The Solar Wind
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The solar wind streams off of the Sun in all directions at speeds of
about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun's
hot corona. The temperature of the corona is so high that the
Sun's gravity cannot hold on to it. Although we understand why this happens we do not
understand the details about how and where the coronal gases are accelerated to these high
velocities. This question is related to the question of
coronal heating.
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Solar Wind Variations
The solar wind is not uniform. Although it is always directed away from
the Sun, it changes speed and carries with it magnetic
clouds, interacting regions
where high speed wind catches up with slow speed wind, and composition variations. The solar wind
speed is high (800 km/s) over coronal holes and
low (300 km/s) over streamers. These high and
low speed streams interact with each other and alternately pass by the Earth as the Sun
rotates. These wind speed variations buffet the Earth's magnetic field and can produce
storms in the Earth's magnetosphere.
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The Ulysses spacecraft
completed two orbits through the solar system during which it passed over the Sun's south
and north poles. Its measurements of the solar wind speed, magnetic field strength and
direction, and composition have provided us with a new view of the solar wind.
Ulysses was retired on June 30, 2009.
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The Advanced
Composition Explorer (ACE) satellite was launched in August of 1997 and placed into an
orbit about the L1 point between the Earth and the Sun. The L1 point is one of several
points in space where the gravitational attraction of the Sun and Earth are equal and
opposite. This particular point is located about 1.5 million km (1 million miles) from the
Earth in the direction of the Sun. ACE has a number of instruments that monitor the solar
wind and the spacecraft team provides real-time information on solar wind conditions at
the spacecraft.
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Solar Wind Web Links
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