The occurrence of periodically structured Pc 1 (pPc 1) during several intense geomagnetic storms is examined in detail. Unlike unstructured or quasi-structured Pc 1, pPc 1 strongly tend to occur only during a later epoch of the recovery phase of geomagnetic storms. The available pPc 1 data, together with satellite and ionospheric data, are consistent with the idea that pPc 1 activity originates in the proton electromagnetic cyclotron instability that occurs when the high-energy tail of the ring current is brought to the stable trapping limit. This occurs in the late recovery phase when the plasmasphere has filled sufficiently from the ionosphere. Some characteristics of the observed pPc 1 activity are found to be dependent on the character of minor substorm activity occurring on the ring current recovery phase.
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Heacock et al. (1973) studied this question.
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