Resolved and unresolved reasons for magnetic storms

P. C:son Brandt, S. Ohtani, M. -C. Fok, D. G. Mitchell, M. Liemohn, 
A. Ridley, E. C. Roelof, R. Demajistre


IMAGE/HENA has imaged more than a two dozens of geomagnetic storms of
various strengths since its launch in March 2000. In all storms the
peak of the ion differential flux during the mainphase has been
concentrated to the midnight and even the post-midnight region. The
classical picture of the E-field of the inner magnetosphere has been a
sum of the solar wind convectional and corotational field, which
produces a pattern with a potential minimum around dusk. Therefore the
peak of the ion differential flux was expected to be located around
dusk. In this presentation we have investigate 19 relatively
long-lasting main phases for storms with a minimum Dst <= -50
nT. The local time location of the peak ion differential flux extends
from 22 magnetic local time (MLT) to as far as 06 MLT. The local time
angle is found to depend on the clock angle of the interplanetary
magnetic field (IMF), with the most severe rotation towards dawn
occuring when IMF B_z is strongly negative and IMF B_y is strongly
positive. The morphology of the ion distribution is about the same in
the entire 10-200 keV range, which suggests that this pattern can be
explained in terms of a strong (1-10 mV/m) electric field at L<5,
that focus the ions to the post midnight sector by E X B drift. 
Substorm injections during the main phase decay away during $\sim$1 
in the 27-60 keV range, which suggests that the ions exit the
magnetopause around dusk or afternoon local time. Strong E-fields in
the inner magnetosphere has been suggested by in-situ observations by
CRESS and ground-based radar observations by the Millstone Hill
facility (J. Foster, Haystack Observatory, MIT, MA). We present
kinetic model runs by M. -C. Fok (this conference) and M. Liemohn and
A. Ridley (this conferene) that self consistently calculates the
E-field set up by the closure of the partial ring current in the
low-latitude ionosphere, where the conductivity is low. The potential
drop in the ionosphere therefore becomes high, which then feeds back
out into the equatorial magnetosphere. We briefly discuss the effects
of stagnation of the earthward when it reaches a region where the
magnetic drift cancels the electric drift.

_______________
To be presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002