Is the longitudinal distribution of solar flares nonuniform?

Two methods of searching for periodicity in weighted time point series are applied to earlier major solar flare data during three decades (1956–1985). The results of this analysis are tested against new solar flare data during the four years 1989–1993. Active longitudes rotating with a constant synodic period of 22.d069± 0.d012 could explain the complete data set over both hemispheres. The nonuniform distribution of the flare activity with this period displays two active longitudes separated by 180◦, but also shows other detailed features. No significant periodicity is detected on the southern solar hemisphere, whereas active zones rotating with a constant synodic period of 26.d722± 0.d015 are present in the earlier and new data on the northern hemisphere. The main peculiarity is that two highly significant periodicities (22.d07/26.d72) are detected in the same data. The phenomena detected are interpreted as a signature of a long–lived nonaxisymmetric component of the solar magnetic field below the visible surface.