Download Earths-Magnetic-Field

INTRODUCTION
Have you ever used a compass
like the car compass in the
figure below? The pointer in a
compass always points north
because Earth is a giant
magnet.
A compass pointer is aligned by Earth’s magnetism to point north.
MAGNET EARTH
Imagine a huge bar magnet
passing through Earth’s axis, as
illustrated in the figure below.
This is a good representation of
Earth as a magnet. Like a bar
magnet, Earth has north and
south magnetic poles and a
magnetic field.
Earth is like a giant bar magnet.
EARTH’S MAGNETIC POLES
Although a compass always points north, it doesn’t point to
Earth’s geographic north pole, which is located at 90° north
latitude (see the figure below). Instead, it points to Earth’s
magnetic north pole, which is located at about 80° north latitude.
Earth’s magnetic south pole is also located several degrees of
latitude away from the geographic south pole.
Earth’s magnetic north pole is close to the geographic north pole.
A compass pointer has north and south poles, and its north pole
points to Earth’s magnetic north pole. Why does this happen if
opposite poles attract? Why doesn’t the compass needle point
south instead? The answer may surprise you. Earth’s magnetic
north pole is actually the south pole of magnet Earth! It’s called
the magnetic north pole to avoid confusion. Because it’s close
to the geographic north pole, it would be confusing to call it the
magnetic south pole.
EARTH’S MAGNETIC
FIELD
Like all magnets, Earth has a magnetic
field. Earth’s magnetic field is called the
magnetosphere. It is a huge region that
extends outward from Earth for several
thousand kilometers but is strongest at the
poles. You can see the extent of the
magnetosphere in the figure below.
The magnetosphere extends outward from Earth in all directions.
MAGNETIC FIELD REVERSALS
Do you like to read science fiction? Science fiction writers are really creative. For example, an author might
write about a time in the distant past when compasses pointed south instead of north. Actually, this idea
isn’t fiction—it’s a fact! Earth’s magnetic poles have switched places repeatedly over the past hundreds of
millions of years, each time reversing Earth’s magnetic field. This is illustrated in the figure below.
We think of today’s magnetic pole orientation as “normal” only because that’s what we’re used to.
Scientists don’t know for certain why magnetic reversals occur, but there is hard evidence showing that
they have occurred. The evidence comes from rocks on the ocean floor. Look at the figure below which
shows a ridge on the ocean floor. At the center of the ridge, hot magma pushes up through the crust and
hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in
place forever. The newly hardened rock is then gradually pushed away from the ridge in both directions as
more magma erupts and newer rock forms. Rock samples from many places on the ocean floor reveal that
magnetic domains of rocks from different time periods are aligned in opposite directions. The evidence
shows that Earth’s magnetic field reversed hundreds of times over the last 330 million years. The last
reversal was less than a million years ago. What might happen if a magnetic reversal occurred in your
lifetime? How might it affect you?
The alignment of magnetic domains in rocks on the ocean floor provide evidence for Earth’s magnetic reversals.
WHY IS EARTH A
MAGNET?
The idea that Earth is a magnet is far from new. It was first proposed in
1600 by a British physician named William Gilbert. However, explaining
why Earth acts like a magnet is a relatively recent discovery. It had to
wait until the development of technologies such as seismographs,
which detect and measure earthquake waves. Then scientists could
learn about Earth’s inner structure (see the figure below). They
discovered that Earth has an inner and outer core and that the outer
core consists of liquid metals, mainly iron and nickel. Scientists think
that Earth’s magnetic field is generated by the movement of charged
particles through the molten metals in the outer core. The particles
move as Earth spins on its axis.
Charged particles flow through Earth’s liquid outer core, making Earth a giant magnet.
BENEFITS OF EARTH’S
MAGNETIC FIELD
Earth’s magnetic field helps protect Earth and its organisms from harmful
particles given off by the sun. Most of the particles are attracted to the north and
south magnetic poles, where Earth’s magnetic field is strongest. This is also
where relatively few organisms live.
Another benefit of Earth’s magnetic field is its use for navigation. People use
compasses to detect Earth’s magnetic north pole and tell direction. Many
animals have natural "compasses" that work just as well. Birds like the garden
warbler in the figure below use Earth’s magnetic field to guide their annual
migrations. Recent research suggests that warblers and other migrating birds
have structures in their eyes that let them see Earth’s magnetic field as a visual
pattern.
The garden warbler flies from Europe to central Africa in the fall and returns to Europe in the spring. Its internal
“compass” helps it find the way.