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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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