Earth's Magnetic Field and its Changes in Time

A compass tells you what direction is 'North', but have you ever wondered how it can do that? The answer has to do with something called MAGNETISM!


Every magnet produces an invisible area of influence around itself. When things made of metal or other magnets come close to this region of space, they feel a pull or a push from the magnet. Scientists call these invisible influences FIELDS. You can make magnetic fields visible to the eye by using iron chips sprinkled on a piece of paper with a magnet underneith.

On the Sun, our nearest star, you can see the same kinds of magnetic fields as they pop out of the surface of the sun. This picture shows the lines of magnetism near a sunspot.

The entire planet Earth would easily fit under the archway formed by the magnetic field loop.

We can see these loops of magnetism on the sun because very hot gases flow along them and light them up!

A compass works the way it does because Earth has a magnetic field that looks a lot like the one in a magnet. The Earth's field is completely invisible, but it can be felt by a compass needle on the Earth's surface, and it reaches thousands of miles out into space.

If you were to take a rocket ship into space and study the Earth's invisible magnetic field, it wouldn't really look like a bar magnet at all. Earth's magnetic field gets stretched out into a comet-like shape with a tail of magnetism that stretches millions of miles behind the earth, opposite from the sun. The sun has a wind of gas that pushes the earths field from the left to the right in the picture. Scientists have studied many different parts of the Earths magnetic field and have given them names so that they can talk to one another about them. Each part is part of the vast 'geomagnetic system' in which many amazing things have been seen and discovered.

In your classroom, you can make a magnetic field by letting a current flow through a piece of wire wrapped around a nail. When you attach the battery, the nail becomes an ELECTROMAGNET and you can use it to lift paperclips.

The core of the Earth is also an electromagnet. Although the crust is solid, the core of the Earth is surrounded by a mixture of molten iron and nickle. The magnetic field of Earth is caused by currents of electricity that flow in the molten core. These currents are hundreds of miles wide and flow at thousands of miles per hour as the earth rotates. The powerful magnetic field passes out through the core of the earth, passes through the crust and enters space. This picture which was created by a computer from a mathematical model, shows the solid inner core region ( inner circle) surrounded by a molten outer core (the area between the two circles). The currents flow in the outer core, and the lines of force shown in yellow, travel outwards through the rest of the earth's interior. If the earth rotated faster, it would have a stronger magnetic field. If it had a larger liquid core it would also have a stronger magnetic field.

By the time the field has reached the surface of earth, it has weakened a lot, but it is still strong enough to keep your compass needles pointed towards one of its poles. All magnets have two POLES: a North Pole and a South Pole.

There is another thing that we know about magnets and magnetism. When you put like poles together ( South facing South or North facing North) they repell each other. You can feel this force of repulsion yourself! When you put un-like poles together (South facing North) you can feel magnetic attraction.

In the Northern Hemisphere, your compass needle points North, but if you think about it for a moment, you will discover that the magnetic pole in the Earth's Northern Hemisphere has to be a South polarity. This is so, because the North-type magnetism of the compass needle has to be attracted by a south-type magnetism in the Earth in order to 'seek out' the north pole. If the Earth's magnetism has a south-type polarity in the Northern Hemisphere, where does it have a north-type polarity? Answer, in the Southern hemisphere!

The magnetic poles of earth are not fixed on the surface, but wander quite a bit as the map shows. The pole in the Northern Hemisphere seems to be moving northwards in geographic latitude by about 10 kilometers per year, but the motion is only an averge.

On any given day, it moves erratically by many tens of meters because of changes in the currents inside earth's core, as well as the influence of electrical currents in the ionosphere, and the changing space environment due to solar storms and winds.

If you were to stand directly over the magnetic pole with a compass, the needle would point straight downward. At other points on the globe, it points horizontally, but if you could observe it carefully, you would also see a slight dip in the needles tip. As you approach the magnetic pole, this dip becomes more and more obvious.

Studies of the Mid-Atlantic Ridge in the Atlantic Ocean half-way between North America and Europe have shown that as the fresh rock cools, it records the polarity of the earth's field. By dating the rocks on either side of the ridge, geologists discovered that the polarity of the Earth's field changes over the course of thousands of years. This was an exciting discoverery that not only verified the theory of Continental Drift, but demonstrated that earth's magnetism isn't constant over millions of years. The magnetic field of earth actually changes its polarity over time. They are called Polarity Reversals, but should not be confused with the rotation axis of earth actually changing.

There have been about 170 of these reversals during the last 76 million years according to geological evidence. The time between reversals seems to be growing longer, and is currently about 300,000 years or so. The last one of these happened about 770,000 years ago (0.77 on the graph). We are currently living during a period that has been called the Brunhes Magnetic Chron when the South Magnetic Pole is in the Northern Hemisphere. During the previous Matumaya Magnetic Chron, the North Magnetic Pole was in the Northern Hemisphere! Note that, from the polarity figure, at a time 0.94 million years ago (940,000 years ago) the magnetic field reverse itself by going nearly to 'zero' but then after a few thousand years it recovered and began to increase in strength. During the next 150,000 years it rose to a maximum strength and then began to decline. Notice, also, how fast the magnetic field recovers after it reaches 'zero', in some cases much less than 10,000 years.

Presently, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate!