Ring current and the magnetosphere-ionosphere coupling during the superstorm 
of 20 November 2003

Y. Ebihara, M.-C. Fok, S. Sazykin, M.F. Thomsen, M.R. Hairston, D.S. Evans, 
F.J. Rich, and M. Ejiri1


We investigated the impact on the terrestrial ring current of a coronal mass
ejection (CME) and the associated magnetic cloud that severely disturbed the
EarthÕs magnetosphere on 20 November 2003. This CME decreased the Dst index to
-472 nT, which makes it the second largest storm, based on the minimum Dst index
values, observed between 1957 and 2004. Data from the DMSP, NOAA, and LANL
satellites showed the unique characteristics of this storm; a polar cap
potential that increased to at least 200 kV, a polar cap boundary that moved as
low as about 60  MLAT, a plasma sheet density that increased to 5 cm^3 at L =
6.6 when the Dst index was near its minimum, and the inner edge of the plasma
sheet ion population that penetrated into a region for which L<=1.5. In order to
study the dynamics of the ring current and the associated
magnetosphere-ionosphere coupling, we performed a ring current simulation that
computed the evolution of the phase space density of the ring current ions and
the closure of the electric current between the magnetosphere and the
ionosphere. Major results were as follows: (1) The ring current, in terms of the
Dst index and the inner edge of the plasma sheet, can result from the
enhancement of the convection electric field, given the polar cap potentials
used in the model; (2) The solar wind particles probably penetrated quickly into
the geosynchronous altitude on the nightside with a lag of about 80 min,
resulting in further enhancement of the ring current; (3) Dense geocoronal
neutral hydrogen or a large coefficient of pitch angle diffusion (>10 4 s 1) is
probably needed to account for the rapid motion of the inner edge of the plasma
sheet (or the ring current) population to a higher L value; (4) Both the
simulated and observed field-aligned current (FAC) distributions show multiple
current sheets, rather than the normally expected two current sheets.
Fluctuations in the polar cap potential and the plasma sheet density are
believed to cause the multiple sheets of field-aligned currents; (5) The
equatorward edge of the Region 2 type field-aligned currents was observed to
expand as low as 40  MLAT, which is consistent with the simulation; and (6) The
convection pattern can be much more complicated than an average one due to a
strong Region 2 FAC. A noticeable feature was the reversal of the zonal
ionospheric plasma flow that emerged on the dawnside. In particular, a westward
flow was observed in the equatorial region of the eastward plasma flow at dawn.
Its speed had a local maximum of about 5  equatorward of the flow reversal. The
flow reversal is thought to have resulted from the relatively strong shielding
electric field.

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Journal of Geophysical Research, 110, A09S22, doi:10.1029/2004JA010924, 2005