Inside Sources

Although the shape of the Earth's main field is much like that of a giant dipole magnet (Figure 1.5), there are five good reasons given by scientists to show that a solid magnet cannot cause that field:

1. Crustal magnetization cannot give rise to the main field. An inventory of all magnetized materials in the Earth's crust shows them to be of insufficient magnitude to account for the Earth's main field.

2. The Earth becomes more conducting and hotter toward the center at about 6371 km (3959 miles) deep. Approximately 25 km (16 miles) into the crust, the Curie Temperature (see Section 2.1.2, p. 31) is reached for iron and magnetite. Recall that this is the temperature at which any large-scale solid-magnet properties are scrambled.

3. Since the time when the first records of magnetic declination were kept, it has been apparent that the pole locations have been drifting westward; a rigid Earth magnet cannot model such a change.

4. Paleomagnetic evidence (see Section 2.1.3, p. 35) has shown that the north and south magnetic poles have reversed many times over the last hundred-million-year record of continental Earth history. Fluids could cause this, but not solid material.

5. A dipole field has the same form whether it comes from a solid bar magnet with separated north and south poles or from a ring of current flowing about a given area. Although a simple reversal of ring current in the conducting Earth material can reverse the poles, a solid magnet in the Earth cannot move itself.

There is sufficient reason to believe that the main magnetic field we observe at the Earth's surface and in near space must be due to current systems deep within the Earth. Using the recordings of earthquake signals that have traveled through the Earth, the seismologists have been able to prove that the outer-core region of the Earth, between depths of about 2700 and 5200 km (1700 to 3200 miles), is a hot and dense liquid of highly conducting nickel-iron (Figure 3.1). Paleomagneticians have devised a dynamo theory for the generation of the main field in this region. They describe a gravitational accretion process near the core-mantle boundary (CMB) that drives electric currents to become organized into a giant loop by the Earth's spin and spherical shape. The field from such an outer-core current loop has the same form as a dipole magnet (Figure 3.2 and Plate 8). Researchers continue to create elaborate computer models of the Earth's internal dynamo region, but the absence

FIGURE 3.1 ► Regions of the Earth's interior are identified from seismic evidence of the density (given in megagrams per cubic meter).

of critical information about the outer core and lower mantle regions only encourages disputes.

Planets and moons in our solar system display magnetic dipole fields when they are spinning and have fluid core regions. Planets Mercury, Jupiter, Saturn, Uranus, and Neptune all have main fields and magnetospheres. Although Mars has no such field now, rock samples indicate an internal dynamo main field existed in that planet's early history. Venus also has no permanent main field; present surface temperatures there preclude the gathering of rock samples to provide the necessary evidence of ancient magnetism. Our Moon has no liquid core and no main field. However, Ganymede, a moon of Jupiter, seems to display such a field.

The current generated in the Earth's outer core is slowly varying, with a direction dependent on the initial startup conditions. Disruptive eddy currents within that region can, in time, destroy the organized loop current. Using the more recent main field evidence, scientists have found that, in cycles averag

FIGURE 3.2 ► A loop of westward electric current at the liquid outer core creates a southward field within the Earth, which forms the northward-directed dipole main field of the Earth.

FIGURE 3.2 ► A loop of westward electric current at the liquid outer core creates a southward field within the Earth, which forms the northward-directed dipole main field of the Earth.

ing 200 to 250 thousand years, a major outer-core current can arbitrarily form again with effective current flowing either westward or eastward—defining the same or opposite polarity pole to which our compass needle now responds. We are now in a period of declining main dipole field strength and overdue for a reversal; possibly the event will occur within the next few thousand years.

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