The collapse of the Earth's magnetic field--which guards the planet and guides many of its creatures--appears to have started about 150 years ago, the New York Times reported last week. The field' s strength has decreased by 10 or 15 per cent so far and this has increased the debate over whether it signals a reversal of the planet' s lines of magnetic force. During a reversal, the main field weakens, almost vanishes, and reappears with opposite polarity (极). The transition would take thousands of years. Once completed, compass needles that had pointed north would point south. A reversal could cause problems for both that rely on the magnetic field for navigation would find migration confusing. But experts links between past field reversals and species extinctions. Although a total transition may be hundreds or thousands of years away, the rapid decline in magnetic strength is already affecting satellites. Last month, the European Space Agency approved the world's largest effort at tracking the field's shifts. A group of new satellites called Swarm is to monitor the collapsing field with far greater precision. "We want to get some idea of how this would evolve in the near future, just like people trying to predict the weather," said Gauthier Hulot, a French geophysicist working on the satellite plan. "I'm personally quite convinced we should be able to work out the first predictions by the end of the mission." No matter what the new findings, the public has no reason to panic. Even if a transition is coming on its way, it might take 2,000 years to mature. The last one took place 780,000 years ago, when early humans were learning how to make stone tools. Deep inside the Earth flow hot currents of melted iron. This mechanical energy creates electromagnetism. This process is known as the geophysical generator. In a car's generator, the same principle turns mechanical energy into electricity. No one knows precisely why the field periodically reverses. But scientists say the responsibility probably lies with changes in the disorderly flows of melted iron, which they see as similar to the gases that make up the clouds of Jupiter.
单选题
According to the passage, the Earth's magnetic field has______ A. misguided many a man and animals B. begun to change to it's opposite direction C. caused the changes on the polarities D. been weakening it's strength for a long time
单选题
During the transition of the Earth's magnetic field______ A. compass will become useless B. man and animals will be confused in directions C. the magnetic strength of the Earth will disappear D. the magnetic strength of the Earth will be stronger
单选题
The author says "... the public has no reason to panic" because______ A. the transition is still thousands of years from now B. the transition can be precisely predicted by scientists C. the process of the transition will take a very long time to fnish D. the new transition will come 780,000 years later
单选题
The cause of the transition of the Earth's magnetic field comes from______ A. the movement deep inside the Earth B. the periodically reverses of the Earth C. the force coming from outer space D. the mechanical movement of the Earth