The Science of Solar Flares

You probably already know that the sun is, ultimately, the source of almost all life on Earth - but sometimes the Sun causes havok!  Stuart Clarke is the author of "The Sun Kings" - The story of Richard Carrington and the Birth of modern astronomy, and he joins us in the studio now.

Carrington observed something now called the Carrington Flare in 1859 - but observations of the Sun have come a long way in those 150 years.  Chris Davis from the Rutherford Appleton Laboratory joins us to tell us a bit more about how we study the Sun, including the innovative STEREO programme - using two spacecraft to get stereoscopic, so 3D, images of the Sun...

Chris - First of all, let me introduce you to Stuart. Hi Stuart. Thank you for coming in.

Stuart - It's my pleasure.

Chris - Tell us about this story. We view the sun as this highly predictable and stable thing. You get up in the morning and it's always there. The evidence is, it's not as stable as we might think.

Stuart - Indeed. The sun is a highly variable cauldron of the most intense magnetic activity. If you go back in history you can find times when there have been extraordinary events that have taken place on the sun via its magnetic field. The Sun Kings is about what appears to be the most violent of these storms in history. That took place on September the 2nd, 1859.

Chris - What happened?

Stuart - Well, imagine this: two thirds of the Earth's skies erupt with the most incandescent, blood-red aurora. The kind of things that you see in the sky usually when you go up to high polar latitudes.

Chris - So the whole sky just goes bright red like it's made of blood?

Stuart - In this case, yes. That's what most of the eye-witness reports suggest. Worse than that is there seem to be bolts of white light flaring upwards and exploding with silent brilliance in the heavens. It was just the most amazing cataclysm that engulfed most of the Earth's skies.

Chris - This was the 1800s. People must have thought the Earth was coming to an end?

Stuart - They had absolutely no idea what was going on, at all. There was a sinister side to this story as well. At the time that the aurora lit the sky the telegraph network across the world went down. It stopped functioning and it stopped functioning in the most spectacular of fashions. Electrical currents surged along the wires into the offices. The sparks flew from the equipment, offices were set on fire, some of the operators were stunned unconscious. The whole of the telecommunications network (as the telegraph was at the time) was taken down. At the same time, every compass on the Earth went haywire. In an instant and for reasons unknown to anybody global communications and navigations just stopped.

Chris - Did people have any clue as to what was going on? There must have been massive speculation. People must have thought, 'the end of the world is nigh,' here.

Stuart - One person was in the right place at the right time. The kind of coincidence you couldn't get away with writing in fiction. That person was an English amateur astronomer called Richard Carrington. He was working in his observatory at Redhill and he was engaged in his mission for a full 11 years and chart the comings and goings of dark blemishes on the surface of the sun, called sunspots. It was known that the more sunspots there were on the sun, the more unreliable the compass readings were. There was this clue that somehow the sun was magnetic and that magnetism could reach out across space and affect the Earth,

Chris - So how did Carrington connect this amazing event that you're describing with the sunspots that were going on?

Stuart - What happened was that, just before noon, on the 1st of September he was looking at a massive sunspot about ten times the diameter of the Earth. He saw two brilliant beads of white light appear above the sunspot. He realised that this kind of explosion, for want of a better word, had certainly not been charted or recorded before. As he traced this it lasted for a few minutes, it passed across the top of the sunspot and then disappeared. He went to try and find other astronomers to see if they'd seen something like this. Although he couldn't find instantly anybody else who had seen it, at the Kew observatory in Richmond they had magnetic needles studying the Earth's magnetic field and at exactly the moment that Carrington saw his flare so the magnetic needles had jumped. It was just as if the Earth's magnetic field had been struck by some mighty fist or it was like a bell being struck by a hammer and it was still ringing. That night the magnetic needles continued to jitter and get stronger and then suddenly the skies burst into this aurora.

Chris - How long did this phenomenon last for? Boats not knowing where they were going, people getting electrocuted, offices catching fire?

Stuart - The Earth had been in the grip of magnetic squalls, for want of a better word, on and off for the best part of a week. This one lasted for over 24 hours.

Chris - So Chris Davies, from the Rutherford Institute, what was going on this time and what else was happening?

Chris D - It sounds astonishing, doesn't it? Thanks Stuart, for giving me the most exciting I've ever had, I think! What happens, we now know, is that the sun has a magnetic field but the sun is a fluid. So unlike the Earth which is a solid body as the sun rotates it churns and twists that magnetic field with it. The sun also has a stream of particles coming away from the sun called the solar wind. Part of the sun is blowing out into space the whole time. When you twist and wrap up the magnetic field, which is what's happening during the solar cycle, you actually store energy in the magnetic field because you're stretching it. It's like winding a propeller on a rubber band on a model aircraft. You're storing energy by twisting it and stretching it. When that magnetic field is twisted and contorted and can take no more the magnetic field reconfigures in some way. That can release energy in the form of light which is the flare. The flare is given off by particles as they're accelerated very rapidly during that process. It flings a very hot, energised, electrified gas into space along with the sun's magnetic field. Of course, if that arrives at the Earth it can interact with the Earth's magnetic field and allow that material to interact with the Earth's atmosphere which is what causes the aurora. At the same time electrical particles flow in the upper atmosphere because there's very little resistivity in the upper atmosphere and so the particles can flow very readily. Those induced currents in the telegraph wire that are on the ground and that causes surges of electricity which we've heard caused such a disaster at the time.

Chris - Stuart, as people experienced this how did their understanding build from 1859 when this event happened so they began to get towards understanding what they'd experienced?

Stuart - Well, it was a long process and many of the people at the time - their first thoughts were just utter disbelief. They had become extremely used to the idea that gravity was the only force that could really communicate itself across space. Here was what seemed like the suggestion that magnetism could do the same. This was flare, the Carrington Flare and the subsequent magnetic storm was seen before they even had a working theory of electromagnetism. That came along with James Clark Maxwell and here's the fascinating point about this: Maxwell's field theories seemed to prove categorically, once and for all, that this magnetic energy couldn't come from sun. The sun couldn't radiate enough energy to cause the aurora and the movement of the compass needles that were being seen on the Earth. That only became unpacked in the decades of the early 20s when people started to realise that particles could carry electrical or magnetic fields and that you didn't have to radiate huge waves of the energy all throughout space. In fact, you could separate in specific directions like little magnetic cannon balls.

Chris - Have we had anything similar to this happening since?

Chris D - That's certainly the biggest storm. There is another storm more recently though, 1989, I think. There was a storm which had similar effects: large auroral disturbances and induced currents. Not in the telegraph system of course because by this time we're not using the telegraph. We did have a national grid and those countries such as Canada and Alaska whose national grid stretches over large areas have long cables. It induced currents in things which again blew up transformers and large areas of Quebec, for example, were without electricity for many days.

Rayce Stepanovic - How do these pulses do the damage that they do?

Chris D - The reason is that when you have these magnetic fields releasing it releases an intense storm of particles. It's called a coronal mass ejection to scientists which is a very dull name for anything. Actually it's a billion tonnes of material travelling at a million miles an hour. It contains about 100 times the energy of the world's entire nuclear arsenal. Although it's spread out over an extremely wide region in space if that magnetic field is in the opposite sense than the magnetic field on Earth then we all know from school that magnets of opposite polarities attract. Magnets of similar polarities repel. If they're opposing polarity the two magnetic field can interact and that allows all this hot material to interact with the Earth's atmosphere .

Chris - Stuart, something similar happened in 2003, didn't it? That time, luckily, it missed us.

Stuart - Yes, there were the Halloween Flares of 2003.

Chris - How appropriate!

Stuart - It really was. It took place over about a 14 day period. There was one flare on November the 4th which was colossal. Luckily it happened to be pointed virtually away from us in space. There were two flares a little bit earlier on October the 28th and October the 30th. They were large and they were pointed towards the Earth. They were perhaps five times smaller than say the Carrington Flare but it was the first time the civil aviation authorities actually diverted aircraft just in case, as a precaution.

Chris - Aircraft were brought down into a lower altitude, for example?

Stuart - Yes they were brought down into lower altitude so there was more air about them to act as a cushion. Also they were brought away from the polar regions. In the polar regions that's where the Earth turns back into the planet and the core where it's generated. That's where collar particles from these ejections can be funnelled into the atmosphere. That's what causes the aurora so the aircraft are brought away, just as a precaution.

Paul Taylor - I understand the sun gets its power from hydrogen fusion and gradually fusing together bigger and bigger atoms. Towards the end of its life and particularly if it goes supernova it blasts all of the heavier elements out into the surrounding space. I've often heard it said it's what feeds the next generation of stars. Is it the spent star fuel that goes on to create the new star?

Chris D - It certainly happens if you burn hydrogen to form helium the star is the consequence of an equilibrium of forces. You've got the gravitational collapse of that body of gas, pulling inwards on it and heat generated by the nuclear reaction in it pulling the star out. You've got to have an intense amount of pressure in the middle to force hydrogen nuclei close enough together to form helium. There are various other burning cycles. You can burn helium to produce carbon but eventually when all the fuel is used up in the core there is no forcing out of the star. Gravity wins out and it collapses the star in on itself. When this happens, very briefly you get a large increase in density in the star's core. That can generate these much heavier elements and then the star will explode.

Chris - So you can look at it as the stars are the uteruses of the universe, they give us everything that we're made of?

Chris D - That's true. The star isn't going to be completely burned to the other element. There is going to be a large amount of hydrogen. Also, the universe is still very much dominated by hydrogen gas which is the primal fuel for stars. That dust, that matter, the heavier elements will be spread out into space. Some of it will contaminate the next generation of stars. As another cloud starts to collapse together under its own [gravity] you'll get some of that heavier element polluting, if you like, the new star that's formed. It won't be pure hydrogen to start with and you'll still have the majority of the gas will be hydrogen but with the heavier elements surviving.