# THE MAGNETIC FIELD

## What is Earth's magnetic field?

Electromagnetic force permeates our planet. In essence, Earth itself acts like a giant spherical magnet. This is caused primarily by the motion of electrical currents within Earth, probably through the liquid metallic part of Earth's core. Combined with Earth's rotation, the core acts like an electric dynamo, or generator, creating a magnetic field.

Earth's magnetic field extends thousands of miles outward into space. Magnetic field lines, carrying and projecting electromagnetic force, anchor at Earth's magnetic poles (north and south) and bulge outward, usually in large loops. Occasionally, they stream outward into space. The magnetic north and magnetic south poles of Earth's magnetic field are very close to the geographical north and south poles, which mark the axis of Earth's rotation. (Be careful, by the way. There are two ways to define Earth's magnetic poles—the "magnetic north pole" is on an island in Canada, but the "geomagnetic north pole" is actually on Greenland, and the "geographic north pole" is on an ice shelf floating on the ocean, hundreds of miles from any land.)

## How did people discover that Earth has a magnetic field?

The ancient Chinese were the first to use magnets as compasses for navigation. Though they did not know it, these "south-pointing needles" worked because the magnets aligned themselves with Earth's magnetic field. Since Earth's magnetic poles have been very close to the rotational north and south poles, compasses point almost exactly north and south in most parts of the world.

Over time, scientists started making a connection between lodestones (permanent magnets) and the nature of Earth itself. The English astronomer Edmund Halley (1656-1742), for example, spent two years crossing the Atlantic on a Royal Navy ship, studying Earth's magnetic field. Later, the German mathematician and scientist Karl Friedrich Gauss (1777-1855) made important discoveries about how magnets and magnetic fields work in general. He also created the first specialized observatory for the study of Earth's magnetic field. With his colleague Wilhelm Weber (1804-1891), who was also famous for his work with electricity, Gauss calculated the location of Earth's magnetic poles. (Today, a unit of magnetic field strength is called a gauss in his honor.)

## Why is Earth's magnetic field important to life on Earth?

Earth's magnetic field extends out into space, creating a structure called a magnetosphere, which surrounds our planet. When the magnetosphere is hit by charged

A graphic from NASA depicts the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission to study the Earth's magnetic fields (blue lines) and substorms (aurora that result from intense space storms).THEMIS's orbit changes to varying degrees in order to better pinpoint the location of substorms. (NASA)

particles from space, such as from the solar wind or from a coronal mass ejection, it deflects these particles away from Earth's surface, significantly reducing the amount that strikes life forms down on Earth's surface. This protects us from the hazards of being hit by too many such particles.

## What role did Walter Maurice Elsasser play in discovering the magnetic field?

German-born American physicist Walter Maurice Elsasser (1904-1991) was the scientist who discovered that the Earth's magnetic field is generated as a result of how the planet's hot core acts like a dynamo. He also brilliantly discovered that analysis of rock particles reveal clues about the orientation of the Earth's magnetic field over time, which in turn contributed to the science of plate tectonics and the history of Earth's climate.

## How does the magnetosphere help animal behavior on Earth?

Earth's magnetic field is also very important to animals on Earth that migrate or otherwise travel long distances. Some animals have impressive built-in magnetic sensors; biologists have shown that many migratory birds figure out where to fly by using Earth's magnetic field to guide them. Humans, too, benefit from the magnetosphere by using magnetic compasses to figure out which way is north or south.

## How strong is Earth's magnetic field?

On typical human scales, it is pretty weak; at Earth's surface, it is about one gauss in most places. (A refrigerator magnet is typically 10 to 100 gauss.) However, the

How do we know Earth's magnetic field can flip upside down?

In 1906 French physicist Bernard Brunhes (1867-1910) found rocks with magnetic fields oriented opposite to that of Earth's magnetic field. He proposed that those rocks had been laid down at a time when Earth's magnetic field was oriented opposite to the way it is today. Brunhes's idea received support from the research of Japanese geophysicist Motonori Matuyama (1884-1958), who in 1929 studied ancient rocks and determined that Earth's magnetic field had flipped its orientation a number of times over the history of our planet. Today, studies of both rock and the fossilized microorganisms imbedded in the rock show that at least nine reversals of Earth's magnetic field orientation have occurred over the past 3.6 million years.

The exact cause of the polarity reversal of Earth's magnetic field is still unknown. Current hypotheses suggest that the reversal is caused by Earth's internal processes, rather than external influences like solar activity.

energy in a magnetic field depends strongly on its volume; so since the field is bigger than our entire planet, overall, Earth's magnetic power is formidable.

## Does Earth's magnetic field ever change?

Yes, the magnetic field is constantly changing, though very slowly. The magnetic poles actually drift several kilometers each year, often in seemingly random directions. Over thousands of years, the strength of the magnetic field can go up and down significantly. Even more amazing, Earth's magnetic field can reverse directions—the north magnetic pole becomes the south magnetic pole, and vice versa. According to scientific measurements, our planet's magnetic field last had a polarity reversal about 800,000 years ago.

## What will happen when Earth's magnetic field flips upside down?

Probably not much will happen to our daily lives when Earth's magnetic field undergoes a polarity reversal. Measurements over the years show that there has been about a six percent reduction in the strength of Earth's magnetic field in the past century, so some scientists think that a polarity reversal on Earth will likely happen sooner rather than later. Some non-scientific hypotheses have been put forth, suggesting that there will be an environmental catastrophe as a result. There is no scientific reason to believe, however, that such disasters will occur.

## Do any other objects in the solar system have magnetic fields that flip upside down?

Yes, all planets and stars with magnetospheres are thought to undergo magnetic polarity reversals. The Sun, for example, undergoes a magnetic field polarity reversal every 11 years. Astronomers can see and study this effect in other astronomical bodies, and from them, learn more about the changes in Earth's own magnetic field.