In spite of its nearly-cyclic nature, the solar cycle is characterized
by amplitude, period, and shape, which vary irregularly.
These irregularities appear to be an intrinsic feature of the solar
cycle being observed in many other solar observables including
irradiance, surface flows, and polar faculae counts. Anyway, the
origin of these irregularities is still unclear.
Using the temporal and latitudinal distribution of sunspots
recorded since 1874, it was proposed (Mininni et al. 2002) that
the solar magnetic cycle, investigated by means of the butterfly
diagram, might be interpreted as being the result of the superposition
of two oscillations, characterized by constant amplitude
and phase and by a period close to 22 years, on a stochastic
background. This suggested that the spatio-temporal irregularities
observed in the solar magnetic cycle were not a plain manifestation
of low-dimensional chaos being due to the interaction
of two superposed antisymmetric modes with a stochastic background
(Mininni et al. 2002, 2004). Similar results were found
by Lawrence et al. (2005), who investigated the 22 year solar
cycle by NSO Kitt Peak synoptic maps, and by Vecchio et al.
(2005), who studied the spatio-temporal features of 11-year solar
activity using the green coronal emission line at 530.3 nm.
In contrast, Letellier et al. (2006) showed that the 22-year solar
magnetic cycle, as reconstructed by sunspot time series, is in
agreement with a low-dimensional chaotic dynamics. In detail,
they showed that the phase-space diagram (the phase portrait) of
the sunspot number resembles that of a Rössler dynamical system.