1.2 What is a “model”?
The review’s very title demands an explanation of what is to be understood by “model”.
A model is a theoretical construct used as thinking aid in the study of some physical system
too complex to be understood by direct inferences from observed data. A model is usually
designed with some specific scientific questions in mind, and researchers asking different questions
about a given physical system will come up in all legitimacy with distinct model designs. A
well-designed model should be as complex as it needs to be to answer the questions having
motivated its inception, but no more than that. Throwing everything into a model - usually in the
name of “physical realism” - is likely to produce results as complicated as the data coming
from the original physical system under study. Such model results are doubly damned, as they
are usually as opaque as the original physical data, and, in addition, are not even real-world
The solar dynamo models discussed in this review all rely on severe simplifications of the set of
equations known to govern the dynamics of the Sun’s turbulent, magnetized fluid interior. Yet most of them
are bona fide models, as defined above. Since direct numerical simulations of the solar dynamo in
the appropriate parameter regime are not forthcoming, the simplified models discussed in this
review are likely to remain our primary interpretative tool for solar and stellar cycles in years to