Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for ''negative storms'' (decrease in the peak electron density, NmF2) in summer and ''positive storms'' (increase in NmF2) in winter. A three-dimensional, time-dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule-rich air to higher levels. Once created, the ''composition bulge'' of increased mean molecular mass is transported by both the storm-induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer-to-winter circulation at solstice transports the molecule-rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral-chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a ''negative storm''. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic ''positive storm''. ¿ American Geophysical Union 1996