Facultad de Ciencias de Orense, Universidad de Vigo, Campus Universitario de Orense, E-32004 Orense, Spain
"Mechanisms producing long-term changes
in solar and stellar activity"
A striking feature in the history of surface solar activity is a
prolonged minimum between 1645 and 1715, known as the {\it
Maunder Minimum}. Tracing back solar activity by means of proxy
data (abundances of cosmogenic isotopes $^{10}$Be and $^{14}$C)
has shown that grand minima in magnetic activity are a recurrent
feature. Grand minima seem also to occur in other cool stars
with solar-like magnetic activity.
The mean solar magnetic field behaves irregularly in time.
Grand minima are the most prominent manifestation of this
irregularity, albeit not the only one: it is also apparent in
the length and amplitude variations of the 11-year sunspot
cycle. Although the statistical and spectral properties of solar
activity --as measured by the sunspot number-- are still not
well known, there is evidence for various modulations of the
solar cycle. The question whether the observations and the
historical sunspot record allow to distinguish between a
stochastic or a deterministic origin of the irregularity in the
solar cycle seems to be as yet unresolved.
Most of the computations aiming at describing intermittent
activity are based on low-order systems of ordinary differential
equations (ODE) which can be considered as truncated expansions
of the partial differential dynamo equations (PDE), but also
models based on full PDE may lead to intermittent behaviour.
Although all theoretical models of the solar dynamo rely on
mean-field dynamo theory, in which the small-scale turbulent
motion is parametrized in order to compute large-scale effects,
there are significant differences in the physical assumptions
considered by different authors. Various alternative scenarios
for the long-term magnetic activity will be critically
reviewed.