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Otago University Space Physicist Associate Professor Craig Rodger explains what a coronal mass ejection (CME) is. He then goes on to describe the impact such an event could have on the Earth’s magnetic field. Some of these impacts at ground level could induce rogue electric currents in electrical transmission lines, disrupting electrical supply.


There is a lot of terminology in space physics, there’s a lot of jargon, and one of the cool terms that are out there is a CME. It doesn’t sound like a cool term, but bear with me. It stands for coronal mass ejection – CME. It all starts from the Sun – most interesting things start from the Sun.

So the Sun is a huge ball of plasma, it’s bigger than you can imagine, and every now and again there is an explosion just under the surface of the Sun, and that explosion just under the surface of the Sun can [makes sound effect], throw out a huge mass of plasma, and that’s a coronal mass ejection. It’s millions of tonnes of matter being ejected from the Sun and spat out into space.

If the CME is directed towards Earth, then we’re interested. You’ve got this huge cloud of plasma – imagine a gigantic plasma pillow coming out at 800 kilometres a second from the Sun towards Earth. Now that cloud of plasma has embedded in it the magnetic field of the Sun, it’s actually still attached to the magnetic field of the Sun, 150 million kilometres away from the Earth, and when it arrives at the Earth, it strikes the Earth’s magnetic field. It never gets to the Earth’s atmosphere directly, and the reason it never gets to the Earth’s atmosphere directly is that we have a magnetic field around the Earth and the CME has a magnetic field in that – the magnetic field lines won’t cross. But nonetheless, the CME strikes, and it squeezes the Earth’s magnetic field – it’s just like being hit with a hammer, I suppose, you will end up compressing the thing that you hit. So this 800, 850 kilometre a second, million tonne mass strikes the magnetic field of the Earth, and the magnetic field of the Earth contracts and squeezes down.

When you’re squeezing the magnetic field, that means you’re changing magnetic fields. Physics tells you that a changing magnetic field will induce currents in conductors, and the Earth is a conductor. A changing magnetic field induces a current in the Earth that we’re not expecting, and those currents travelling in the Earth’s surface can end up getting into the technological infrastructure that we have, getting into the electrical lines, in the electrical network. When it gets to the transformers that are at the end of the wire, the transformer either becomes damaged, or in the really, really bad case, it burns out.

Associate Professor Craig Rodger, University of Otago, Department of Physics

NASA Earth Observatory
NASA/Goddard Space Flight Center
U.S. Geological Survey

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