The initial ionospheric storm positive phase often appears as a large enhancement in F region electron density at middle latitudes in the evening hours following a storm sudden commencement. The cause of this dusk effect is still not well understood, though several mechanisms have been proposed. This phenomenon is studied using incoherent scatter radar data collected at Millstone Hill and total electron content data from a north-south chain of stations during the May 26--27, 1990, storm. Nine-position Millstone Hill radar data allows estimation of gradients in electron density and in velocity components above the station. From these the motion term in the continuity equation is calculated and used along with wind, electric field data, and modeling to assess the relative importance of the proposed mechanisms during this storm. It is shown that a combination of mechanisms appear to have been involved, including a traveling atmospheric disturbance, advection of high-density plasma from lower latitudes, and neutral composition changes. ¿ American Geophysical Union 1995