To test various hypotheses about positive ionospheric storm development, we numerically simulated the magnetic storm of 24--27 January 1974 to obtain the global pattern of its thermospheric and ionospheric effects. We use a global self-consistent numerical model of the thermosphere-ionosphere-magnetosphere system which calculates all plasma parameters as well as electric fields both of magnetospheric and thermospheric dynamo origin. The predicted neutral composition changes are consistent with observations of the AE-C and ESRO-4 satellites. The storm-to-quiet density ratio of [O>/[N2> (at a fixed height) does not change significantly at middle and low latitudes while the absolute density of all neutral constituents is enhanced. The density ratio at a fixed pressure level can be slightly enhanced under certain conditions. Positive ionospheric storm effects during the main phase of the storm are shown to be mainly due to the uplifting of the ions along the field lines by an equatorward directed disturbance wind component at middle latitudes and their equatorward (convergent) movement at low latitudes. This is driven the passage of travelling atmospheric disturbances and by the large-scale disturbance wind circulation. Under daylight conditions minor contributions arise from the general thermal density enhancement of all constituents favoring positive ionospheric response due to a stronger ion production rather than loss. Height dependent deviations from diffusive equilibrium of minor thermospheric constituents are due to the relatively rapid (in comparison with their diffusion) vertical bulk motion. ¿ 1999 American Geophysical Union