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The Solar WindAn Expanding Coronal Gas that Fills Interplanetary Space
Oct 15, 2010 © Harry P. Schlanger
The Sun's high surface temperature causes solar material to accelerate into space and escape as a solar wind, which affects Earth and other heavenly bodies.
A previous article described the Sun's
internal solar structure, which includes a central core, a radiative layer, a convective
layer and its surface layers. Other articles introduced
solar activity observed at the surface, which includes an 11-year sunspot cycle, or an
equivalent 22-year magnetic cycle, and the origin of the
Sun's magnetism.
To recap, the Sun consists of a huge fiery hydrogen gas ball. The gas is in a state called "plasma", meaning that the high solar energy has
stripped the hydrogen atoms of their electrons and particles are now charged, or ionized. Plasma at the Sun's core is very dense - a suitable
environment for the thermonuclear fusion of hydrogen into helium, a process that releases tremendous energy.
How Solar Wind Escapes the Sun
It was also shown that gas density decreases away from the Sun's centre. Likewise, temperature starts at
a few thousand degrees and decreases away from the centre. However, at the surface (corona), temperature rises spectacularly to about 2 million degrees.
Now, the average velocity of particles depends on temperature. Therefore, particles such as protons and electrons are accelerated
outwards, radially of the Sun, and create a high pressure difference. The loss of solar material streaming into space can be continuous or discrete
and is the source of the so-called "solar wind ", believed to cause
comet tails to point away from it.
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How ionized material leaves the Sun's poles as solar wind. Material tends to follow magnetic
field lines as shown, opening up like a jet nozzle (Department of Physics, The University of Wales, Aberystwyth)
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Solar wind escapes primarily through coronal holes, which are regions of open magnetic field, such as are found predominantly near the Sun's poles.
In the equatorial plane, magnetic field lines are more likely to close on themselves and entrap the hot coronal gases, therefore suppressing contributions
to the solar wind.
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The Sun flings out solar material as in a spiral, just like a sprinkler (NASA)
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As the Sun rotates, the solar wind takes the pattern similar to that produced by a spinning lawn sprinkler. Unlike the jet from a sprinkler, the solar wind
is tied to the magnetic field. This effect was discovered by American solar astrophysicist, Eugene Parker (1927 - ) and is known as the Parker Spiral.
Effect of the Solar Wind
Solar wind affects the magnetic fields of all the planets in the solar system. In particular, it contributes to fluctuations in celestial radio waves
observed on the Earth (Wikipedia).
As the solar wind approaches the magnetic field of a planet, particles are deflected around it. This region is called the magnetosphere.
Planets that have a weak or non-existent magnetosphere are subject to atmospheric stripping by the solar wind.
On Earth, the solar wind is observable when it is strong enough to produce phenomena such as the Aurora and geomagnetic storms.
The solar wind also affects incoming cosmic rays, known to interact with the atmosphere of planets.
References:
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Unravelling the Secrets of the Solar Wind.
University of California Riverside. The Institute of Geophysics and Planetary Physics. (eBook).
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Solar Wind. Wikipedia.
The copyright of the article The Solar Wind: An Expanding Coronal Gas that Fills Interplanetary Space is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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