On the association between northward turnings of the interplanetary magnetic field and substorm onsets

Abstract

We re‐examine whether substorms are triggered by solar wind fluctuations or an internal magnetospheric instability by comparing the statistical associations between substorm onsets and (1) an external trigger definition, (2) a simple internal trigger definition of only prior loading of solar wind energy that is a subset of the external trigger definition. Statistical associations are calculated both for observed substorm onsets and onsets generated by a Minimal Substorm Model in which substorms are purely internally triggered. Thence we argue that a minimum interval of prior loading is a necessary condition for substorm onset, a subsequent northward IMF turning is not necessary, and consequently that an internal trigger from a magnetospheric instability is a necessary and sufficient condition for substorm onset. We discuss how this result may explain a report that externally triggered substorms are systematically larger than non‐externally triggered substorms.