The Magnetospheric Response to the October 4-6, 2000 Storm

Reeves, G. D., M. G. Henderson, R. M. Skoug, M. F. Thomsen,
J. M. Jahn, and P. C. Brandt,

IGPP Workshop on the Nightside Magnetosphere, Yellowstone National
Park, Wyoming, October 1-5, 2001.

The October 4-6 geomagnetic storm reveals several interesting
behaviors that illuminate the injection of plasmasheet material into
the inner magnetosphere and the subsequent ring current and auroral
responses. The storm began on October 4 in the second half of a
Noth-then-South magnetic cloud. For nearly all of October 4 the IMF
was only moderately southward with Bz around -10 nT. During that
period of modest solar wind energy input geosynchronous satellites
saw a series of "sawtooth injections" which have a very
characteristic temporal flux profile indicative of injection followed
immediately by renewed plasmasheet thinning. Each sawtooth injection
involves a large portion of the nightside and dusk-side
magnetosphere. The injections are each accompanied by brightening of
the auroral over a broad and expanded auroral oval. The
geosynchronous injections (of electrons and ions) are also apparent
in simultaneous ENA images from the POLAR and IMAGE spacecraft which
show that there is a true injection of fresh plasmasheet material
into the inner magnetosphere and not simply an adiabatic
reconfiguration of the magnetic field. During the period of sawtooth
injections the Dst index decreased slowly to nearly -150 nT over an
approximately 20-hour period. During that 20-hour decrease of Dst
there is no evidence of trapping of injected ring current particles
and nearly the entire build-up appears to be partial or untrapped
ring current. The long and slow main phase provides an unusual
opportunity for spacecraft to observe the dynamics during this event.

The remainder of the three-day interval is equally interesting. On
October 5 Bz decreased to below -20 nT following a shock passage.
This drove the magnetosphere into a dynamic state more typical of
geomagnetic storm conditions with frequent but more incoherent
periods of injections and auroral activity. Dst did not reach its
minimum value of -187 nT until 13 UT on October 5 when it then began
a gradual and fairly "classic" two-phase recovery. The dynamics on
October 5 which appear typical of geomagnetic storms provide an
interesting contrast with the dynamics on October 4 which share many
of the features of a "steady magnetospheric convection" event in
spite of the fact that there was nothing steady about the response in
the inner magnetosphere.