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 (<25%) of the substorms stud- ied are associated with northward turnings of the IMF. How- ever, the majority of the observed onsets are associated with a growth phase characterised using a subset of the criteria employed to define northward-turning IMF triggers. Based upon a case-by-case investigation and the results of an anal- ysis using the statistics of point processes, we conclude that northward-turning structures in the IMF, while sometimes coinciding with the initial phase of individual substorms, are not required to trigger the magnetospheric instability associ- ated with substorm expansion phase onset.