On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field

Some studies over the last decade have indicated that the instability responsible for substorm expansion phase onset may require an external trigger such as a northward turning of the interplanetary magnetic field (IMF). Statisti- cal investigations have lead to contrasting interpretations re- garding the relationship between proposed solar wind trig- gers and substorm onsets identified from geomagnetic data. We therefore present the results of a study into the possible triggering of 260 substorms between 2001–2005, exploiting data from the Cluster and IMAGE satellite missions. We find that only a small fraction (