External vs Internal Triggering of Substorms: An Infomation-theoretical Approach
Authors: Jay R. Johnson, Simon Wing
Abstract: The role of external triggering of substorms through northward turning of the interplanetary magnetic field has been examined in a number of recent studies [Hsu and McPherron, 2002; Morley and Freeman, 2007]. 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 turnings and substorm onsets are con-incidental 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 [Freeman and Morley, 2004], which has no requirement of northward turning. We examine an expanded list of substorms [Frey et al., 2004; Frey and Mende, 2006] 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 datasets 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.
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Published in: Geophysical Research Letters 41:16
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