Tail-dominated storm main phase: 31 March 2001

R.M. Skoug, M.F. Thomsen, M.G. Henderson, H.O. Funsten, G.D. Reeves, 
C.J. Pollock, J.-M. Jahn, D.J. McComas, D.G. Mitchell, P.C. Brandt, B.R. Sandel, 
C.R. Clauer, and H.J. Singer 


On 31 March 2001 a fast solar wind transient with strong southward
interplanetary magnetic field Bz produced a large geomagnetic storm at Earth,
with a drop in the Dst index to -350 nT between 0400 and 0900 UT. The EarthÕs
magnetosphere was very compressed during this interval, with the bow shock
crossing geosynchronous orbit on at least two occasions. Here we present
space-based and ground-based observations demonstrating that tail currents,
rather than ring currents, were the dominant contributor to the Dst index during
the main phase of this storm. The plasma sheet during this interval was
exceptionally dense and penetrated very deeply towards the Earth, leading to
extremely strong tail currents flowing quite close to the Earth. These tail
currents produced a very distorted magnetosphere, with strong stretching of the
magnetic field lines in the nightside plasma sheet. Energetic neutral atom (ENA)
images from the MENA and HENA instruments on IMAGE show a very narrow spatial
distribution, with ENAs confined to the nightside until a magnetic field
dipolarization at ~0630 UT when Dst was 250 nT. Ground magnetometer measurements
confirm that the disturbance was localized on the nightside and dominated by
tail currents up until the field dipolarization. Following the dipolarization,
higher energy ENAs began to drift toward dusk, forming a partial ring current.
Even at that time, low-energy ENAs were not observed on the dayside, either due
to inhibited access or to strong charge exchange losses.
 
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Journal of Geophysical Research, 108(A6), 1259, doi:10.1029/2002JA009705, 2003.