The Earth´s Magnetic Field
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Source: CCTV.com
01-30-2007 07:56
As we all know, the Earth is the only one of the nine planets in our solar system that supports life. And nearly 2 million species inhabit the Earth. Water, the right temperature and the atmosphere are all factors supporting life. Perhaps less well-known is the role played by the Earth’s magnetic field.
Human beings have developed instruments and identified landmarks that help them travel across the world. But how do the sea turtles navigate across the vast expanses of featureless ocean, to reach their feeding and breeding sites?
The green sea turtles perform one of Nature’s longest migrations. In swimming back to the islands of its birth, this green turtle is undertaking an incredible journey; one that started from this same beach several years ago. At the very beginning of their life’s journey, the hatchlings make straight for the ocean, guided by the bright moonlit sky.
Once in the ocean, they know exactly where they are going. Over the next 20 years, they will travel approximately 10,000 miles, to their breeding grounds, and then back again to precisely the same beach where they were hatched. But how they achieve this remarkable feat of navigation has long been one of the great mysteries of the seas. Evidence uncovered recently, suggests that the sea turtles rely at least partly upon an invisible map of landmarks created by the Earth’s magnetic field.
Scientists now believe that magnetite in a turtle’s brain works as a magnetic compass, in just the same way as compass needles work for us. New research shows that sea turtles can detect and distinguish among the magnetic fields in different locations, enabling them to compile a "magnetic map" in their brains, by which they navigate to specific feeding and nesting areas. Even after 20 years, this map will lead them back to their birthplace. So when a female green sea turtle, once she has matured, swims back, she is effectively using her memory.
Research has shown that dolphins and whales use the same system in getting about. In fact, the Earth’s magnetic field provides landmarks for most forms of marine life.
Sharks are one of nature's ultimate designs, tested over 400 million years — confident, sleek and lethal. Studies show some sharks are sensitive to a change in an electric current as tiny as five-billionths of a volt – and to the magnetism it produces. They use this ability to prowl the ocean to seek out their prey – even if it is hidden under sand.
The mole is not a migratory animal. It lives in underground tunnels extending for up to 200 metres. At the end of a tunnel, the mole will build its nest. And it is always accessible only from the south. And if the direction of the magnetic field is changed, the mole will realign his nest.
The origin of the Earth's magnetic field is still not completely understood. But it is thought to have something to do with the electrical currents generated by the spinning of molten iron and nickel in the outer part of the Earth’s core.
The magnetic field reaches out and surrounds the planet. It extends from the magnetic poles, forming semi-circular loops between them, which are the Earth’s magnetosphere.
Many species of animals possess a magnetic sense. They use the magnetic field to orientate themselves when, for example, in times of danger they have to locate the direction of the nearest place of safety.
Perhaps a better-known phenomenon is the migration of birds. Using magnetic clues and the position of the stars and sun, they can find their way south in the autumn and north in spring.
Thus a snow goose can fly the 1800 miles from Greenland to Islay in Scotland. Other migratory birds, such as the House Martin, can migrate – sometimes alone – over such an extremely long distance as that from Africa to Europe. There are no signposts in the endless Sahara or the vast Mediterranean. They rely on the sun and the magnetic field to make sure of their route.
Pigeons are well known for their homing ability. It is common for them to find their way home over distances of 1500 miles. Research shows that pigeons have the ability to detect the angles of the magnetic force lines that surround the earth rather like the magnetic field around a bar magnet. They use these force lines to identify the magnetic poles, and are thus guided on their journey, especially in bad weather. If these lines ever disappear, the pigeon will become confused, even desperate.
In 1988, most of the 5000 pigeons flying from France to England in a race, lost their way. Subsequent investigation revealed that the magnetic field in the area had been badly disrupted, by a well-known but rarely seen natural phenomenon. .
In summer nights, people living in the polar regions are fortunate to have an occasional sight of the aurora – brilliant curtains of light in the night sky. For centuries this phenomenon filled those who witnessed it with awe and wonder. Serious study of the aurora was begun only a hundred years ago, and now scientists have uncovered its secrets.
What they have revealed is that the auroras, though undoubtedly beautiful, pose hidden dangers for our planet.
The fact is that the Earth’s magnetic field, apart from serving as a basic condition for life, also shields most of the inhabited parts of the planet from charged particles emanating from the sun.
The magnetic field acts like a cloak, protecting the Earth from these solar events. But some particles do reach Earth. The auroras are produced by blasts of solar particles arriving at the poles. Since the particles are charged, when they come into contact with the Earth’s magnetosphere, they make the gases in the upper atmosphere fluoresce, like a fluorescent light. Depending on the amount of energy in the air molecules, the aurora displays will glow with different colours, although green is the most common.
Certain effects of the auroras can be quite unpleasant. Particle streams can influence the magnetic field, which may disorientate animals using it in their migration. They may also damage satellite and radio communications, impair navigation systems, and in extreme cases even cause electrical blackouts. Such incidents have been known to happen in Canada and Northern Europe, where the auroras are more common than in other parts of the world.
And they are becoming more frequent and are being seen in more places. Which raises questions about the Earth’s magnetic field, and its ability to continue protecting us.
When molten lava erupts onto the Earth's crust and hardens, it creates a snapshot of the Earth's polarity, in much the same way that iron filings on a piece of cardboard align themselves to the field of a magnet held beneath it. Such "magnetic fossils" document how the magnetic field has changed during the history of our planet.
In certain areas of the planet, such as the United States and the southern Antarctic, the intensity of the magnetic field is fading quickly. The fact is, the magnetic poles are moving, at a rate that has increased by 50 percent in the past few years. This means that, at the current rate of 86 metres every day, the north pole will arrive in Siberia in 50 years.
Research has shown that the strength of the Earth’s magnetic field has decreased by 10 percent over the past 150 years. This raises concerns that it may collapse, or that its polarity may be reversed.
The possibility is a remote one, but a number of abnormal phenomena are suggesting that it may yet happen.
Humpback whales are known to use their magnetic sense in moving between the warm tropical or subtropical waters where they spend the winter, and the polar regions where they pass the summer. Inevitably, some lose their way. But in the past decade, the number of whales stranded on beaches has doubled, and scientists think they may have been confused because of the weakening of the magnetic field.
This weakening, evident since 1845, may be just the latest in a series of lulls – so-called excursions – in the magnetic field that have been known to last several centuries. But some scientists speculate that the changes may be far more significant.
The Earth's geological history shows that the magnetic field goes through a reversal, or a flip, on average about once every 200,000 years. However, the time between these flips varies considerably. The Earth would seem to be well over due for another magnetic flip, since the last one occurred way back, 780,000 years ago.
The fact is nobody really knows what’s going on – although there are already signs of what may be in store for our planet, in the event of the worst happening.
Over the southern Atlantic Ocean, the weakening of the magnetic field has diminished its effect as a shield protecting the Earth from the natural radiation bombarding our planet from outer space.
In what is known as the South Atlantic anomaly, the signals from satellites passing over the region are from time to time interrupted.
Even the Earth’s atmosphere is at risk. Without magnetic protection, the solar wind emitted by the Sun may reach the atmosphere, and this raises the frightening prospect of the atmosphere being stripped away – although that won’t happen any time soon, more likely in a few billion years. On Mars, the failure of the magnetic field several billion years ago led to its atmosphere virtually being boiled away.
But whatever may be happening way up in our atmosphere, there’s little evidence that life on Earth is being affected. Not yet, at least. It’s summer, and the next generation of turtles are about to set out on their life’s great journey.
Editor:Lin Lin