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Ann. Geophys., 25, 507-517, 2007
www.ann-geophys.net/25/507/2007/
© European Geosciences Union 2007
An unusual giant spiral arc in the polar cap region during the northward phase of a Coronal Mass Ejection
L. Rosenqvist, A. Kullen, and S. Buchert
Swedish Institute of Space Physics, Uppsala, Sweden
Abstract. The shock arrival of an Interplanetary Coronal Mass Ejection (ICME) at ~09:50 UT on 22 November 1997
resulted in the development of an intense (Dst<−100 nT) geomagnetic storm at Earth.
In the early, quiet phase of the storm, in the sheath region of the
ICME, an unusual large spiral structure (diameter of ~1000 km) was observed at very high latitudes
by the Polar UVI instrument. The evolution of this structure started as a polewardly displaced auroral bulge which
further developed into the spiral structure spreading across a large part of the polar cap.
This study attempts to examine the cause of the chain of events that resulted in the giant auroral spiral.
During this period the interplanetary magnetic field (IMF) was dominantly northward (Bz>25 nT) with a
strong duskward component (By>15 nT) resulting in a highly twisted tail plasma sheet.
Geotail was located at the equatorial dawnside magnetotail flank and observed accelerated
plasma flows exceeding the solar wind bulk velocity by almost 60%. These
flows are observed on the magnetosheath side of the magnetopause and the acceleration mechanism is proposed to
be typical for strongly northward IMF.
Identified candidates to the cause of the spiral structure include a
By induced twisted magnetotail configuration,
the development of magnetopause surface waves due to the enhanced pressure related to the accelerated
magnetosheath flows aswell as the formation of additional magnetopause deformations due to
external solar wind pressure changes. The uniqeness of the event indicate that most probably a
combination of the above effects resulted in a very extreme tail topology. However, the data coverage
is insufficient to fully investigate the physical mechanism behind the observations.
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