The temporal behaviour of solar activity (as manifested in sun-spots) has long been debated. The 11-year periodicity in the total number of sunspots is well established observationally, as is a periodicity of 152-158 days in the occurrence of high-energy solar flares that was seen during cycle 21 (refs 1-7). The cause of the latter periodicity is not clear, although several mechanisms have been proposed. Here we report a time-frequency analysis, using the wavelet technique, of sunspot areas between 1874 and 1993, which reveals a 158-day periodicity coincident with that of energetic solar flares. The signature of this periodicity is strongest in cycle 19, which was the most intense cycle of the century. The periodicity disappears after cycle 21. The analysis shows that the 158-day periodicity in both high-energy solar flares and sunspots is related to a periodic emergence of magnetic flux which only appears near the maxima of some solar cycles.
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