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Team discovers 'throttle' for solar wind (Forwarded)



 
 
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Old June 7th 07, 08:41 PM posted to sci.space.news
Andrew Yee[_1_]
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Default Team discovers 'throttle' for solar wind (Forwarded)

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May 17, 2007

Team discovers 'throttle' for solar wind

Helium may act as a "throttle" for the solar wind, setting its minimum
speed, according to new results from an MIT-led team using NASA's Wind
spacecraft.

The solar wind is a diffuse stream of electrically conducting gas (plasma)
constantly blowing from the sun. "This result gives us another clue about
how the solar wind is accelerated, which may help us better understand space
weather," said Justin Kasper, a research scientist at MIT's Kavli Institute
for Astrophysics and Space Research and lead author of a paper on this
research that appeared in the Astrophysical Journal May 1.

When turbulent solar wind hits Earth's magnetic field, it can cause magnetic
storms that overload power lines and radiation storms that disrupt
spacecraft.

The new research could also lead to a deeper understanding of plasma
physics, which is of interest because stars are made of plasma and plasma is
used in advanced devices like plasma TVs and experimental fusion reactors.

The sun's atmosphere, or corona, can be seen from Earth during the peak of a
total solar eclipse, when it appears as a shimmering halo around the moon.
At the beginning of the space age, scientists discovered that the corona is
being blown into space as the solar wind, so we are actually embedded in the
atmosphere of the sun. Later observations revealed the solar wind blows at a
minimum speed of about 260 kilometers per second (161 miles/second). No one
knows why this particular speed is in effect.

Hydrogen, the most common element in the universe, makes up most of the sun
and the solar wind. Helium is the second most abundant element, but it is
much rarer in the solar wind than it is elsewhere in the universe. The team
discovered that the abundance of helium increased as the solar wind speed
increased, from near zero around the minimum speed to more than four helium
atoms for every 100 hydrogen atoms at speeds greater than about 500
kilometers per second (310 miles/second).

Because helium nearly vanishes from the solar wind at its minimum speed, the
researchers believe helium might somehow set the minimum speed. Helium is
not accelerated efficiently by any process thought to be propelling the
solar wind. Instead, it has to be dragged along by the hydrogen: Solar wind
hydrogen atoms exert a small electric field that drags the helium out along
with it, according to the team.

When helium hitches a ride with hydrogen, it slows down the hydrogen atoms.
"At the minimum speed -- the speed where the solar wind is no longer able to
drag out helium -- the solar wind itself can't escape either," said Dr.
Keith Ogilvie of NASA's Goddard Space Flight Center, Wind Project Scientist
and a co-author on the paper.

"It's still not clear exactly how the helium sets the minimum speed at its
particular value of around 260 kilometers per second, or why more helium is
found as the solar wind speed increases, but it's a clue that we are missing
something fundamental about what makes the solar wind blow," said Kasper.

It's also unknown what gets the solar wind blowing again once it falls below
its minimum speed, but there are hints the process may be related to violent
eruptions of plasma from the sun called coronal mass ejections, or CMEs.
CMEs have five to 10 times the amount of helium seen in the solar wind,
according to the team. As the solar wind stagnates, helium builds up until
the plasma is explosively released as a CME in this scenario. Earthbound
CMEs also cause disruptions in satellites, power systems and radio
communication, including the Global Positioning System.

The team used the solar wind experiment (SWE) instrument on board the Wind
spacecraft to sample the solar wind. The SWE instrument uses an electric
field to measure the speed, density and temperature of hydrogen and helium
in the solar wind. The results were compiled from about 2.5 million
measurements by the instrument over more than 10 years. "The SWE instrument
has been extremely stable over all this time, so we know the changes we see
in the solar wind are real and not just from changes in the instrument,"
said Ogilvie.

Additional authors of the Astrophysical Journal paper are Michael L.
Stevens, a graduate student in MIT's Department of Physics; Alan J. Lazarus,
a senior research scientist in physics; and John T. Steinberg of Los Alamos
National Laboratory.

This work was funded by NASA and the National Science Foundation.

RELATED LINKS

* MIT Kavli Institute for Astrophysics and Space Research
http://space.mit.edu/
* The Astrophysical Journal
http://www.journals.uchicago.edu/ApJ/

IMAGE CAPTIONS:

[Image 1:
http://web.mit.edu/newsoffice/2007/s...1-enlarged.jpg (49KB)]
Research scientists Justin Kasper and Alan Lazarus display a Faraday Cup, a
device used to measure the current in a beam of charged particles. Photo:
Donna Coveney

[Image 2:
http://web.mit.edu/newsoffice/2007/s...2-enlarged.jpg (38KB)]
These are false-color images of ultraviolet light emitted by the solar
atmosphere taken with SOHO's Extreme-ultraviolet Imaging Telescope. Note how
more structures appear in the atmosphere as the solar maximum of 2000
approaches, from the left image in early 1997 to the right image in late
1999. These changing structures regulate the escape of helium in the solar
wind. Image courtesy: ESA and NASA
 




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