We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well-defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67¿ &Lgr;m with half-maximum widths of 3 hours of MLT and 2¿ &Lgr;m, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx>0 or Bz0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ~2200 MLT, whereas in winter they tend to occur near midnight at ~2300 MLT. The average summer-winter difference in the onset location is ~1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By>0 and toward midnight for By0 in summer and latest (2330 MLT) for By0 the onset location shifts toward dusk when By>0 but toward dawn when By<0; the sense of this shift reverses for Bx<0. An implication of the results is that auroral breakup is not conjugate. ¿ 2001 American Geophysical Union