  Simulation results from the NRL ionospheric model SAMI2 indicate that the
changes in the F-region over Millstone Hill during the geomagnetic storm
beginning on 3 April 2004 are primarily due to the influence of a long-lasting
eastward electric field, as was previously suggested by {\it Huang et al.}
[2005]. A simulation of the storm day agrees well with the observational data
and shows that, compared with the ionosphere of the previous quiet day, the
peak electron density in the F-region (NmF2) increased by a factor of $\approx
2$, the altitude of the peak density (hmF2) rose by $\approx 80$ km, and the
F-region electron temperature decreased by $\approx 1000$ K. Further
simulations in which either the neutral atmosphere and winds or the electric
field were replaced by their quiet day counterparts clearly suggest that the
electric field played the dominant, although not exclusive, role in producing
these effects.

Space Physics