dipole_magnetic_field

dipole_magnetic_field - Figure 1: Cutaway of Earth’s...

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Unformatted text preview: Figure 1: Cutaway of Earth’s interior. 1 Earth’s Structure and Magnetic Field 1.1 The Earth’s Interior The Earth is roughly spherical in shape with a radius of approximately 6370 km (4000 mi). Since it is only possible to drill down to less than 8 miles into the Earth’s crust, most information about the earth’s interior is obtained indirectly. By measuring vibrations caused by earthquakes and man-made explosions, and then using our understanding of wave propagation in solid and fluid materials, it is possible to deduce the basic properties of the layers making up the interior of the Earth. Figure 1 displays a cut-away view of the Earth (from http://www.nasa.gov/worldbook/ earth worldbook prt.htm). As can be seen, the interior is composed of several distinct re- gions. The center of the planet is composed of a nickel-iron core approximately 3500 km in radius. Thus, it is slightly larger than half the radius of the planet. The core occupies 16% of the volume of the planet, and about 36% of the mass. In the inner portion of the core (out to approximately 1300 km the temperature can be as hot as 7000 o C, but because 1 Figure 2: Illustration of Earth’s dipole magnetic field. of the extreme pressure, the rock remains solid. The outer portion of the core is somewhat cooler (7000 to 8000 o C, and as a consequence is in a liquid state. A differential rotation of the fluid outer core with respect to the rest of the planet is believed to be the cause of the currents that generate the Earth’s geomagnetic field. This is referred to as the geodynamo theory . Outside the core, and continuing almost to the planetary surface is the mantle . The mantle is solid, but behaves like a pliable plastic that can flow slowly. The Earth’s crust is composed of distinct plates that can float on the mantle surface, resulting earthquakes, volcanos, and mountain ranges. 1.2 The Geomagnetic Field The fact that the earth is effectively a giant bar magnetic has been known for centuries. Gilbert showed in the early 1600s that the field near the surface of the earth was dipolar in character (See Figure 2, from http://en.wikipedia.org/wiki/Image:Dipole field.jpg). It was also recognized early on that the terrestrial magnetic field was not constant. There are secular changes occurring with time scales of centuries or longer, which are believed to be due to the geodynamo within the earth. When rocks cool down from a volcanic eruption or other violent events, the geomagnetic field can be ”imprinted,” thus allowing for a deduction of the strength and orientation of the field at the time of cooling. Coupled with 2 standard radiometric dating, a history of the geomagnetic field can be produced. The results indicate that magnetic field direction has varied considerably over time, and even changed polarity (i.e., going to zero then reversing sign) on numerous occasions. During the past 100 million years, the reversal rates have varied considerably. Recent rock records (i.e., the lastmillion years, the reversal rates have varied considerably....
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This note was uploaded on 09/10/2010 for the course ECE 107 taught by Professor Fullterton during the Spring '07 term at UCSD.

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dipole_magnetic_field - Figure 1: Cutaway of Earth’s...

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