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IMAGES:
The links below take you to a page where you can download high resolution images.
This color picture is a computer representation of one of nearly
10 million modes of sound wave oscillations of the Sun as seen by the
GONG network of telescopes. Areas of the solar surface moving away from
us are shown in red tones and approaching regions are shown in blue. By
monitoring these oscillation patterns on the solar surface, GONG allows
astronomers to infer much about conditions in the solar core, leading to a
greater understanding of our nearest star.
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These cutaway images show snapshots of the changing solar rotation at two different times about six months apart, on the surface (left side of images) and below the solar surface. Near the surface the rotation is faster than average in the red areas and slower in the green; deeper down red indicates faster than average rotation and blue indicates slower rotation.
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The GONG network of telescopes consists of six observing stations around the
world, positioned so the Sun never sets on GONG. The GONG instruments
are housed in rectangular trailers as shown here. A world map of
all GONG sites is found at the GONG web site.
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Time variations of solar rotation, as derived from 4.5 years of GONG
data, are shown in this graph of latitude versus date. Faster rotation is
shown as red/yellow; slower rotation is shown as green/blue. Bands of
faster and slower rotation are seen to be migrating toward the equator.
These flow bands extend below the solar surface through the convenction zone.
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Image Credit: AURA/NSO/National Science Foundation
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The Beat Goes On - Inside The Sun
Astronomers from the National Science Foundation's National Solar Observatory
are announcing in today's issue of Science that their discovery of
periodically varying gas motions below the solar surface provides an
important clue toward solving the biggest secret of the Sun - the origin
of the 11-year cycle of solar activity.
Lead author Rachel Howe, with Frank Hill and Rudi Komm from the National
Solar Observatory (NSO) in Tucson, AZ, and colleagues from other
institutions, have analyzed 4.5 years worth of observations from the
National Science Foundation's Global Oscillation Network Group (GONG),
a world-wide network of telescopes providing continuous observations of
the Sun, to make this intriguing discovery.
Howe and colleagues have discovered variations in the motion of a region
far below the solar surface possibly connected to the solar activity seen on
the surface. Probing the depths of the solar interior with a technique
known as "helioseismology" which uses sound waves trapped inside the
Sun, Howe and her colleagues are able to measure the speed of the gas at
different locations below the surface. They then searched for organized,
systematic changes in the speed of the moving material. They
found that near the base of the convective layer, approximately
30% of the Solar radius or 210,000 km beneath the surface, the rotation rate
periodically varies with a cycle of 15 months. Their systematic search
looked from the solar surface to nearly half way to the solar core, and
from the solar equator to near the pole.
The surface of the Sun rotates about once every 27 days at the equator.
Unlike the Earth, however, all points on the solar surface do not rotate at
the same rate. The solar equator rotates more rapidly than the poles where
the rotation rate is once every 35 days. This "differential" rotation, long
a mystery of solar physics, extends throughout the convective layer and
then vanishes immediately below it. It is at this transition point that
Howe and her colleagues observe the 15-month rhythmic variation of
the rotation.
How this pulse in the rotation deep below the surface affects the solar cycle,
and why it varies every 15 months instead of 11 years is "another piece of the
puzzle for the theorists", explains Howe. "At first we were skeptical of the
pattern. Knowing the complexity of models used to explain the solar magnetic
field and its connection to observed solar activity, we were expecting
nothing, or chaos, in our observations at that location. Using our status as
Associate Investigators on NASA's SOHO mission," Howe continues, "we were able
to confirm the existence of the periodic variation using an entirely separate
instrument and analysis technique. We do believe the pulse is real. We've
looked twice in very different ways and see it both times. It's definitely
there."
Understanding the solar cycle, which has been recorded since 1750, is
a fundamental, and unanswered, question in solar astronomy. Every 11
years the Sun goes from being a relatively quiet place to exhibiting
a high level of activity which includes sunspots, solar flares, and
coronal mass ejections. At the end of the cycle, the Sun returns to
its quiet state. We are at the peak of activity for the current solar
cycle, with predicted maximum occurring the Spring of 2000.
The GONG network provides the most systematic, continuous observations of
the oscillations of the Sun yet available.
Prior to the development of the helioseismology technique central to GONG, most
of what we knew about the Sun came from observing the solar surface
(photosphere) and the layers immediately above the surface.
GONG monitors the surface of the sun 24-hours a day and
tracks tiny oscillations on the Sun's surface. These oscillations are
visual evidence of sound waves traveling through the sun's interior; by
analyzing patterns in the sound waves we can develop an understanding of
the conditions of the material the sound waves are traveling through.
Many techniques used in current helioseismology are derived from methods
developed for studying the effect of earthquakes in a terrestrial setting.
In both disciplines, watching the waves of energy being transported through
the host sphere, Sun or Earth, gives clues to the density and composition
of the material being traversed.
"We listen to the Sun's heartbeat to understand what is happening in its
core", explains Frank Hill, NSO astronomer and coauthor on the Science
paper. "We can't hear these sound waves on earth because there is no
air in between the Sun and earth to carry the sound. (Even if there were,
the frequencies are too low to be heard by the human ear.) But, we can see
these sound waves in the oscillation patterns on the solar surface."
Howe continues, "Different sound wave patterns occupy different regions
in latitude, and different depths within the Sun. By analyzing groups
of sound waves in the Sun, we can build up a two-dimensional
picture of the rotation inside the Sun".
GONG is an international project led by the U.S. National Science Foundation
with the participation of twenty nations. GONG observing sites are
located at Big Bear Solar Observatory, California; the High Altitude
Observatory station at Mauna Loa Solar Observatory in Hawaii;
Learmonth Solar Observatory in Australia; Udaipur Solar Observatory in
India; the Teide observing station of the Instituto de Astrofisico de
Canarias in the Canary Islands, Spain; and Cerro Tololo Interamerican
Observatory in Chile. SOHO is a project of international
cooperation between European Space Agency and NASA. GONG is a division
of the National Solar Observatory (NSO). The National Solar
Observatory is operated by the Association of Universities for Research
in Astronomy (AURA), Inc. under cooperative agreement with the National
Science Foundation.
Related web sites:
GONG URL:
www.gong.noao.edu
Rachel Howe's web page:
www.nso.noao.edu/press/tach/
Helioseismology in the classroom:
www.noao.edu/education/ighelio/solar_music.html
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For more information:
Dr. Frank Hill
NSO Astronomer
(520) 318-8138
fhill@noao.edu
Dr. Rachel Howe
NSO Astronomer
(520) 318-8573
rhowe@noao.edu
Dr. Rudi Komm
NSO Astronomer
(520) 318-8428
rkomm@noao.edu
Suzanne Jacoby
NOAO Press Officer
(520) 318-8364
sjacoby@noao.edu
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