4 minute read
Earth’s magnetic fi eld
SPACE
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Earth’s magnetic fi eld
Just where did our protective magnetosphere come from?
Before we can understand how Earth’s magnetic fi eld works, we need to fi rst have a basic understanding of magnetism. Magnetic fi elds are formed when electric charges move through magnetic materials like iron.
Any magnetised material is dipolar, which means it has a north and south pole, and the magnetic fi eld lines run from north to south. The magnetic fi eld lines at the north pole swing back round to the south pole, creating an external magnetic fi eld outside the material that can infl uence other things that get too close.
You’re probably familiar with a bar magnet, and in essence Earth’s magnetic fi eld is very similar to that; imagine a giant bar magnet running through the core of Earth from pole to pole and you’ll get the picture. However, Earth’s core is molten, so our planet’s magnetic fi eld is induced by a circulating electric current at the core. One of the outcomes of this is that, on rare occasions, Earth’s magnetic fi eld can fl ip. This is believed to happen once every 200,000 years on average.
Taking the ‘bar magnet’ through Earth analogy further, it just so happens that the south pole of Earth’s magnetism is at the geographic north pole, and the north pole is at the geographic south pole. When someone refers to ‘magnetic north’, they’re actually referring to the south pole of Earth.
Earth’s magnetic fi eld is also not perfectly aligned with the rotation of the planet, instead being tilted at an angle of 11 degrees. It’s also not stationary; the magnetic poles are constantly moving, and indeed the south magnetic pole (at geographic north) has drifted up to 1,100 kilometres (684 miles) across the Canadian Arctic over the past four centuries.
Interestingly, though, despite the size of Earth, the magnetic fi eld is weaker than a fridge magnet. However, that’s still enough to protect us from harmful radiation from the Sun and elsewhere in the galaxy, and helps our planet retain its atmosphere.
Core
Earth’s liquid core creates an electric current that in turn forms a magnetic fi eld around the planet.
Solar wind
Incoming solar radiation is defl ected by Earth’s magnetic fi eld, protecting us from harm.
Weakest point
The magnetic fi eld is weakest at the equator where the fi eld lines are most spread apart.
Strongest point
The magnetic fi eld is strongest near the poles where the fi eld lines converge.
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DID YOU KNOW?
The last geomagnetic reversal of Earth’s magnetic field is believed to have occurred 41,000 years ago
Polarity
Geographic north pole is actually south polarity in terms of magnetism, and vice versa.
Tilt
Earth’s magnetic fi eld is actually tilted at an angle of 11 degrees from the spin axis of the planet.
Origin of the magnetic fi eld
Future of the magnetic fi eld
As already mentioned in the main text, Earth’s magnetic fi eld is the result of moving electric fi eld in the liquid molten iron core. Compared to the surface, the magnetic fi eld at the core is about 50 times stronger.
It’s likely that Earth has had a magnetic fi eld for pretty much the entirety of its 4.5 billion-year lifetime. However, when Earth fi rst formed, it’s likely that the entire core was liquid; at the moment, only the outer core is liquid, with the inner core being solid due to the intense pressure. This means that Earth’s early magnetism was likely stronger than it is now. Exactly how much stronger we can’t be sure, but it’s believed this strong magnetic fi eld helped Earth retain an atmosphere early in its life, in the opposite way that Mars has lost its atmosphere as its magnetic fi eld has dissipated.
Earth’s magnetic fi eld is weakening, but the exact reason why is poorly understood. However, this is no cause for concern; records suggest it decreases and increases in intensity relatively frequently. Since German mathematician Carl Friedrich Gauss fi rst measured its strength in 1845, it has dropped about ten per cent.
If the magnetic fi eld drops signifi cantly further, there is a chance the magnetic fi eld could fl ip. Contrary to popular belief, however, this will not signify the end of the world. The magnetic fi eld has been known to fl ip many times over the last billion years, and life has survived. Therefore it’s unlikely another fl ip would cause any devastating effects.
The only true danger is if the magnetic fi eld were to disappear completely. As long as Earth has a liquid core, though, it will continue to have a magnetic fi eld. Unless you’re still around in a few billion years when such an event could occur, you haven’t got much to worry about.
Magnetosphere
The magnetic fi eld creates a protective cone of infl uence known as a magnetosphere around our planet.
