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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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