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THE
EARTH'S MAGNETOSPHERE
A spherical magnet in an otherwise empty region of
space would have a magnetic field approximately modeled
in the figure below.
The Earth's magnetic field close to the
Earth can be thought of approximately as a spherical
magnet. More than 90% of the Earth's magnetic field
measured is generated internal to the planet in the
Earth's outer core. (see A
Magnet in Space) This portion of the geomagnetic
field is often referred to as the Main Field. The
Main Field creates a cavity in interplanetary space
called the magnetosphere, where the Earth's magnetic
field dominates in the magnetic field of the solar
wind. The magnetosphere is shaped somewhat like a comet
in response to the dynamic pressure of the solar wind.
It is compressed on the side toward the Sun to about
10 Earth radii (RE is 6400 km) and is extended tail-like
on the side away from the Sun to more than 100 Earth
radii. Click here to
view a short animation showing the Earth's magnetosphere.
For a three-dimensional model of the
Earth's magnetic field visit the below link. It may
help you to visualize the field. +
Website for Model of Earth's Magnetic Field
The shape of the Earth's magnetic field
is formed by the interaction of several important features.
One feature is, of course, the Earth's internal magnetism.
(see A Magnet in Space)
Another feature is the interplanetary magnetic field.
This magnetic field arises at the Sun and extends into
interplanetary space. The interplanetary magnetic field
is formed by currents of
plasma within the Sun and within the solar wind. This magnetic field pattern
spirals outward from the Sun to fill space throughout the solar system. (see Interplanetary
Magnetic Field Lines) The third significant feature contributing to the
shape and activity of the Earth's magnetic field is the solar wind, the
plasma streaming constantly from the Sun in all directions.
The solar plasma interacts with the magnetic
field of the Sun. The plasma and the magnetic field
typically behave as if they are "frozen" together.
This lets one visualize the solar wind as carrying
the magnetic field outward from the Sun. The plasma
on the magnetic field lines connected to the Earth
has its own source in the low altitude ionosphere.
In the 1930's Chapman
and Ferraro predicted that the plasma and magnetic
fields from the Sun and the plasma and magnetic fields
of the Earth would not mix. They thought that the magnetic
field of the Earth could create a complete barrier
to the solar wind. The boundary between the interplanetary
magnetic field and the region dominated by the Earth's
magnetic field is called the Magnetopause. In the Chapman-Ferraro
model, the plasma of the solar wind and the magnetic
fields of the Sun slide over and around the Earth's
magnetosphere without any mixing.
Other aspects of magnetic fields are
important to understanding the Sun-Earth interaction.
Scientists know that oppositely directed magnetic fields
produce a weakened magnetic field and can even cancel
the field. The point at which the magnetic field weakens
and cancels is called the neutral point. At the neutral
point magnetic field lines form new connections. In
the 1960s James Dungey realized the importance of this
information to the Sun- Earth magnetic system. The
Earth's magnetic field runs from geographic south to
north. When the Sun's magnetic field is in the opposite
direction, a neutral point forms on the Sun side of
the Earth's magnetosphere. Another neutral point forms
in the tail of the Earth's magnetosphere where the
lines are pressed together by the solar wind.
 Magnetic
reconnection can occur in the two magnetospheric regions
indicated here in gray: at the magnetopause and in
the magnetotail. When the Sun's magnetic field (blue)
points southward (downward), it can reconnect at the
magnetopause with Earth's closed field (green). In
the magnetotail, reconnection occurs when Earth's open
field (red) is squeezed together. The sketch is not
to scale. Courtesy
Charles Day and Physics Today
The neutral point that forms on the Sun
side of the magnetosphere allows the magnetic field
lines of the Sun to connect with the Earth's field.
This process involves much more than just forming a
new connection. At the neutral point magnetic energy
is converted into kinetic and thermal energy of the
plasma associated with the magnetic field lines. Dungey
called this process reconnection. One end of the field
line is still in the solar wind. The other end is connected
to the Earth. On the side of the Earth away from the
Sun (the night side), the magnetospheric field becomes
elongated with the neutral point again separating closed
magnetospheric field lines from field lines in the
solar wind. This compression in the magnetotail can
store considerable energy, when there are large bursts
of solar plasma. Very energetic solar events, such
as Coronal Mass Ejections aimed at the Earth, bring
enormous amounts of energy to the magnetosphere. During
these high-energy events the tail of the magnetosphere
becomes very compressed. As you can see in the following
series of images, part of the magnetic field between
the Earth and the original neutral point gets "pinched
off" and the field and the plasma with it "snaps
back" toward the Earth.
Sketches showing the development of the
night side tail magnetic field during the growth and
expansion phases of a substorm.  Sketch
(a) shows the development of distended field lines
in the near-Earth tail during the growth phase. Sketch
(b) shows the onset of the field disruption at the
beginning of the expansion phase and the rapid return
of the inner tail field. Sketch (c) shows the down-tail
movement of this disruption. The separate elliptical
region in the end of the tail contains plasma and will
flow out of the tail and into the solar wind. Courtesy
of A
Beginner's Guide to the Earth's Magnetosphere by
S. Cowley
As the magnetosphere snaps back toward
the Earth, it introduces more energetic plasma closer
to the Earth. The dynamic interaction of the plasma
and magnetic field produces electrical fields and currents
in the North and South polar regions. The electrical
fields accelerate charged particles down into the atmosphere.
Movement of the charged particles of the plasma becomes
very complicated near to the Earth. More information
can be obtained at Trapped
Radiation and Principles
of Magnetic Trapping of Particles. For another
explanation of the interaction between the Sun's magnetic
fields and that of the Earth see The
Tail of the Magnetosphere or the POETRY website, "Exploring
the Earth's Magnetic Field", Chapter 2.
When these energized charged particles
enter the Earth's upper atmosphere  ,
several events can occur. One of these is the aurora,
spectacular sheets of color moving in the night sky
at altitudes between 70km and 250km. (Photo: March
5, 2000 Credit and Copyright© Dick Hutchinson)
 (Photo:
Aurora near Circle, Alaska on the Yukon River. April
5, 2000 Credit and Copyright© Dick Hutchinson)
Auroras occur when high energy electrons
collide with oxygen and nitrogen molecules. The high
altitude red color is light emitted from excited oxygen.
The red color at high altitudes is rare and is present
only during high-energy magnetic storms. Excited oxygen
molecules also are the sources of the more common green
color. Excited nitrogen molecules produce a blue-violet
hue that is sometimes difficult to see. Excited nitrogen
can also produce the red color seen at low altitudes.
Highly energetic solar events, such as
Coronal Mass Ejections directed at the Earth, increase
the intensity of electrons impacting the atmosphere.
Auroras become more active over larger areas during
large solar storms. The large magnetic storms also
cause significant electrical currents in long conducting
objects near the Earth's surface. In the late 1800s,
telegraph operators used batteries to send electrical
impulses as code along wires. When auroras were most
active, telegraphers found that they didn't need to
use batteries. Solar storms were generating voltages
in the wires so strong that some operators were nearly
electrocuted. Solar storms in 1940, 1958, 1972 and
1989, generated electric currents in power lines so
strong that transformers damaged, plunging large areas
into darkness. In one storm a transformer exploded.
See Lessons Learned from Solar Cycle 22.
Electric currents can also be generated
in gas and oil pipelines during magnetic storms. The
effect pipeline increases the rate of corrosion. Ongoing
tests of the Alaska oil pipeline indicate that it is
corroding faster than originally anticipated. In June
of 1989 a natural gas pipeline in Siberia leaked gas
that was ignited by passenger trains. The resulting
explosion killed hundreds of people. One interpretation
of the evidence is that an increased rate of corrosion
due to electric currents caused the leak.
The effects of solar storms are considered
so important that scientists are now calling interactions
between solar wind and the Earth's magnetosphere "space
weather". Space weather predictions are being
sought to anticipate storms that could cause satellite
damage, auroras, power outages, and increased pipeline
corrosion. But does this variation in solar activity
affect weather at the Earth's surface? Read
Solar activity and the Weather - Is There a Connection? And find out.
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