1. Introduction
The Sun has been observed through telescopes for almost four
centuries. During this period, our understanding of the Sun and
of its dynamics has undergone a profound revolution. However,
it is only in the past few decades that the traditional view of many
solar features has been completely reconsidered in the light of
both theoretical advances and high-resolution ground and space
observations.
Solar activity is traditionally estimated by the relative
sunspot number R, also known as the Wolf sunspot number,
which represents a qualitatively reliable proxy of the activity itself.
The Wolf sunspot number R is important because it is one
of the oldest and longest running direct record of solar activity,
with reliable observations starting in 1610.
However, a more informative characterization of the solar
cycle in terms of sunspots is provided by the spatio-temporal diagram
built by plotting, for each solar rotation cycle (Carrington
cycle), the total area of observed sunspots as a function of latitude.
This diagram, known as butterfly diagram, shows that
sunspots are not randomly distributed over the solar surface but,
at any stage of the cycle, they are concentrated in a latitudinal
band across the solar equator. Since sunspots represent a surface
manifestation of the emergence of the toroidal magnetic field residing
in the solar interior, the butterfly diagram also represents
a spatio-temporal map of the solar internal large-scale toroidal
magnetic field.
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 dist