The Millstone Hill Ionospheric Model is used to simulate the April 10--12, 1997 ionospheric storm. For the first time determinations of the height-varying wind from incoherent scatter radar measurements are used to simulate the ionospheric plasma. Results indicate that the observed dusk effect is due to a rise in HmF2 combined with an enhanced downward flux of ionization, associated with an altitude variation in the neutral wind. The negative phase on April 11 is fully explained by increased recombination due to the increased neutral composition ratios N2/O and O2/O given by the MSIS model, except that reproduction of the observed G-condition during the morning hours requires these ratios to be larger than those in MSIS-86. This is consistent with the concept of a composition disturbance zone rotating into the morning sector following a storm which commenced during the nighttime hours. ¿ 1999 American Geophysical Union