The unusual sunspot minimum: Challenge to the solar dynamo theory

Abstract

The last cycle 23 was low, long, complex, and very unusual. The “peculiarity” of the minimum was that the field was weak, but also that the morphology of the heliosphere was very complex. A large number of features of intermediate scale—neither global nor local—were observed. There are reasons to believe that the amplitude and the period of a cycle are determined by the large-scale meridional circulation which, in turn, may be modulated by planetary tidal forces. There are evidences that at present the deep meridional circulation is very slow, from which a low and late maximum of cycle 24 can be predicted. Calculations of the planetary tidal forces indicate that cycle 25 will be still lower, and therefore cycle 24 is the beginning of a secular solar activity minimum. Various prediction methods are summarized, all indicating that we are entering a period of moderately low activity, and the possibility of a Maunder-type minimum is very small. Arguments are also presented in favor of a near-surface dynamo.