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Team discovers 'throttle' for solar wind (Forwarded)



 
 
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Old June 7th 07, 09:30 PM posted to sci.astro
Andrew Yee
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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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