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A
MAGNET IN SPACE
 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  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 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.
 The
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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