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Sun Structure
A cross section of the Sun reveals its various layers. The Core is the hottest part of the
internal sun and is the location of nuclear fusion. The heat and energy produced in the
core is radiated out through the Radiative Zone. When the energy reaches the Convective
Zone the energy is transferred by the process of convection, i.e., hot gas below rises and
the cooler surface gases sinks. At the top of the Convective Zone is the Photosphere, the
part of the Sun we see from Earth. In the Photosphere, you can see the granules, areas of
bright (hotter) and dark (cooler) gas. These granules reflect the motion occurring in the
Convective Zone. Sunspots, solar flares, and prominences occur in the Photosphere.
Through spectral analysis, the surface temperature is determined to be about 5800o K.
Above the Photosphere is the solar atmosphere, which consists of the Chromosphere and
the Corona. The Chromosphere is the lower layer of the Sun’s atmosphere. It is virtually
invisible due to the overwhelming brilliance of the Photosphere, however it can be
observed during a solar eclipse (remember never look at the sun directly, even during an
eclipse). The upper layer of the Sun’s atmosphere is the Corona, which is also invisible
except during a solar eclipse. The temperature rises dramatically with elevation through
the Corona reaching a temperature of 1,000,000o K. This swift increase in temperature is
not fully understood. Solar wind and Coronal Mass Ejections (CMEs) are generated in
the Corona.
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Solar Wind
Solar wind is created as ionized solar particles, protons and electrons (plasma), leave the
Sun’s corona in steady streams. Due to the extremely hot temperature and high energy,
many of these particles attain sufficient velocity to escape the Sun’s gravity. Solar wind
leaves the Sun’s corona through coronal holes. The dark X-ray image from the sun
below illustrates the dark corona hole moving over one-day intervals.
Sunspots
Sunspots are cool areas of photospheric gases. The sun can have anywhere from 100’s of
sunspots to no sunspots at all. Sunspots will often “grow” over several days and can last
through several solar rotations. The dark areas on the sun below are examples of
sunspots.
Scientists feel that sunspots are cooler than their surrounding area because they have
abnormally high magnetic fields, which would block the convective flow of the hotter gas
from below. Sunspots are usually found in magnetically opposite pairs (one aligned
north and the other south), which is further evidence of their magnetic nature.
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Solar Cycle
Sunspots occur in a regular cycle of 22 years. During the first 11 years, the location and
number of sunspots start to follow a somewhat regular pattern. As the cycle begins,
sunspots develop in the upper latitudes of the Sun. Their numbers increase yearly and
migrate downward toward the Sun’s equator at which time solar maximum is reached.
During this 11-year span, all the sunspots in one solar hemisphere have opposite polarity
to the ones in the other hemisphere. During the next 11 years, the solar hemispheres
reverse polarity.
The picture on the left below show the “butterfly pattern” of the number of sunspots and
their location over the 11 year cycle, and the graph to the right the number of sunspots
over time.
Solar Prominences
Solar prominences are large looping ejections of energetic particles into the lower corona,
as shown in the top right of the picture below. These events tend to follow sunspot
occurrences and are associated with magnetic instability. They can last for days or
weeks.
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Solar Flares
Solar flares are also associated with magnetic instability, but are more violent than a
prominence. They can release as much energy as a prominence in only minutes or hours.
Coronal Mass Ejections (CME)
CMEs are generated in the corona and not in the photosphere, as are flares and
prominences. They represent the most violent solar particle eruptions. Up to 10 billion
tons of plasma (30x’s more massive than the earth) can be released in a CME. It is
unclear why CMEs occur and prediction effects have been unsuccessful. CMEs create
the most adverse effect on Earth’s magnetosphere, leading to the most severe
geomagnetic storms. The picture below has a disk blocking the sun allowing the CME to
be visible.
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For more information…
1. http://sohowww.nascom.nasa.gov/explore/poster.html
This is a terrific site to explore the structure of the sun.
2. http://www.sec.noaa.gov/info/Origami.pdf
Using origami to explain energy transfer through and from the sun is a
creative activity for the classroom.
3. http://www.sec.noaa.gov/Education/
NOAA space environment center is an excellent location to look up information on
the solar environment.
4. http://www.millennium-ark.net/Geo_Solar/Solar_Links.html
This site has great links to just about any information about the sun. It includes real
time data and movies.
5. http://www.chabotspace.org/vsc/exhibits/solarb/thesun/default.asp
Solar information and review quizzes make this site a good location to test your solar
knowledge.
6. http://umbra.nascom.nasa.gov/images/latest.html
Current solar images taken in various wavelengths
7. http://sohowww.nascom.nasa.gov/
Another site with current solar data including solar wind information
8. http://www.bbso.njit.edu/arm/20020719/wl_fd.htm
Excellent site for checking daily sunspot activity