In our daily experience, the Sun is a quiet companion that constantly
provides us with light and warmth. However, observations of the
different structures in its atmosphere show that much activity takes
place. Waves and turbulence heat the magnetized plasma of the corona to
more than a million kelvins. The magnetic field produces sunspots and
active regions, and sustains relatively cool and dense prominences in
the hot and tenuous corona, which expands towards a continuous stream of
solar wind. More dramatically, instabilities of the magnetic field lead
to eruptions, which can have important consequences on the heliosphere
in general and on Earth in particular, due to short-term variability in
light emission, magnetic field, and accelerated particles. In an
increasingly technological society, human activities can be affected and
space weather services, including the prediction of eruptions, become a
necessity.