The periodic behaviour of solar activity - The near 155-day periodicity in sunspot areas

The historical record of daily sunspot areas (1878-1982), covering cycles 12 to 21, has been analyzed, looking for the periodicity around 155 d found in other indicators of solar activity, mainly solar flares. The results indicate that a periodicity between 150-160d (77-72 nHz) seems to be significant during solar cycles 16-21, while it cannot be detected in solar cycles 12 to 15. However, a sliding-window analysis reveals some time intervals within each of most of the cycles considered, in which a strong periodicity appears at 155.6 d. This feature suggests an intermittent character. Also, some evidence is found suggesting that the periodicity appears or is more important in the hemisphere where sunspot areas have been dominant during the solar cycle.