Living Reviews in Solar Physics: "Solar cycle prediction"
Petrovay, K. Solar cycle prediction. Living Rev Sol Phys 17, 2 (2020). https://doi.org/10.1007/s41116-020-0022-z
Open Access | Review Article
Published: 23 March 2020
This article is a revised version of https://doi.org/10.12942/lrsp-2010-6.
Major revision, updated and expanded: The review has been significantly expanded and reorganized. Six new figures have been added and most of the old figures have been updated or redrawn completely. One new table was also added and the content of the table of selected forecasts has been replaced by early forecasts for Cycle 25. The most extensive additions discuss the revision of the sunspot number series that took place in 2015, exciting new research on the origins of the drop in solar activity level and on the polar precursor method, as well as cycle prediction based on surface flux transport models and on non-axisymmetric dynamo models. The number of references has increased from 209 to 423.
A review of solar cycle prediction methods and their performance is given, including early forecasts for Cycle 25. The review focuses on those aspects of the solar cycle prediction problem that have a bearing on dynamo theory. The scope of the review is further restricted to the issue of predicting the amplitude (and optionally the epoch) of an upcoming solar maximum no later than right after the start of the given cycle. Prediction methods form three main groups. Precursor methods rely on the value of some measure of solar activity or magnetism at a specified time to predict the amplitude of the following solar maximum. The choice of a good precursor often implies considerable physical insight: indeed, it has become increasingly clear that the transition from purely empirical precursors to model-based methods is continuous. Model-based approaches can be further divided into two groups: predictions based on surface flux transport models and on consistent dynamo models. The implicit assumption of precursor methods is that each numbered solar cycle is a consistent unit in itself, while solar activity seems to consist of a series of much less tightly intercorrelated individual cycles. Extrapolation methods, in contrast, are based on the premise that the physical process giving rise to the sunspot number record is statistically homogeneous, i.e., the mathematical regularities underlying its variations are the same at any point of time, and therefore it lends itself to analysis and forecasting by time series methods. In their overall performance during the course of the last few solar cycles, precursor methods have clearly been superior to extrapolation methods. One method that has yielded predictions consistently in the right range during the past few solar cycles is the polar field precursor. Nevertheless, some extrapolation methods may still be worth further study. Model based forecasts are quickly coming into their own, and, despite not having a long proven record, their predictions are received with increasing confidence by the community.
Kristóf Petrovay is Professor and Head of the Department of Astronmy at Eötvös Loránd University, Budapest, Hungary. His research interests in solar physics include dynamo theory, convection, sunspots, and turbulence.