The present study gives an overview of the Millstone Hill ionospheric model, which solves continuity and momentum equations for O+(4S), O2+, NO+, and N+. Photochemical equilibrium is assumed at the lower boundary (120 km in the present study) and for estimation of the ion concentrations of N2+ and the metastable states of O+. Inputs to the model are the ion drifts, electron temperatures, and ion temperatures measured by the Millstone Hill incoherent scatter radar, the Mass Spectrometer Incoherent Scatter 1986 (MSIS-86) neutral atmosphere, the observed electron density at the upper boundary (500 km), and an empirical model for NO. The model is extremely flexible in selection of sets of cross sections, EUV flux models, and models for ionization by secondary photoelectron impact. The Millstone Hill ionospheric model is used to simulate the May 26--27, 1990, ionospheric storm, which has been previously studied by Buonsanto [1995a>. The main feature of this storm is an observed large enhancement in the F region electron density above Millstone Hill, the so-called dusk effect. Results show that the effects of the electric fields and height variations in the neutral wind associated with a traveling atmospheric disturbance played a crucial role in the formation of the observed dusk effect. Advection of higher-density plasma from the south apparently also contributed. Vibrationally excited N2 and O2 are found to be important in simulating the electron density during these disturbed summer solar maximum conditions. It has not been found necessary to change the MSIS-86 neutral density in order to bring about good agreement between the observed and calculated NmF2 and HmF2. ¿ 1999 American Geophysical Union