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* *
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	  	*The Solar
Wind *

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*The Sun makes itself known throughout much of the Solar System by the
influence the / solar wind/ of high-speed charged particles constantly
blowing off the Sun. The solar wind may be viewed as an extension of the
outer atmosphere of the Sun (the corona) into interplanetary space. *

*Components of the Solar Wind*

*The solar wind contains roughly equal number of electrons and protons,
along with a few heavier ions, and blows continously from the surface of
the Sun at an average velocity of about 400 km/second. This is a
remarkable velocity: particles in the solar wind from the Sun's surface
travel at a speed that would allow them to go from Knoxville to Memphis
in less than 2 seconds! This wind leads to a mass loss of more than 1
million tons of material per second, which may seem like a large number,
but is insignificant relative to the total mass of the Sun. *

*The Role of the Coronal Magnetic Field*

* *
<corona_dipole.jpg>
Magnetic field lines
in the solar corona

*The solar wind escapes primarily through coronal holes <corona.html>,
which are found predominantly near the Sun's poles; in the equatorial
plane the magnetic field lines of the Sun are more likely to close on
themselves, particularly in periods of low solar activity. These closed
field lines trap the hot coronal gases, leading to enhanced X-ray
emissions from these hotter regions, but suppressing contributions to
the solar wind. *

*The adjacent image shows an enhanced image of the solar corona. In this
image the magnetic field lines of the corona correspond approximately to
the boundaries between regions of different color (more info)
<http://www.oma.be/BIRA-IASB/Project_educatif/Vent_sol/vent_sol.angl.html>.
Notice that in the equatorial regions the field lines traced by these
color boundaries tend to form closed loops, indicating the trapping of
coronal gas.
*

*Influence of the Solar Wind on the Earth*

*As we have already discussed in the section on the Earth
<../../../astr161/lect/earth/earth.html>, the solar wind can have a
large influence on our planet, particularly in times of the active Sun
(near sunspot maximum) when the wind is strong and can contain bursts
corresponding to flares and coronal mass ejections from the Sun. The
solar wind has a significant influence on our ionosphere
<../../../astr161/lect/earth/atmosphere.html>, the Earth's magnetic
field <../../../astr161/lect/earth/magnetic.html>, on Earth's auroras
<../../../astr161/lect/earth/aurora.html>, and on telecommunication
systems. For example, there is reason to believe that a burst of
particles from a coronal mass ejection detected 5 days earlier by SOHO
may have killed the Telstar 401 communications satellite on January 11,
1997 (News Story <http://cnn.com/TECH/9701/21/cosmic.chaos/index.html>). *

*Earth's Ionosphere and the Sun*

*The level of activity on the Sun can have a large influence on the
Earth's ionosphere, as illustrated in the following animations. *

*Electron Density*

The adjacent animations simulate the variation by month of the
ionosphere for two different years:

1. The year 1990 (upper image), which was a period of high solar
activity with many (150) sunspots.
2. The year 1996 (lower image), which was a period of low solar activity
with few (10) sunspots.

The plots show electron density contours, which are an indication of the
amount of ionization in the atmosphere. Yellows and reds indicate larger
ionization and blues and greens indicate smaller ionization. Notice the
substantial differences in these two animations, with much stronger
atmospheric ionization in the upper image (the active Sun of 1990) than
the lower image (the quiet Sun of 1996).

The adjacent images are based on these electron density contour maps
<http://www.ngdc.noaa.gov:8080/GOIN/IONO/ionocontour_90.html> of the
ionosphere <../../../astr161/lect/earth/atmosphere.html> for months in
the year 1957 to the present. Additional animations may be found in this
NOAA directory <http://www.ngdc.noaa.gov:8080/GOIN/IONO/CONTOUR>.

*The level of solar activity has similar effects on related phenomena
such as Earth's auroras. *