. Milan5, M. Lester5, W. F. Denig6, and R. J. Barnes2
1Department of Physics, University of Oslo, Oslo, Norway
2Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA
3also at: Arctic Geophysics, University Centre on Svalbard, Longyearbyen, Norway
4Air Force Research Laboratory, AFOSR/CA, Arlington, Virginia, USA
5Department of Physics and Astronomy, University of Leicester, Leicester, UK
6Space Vehicles Directorate, Air Force Research Laboratory, VSBXP, Hanscom AFB, MA, USA
Abstract. In this paper we focus on flux transfer events (FTEs) and poleward moving
auroral forms (PMAFs) in the cusp region, combining data from the EISCAT
Svalbard radar, SuperDARN HF radars, ground-based optics, and three
low-altitude polar-orbiting spacecraft. During an interval of southward
interplanetary magnetic field the EISCAT Svalbard radar tracked a train of
narrow flow channels drifting into the polar cap. One 30-60 km wide flow
channel surrounded by flow running in the opposite direction is studied in
great detail from when it formed equatorward of the cusp aurora, near
magnetic noon, until it left the field-of-view and disappeared into the polar
cap. Satellite data shows that the flow channel was on open field lines. The
flow pattern is consistent with field-aligned currents on the sides of the
flow channel; with a downward current on the equatorward side, and an upward
current on the poleward side. The poleward edge of the flow channel was
coincident with a PMAF that separated from the background cusp aurora and
drifted into the polar cap. A passage of the DMSP F13 spacecraft confirms
that the FTE flow channel was still discernable over 15 minutes after it
formed, as the spacecraft revealed a 30–40 km wide region of sunward flow
within the anti-sunward background convection. From the dimensions of the
flow channel we estimate that the magnetic flux contained in the event was
at least 1 MWb. This data set also shows that Birkeland current filaments
often seen by low-altitude spacecraft in the cusp/mantle are really
associated with individual FTE events or a train of FTEs in progress. As the
region 0 or cusp/mantle current represents the statistical average
consistent with the large-scale flow pattern, we therefore introduce a new
term – FTE currents – to denote the unique pair of Birkeland current
sheets that are associated with individual meso-scale FTE flow disturbances.
The poleward moving auroral forms (PMAFs), often referred to in the
literature, are the optical signature of the upward FTE current.
Keywords. Magnetospheric physics (Current systems;
Magnetopause, cusp and boundary layers) – Ionosphere
(plasma convection)
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