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WHY
DO SUNSPOTS AND CMES OCCUR?
The causes of solar events such as sunspots, solar
wind, prominences, streamers, plumes and coronal mass
ejections (CME's) have become better understood because
of NASA missions
such as Ulysses, ACE
(Advanced Composition Explorer) , Yohkoh, SOHO
(Solar and Heliospheric Observatory), and TRACE
(Transition Region and Coronal Explorer). However
there is much that scientists hope to learn from future
missions such as Solar-B, STEREO
(Solar Terrestrial Relations Observatory), SDO
(Solar Dynamics Observatory), and Sentinels.
The solar interior is very dense (see Amazing
Structure of the Sun), and its sphere rotates
rigidly like a solid with a period of rotation of
27 days. However, the period of rotation of the ionized
gases of the outer regions, such as the convective
zone...
 ...and
photosphere changes from the equator to the poles.
The convective zone has a rotation of 25 days at the
equator. At the poles the convective zone takes 35
days for one revolution.
The convective zone rotates more slowly
than the radiative zone at the poles. However, the
convective zone rotates more slowly than the radiative
zone at the equator. The differences in rotation cause
tremendous shear forces in the thin region between
the radiative zone and the convective zone. These zones
are made of plasma. The flow of electric charge (see Electricity),
which is an electric current, creates magnetic fields
(see Magnetism and Electromagnetism).
The intricate interactions of rotation,
convection and shear create complex dynamics within
the Sun's magnetic field. The following animation illustrates
these complex dynamics.
The lines drawn on the Sun represent
magnetic field lines. As the Sun rotates the magnetic
field lines are twisted. With sufficient twisting they
pop out and release tremendous energy. For more information
see Magnetic Personality or The
Solar Dynamo.
These complex dynamics cause sunspots
to appear and disappear in time-periods of days to
months. Variations in the number of sunspots occur
in eleven-year cycles. (see Sunspot
Cycle) Sunspot maxima and minima occur in 11-year
cycles as shown below.
 Scientists
discovered that the Sun's magnetic field reverses itself
every eleven years at the time of the maximum. Since
the period of reversal of magnetic fields is 22 years,
we can guess that there are larger cycles in the solar
system. The following diagram pulls theory and observation
together.
The green and white plane in the foreground
is from the solar corona from the Extreme Ultraviolet
Imaging Telescope (EIT) instrument. The solar corona
is a region of hot, electrically charged gas streaming
from the surface of the Sun. The image shows different
amounts of coronal material at a temperature of about
1.5 million degrees Celsius (2.7 million degrees Fahrenheit).
Whiter areas represent more material at this temperature
and darker areas represent less. The black and white
spots represent magnetic field concentrations with
opposite orientations, called polarity. Each spot is
roughly 5,000 miles across.  These
concentrations make up the solar "magnetic carpet" that
is believed to be responsible for the extreme heating
of the corona.
Between pairs of opposite polarity, magnetic
field connections exist, represented here by lines
based on computer calculations. These horseshoe shaped
loops extend above the surface into the corona. Although
small relative to the Sun, they range between a few
thousand to several tens of thousands of miles in length.
The smallest would still fit around the Earth. Each
one of these loops carries as much energy as a large
hydroelectric plant, such as the Hoover dam, would
generate over a million years.
There is a direct connection between
this magnetic carpet model and sunspots. Sunspots usually
come in groups of two spots. One spot has a north magnetic
pole and the other is a south magnetic pole. Sunspots
appear to occur where magnetic fields suppress convection
of hot matter to the surface. Flares occur near sunspots,
usually along the dividing line (neutral line) separating
opposite magnetic fields. At times the strength of
the magnetic field is quite small and there is relatively
little solar activity. When the magnetic field strength
increases the solar activity (flares prominences and
CMEs) becomes more intense.
All of the Sun's more dynamic features
seem to be associated with magnetic fields. Additional
background information can be found on Electricity, Magnetism and Electromagnetism by
clicking on the words highlighted in this sentence.
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