Where Do Geomagnetic Field Lines Go During Great Space Storms ?

Strong geomagnetic storms are relatively rare events, represented by only a
small fraction of data used in the derivation of existing empirical models of
the magnetosphere.  Hence, using those models for the mapping of the storm- time
magnetosphere is at most an extrapolation, based on trends obtained from quiet
and moderately disturbed data.  To overcome that limitation, a set of data was
created, containing only clear-cut events with Dst<-65 nT, with the goal to
derive models of the inner and near geomagnetic field (R<15Re), representing
strongly disturbed magnetosphere and its evolution during the storm cycle.  The
final data set included about 143,000 records with 5-min average B-vectors,
covering 37 major storms between 1996 and 2000.  Most of the data came from
GOES-8, -9, -10, Polar, and Geotail spacecraft (two storms in February-March of
1998 were also partially covered by the data of Equator-S).  In all cases, only
those storms were selected for which concurrent solar wind and IMF data were
available for the entire duration of the event.  Interplanetary medium data were
provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail.  The inner
magnetospheric field was represented using our new model [J.  Geophys.  Res,
v.107, No.A8] with a duskside partial ring current, having variable amplitude
and scale size, an essential part of the storm-time current system.  The
modeling revealed an enormous distortion and dawn-dusk asymmetry of the inner
magnetosphere during the peak of the storm main phase, caused by the combined
effect of the symmetric and partial ring currents, cross-tail current, and
Birkeland currents.  We found that during storms with Dst<-250 nT the tail-like
deformation of the nightside field penetrated so close to Earth that the
quasi-dipolar approximation broke down at distances as small as 3-4Re.  This
finding yields a quantitative answer to the question of why the auroras expand
to unusually low latitudes during extremely strong storms.  It also may
naturally explain the observed impulsive injections and energizations of charged
particles on the innermost L-shells.  Finally, it questions the validity of
using the dipole or quasi-dipole approximation in numerical simulations of
storms in the inner magnetosphere.