Subauroral ion drifts (SAID) are narrow layers of intense westward ionospheric flow observed during substorms. We present numerical simulations showing that the combined effect of thermo-electric and convection electric fields in a magnetospheric current sheet - when mapped down to the ionosphere - can account for the westward direction of the ion drift, the width and intensity of the drift speed peak, and the lifetime of SAID. The model can also explain why SAID occur mainly in the pre-midnight sector.