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FANTASTIC
VOYAGE INSIDE THE SUN REVEALS HIDDEN WORLD OF SURPRISING COMPLEXITY
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up of sunspot and whole image of Sun with sunspot | Scientists
have peered beneath the surface of the Sun to discover how large areas of stormy
solar activity, called active regions, form and grow. Additionally, they've got
their best look yet at why sunspots -- dark blotches on the solar surface, often
grouped in active regions -- sometimes go for a spin. "These
discoveries are showing us that the Sun's interior is much more complex and dynamic
than we thought," said Prof. Philip Scherrer of Stanford University who is
Principal Investigator of the SOHO/MDI project. "This emerging picture tells
us that understanding violent solar activity, which is driven by turbulence within
the Sun, will be more challenging." Two
teams of scientists used the Michelson Doppler Imager (MDI) instrument on board
the Solar and Heliospheric Observatory (SOHO) spacecraft to infer the sub-surface
structure of selected areas on the Sun by analyzing sound-generated ripples on
its surface, using a technique similar to ultrasound diagnostics at a medical
laboratory. They were able to construct a picture of magnetic structures inside
the Sun because sound travels faster in solar regions with a strong magnetic field. The
results are the topic of a press conference scheduled for December 10 at 10:00
A.M. PST in the Moscone Convention Center, San Francisco, during the fall meeting
of the American Geophysical Union. Active
regions are sites of fierce activity, generating explosions called solar flares
and eruptions of electrified and magnetized gas (plasma) called Coronal Mass Ejections
(CMEs). Scientists know this activity is driven by distorted magnetic fields that
suddenly snap to a new, less energetic configuration, and that active regions
are sites of strong magnetic fields.
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on image for animation. | By
peering beneath the surface of "AR 9393," the largest active region
in the current solar cycle, a team led by Dr. Alexander Kosovichev of Stanford
University found that such regions are comprised of many small magnetic structures
that rise quickly from deep within the Sun. At one point last year, AR 9393 stretched
150,000 miles (240,000 kilometers) across the Sun, more than 18 times the diameter
of the Earth. "We
thought active regions had a simple structure," said Kosovichev. "But
instead of one large tube-like magnetic structure that rises from deep inside
the Sun, we find that active regions are made up of many small magnetic structures
emerging at adjacent locations. Moreover, the magnetic structures are replenished
by others as they emerge, which makes the active region grow." While
clarifying the structure of active regions, the new details engender many more
questions. It's not yet known why a given region on the solar surface can suddenly
erupt with magnetic structures and become active, or what causes the active region
to be replenished by magnetic "reinforcements". According to the researchers,
their data extends about 62,000 miles (100,000 kilometers) inside the Sun -- to
the limit of the MDI -- but the generation and storage of the magnetic structures
probably occurs at the bottom of the Sun's convection zone, called the tachocline,
which extends another 62,000 miles down, or 124,000 miles beneath the surface. A
second team led by Junwei Zhao, also of Stanford, used SOHO MDI to explore beneath
a Sunspot to understand why they sometimes start rotating. The Sunspot was located
in the Sun's northern hemisphere, in an active region designated AR 9114. Although
an average-sized spot at about 18,600 miles (30,000 kilometers) across, it exhibited
unusually pronounced rotation, spinning more than 200 degrees counter-clockwise
in less than three days. Zhao's team discovered that there was a strong plasma
vortex beneath the rotating Sunspot and that the magnetic fields lacing the Sunspot
appeared to be twisted beneath the surface. Like
Kosovichev's research, Zhao's observation raises new questions. "Now we have
a dilemma similar to the 'Which came first -- the chicken or the egg' question,"
said Zhao. "Is it the vortex that twists the magnetic field or does the twisted
magnetic field somehow create the vortex?" Discovering
the cause of twisted solar magnetic fields is important because it might eventually
help predict stormy solar activity. Twisted magnetic fields on the Sun can suddenly
snap to a new configuration with less energy. The excess energy is released in
violent solar activity as solar flares and CMEs. These events occasionally disrupt
satellites, power systems, and radio communication at Earth. Kosovichev's
team made its observations from March to May in 2001, and Zhao's team made its
observations August 7 -11, 2000. SOHO is a cooperative project between the European
Space Agency and NASA. HIDDEN
STRUCTURE OF ACTIVE REGIONS MOVIE (Click
on link for .mov version) Click
here for the mpeg version Credit:
NASA/ Walt Feimer, Max-Q Digital. This
is a computer animation illustrating how emerging magnetic structures form areas
of stormy solar activity, called active regions, on the Sun's surface. Instead
of one large tube-like magnetic structure that rises from deep inside the Sun,
scientists found that active regions are made up of many relatively small magnetic
structures (represented by white loops) emerging at adjacent locations. Active
regions are sites of fierce activity, generating explosions called solar flares
and eruptions of electrified and magnetized gas (plasma) called Coronal Mass Ejections
(CMEs). Scientists know this activity is driven by distorted magnetic fields that
suddenly snap to a new, less energetic configuration, and that active regions
are sites of strong magnetic fields. For more details, see the captions accompanying
the still images from this animation. CREDIT:
NASA THE
HIDDEN STRUCTURE OF STORMY REGIONS ON THE SUN This
image sequence is taken from a computer animation illustrating how emerging magnetic
structures form areas of stormy solar activity, called active regions, on the
Sun's surface. Active regions are sites of fierce activity, generating explosions
called solar flares and eruptions of electrified and magnetized gas (plasma) called
Coronal Mass Ejections (CMEs). Scientists know this activity is driven by distorted
magnetic fields that suddenly snap to a new, less energetic configuration, and
that active regions are sites of strong magnetic fields. Instead
of one large tube-like magnetic structure that rises from deep inside the Sun,
scientists found that active regions are made up of many relatively small magnetic
structures emerging at adjacent locations. In this image, the magnetic structures
are represented as white arches emerging from the solar surface. The dark blotches
at the bases of the arches are sunspots, which are relatively cool, dark areas
on the surface of the Sun that form when solar magnetic fields become concentrated.
This
image takes the viewer beneath the solar surface to see a group of magnetic structures
(white loops) rising up from deep within the Sun.
In
this image, the rising magnetic structures (white loops) pierce the surface of
the Sun, forming sunspots (dark blotches) where the structure's concentrated magnetic
field intersects the solar surface.
This
is how the emerging magnetic structures appear above the surface of the Sun. These
structures are normally invisible, but sometimes electrified gas (plasma) is channeled
by their magnetic field. When this happens, the magnetic structures appear as
beautiful arches and loops when observed by spacecraft, such as NASA's Transition
Region and Coronal Explorer, that are capable of detecting the radiation emitted
by the multimillion-degree plasma.
As
the magnetic loops rise higher, some become stretched and distorted. This is represented
by the loop that appears narrow in the center, as if it's being pinched.
When
the two sides of the loop get close enough, it snaps to a new configuration with
less energy via a process called magnetic reconnection. The loop splits in two,
forming a smaller arch at the solar surface and a separate loop in the Sun's atmosphere.
The excess energy is released in explosive events like flares and coronal mass
ejections (CMEs). The white flash represents a flare generated in the small arch
as electrons are accelerated down its magnetic field and slam into the denser
plasma near the solar surface, releasing high-energy radiation. The orange mist
represents plasma trapped in the atmospheric loop. This loop quickly rises and
expands, propelling the CME plasma away from the Sun.
IMAGE CREDIT: NASA Back
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