Comparison of activity in solar cycles 18, 19, and 20

Abstract

Although solar activity cycles are in general remarkably similar, cycles 18, 19, and 20, with maximums in 1947.5, 1958.2, and 1968.9, respectively, differed in the heights of their maximums, in the patterns of their development, and to a certain extent in the character of their manifestations of solar activity. Of the 20 solar cycles since 1755, the maximums in sunspot numbers for cycles 18, 19, and 20 rank as 3, 1, and 10, respectively. Cycle 18, with a maximum smoothed monthly sunspot number of 151.8, was the cycle of ‘giant’ spots. Cycle 19, with the highest recorded maximum, 201.3, included many large spots but failed to produce spots with areas equal to those of cycle 18. The highest known daily 2800‐MHz flux occurred in cycle 18, not in cycle 19. Cycle 20, with a sunspot maximum of only 110.6, appears to have been close to the average cycle of the last 220 years. An attempt is made to compare the general levels of flare production in the three cycles. Cycle 19 is estimated to have been more flare‐rich than cycle 18 by a much smaller factor than the original data implied. In cycle 20, general flare production drops conspicuously. Two types of flare‐rich centers of activity are recognized, viz., those with extraordinarily high numbers of flares of Hα importance ⪖1 and those with unusually high frequency of flares with comprehensive indices of 11 or greater (great ionizing and radio frequency emission as well as Hα area). A list of the most flare‐rich regions in cycles 18, 19, and 20 is presented. In addition, the times and locations of the ∼100 greatest individual flares in the three cycles have been tabulated. North‐south asymmetry of sunspots and the shapes of the mean sunspot curves were not the same for the three cycles. The minimums between the cycles differed in duration and residual activity. Solar magnetic measurements exhibit the 22‐year period and make polarity patterns alike in cycles 18 and 20. Summarized coronal measurements from the Pic‐du‐Midi Observatory for 1944–1973 suggest that the relationships between coronal intensities and other measures of solar activity are complex. Emphasis on times of the greatest solar activity in cycles 18–20 leads to a certain sense of positive relationship between events on the sun and geophysical phenomena. Geophysical uncertainties abound, however, if consideration drops to just above average solar circumstances. Comments are made on certain problems in solar‐terrestrial relationships.