Abstract:
Observations of solar Ca(+) K daily plage area from 1951 to 1977 are analyzed to search for short-period (12-13 day) variations. The 27-day periodicity due to solar rotation modulation is found at all latitudes. Using an autocorrelation technique a 12-13 day periodicity is found only in three of the 57 data sets examined. However, a weak peak around 12-13 days is found in the power spectrum of all of the data sets. It is suggested that two distinct active longitudes separated by about 140-170 degrees are the cause of the 12-13 day periodicity in the autocorrelation.
