Periodic behaviour of solar flare activity

Abstract

The periodic nature of solar activity has been studied using parameters such as the sunspot Wolf numbers, calcium plage areas and flare indices. The magnitude of the solar activity based on these parameters reveals periodicities other than the most pronounced 11-yr one. Any absolute detection of periodicity in active phenomena would have fundamental significance for our understanding of solar activity. Here we investigate the temporal variation of the flare activity of the Sun using the data of 8,821 Hα flares which occurred during the period January 1965 to February 1984, and show new evidence for 155-day and 17-month periodicities of the flare activity. The 155-day periodicity is examined by taking into account the location of the flare on the Sun. It is suggested that the 155-day period may be related to the timescale for the storage and/or the escape of the magnetic field.