VOA常速英语:Scientists Take on Earth’s Magnetic Field

WASHINGTON—Most of us hardly ever think about the Earth's magnetic field. We might be aware that it helps guide birds as they migrate and ensures that our compasses point north, but not realize that it is one of the crucial components that make life on Earth possible. Yet, the processes in the Earth’s core that create the magnetic field are poorly understood.

At a lab of University of Maryland, scientists are trying to replicate the conditions that reign 3,000 kilometers below our feet.
 
Deep inside our planet, a 3,400 kilometer-wide sphere of molten iron and nickel, with a smaller solid iron ball at its center, creates the electric currents responsible for Earth’s magnetic field.
 
The most immediate benefit is its role in navigation.
 
But there’s more, says University of Maryland physics professor Daniel Lathrop.
 
“It’s an important part of what makes the Earth a habitable planeta because it shields us from a lot of the worst radiation from the sun,” he said.
 
Evidence shows that the Earth’s magnetic field is not stationary. It wanders around and sometimes even ventures [expands?] far away.
 
“We do know the Earth’s magnetic field has reversed north-south hundreds of times in Earth’s history," said Lathrop. "Of course, the spin of the Earth stays the same, but where the magnetic pole is - moves and has actually reversed.”
 
In their laboratory, funded by the National Science Foundation, Lathrop and his team are trying to find out how and why that happens.
 
Spinning a ball of molten iron being somewhat impractical, researchers built a suitable substitute: a 3-meter wide spinning steel ball filled with 12.5 tons of sodium, a soft metal that melts at just under 100 degrees Celsius.
 
A 1-meter-wide inner non-magnetic steel sphere substitutes for the solid core. Both can be rotated independently.
 
Spinning them slowly has not created the so-called dynamo effect, thought to be responsible for creating magnetic fields.
 
“We only see them when we impose small magnetic fields from the outside. But imposing small magnetic fields from outside we get a factor of 10 larger magnetic fields induced by the flow. We get large gain without the so-called dynamo. Now we’re aiming to go full speed,” said Lathrop.
 
Will these experiments prepare us for the possible reversal of the Earth’s magnetic field? 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,” he said.
 
Lathrop notes that the intensity of our planet’s magnetic field has dropped about 10 percent in the last 170 years, so the reversal process may have already started.
 
Fortunately for us, the change is too slow to affect our regular lives.