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Some studies over the last decade have indicated that the instability\udresponsible for substorm expansion phase onset may require an external\udtrigger such as a northward turning of the interplanetary magnetic field\ud(IMF). Statistical investigations have lead to contrasting interpretations\udregarding the relationship between proposed solar wind triggers and substorm\udonsets identified from geomagnetic data. We therefore present the results of\uda study into the possible triggering of 260 substorms between 2001–2005,\udexploiting data from the Cluster and IMAGE satellite missions. We find that\udonly a small fraction (<25%) of the substorms studied are associated with\udnorthward turnings of the IMF. However, the majority of the observed onsets\udare associated with a growth phase characterised using a subset of the\udcriteria employed to define northward-turning IMF triggers. Based upon a\udcase-by-case investigation and the results of an analysis using the\udstatistics of point processes, we conclude that northward-turning structures\udin the IMF, while sometimes coinciding with the initial phase of individual\udsubstorms, are not required to trigger the magnetospheric instability\udassociated with substorm expansion phase onset

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