Discovery of the near 158 day periodicity in group sunspot numbers during the eighteenth century

A new record of solar activity, made by compiling the daily number of sunspot groups visible on the Sun's surface between 1610 and 1995, has recently been made available by Hoyt & Schatten. Wavelet analysis of this record shows that an episode of the periodicity near 158 days occurred during the eighteenth century, around the maximum of solar cycle 2, and that episodes of the periodicity, much weaker than that in solar cycle 2, have appeared around the maxims of solar cycles 16-21 (covering the interval 1923-1986). The presence of the periodicity in the group sunspot number confirms that it is caused by a periodic emergence of magnetic flux. On the other hand, periodogram analysis allows one to compare the behavior of the periodicity in both sunspot groups and sunspot areas, and the results suggest that, at least during the twentieth century, the periodic emergence of magnetic flux has adopted two different forms. In solar cycles 16 and 17, new sunspot groups were periodically formed, simultaneously increasing the number of sunspot groups and the total sunspot area on the Sun's surface, while during solar cycles 18, 19, 20, and 21 the periodicity has occurred within already formed sunspot groups, increasing sunspot areas only. We point out that this second type of emergence, which enhances the magnetic complexity of sunspot groups, is responsible for the appearance of the periodicity in high-energy solar flares as detected by the Solar Maximum Mission during solar cycle 21.