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 C. 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 ≈2, the altitude of the peak density (hmF2) rose by ≈80 km, and the F-region electron temperature decreased by ≈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.