why do scientists think that earth has a magnetic field
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 Earths 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 Earths core. The core creates the electric currents responsible for Earths 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 Earths 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 Earths 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 Earths 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 Earths 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. Im Jonathan Evans. VOAs George Putic reported this story from Washington. Jonathan Evans adapted it. Caty Weaver was the editor. ______________________________________________________________ Words in This Story benefit n. a good or helpful result or effect component n. one of the parts of something such as a system or mixture; an important piece of something navigation n. the act, activity, or process of finding the way to get to a place when you are traveling in a ship, airplane, car, etc. mock adj. done or performed to look like the real thing reversal n. a change to an opposite state, condition, decision, etc. shield v. to cover and protect someone or something The Earth's magnetic field, which protects us from potentially dangerous solar radiation, is gradually losing its stability. No need to move underground or build space colonies just yet, though: the changes are taking place over millions of years. You might assume that compasses will always point north, but in fact the magnetic poles have swapped places many times in the Earth's history.
Earth scientists have long suspected that these flips are becoming more frequent, and that the magnetic field was less prone to pole reversals in the distant past. Now the most detailed analysis of the geological evidence to date suggests that the field really is slowly destabilising. Whereas in the distant past it reversed direction every 5 million years, it now does so every 200,000 years. Earth's magnetic field is powered by the heart of the planet. At its centre is a solid inner core surrounded by a fluid outer core, which is hotter at the bottom. Hot iron rises within the outer core, then cools and sinks. These convection currents, combined with the rotation of the Earth, are thought to generate a " " that powers the magnetic field. The last major reversal was 781,000 years ago Because of changing temperatures and fluid flows, the strength of the magnetic field varies, and the positions of the north and south magnetic poles shift. These shifts leave traces in rocks. When lava cools, metal oxide particles within the rock become frozen in the direction of the prevailing magnetic field. So scientists can work out the historic positions of the magnetic poles by examining and dating lava samples. As a result we know there have been about 170 magnetic pole reversals during the last 100 million years, and that the last major reversal was 781,000 years ago. So are these reversals becoming more or less common? In theory, it depends on what is happening to Earth's core. Researchers believe the inner core is slowly growing, as the outer core cools and solidifies. That should mean more frequent flips. Simulations by of the University of California, Santa Cruz and his colleagues, suggest that a bigger inner core would be more of an obstruction to currents in the outer core, making for a more unstable magnetic field. But it is hard to verify this, because in older rocks the evidence of magnetic field direction is less well preserved.
So of the University of Helsinki in Finland, assembled a swathe of existing data from rock samples between 500 million and 3 billion years old. First, Veikkolainen weeded out all the less reliable data. For example, he rejected all samples containing hematite, because it can form a long time after the rest of the rock, leading to muddled data. He also left out slow-cooling rocks like granite, and threw out samples that were known to have tilted, unless they could be accurately corrected based on other evidence. Having whittled down around 300 data sets to 55, Veikkolainen estimated. He found that reversals happened about once every 3. 7 million years between 500 million and 1. 5 billion years ago. But in an earlier time, between 1. 5 billion and 2. 9 billion years ago, the magnetic field only flipped once every 5 million years. That is much less often than in the last 150 million years, when the field has flipped every 600,000 years. In the last 10-20 million years it has sped up even more, to once every 200,000-250,000 years. "The evidence points to a more stable field in the very far past and fewer reversals," says Veikkolainen. "This looks to me to be the most thorough study that has been done so far, and it does reinforce many of the conclusions of earlier work, so I think it's pretty good evidence," says of the University of California, Santa Cruz. Are we due for another flip? It's hard to say. Data collected by the European Space Agency's satellite array reveals the Earth's magnetic field has recently been at a rate of around five per cent per decade. The field does change all the time, but a rate of five per cent century is more normal,. It is unclear precisely what would happen if the field weakens greatly or disappears for a time during a reversal. However, scientists believe power grids and communications systems are potentially at risk.
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