Very good question! And
a difficult topic. The solar cycle is thought to arise ultimately
from the Sun's dynamo process. During the solar minimum Sun's
magnetic field is most simple, resambling a dipole field (like in the
Earth's magnetosphere). The magnetic field of the Sun is generated in
the convection zone (or rather in the boundary of it). Sun rotates
faster at equator than at poles and this uneven rotation twists
north-south orientated field lines around the Sun. The resulting
magnetic tubes rise to the surface and one manifestation are
sunspots. Large-scale solar flows (so-called meridional flows) closer
to the surface transport magnetic field to the poles where it
interacts with the existing field and changes it sign slowly. During
this time when the north and south poles of the Sun are changing the
Sun is most active (e.g. you see plenty of solar eruptions) and its
magnetic field is most complex. Then situation returns back to solar
minimum dipole configuration.
This cycle from minimum to
minimum takes approximately 11-year. Why it is 11-years I do not
know, but I have understood that at least speed of the large-scale
flows that transport the
magnetic flux to poles and back affect strongly to solar cycle
length. For example, according to some models the changes in these
flows would be responsible for the recent deep and long solar
minimum. Also, if one looks sunspots statistics all cycles are not
11-years, it varies approximately between 10 and 13 years.
And to make things even
more complicated it seems that Sun's south and north hemispheres do
not evolve always in phase! You can read about this for example here: http://www.nasa.gov/missi...pole-asymmetry.html. So there is a lot we do
not still understand about solar cycles.