The periodicities in the occurrence rate of whistlers observed at Moshiri (geomag.lat.:35'N; L=1.6) have been investigated. The short periods of the order of hours have been studied by means of the cepstrum analysis for the continuous recording data, and it is found that there are a major peak just below 2 hours and a minor peak at about 4 hours. These periods are interpreted in terms of the cyclic formation of whistler ducts. Then, the long period of the order of days have been examined by

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... s of the same cepstum analysis for the routine-based data of occurrence rate every hour during 14 years, and we have found that this period is likely to be caused by the modulation due to planetary waves.