A subauroral ion drift (SAID) layer is characterised by a narrow peak of westward ion drift speed exceeding 1000 m/s. It is confined in less than 1 degree in latitude and located equatorward of the nightside auroral zone but poleward of the plasmapause. We propose a mechanism for the narrow peak electric field driving this phenomenon: an electrostatic potential is produced by thermo-electric charge separation across the front edge of a hot plasma cloud moving inward from the tail and penetrating into the colder background plasma in the plasmatrough and at the plasmapause. Quantitative calculations corroborate this scenario.