External versus internal triggering of substorms: An information-theoretical approach

The role of external triggering of substorms through northward turning of the interplanetary magnetic field has been examined in a number of recent studies. While Hsu and McPherron (2002, 2004) argue that the strong association between external triggers defined by Lyons et al. (1997) and substorm onsets could be responsible for most substorms, Morley and Freeman (2007) argue that the association between northward interplanetary magnetic field (IMF) turnings and substorm onsets are coincidental rather than causal, because the same external triggers are also closely associated with an artificial list of substorm onsets generated with the Minimal Substorm Model, which has no requirement of northward IMF turning. We examine an expanded list of substorms using conditional redundancy, an entropy-based measure of conditional dependency, to examine whether northward IMF turning as an external trigger provides any additional information about substorm onset beyond knowing that there has been a period of sustained loading of energy flux (southward IMF). Our analysis reveals that only a few percent additional information is provided by the northward turning criterion, which is consistent with the statistics of surrogate data sets of external triggers constructed to coincide with 2% of substorms. We therefore conclude that northward turning of the IMF is, in general, coincidentally, rather than causally, associated with substorm onsets.