4 Model-Based Predictions

“Progress in dynamo theory is extremely difficult, as it can be made only by understanding the interaction of turbulent plasma motions with magnetic fields. Indeed, the extreme conditions within the solar interior make this a formidable task. ... Any predictions made with such models should be treated with extreme caution (or perhaps disregarded), as they lack solid physical underpinnings.”
(Tobias et al., 2006)

While attempts to predict future solar cycles on the basis of the empirical sunspot number record have a century-old history, predictions based on physical models of solar activity only started a few years ago. The background of this new trend is, however, not some significant improvement in our understanding of the solar dynamo. Rather, it is the availability of increasingly fast new computers that made it possible to fine-tune the parameters of certain dynamo models to reproduce the available sunspot record to a good degree of accuracy and to apply data assimilation methods (such as those used in terrestrial weather prediction) to these models. This is not without perils. On the one hand, the capability of multiparametric models to fit a multitude of observational data does not prove the conceptual correctness of the underlying model. On the other hand, in chaotic or stochastic systems such as the solar dynamo, fitting a model to existing data will not lead to a good prediction beyond a certain time span, the extent of which can only be objectively assessed by “postdiction” tests, i.e., checking the models predictive skill by trying to “predict” previous solar cycles and comparing those predictions to available data. Apparently successful postdiction tests have led some groups to claim a breakthrough in solar cycle prediction owing to the model-based approach (Dikpati and Gilman, 2006Jump To The Next Citation PointKitiashvili and Kosovichev, 2008Jump To The Next Citation Point). Yet, as we will see in the following discussion, a closer inspection of these claims raises many questions regarding the role that the reliance on a particular physical dynamo model plays in the success of their predictions.

 4.1 The solar dynamo: a brief summary of current models
 4.2 Is model-based cycle prediction feasible?
 4.3 Explicit models
 4.4 Truncated models
 4.5 The Sun as an oscillator

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