The temporal variations of the geomagnetic field at Alert are studied for a two-year period during a minimum of the 11-year sun-spot cycle. During this period (27 August 1962 to 21 September 1964), a recurrent geomagnetic storm is observed, persisting for 29 solar rotations. It produces lines in the geomagnetic power spectrum at frequencies corresponding to the harmonics of 27 days. A further 13 lines, equispaced in frequency by (1/27) cycles/day, are associated with sidebands of the daily line and are believed to result from amplitude modulation of the SqP/DS current system.

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