Predicted intensity of the solar maximum

Abstract

IT is desirable to know beforehand, for planning geophysical and other studies, the intensity of a future period of solar activity. Ohl has shown that the level of geomagnetic activity (as indicated by K_p index) in the descending branch of a solar cycle was well correlated with the height of the following cycle and predicted a maximum annual average Wolf's number of 140–180 for cycle 21. Sargent² has shown that a similar analysis using the ‘aa’ index for the past 100 yr (ref. 3) and sunspot number at the minimum of the cycle gave a multiple regression equation:

R_max(n+1)=3.91+8.56(X₁−0.92 X₂)

where X₁ is the average value of the monthly mean aa indices for the 36 months preceding R_min(n) and X₂ is the value of R_min(n). Sargent predicted a Wolf's number of 150 or more for cycle 21. We describe here a simpler version of the same method—correlating R_max with the minimum annual aa index in the preceding years.