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Evidence is presented that suggests that geomagnetic bay disturbances occur in two stages. The onset of each stage is accompanied by a Pi 2 micropulsation; the fact that a Pi 2 occurs almost synchronously over the earth's surface allows the onset of this micropulsation to be used to define the onset of each stage of a bay. The time lag between the onsets of the two stages ranges from about 10 to 30 minutes. The delay between the two stages is suggested to be the time required for the transfer of information from the front of the magnetosphere to the region of reconnection in the magnetotail. The recognition of the first stage of a bay as a precursor can facilitate high-resolution studies of auroral phenomena.
G. Rostoker (Mon,) studied this question.