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

Abstract

A 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 ($aa_{ \mathrm{H}}$), which is further smoothed by 121 points (363 days). The minimum $aa_{\mathrm{H}}$ ($aa_{\mathrm{Hmin}}$) around the beginning of the current cycle 25 [$aa_{\mathrm{Hmin}}(25)$] is used as a reference to select the similar cycles to cycle 25: $n_{\mathrm{s}}=16, 15, 24$, and 12 (in descending order of weights). The maximum $aa_{\mathrm{H}}$ ($aa_{\mathrm{Hmax}}$) 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 $aa_{\mathrm{Hmin}}(n_{\mathrm{s}})$ and $aa_{\mathrm{Hmin}}(25)$. Based on this method, the maximum amplitude of $aa_{\mathrm{H}}$ for cycle 25 is predicted to be $aa_{\mathrm{Hmax}}(25)=73.8\pm 4.4$ nT, about 14% higher than that in cycle 24 (64.81 nT).