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A MAGNET IN SPACE

Diagram of Earth's magnetic field. The Earth is sometimes referred to as a magnet in space. To begin our exploration of the Earth's magnetic field, let us assume that the Earth is a spherically shaped magnet. The magnetic field around a spherical magnet is essentially the same as if a bar magnet were located inside the Earth as seen above. (see Magnetism) To imagine the magnetic field lines around a spherical magnet, we can use the field lines around a circular magnet (see to right) and then rotate the circular magnet about the vertical axis, creating donut shaped fields layered one on another.

The drawing below represents a simplification of the situation that exists around the Earth. The inner circle represents the magnetic core of the Earth. The outer circle represents the surface of the Earth. The north end of a magnet is defined historically as that end pointing to geographic North. But a magnet's north pole is attracted to a south magnetic pole. Therefore, the south magnetic pole of the Earth's core must be in the geographic North. Notice also that on the surface of the Earth the magnetic field is horizontal only at the equator. North and South of the equator the field would pull a magnet downward as well as toward the pole because a compass needle always points in the direction of the Earth's magnetic field lines.

There are some complications to this model. First, the axis of rotation of the Earth and the axis of the idealized magnet at the core are not aligned. This difference, called declination or magnetic variance, is measured as the angle between true North and magnetic North and may be as large as 20 degrees. This angle varies by small amounts daily due to variations in the Sun's magnetic field. It varies greatly over large periods of time due to changes in the Earth's core. For example, the declination for Halifax, Nova Scotia has changed by about 11 degrees in 250 years. The POETRY website offers activities to plot changes in the Earth's magnetic pole in "Exploring the Earth's Magnetic Field", Activity V.

A second complication occurs because the "magnet" of the Earth is not a solid sphere. The above models assume a solid magnet such as a bar magnet spherical magnet. Such magnets are examples of ferromagnets. The name comes from iron (ferric), the most common element to display this behavior, yet nickel, cobalt, chromium and a few other elements are also ferromagnetic. While any atom with an unpaired electron can have a magnetic field, the atoms of these special elements that form ferromagnets act together in groups called domains, locking together their magnetic poles. Each domain, ranging in size from 0.1 mm to 1.0 mm, will become a tiny magnet. When an external magnetic field is applied under the right conditions, all of these domains are induced to line up creating a large magnet. In addition, ferromagnets tend to stay Diagram of Magnetism magnetized long after the external field is removed. (see Magnetism) (Photo: Courtesy of C R Nave HyperPhysics)

Heating opposes this alignment process. Because ferromagnets lose the coordinated alignment of domains at temperatures well below those near the core of the Earth, it is unlikely that such a simple mechanism can explain the magnetism of the Earth. For example, iron loses its ferromagnetism at about 770° while the temperature of the Outer Core/ Mantel boundary is about 3000°.

The Earth's magnetic field appears to arise from complex flows within the molten core of the Earth. (For more detail visit Electromagnetism, The Dynamo Process and Origin of the Earth's Magnetism.) The magnetic field around the Earth has been modeled in a Los Alamos National Laboratory web site and the model is shown below.

A snapshot of the 3D magnetic field structure simulated bye the Glatzmaier-Roberts geodynamo model. A snapshot of the 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo model. Magnetic field lines are blue where the field is directed inward and yellow where directed outward. The rotation axis of the model Earth is vertical and through the center. A transition occurs at the core-mantle boundary from the intense, complicated field structure in the fluid core, where the field is generated, to the smooth, potential field structure outside the core. The field lines are drawn out to a distance equal to two Earth radii.

Plasma FountainThe field line pattern is not as regular and symmetrical as our idealized model. The shape of the field changes significantly with distance from the Earth where the solar wind (see Solar Wind and CME's) from the Sun collides with the Earth's magnetic field.

On the Sun-side the magnetic field is compressed by the solar wind. On the side away from the Sun, the field is stretched. The POETRY web site has activities for modeling the Earth's magnetosphere in "Exploring the Earth's Magnetic Field", Activity III. For more on the Earth's magnetic field go to The Earth's Magnetosphere.

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Last Updated: April 7, 2011