On the 24-day period observed in solar flare occurrence

Time series of daily numbers of solar H alpha flares from 1955 to 1997 are studied by means of wavelet power spectra with regard to predominant periods in the range of ~24 days (synodic). A 24-day period was first reported by Bai (1987) for the occurrence rate of hard X-ray flares during 1980-1985. Considering the northern and southern hemisphere separately, we find that the 24-day period is not an isolated phenomenon but occurs in each of the four solar cycles investigated (No. 19-22). The 24-day period can be established also in the occurrence rate of subflares but occurs more prominently in major flares (importance classes >=1). A comparative analysis of magnetically classified active regions subdivided into magnetically complex (i.e., including a gamma and/or delta configuration) and non-complex (alpha, beta) reveals a significant relation between the appearance of the 24-day period in H apha flares and magnetically complex sunspot groups, whereas it cannot be established for non-complex groups. It is suggested that the 24-day period in solar flare occurrence is related to a periodic emergence of new magnetic flux rather than to the surface rotation of sunspots.