A theoretical model for the onset of the equatorial spread F (ESF) is introduced that considers all the effects of the electric fields that are present together with the effects of the neutral wind velocity, of gravity, and of the density gradient. These impose well-defined topological constraints on the modes that can be excited. The horizontal (east-west) ion velocity is shown to be one of the key factors for the intrinsic vertical localization of modes that are driven unstable by the effective vertical gravity (including the E field contribution). The thickness of the layer of localization is related to the vertical gradient of the ion horizontal velocity, of the neutral wind velocity, and of the ion-neutral collision frequency. The horizontal gravity resulting from the difference between the horizontal ion and neutral velocities is shown to not be a driving factor for the instability, under realistic conditions when the eastward electric field is included in the analysis, but to have an important influence on the mode topology (vertical profile). An explicit analytical solution is found for the relevant dispersion equation, which includes, in particular, the contribution of a vertical ion velocity. The mode vertical profile under conditions that are shown to be realistic is found. It is suggested that the necessary conditions for the intrinsic localization of the modes determine the onset of the ESF. The mode ballooning structure, along the magnetic field, including the effect of the plasma finite resistivity is also described. ¿ 1999 American Geophysical Union