A necessary condition for the formation of thin metallic ion layers in the high-latitude ionosphere, when strong convective electric fields are present, is that these fields have a magnetically westward component. However, observations have shown that proper field direction does not guarantee the occurrence of a layer. A sufficient abundance of metallic ions is also required. We assert that the abundance of metallic ions (and hence occurrence of thin layers) is strongly influenced by large-scale ion transport, which at high latitudes is determined by the spatial and temporal structure of the large-scale convective electric field. A simple model is presented which indicates that on the dayside, ions should be lifted from the nominal background metallic layer below 100 km into the lower F region, where they flow horizontally in a narrow vertical stream (~50 km) toward the nightside. Upon entering the nightside this stream of metallic ions precipitates within a limited band of geomagnetic latitudes, leading to spatially selective enhancements of ion abundance within the E region. Given an appropriately directed convective electric field, the rate of occurrence of detectable thin ion layers should be greater at those locations where precipitation has occurred and, conversely, should be less where precipitation has not occurred. We suggest that this process controls the abundance of metallic ions and therefore explains the fact that layers are sometimes absent in observations, despite the presence of westwardly directed electric fields.¿ 1997 American Geophysical Union