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The sun is our main source of light, but it also gives off
particles, consisting mostly of electrons and protons. Sunlight
takes about eight minutes to travel from the sun to the Earth (the speed of light
is constant and = 300,000 kilometers per second). The solar
particles make up what we call the solar wind that blows outward from
the sun from about 250 kilometers per second up to 2,500 kilometers
per second. Thus, it takes the solar wind particles from 17 hours to 7
days to travel the 150 million kilometers to Earth. |
Solar eclipse March 2006 in Turkey. Photo by Ulrich
Rieth. |
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The visible light is quite uniformly distributed over the surface of the
sun, but the particles making up the solar wind come from many discrete
sources, some of them flaring up near sunspots and others more constant
with time and location across the visible disk. Because the sun rotates
on its vertical axis with a period of approximately 28 days, streams
from the constant sources, called coronal holes, appear in a spiral
pattern called a lawn sprinkler effect. The blasts of particles
associated with solar flares form expanding rings of shock waves in the
solar wind. |
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We can see the solar wind near the sun because the light is bright and
there are lots of particles there. You can see this from the cameras
that watch the space around the sun from satellites between the Earth
and the sun. To track the solar wind most of the way from the sun to the
Earth, we depend on a model of the solar wind called the
Hakamada-Akasofu-Fry, or HAF model after the people who developed it.
This is the model that is used to show the solar wind configuration in
the
Solar Wind Plots. When you click on the start button, you see the
change, as the sun rotates, of the magnetic field lines from the sun in
the plane containing all of the planets, including Earth. The closer
together the lines are, the greater number of particles there are, and
the greater the effect on the coupling of the solar wind to the Earth,
producing the aurora.
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The solar wind carries with it the sun's magnetic field. Increased
solar wind velocity and particle density associated with the solar
particle sources act in two ways to connect the solar wind to the
Earth's magnetic field. First, the sun's magnetic field joins with
the Earth's magnetic field and transfers the solar particles to the
Earth's magnetosphere. Secondly, the electrons and protons are
separated by the Earth's magnetic field to produce a giant electric
field that generates the power for the aurora. Thus, following the
solar wind is important to forecasting the aurora. |
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