A statistical study of sunspot activity during 1969 to 1986 was carried out by using the number of sunspot groups and their areas. We found a 17-month periodicity, which is consistent with the 500-day periodicity of flare occurrence (Ichimoto et aL, 1985).
The short-term periodicity of sunspot activity has been discussed by many authors, commonly using the ZOrich sunspot number on the whole disk. Shapiro and Ward (1962) reported a 26-month periodicity and Rao (1973), 3.5-and 5.6-year periodicities of the sunspot number. Yacob and Bhargava (1968) mentioned a 16.3-month periodicity both in the sunspot number and in the geomagnetic data. Furthermore, Belmont et aL (1966) found a 19-month periodicity of the 10.7 cm solar radio flux and Ichimoto et al. (1985) a 17-month periodicity of flare occurrence. In this paper, we described the time variation of sunspot activity during the period from 1969 to 1986 using the area and the number of sunspot groups for both hemispheres as the indices of sunspot activity.
The data base, including the area of sunspot groups, the locations and the Carrington rotation number of their occurrence was constructed for our purpose with the use of the data of sunspot activity published in Solar Geophysical Data (SGD) during 1969 to 1986. The accuracy of measured area published in SGD has been already examined by Chapman and Groisman (1984). They compared the sunspot area observed at the Sacramento Peak Observatory with those in SGD and estimated that the scatter between both data set was less than 3 %.
Throughout the construction of our data base, the locations and the ZOrich type of sunspot groups were confirmed with the use of the sunspot data accumulated at the Solar Section of Oriental Astronomical Association (directed by one of us, M.S.). We found several errors in SGD data, especially in the date and locations, which may be caused through the compilation processes of the data. Our data base after correcting these errors becomes the more reliable one.
The temporal variation of sunspot activity during cycle 21 was examined in detail with the following indices. (a) The total area (TA): the sum of maximum areas of sunspot groups during a disk passage in one solar rotation. (b) The total number (TN): the total number of sunspot groups observed in one solar rotation. (c) The mean area (MA): the