What causes the 24-day period observed in solar flares?

Previous studies report a ~24-day (synodic) period in the occurrence rate of solar flares for each of the solar cycles studied, Nos. 19-22 (Bai 1987, ApJ, 314, 795; Temmer et al. 2004, Sol. Phys. 221, 325). Here we study the 24-day period in the solar flare occurrence for solar cycles 21 and 22 by means of wavelet power spectra together with the solar flare locations in synoptic magnetic maps. We find that the 24-day peak revealed in the power spectra is just the result of a particular statistical clumping of data points, most probably caused by a characteristic longitudinal separation of about +40 degrees to +50 degrees of activity complexes in successive Carrington rotations. These complexes appear as parallel, diverging or converging branches in the synoptic magnetic maps and are particularly flare-productive.