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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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