New theory about the weakening of the Earth’s magnetic field
The Earth’s magnetic field, which protects the planet and its 1,300 satellites against particles from outer space, is becoming weaker and weaker. A DTU researcher may have pinpointed the reason why.
The Earth’s magnetic field has become progressively weaker over the past 175 years, and today it is nine per cent less powerful than it was in 1840. Researchers have long speculated as to the cause of the ongoing decline, but Chris Finlay, Senior Scientist at DTU Space, has now identified a possible explanation: He believes that decrease in the power of the magnetic field is mainly attributable to the liquid metals that are moved around in a gigantic gyre below the Earth’s crust.
The gyre actually consists of huge volumes of liquid iron and nickel, which move around in a whirl in a 7,000-km-thick belt in the Earth’s outer core.
“The movement is incredibly slow—approximately 10 km per year,” relates Chris Finlay, who has been studying the gyre using data from the satellites Ørsted, Champ and Swarm.
“Compared with the speed at which the tectonic plates move, however, this is remarkably fast, and we have been using the satellite data to establish what it means. As it turns out, the gyre controls a huge section of the Earth’s magnetic field—the area known as the north-south-dipolar field,” he continues.
One aspect of particular interest is an area of the gyre beneath the Indian Ocean, which transports material from the South Pole north towards the equator. Movements such as this one weaken the magnetic field, especially given that the section of the gyre that should move material from the equator towards the South Pole—which is located close to South America—is not equally active.
The gyre moves within an area from 2,900 km to around 5,000 km beneath the Earth’s surface, where the inner, solid core begins. The pressure here is around two million atmospheres (equivalent to 400 elephants standing on a stiletto heel), and the temperature is in the region of 4,000–6,000°C. In these conditions, the metals are as fluid as water at room temperature.
A weakened dynamo
The liquid metal acts as a planet-size dynamo, converting kinetic energy (movement energy) into electromagnetic energy, which thus creates the magnetic field. The movements in the liquid metal are controlled—just like the weather above ground—by convection (movement produced by differences in temperature, ed.), and by the rotation of the Earth, which produces what is known as the magnetic dipole.
Over and above the giant gyre, there are numerous small whirlpools that complicate the situation—again, just like the weather above the surface.
One day, the field will become so weak that the dipole will hit zero. Geological surveys provide evidence of hundreds of pole reversals. Over a relatively short period during these reversals, there will be neither north nor south, but the poles will gradually change places and south will become north. Given the speed at which the magnetic field is currently decreasing, Chris Finlay estimates that it will take around 2,000 years for the Earth to reach this point —so we have plenty of time to prepare for the event. However, there are no guarantees that the current rate will continue unchanged.
Accurate forecasts essential
For the present, though, there is every reason to keep a close eye on the magnetic field because we have around 1,300 satellites in orbit above us, and the weaker the magnetic field becomes, the more exposed the satellites will be to radiation from the sun and other sources.
“If you take a look at a map of where the satellites in space are failing, you will see that it’s typically in places where the magnetic field is weakest, because that’s where particles from outer space come closest to the satellites’ orbit and can disrupt them,” explains Chris Finlay.
He goes on to relate that the safety of the satellites is the biggest worry in the context of developments in the magnetic field—and the area where his research finds practical application. The reason for this is that accurate forecasts of changes in the strength of the magnetic field are essential so that allowance can be made for increased radiation when planning new satellite missions many years into the future.
“Our models indicate that the gyre will continue to cause the strength of the magnetic field to fall over the coming decades. But other than that, the models differ —just like terrestrial weather forecasts. We can make fairly accurate predictions for the next couple of days (or, in this case, decades), but things become more difficult the farther forward we attempt to look,” says Chris Finlay.
source : Technical University of Denmark