Magnetosphere

Earth's magnetosphere

Earth's magnetic field has a strength of about 6×10^-5 tesla at the poles [1]. Above the surface, this field resembles a dipole with distances of several radii of the Earth (R_E). On the sun's side of Earth, the magnetosphere generally extends out to a distance of approximately 10 Earth-radii, while the magnetotail extends several hundred radii in the opposite direction.

The magnetosphere behaves something like a compressible fluid because the Earth's magnetic field traps the low-density conductive plasma of the ionosphere.

The magnetosphere is made to flap and compress by the solar wind. The flapping and compression form electromagnetic storms with direct effects on long-distance electrical lines and radio propagation.

One of the effects of the magnetosphere is to shield the Earth's atmosphere from the solar wind, which might be able to carry away the atmosphere. Scientists disagree on the size and result of this effect. Some scientists believe that without a magnetosphere, the Earth would have lost the majority of its water and atmosphere, and resemble Mars or Mercury.

Another effect is to trap high-speed ions. The magnetic field causes them to circle within the field. The ions are moving fast enough to be hazardous. They form the Van Allen radiation belts.

The magnetosphere as a magnetic trap

When an astronomical object has a magnetic field and an atmosphere, it is commonplace for ions to form a plasma that is trapped in the magnetic field.

Why is it "trapped?" Because plasma conducts electric current. Therefore, when the plasma tries to flow through the magnetic field, the conductive plasma cuts the magnetic field lines. It forms the "winding" of an inefficient natural electrical generator. The electric currents flow in large loops called eddy currents. The eddy currents in turn form magnetic "mirror" fields opposing the Earth's magnetic field. These weaken the Earth's magnetic field where they occur. The energy dissipated in the eddy current and magnetic field comes from the motion of the plasma. The process therefore slows or stops the plasma's motion. The plasma is "trapped" by the magnetic field.

The combination of magnetic field and plasma behaves like a fluid dripping from springy magnetic field lines. Computer simulations of the magnetosphere that show the plasma and field lines have the shape of a hairy comet or spheroid. This combination of magnetic field and plasma is the magnetosphere.

The ions are usually formed as sunlight, especially ultraviolet, hits the upper atmosphere. As planets get farther from the Sun, their magnetospheres therefore become less dense, and less active. Objects that lack a magnetic field, such as the moon, lack a magnetosphere. Objects that lack an atmosphere also lack one, because no ions can form.

A star's solar wind can interact strongly with the magnetospheres of its planets. (See: auroral light)

The inner edge of the magnetosphere is most often called the ionosphere. It is used to reflect radio waves for communications.

See also: magnetic trap

History of magnetospheric physics

The Earth's magnetosphere was discovered in 1958 by Explorer I during the research performed for the International Geophysical Year. Before this, Earth's magnetic field was considered to act like that of a bar magnet, extending hundreds of thousands of kilometers, gradually getting weaker.

Related topics

See also Earth's magnetic field, magnetopause, heliopause, interplanetary magnetic field[?], plasma physics, ring current, Van Allen radiation belt

List of satellites which have provided data on the magnetosphere

For applications to spacecraft propulsion see magnetic sail.

References

• [1] Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)