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 lo cation of 648 well-defined Northern Hemisphere auroral breakups (substorm o nsets) in response to interplanetary magnetic field (IMF) orientation and s eason. The most likely onset location is at 2230 MLT and 67 degrees Lambda (m) with half-maximum widths of 3 hours of MLT and 2 degrees Lambda (m), re spectively. The onset latitude depends primarily on TMF 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 By In summer, substorms tend to occur in the early evening at similar to 2200 MLT, wherea s in winter they tend to occur near midnight at similar to 2300 MLT. The av erage summer-winter difference in the onset location is similar to1 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 (220 0 MLT) for B-y > 0 in summer and latest (2330 MLT) for B-y < 0 in winter. T hese dependencies coincide with those previously reported for the evening s ector ionospheric zonal flow reversal in response to IMF B-y and season, in dicating 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 iden tified: 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.