The Magnetospheric Response to Solar Wind Forcing on 31 March 2001:
SOHO, ACE, CLUSTER, IMAGE,FAST, SAMPEX, and GEO observations of a Major 
Geomagnetic Storm.

Baker, D N,  Burch, J L,  Daly, P W,  Ergun, R E,  Friedel, R,  Fritz, T A, 
Jahn, J M,  Mitchell, D G,  Reeves, G E

A bright active region on the Sun gave rise on 29 March 2001 to a fast
halo coronal mass ejection (CME) event observed by SOHO instruments. On 31
March, a strong interplanetary shock wave ahead of a magnetic cloud
(probably arising from the earlier CME) passed the ACE spacecraft and hit
the Earth's magnetosphere. This driver compressed the subsolar
magnetopause to <= 4 RE geocentric distance and initiated a powerful
geomagnetic storm (Dst ~ -360 nT). The CLUSTER II set of four spacecraft
were located in the midnight sector of the magnetosphere near perigee (r ~
4 RE) at ~ 0635 UT and observed a dispersionless "injection" of energetic
(E>=20 keV) electrons in association with a large magnetospheric substorm
expansion phase onset (AE ~ 1200 nT). A Los Alamos geostationary
spacecraft located at ~ 20 LT (1991-080) observed a strong electron
injection event commencing at ~ 0630 UT, confirming the timing of the
substorm onset. At ~ 0637 UT, the FAST spacecraft at ~ 1800 km altitude
and ~ 19 MLT saw a powerful field-aligned current signature and a
perpendicular electric field of ~ 60 mV/m at an invariant latitude of ~ 60
degrees. Concurrent to these in situ observations, the IMAGE spacecraft
was returning a sequence of global Energetic Neutral Atom (ENA) images
from the medium-energy (MENA) and high-energy (HENA) sensor systems. These
data showed a very prominent injection of substorm ions in the premidnight
(and postdusk) sector of the inner magnetosphere. This event is consistent
with a substorm onset that pushed the substorm "injection boundary" far
inside of geostationary orbit and far toward the dusk sector. The CLUSTER
data, especially, reveal microphysical details of the injection boundary
properties. Together, this armada of spacecraft gave an unprecedented view
both telescopically and microscopically of a powerful magnetospheric
substorm during the main phase development of a major geomagnetic storm.