Repetitive substorms caused by Alfvénic waves of the interplanetary magnetic field during high‐speed solar wind streams

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Substorms sometimes occur repetitively with a period of ∼1–4 hours. In this paper we examine repetitive substorms, identified using particle injections and positive H bays on the nightside, that we find to occur during corotating high‐speed streams associated with coronal holes. The high‐speed streams often last for several days and are accompanied by large amplitude Alfvén waves of the interplanetary magnetic field (IMF). We find that repetitive substorms occur every ∼1–4 hours, regardless of the solar cycle phase, whenever the Earth's magnetosphere is impinged by these high‐speed streams. We further find that a significant number of these substorms are associated with repetitive northward turnings of the Alfvénic IMF, each northward turning preceded by weakly‐to‐moderately southward IMF, i.e., Bz ∼ −3.6 nT for ∼29 min on the average. We present eight example intervals where most of the repetitive substorms were associated with a northward turning. Statistically, for 63.5% of 312 substorms we are able to identify a reasonable association with a northward turning. While limitations of the Weimer‐mapped IMF used here and the spatial structure of the Alfvénic IMF prevent us from estimating a precise figure for the percentage of IMF triggered substorms, our results indicate that many of the repetitive substorms are likely due to repetitive triggering by the Alfvénic IMF.