The solar cycle: predicting the maximum amplitude of the smoothed highest 3-hourly aa index in 3 d for cycle 25 based on a similar-cycle method

similar-cycle method is introduced to predict the maximum amplitude of the geomagnetic index aa for the 11-yr solar cycle. This method is based on the idea that similar cycles tend to have similar cycle parameters. We use the highest 3-hourly aa index in each 3 days’ interval ( aa_H ), which is further smoothed by 121 points (363 days). The minimum aa_H ( aa_Hmin ) around the beginning of the current cycle 25 [ aa_Hmin(25) ] is used as a reference to select the similar cycles to cycle 25: n_s=16, 15, 24 , and 12 (in descending order of weights). The maximum aa_H ( aa_Hmax ) is predicted as the weighted average of the corresponding values in the similar cycles with the weight given by the reciprocal of the deviation between aa_Hmin(n_s) and aa_Hmin(25) . Based on this method, the maximum amplitude of aa_H for cycle 25 is predicted to be aa_Hmax(25)=73.8± 4.4 nT, about 14