Prediction of even and odd sunspot cycles: implications for cycles 25 and 26

Prediction of sunspots has been an everlasting interest in the space science community since the discovery of the sunspot cycle. The sunspot number is an indirect indicator of many different solar phenomena, e.g., total and spectral solar radiation, coronal mass ejections, solar flares and magnetic active regions. Its cyclic variation can even be used as a pacemaker to time different aspects of solar activity, solar wind and resulting geomagnetic variations. Therefore, there is considerable practical interest in predicting the evolution of future sunspot cycle(s). This is especially true in today’s technological society where space hazards pose a significant threat, e.g., to satellites, communications and electric grids on ground have been recognized. Another interest to predicting sunspots arises from the relatively recently recognized influences of variable solar radiation and solar wind activity on Earth’s climate system.

There are a variety of methods developed for predicting sunspots ranging from statistical methods to intensive physical simulations. Some of the most successful, yet relatively simple, methods are based on finding precursors that serve as indicators for the strength of the coming solar cycle. These methods are often based on statistics of all past solar cycles. However, most of these methods do not typically take into account the 22-year Hale cycle of solar magnetism, which is well known in different solar and geomagnetic phenomena.

Here we study the prediction of even and odd numbered sunspot cycles separately, thereby taking into account the Hale cyclicity of solar magnetism. We first show that the temporal evolution and shape of all sunspot cycles are extremely well described by a simple parameterized mathematical expression. We find that the parameters describing even sunspot cycles can be predicted quite accurately using the sunspot number 41 months prior to sunspot minimum as a precursor. The parameters of the odd cycles can be best predicted with geomagnetic maximum geomagnetic aa index close to fall equinox within a 3-year window preceding the sunspot minimum. Cross-validated hindcasts indicate that our method has a very good prediction accuracy. For the coming sunspot cycle 25 we predict an amplitude of 171 +/- 23 and the end of the cycle in September 2029 +/- 1.9 years. We are also able to make a rough prediction for cycle 26 based on the predicted cycle 25. While the uncertainty for the cycle amplitude is large we estimate that the cycle 26 will likely be stronger than cycle 25. These results suggest an increasing trend in solar activity for the next two decades.