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On the 24-day period observed in solar flare occurrence |
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| Authors: | M Temmer A Veronig J Rybák R Braj?a A Hanslmeier |
| Institution: | 1. Institute for Geophysics, Astrophysics and Meteorology, University of Graz, Universit?tsplatz 5, A-8010, Graz, Austria 2. Astronomical Institute/SAS, SK-05960, Lomnica, Slovak Republic 3. Hvar Observatory, Faculty of Geodesy, University of Zagreb, Ka?i?eva 26, HR-10000, Zagreb, Croatia |
| Abstract: | Time series of daily numbers of solar Hα 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 γ and/or δ configuration) and non-complex (α, β) reveals a significant relation between the appearance of the 24-day period in Hα 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. |
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