A Tail Current Dominated Storm Main Phase: March 31, 2001

R M Skoug 1 (505-667-6594; rskoug@lanl.gov)
M F Thomsen 1 
M G Henderson 1 
H O Funsten 1 
G D Reeves 1 
C J Pollock 2 
J M Jahn 2 
D J McComas 2 
D G Mitchell 3 
P Brandt 3 
B R Sandel 4 
C R Clauer 5 
H J Singer 6 

1 Los Alamos National Laboratory, MS D466, Los Alamos, NM 87545, United States 
2 Southwest Research Institute, Instrumentation and Space Research Division, San Antonio, TX 78238, United States
3 Johns Hopkins University, Applied Physics Lab, Laurel, MD 20723, United States 
4 University of Arizona, Lunar & Planetary Lab, Tucson, AZ 85721, United States 
5 University of Michigan, Space Physics Research Lab, Ann Arbor, MI 48109, United States 
6 NOAA, Space Environment Center, Boulder, CO 80305, United States 

On March 31, 2001, a strong shock driven by a coronal mass ejection was
observed by ACE in the solar wind at ~0030 UT. This solar wind transient
produced a large geomagnetic storm at Earth, with a drop in the Dst
index to ­350 nT between 03 and 08 UT. The Earthıs magnetosphere was
very compressed during this interval, with the bow shock crossing the
Los Alamos satellites at geosynchronous orbit. Tail currents rather than
ring currents dominated the geomagnetic disturbances during the main
phase of this storm. Here we present both space-based and ground-based
observations demonstrating the dominant contribution of tail currents to
the Dst index. Energetic neutral atom (ENA) images from the MENA and
HENA instruments on IMAGE show a very narrow ENA distribution confined
to the nightside until ~0630 UT, when Dst was ­300 nT. Not until ~07 UT
did the ENA images (>50 keV) indicate formation of a closed ring
current as ions drifted to the dayside. This drift was associated with a
dipolarization of the geomagnetic field, and a later (~08 UT) northward
turning of the IMF and subsequent turnoff of the magnetospheric
convection electric field. IMAGE/EUV observations at this time show the
disappearance of the plasmasphere from the nightside as Dst drops,
suggesting deep penetration of the plasma sheet. Geosynchronous
observations from the Los Alamos satellites show a very dense plasma
sheet (up to 10/cm^3) with strong convection. GOES magnetometer data
show a very stretched and intense geomagnetic field, consistent with a
strong tail current. Ground magnetometer observations confirm that the
disturbance was localized to the nightside and that tail contributions
dominated the disturbance until ~07 UT.