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　　進入專題：2015年聽力風暴來襲，英語四級每日一聽！

　　Most of us rarely think about the earth's magnetic field. We might know that it helps guide birds as they travel and keeps our compasses pointing north. But, the magnetic field is much more. It is one of the main components that make life on the planet possible.

　　However, the processes in the Earth's core that create the magnetic field are poorly understood. Now, scientists at the University of Maryland are working to reproduce the conditions that lie 3,000 kilometers below our feet.

　　Deep inside Earth is a 3,400 kilometer-wide ball of liquid metals -- iron and nickel. In the center is a smaller ball of solid iron. They combine to form what is called Earth's core. The core creates the electric currents responsible for Earth's magnetic field. The most immediate benefit is its part in navigation.

Daniel Lathrop is a physics professor at the University of Maryland. He runs the laboratory where Earth's magnetic field is studied.

　　"And the Earth's main magnetic field is a very important part of what makes the earth a habitable planet because it shields us from a lot of the worst parts of radiation from the sun."

　　Evidence shows that the earth's magnetic field is not fixed in place. The strength of the field changes direction.

　　"What we do know is the earth's magnetic field has reversed north-south hundreds of times in earth's history. And so, the spin of the earth stays the same, but where the magnetic pole is moves and has actually reversed."

　　Mr. Lathrop and his team are trying to find out how and why that happens.

　　The researchers built a 3-meter-wide steel ball that turns. The ball is filled with 12.5 tons of sodium to reproduce the Earth's outer core. Sodium is a soft metal that melts at just under 100 degrees Celsius.

　　A 1-meter-wide inner non-magnetic steel ball is used to represent Earth's solid iron core. Both can be turned independently.

　　But turning them slowly has not created a process known as the dynamo effect. The dynamo effect is thought to be responsible for creating magnetic fields.

　　"We only see magnetic fields when we impose small magnetic fields from the outside. But imposing small magnetic fields from the outside we get a factor of 10 larger magnetic fields induced by the flow. So we have a very effective half speed gain of magnetic field, without what we called the dynamo... Now we're aiming to go full speed."

　　Mr. Lathrop says it is difficult to predict changes that happen over thousands of years.

　　"One second of operation of the experiment models 5,000 years of evolution of the magnetic field. So the goal is to take data from the experiment to do mock predictions and then you can improve the predictions."

　　Mr. Lathrop notes that the strength of Earth's magnetic field has dropped about 10 percent in the last 170 years. This means the reversal process may have already started.

　　But the change is too slow to affect our everyday lives.

　　I'm Jonathan Evans.

　　VOA's George Putic reported this story from Washington. Jonathan Evans adapted it. Caty Weaver was the editor.

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