Ref: http://www.aip.org/tip/INPHFA/vol-9/iss-6/p18.html

The consistency on Earth of visible solar radiation belies the sun's
dynamic and turbulent state. Just beneath the solar surface, or
photosphere, a layer of ionized hydrogen (along with a little helium
and traces of heavier elements) churns and mixes to a depth of about
200,000 km, convecting heat from the 15-million-kelvin core to the
5,800-K surface. The churning charged particles generate
electromagnetic fields that blossom from the sun's surface in
spectacular patterns, which are observed in the tenuous,
1-million-kelvin plasma of the solar corona. The corona forms the
base of the solar wind, the continuous, even-more-tenuous stream of
charged particles that flows outward from the sun into interplanetary
space. The effects of the interaction of solar charged particles with
Earth's magnetic field are referred to as space weather.

Like terrestrial weather, space weather is characterized by an
average state of relative calm punctuated by bursts of activity.
These solar storms vary in strength and frequency with the 11-year
solar-activity cycle and cause disruptions of various magnitudes on
Earth. During calm periods, the only manifestation of solar weather
may be the auroras (Northern or Southern Lights), caused by the
excitation of atmospheric oxygen and nitrogen by the solar wind's
energetic electrons. Flares and coronal mass ejections (CMEs) are two
types of solar eruptions that can spew vast quantities of radiation
and charged particles into space, potentially causing geomagnetic
storms. If a large flux of charged particles from the sun intersects
the Earth, it can have serious consequences for modern support
systems, including electrical power grids, communications networks,
and satellite operations.

See: http://www.aip.org/tip/INPHFA/vol-9/iss-6/p18.html