Observation of IMF and seasonal effects in the location of auroral substorm onset

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° Λ m with half‐maximum widths of 3 hours of MLT and 2° Λ m , respectively. The onset latitude depends primarily on IMF B z , but also B x : the onset latitude decreases for B x > 0 or B z < 0 and increases for B x < 0 or B z > 0. The onset longitude depends on season and IMF B y . 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 B y effects on the onset longitude occur only when B x and B y are small. Onset locations shift toward earlier local times for B y > 0 and toward midnight for B y < 0. The average onset local time is earliest (2200 MLT) for B y > 0 in summer and latest (2330 MLT) for B y < 0 in winter. These dependencies coincide with those previously reported for the evening sector ionospheric zonal flow reversal in response to IMF B y and season, indicating that auroral breakups are most likely in regions of large velocity shears. A weak dependence of the MLT onset location on the IMF B x is identified: for B x > 0 the onset location shifts toward dusk when B y > 0 but toward dawn when B y < 0; the sense of this shift reverses for B x < 0. An implication of the results is that auroral breakup is not conjugate.