Time Variations in Solar Activity

Abstract

Long-term variations in the height of solar maxima and in the relative spottedness of the northern and southern solar hemispheres are considered. Mean sunspot numbers in the present solar cycle (number 20) are found to be similar to those in cycles with maxima in 1937, 1917, 1860, and 1769.

Time variations within the 11-year cycle are considered. It is pointed out that all solar latitudes and longitudes do not attain maximum levels of activity at the same time. Furthermore, the course of solar activity as measured by sunspot number and 2800 MHz flux is seldom smooth but appears to advance by series of pulses or episodes. The average time interval between onsets of successive Primary Fluctuations, as we have identified them in cycles 19 and 20, was 15 solar rotations. In general, Secondary Fluctuations occurred at intervals of 3 to 5 rotations. These statistical variations in sunspot number corresponded to recognizable episodes in specific solar activity and to maxima in plots of Major Flare and Active Region Indices.

Study of the declining branches of solar cycles 12 through 19 suggests that there will be 5 to 6 Primary Fluctuations of activity on the declining branch of cycle 20 in the years 1970.5–1975. At least half of these pulses can be expected to include significantly flare-rich centers of activity.