Earth’s Magnetospere

Physicist David Southwood on how the magnetic field of Earth causes natural losses of spacecraft, the aurorae and why it’s important to study plasma physics

videos | May 7, 2020

The Earth’s magnetosphere was one of the first discoveries of the Space Age. We knew that the Earth had a magnetic field and we knew it varied a great deal and we knew the Sun controlled the variations in the magnetic field, but really until the Space Age we didn’t know for sure that the Earth lives inside a magnetic cavity called the magnetosphere. The first discoveries of magnetosphere were made with the first spacecraft in 1959 and 1960, a long time ago. We discovered that the Earth’s magnetic field extends out in space and makes a cavity around the Earth, but then puzzlingly enough it’s full of rather unpleasant high-energy radiation. In fact, an early space scientist said, ‘Space is radioactive!’

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So the magnetosphere both protects us but also has this property of containing high-energy radiation that’s potentially dangerous. We have to understand where that radiation comes from and, if you like, how we avoid it having a negative impact first of all on astronauts but also on spacecraft. You could think, ‘Well, spacecraft, they are surely resilient, they’re out in a vacuum, they must be ok’ but no: spacecraft work because of electronics, and high-energy particles can really damage electronics. Indeed, I would say that probably in the 1970s we had a series of losses of spacecraft, one of which could have started World War 3 because it wasn’t known by the Americans. But it was natural damage that had occurred because of the spacecraft being hit by the high-energy radiation and not, let’s say, enemy action from, perhaps, the Soviet Union or some other enemy at the time. Happily, now we do understand things, and we spend a fair amount of time trying to predict what’s going on for exactly that reason.

The science for me is very exciting. When I was a student I had to choose: I wanted to study plasma physics and magnetohydrodynamics. I had two choices: one was to go into the laboratory and work on nuclear fusion because that was the hope of a future electricity supply, it still is; we have big amounts of international activity trying to make a fusion reactor that will be a very clean source of energy. But then the other alternative was to do plasmas in space. For me, I thought space was more exciting. The Earth’s magnetosphere is space very close to home but it gives you the chance with spacecraft to look in detail at processes that could be far off in the cosmos. We have, if you like, astrophysics close to home. For me that’s always been my first love about the magnetosphere of Earth: it has been the physics going on in this fourth state of matter. We know about solids, liquids, gases, and there’s a fourth state called plasma which is very hard to sustain on Earth because it has to have a high temperature.

But actually much of the universe is in the plasma state, so it’s actually normal if you were a truly cosmic being but it’s abnormal if you happen to live on the Earth as we do.

So the Earth’s magnetosphere is the easiest place to get to where it’s occurring naturally and where you can see processes going on. There’s a subtle coupling between the Sun and the Earth through the plasma processes that connect the Sun’s magnetic field with the Earth’s magnetosphere. I worked in a research group at Imperial College which first really opened up the understanding of this, explained how the Earth’s magnetosphere coupled with the Sun.

For me though, the real excitement is again in what the magnetosphere produces just naturally. I myself spent much of my early career working on the complicated waves that are called magnetohydrodynamic waves inside the magnetosphere. The magnetosphere is a closed volume, and so it’s like an echo chamber: there are these complicated waves that get excited when the Sun buffets the magnetosphere of Earth. That made me very happy for the first half of my career I was working in that area.

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Now probably the thing that still remains something of a puzzle is the structure in the aurora, in the northern lights (and the southern lights for that matter). We understand at a basic level how they occur but when I watch them, I just think, ‘It’s so much more complicated than the simple theory suggests’. Simple theories are explaining why you need to go to northern Norway to see it, or to Iceland, or to the North to Novaya Zemlya in Russia, or wherever: you got to go to the very North to see the aurora. But you have the structure of the wave, it’s like curtains in the sky: you see it moving, and you see multiple curtains. I’ve never felt we’ve really cracked the whole problem. It certainly links back to plasma physics occurring far out in the Earth’s magnetosphere.

So I still see the magnetosphere as something we need to go on studying, I still think this is a valid way to spend public money, we should be exploring this. Some people say it’s just because of the impact, because we live in a connected society where we all use electronic equipment, we use phones, televisions, communication systems etc, all of which can be disrupted by magnetospheric weather. That for me is only part of the story. If we still can’t explain the whole system, I still see real basic science to do out there and that basic science by and large I feel has an impact beyond the Earth. My work on magnetohydrodynamic waves had an impact on how we saw waves in the solar atmosphere. Frankly, we’re doing astrophysics close to home. We can put spacecraft inside things we couldn’t dream of anywhere else in the universe, we should go on ensuring we explore this system around us: the Earth’s magnetosphere.

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Emeritus Professor of Physics, Imperial College London
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