ON 26-MONTH PERIODICITY IN QUIET-DAY RANGE OF GEOMAGNETIC HORIZONTAL FORCE AND IN SUNSPOT NUMBER

In view of conflicting and inconclusive evidence on the existence of a quasi-biennial cycle in geomagnetic field a search has been made for the 26-month oscillation using an extensive series of geomagnetic data covering 61 yr from Alibag Observatory. To ascertain if the oscillation in geomagnetic data is of solar origin, the relative sunspot number for the same period has also been analysed in an identical method. It has been concluded that a 26-month oscillation of a small but significant amplitude occurs in the quiet-day range in the horizontal force of the Earth's magnetic field and appears to be of solar origin. The investigation also reveals the existence of a 16-month oscillation of unknown origin, both in the quiet-day range and in the sunspot number. © 1968.