15 Million Degrees

by Lucie Green

  This is where the slight tilt of each sunspot pair comes into play. The twist of the Coriolis force means that the trailing sunspot is closer to the pole already, and in the majority of cases the trailing sunspot is of the opposite magnetic polarity to the nearby pole. So the plasma ‘conveyor belt’ preferentially drags up the magnetic field to a pole that is of the opposite magnetic polarity to that spot. And, if enough magnetic field could be carried up, it could bring about the magnetic field reversal seen around times of solar maximum.

  THE SOLAR DYNAMO

  So now we have a fairly complete model of how the Sun can take its original simple magnetic field and turn it into something amazing and dynamic. This whole system of the plasma flows and movement of the magnetic field is called the ‘solar dynamo’. To recap:

  The simple magnetic field at the bottom of the convection zone is stretched out by the moving plasma and gradually wound around the Sun. This amplifies it and causes flux ropes to form. If the plasma trapped inside these flux ropes becomes less dense in one area, there can be a run-away effect where that part of the rope becomes buoyant and starts to float towards the surface, forming an omega shape. It finally breaks through the surface and forms sunspots where it intersects the photosphere. But, on the way up, Coriolis forces twist the tube, meaning that the trailing part of the tube is slightly closer to the Sun’s nearby pole. Fantastically, the trailing sunspot is of the opposite magnetic polarity to the pole, so when its magnetic field is distributed over the photosphere, and the solar-pole conveyor belt takes its magnetic field up to the pole, it is taking a magnetic field which has the opposite polarity to what was initially there. This accumulates and eventually swamps the previous polarity, causing the magnetic poles to systematically swap. Finally, in turn, this means the next generation of flux ropes forming at the base of the convection zone and later emerging will be the other way around (magnetically speaking) and the cycle continues, for as long as these flows are maintained.

  Phew.

  It is perhaps lucky for those studying the Sun that its magnetic field is variable on timescales that can be appreciated within a human lifetime. The dynamo-driven solar cycle lasts on average 11 years and 36 days. This gives us enough time to collect the data over many cycles, enabling us to understand what processes are at play. I have been studying the Sun long enough to start measuring the passing of time in units of solar cycles. I started my career in solar physics in cycle 23. I watched as the north pole flipped its magnetic polarity in late 2000 and as the south pole reversed its polarity in 2001. There is no reason why they should flip simultaneously if this process is indeed driven by magnetic flux dispersing from sunspots in the respective hemispheres. Sunspots do not form as mirror images in the northern and southern hemispheres. And the meridional flows in the north and south do not have to be exactly the same either.

  I eagerly waited to see the Sun pass into cycle 24. This cycle finally began in January 2008, when the first spots of the cycle were seen in the northern hemisphere. Cycle 24 was a surprisingly slow cycle to start, showing us that despite the advances we have made in understanding the dynamo, the Sun still has the ability to act in a way that we don’t expect. Not all cycles are the same size and it’s hard to predict how strong the upcoming cycles will be.

  And we still have many questions to answer. For example, why does the sunspot cycle last eleven years on average? Why not one thousand, or one million years? Or why not a few months? And in fact the eleven-year cycle isn’t the only pattern that can be pulled out of the sunspot data. The modern understanding is that several cycles are overlaid on top of each other – it’s just that the eleven-year cycle is the most obvious one.

  Some modern ideas are that a memory may exist between solar cycles. The output of one cycle may lead into the next cycle or the cycles after that. The meridional flow may vary significantly from one solar cycle to the next. It could be that changes in the flow speeds have an effect on subsequent solar cycles. For example, if the meridional flow speed increases, more magnetic field is transported across the Sun and the next cycle might be longer.

  Conversely, if the conveyor belt slows down, other processes, such as differential rotation or diffusion of the magnetic field, have more time to come into play. Then it is a case of which of these competing effects dominates to influence the development of the field and the strength of the cycle. Even though nuclear fusion defines our sun as a star in that it generates light, it is the plasma motions that control the dynamo and define the magnetic character of the Sun.

  The key thing here is that the solar cycle, driven by the dynamo, continually brings magnetic flux into the solar atmosphere. It prompts the question: what happens to all this magnetic field? Does it endlessly accumulate over time? And what are the consequences for the Sun’s atmosphere? To answer these questions we need to look at the atmosphere of the Sun into which all the magnetic field is emerging. The first way we can do this from the ground is during times of a total solar eclipse.

  8. Eclipses and Rainbows

  On 22 July 2009, I stood on the deck of a ship sailing eastwards across the Pacific Ocean. I was on my way to see a spectacle that was being heralded as the astronomical event of the century. As the Sun and the Moon dance around the sky, there are occasional chance alignments when the disc of the Moon completely blocks the light coming from the Sun, producing a total solar eclipse. The eclipse of July 2009 would be the longest such alignment this century, creating a full 6 minutes and 39.4 seconds of darkness for those people in the right place on Earth.

  Solar eclipses always attract attention and people will travel around the world to see them. There were 1500 eclipse chasers on my ship, all waiting for those moments of darkness. But the Moon does not actually block all of the light from the Sun and I was there to see the small amount of light that the Moon cannot hide. This light reveals the Sun’s atmosphere – the plasma layers above the photosphere. And there was a very personal reason why I wanted to witness this event.

  After having studied the Sun’s atmosphere for a decade this was going to be the first time that I would actually look at it directly. The Sun’s atmosphere is normally hidden from our view because its faint visible light is lost in the dazzling glare of the photosphere. But this isn’t the case during a total eclipse. The Moon is big enough to block the photosphere so that we cannot see light coming from the Sun’s visible surface, but it’s not large enough to prevent us from seeing the atmosphere. For the previous ten years I had relied on specialist telescopes above the Earth’s atmosphere to view the Sun’s atmosphere in wavelengths that our eyes cannot see. But now I was about to witness an event that reveals the normally hidden solar atmosphere at a time when it’s perfectly safe to see without any specialist equipment. I would see the atmosphere with my own eyes.

  The best location to view the totality of an eclipse varies from event to event and can be almost anywhere on Earth. So I wasn’t complaining when I found out that this one was best viewed from a cruise ship on the sunny Pacific Ocean. On the morning of eclipse day, we sailed past the volcanic island of Iwo Jima, which was the location of a bloody battle during the Second World War and has now reverted to its pre-war name of ‘Iwo To’. It looked so peaceful surrounded by the calm ocean that morning. But, as noon approached, the calm was broken as the ship’s deck became thronged with people. The eclipse was imminent.

  As I stood looking at the Moon sliding into position, I couldn’t help but marvel at the fact that total eclipses occur at all. They only happen because of an astronomical coincidence. Not only does the Moon pass in front of the Sun, but it is also exactly the right si
ze to cover it up. Actually, the Moon passing in front of the Sun is not that surprising if you remember how the Solar System was formed: the Earth, Moon, Sun and other planets formed from the same flattened nebula disc, so they all orbit in the same plane.

  Well, almost. If the Moon orbited the Earth in exactly in the same plane as that in which the Earth orbits the Sun, we would see a solar eclipse once every lunar orbit. Unfortunately for solar eclipse fans, the Moon’s orbit is tilted by about 5 degrees to our orbit around the Sun. So the Moon tends to pass above or below the Sun from our point of view. Only occasionally does it happen to go directly between the Earth and the Sun.

  The truly lucky coincidence, though, is the relative size of the Sun and the Moon. The Sun is 400 times larger than the Moon but it is also 400 times further away from us. So from our perspective both are the same size in the sky. This means that when the Moon moves in front of the Sun it is just the right size to cover it up. If the Moon were smaller/further away it would not block all of the Sun from our point of view (hence annular eclipses occur at the Moon’s apogee); if it were bigger/closer it would block the Sun’s atmosphere as well as the photosphere.

  We actually live at the perfect time in the life of the Solar System for this alignment to work. The Moon is moving away from us at a rate of about 4 centimetres every year, about the same speed that your nails grow – imperceptible to us but, given enough time, the consequence of this will be significant. It means that if there are any humans on the Earth in a billion years’ time the Moon will be too small in the sky to produce a total solar eclipse. For our distant descendants, eclipses will be seen as a black disc that covers most of the Sun but not all. Or maybe humans will have moved to a new planet orbiting another star with better eclipses by then!

  Even when everything is perfectly aligned, the Moon only casts a small shadow on the Earth. For any total eclipse, the Moon blocks the view of the Sun for about just 1 per cent of the Earth’s surface. And the shadow that is cast on the Earth is moving! Because the Moon is moving and the Earth is rotating, the shadow sweeps across the planet. So to see an eclipse you need to be in the right place at just the right time. You have to travel to see a solar eclipse; it will rarely come to you.

  The central part of the shadow, the umbra, sweeps along what’s known as the ‘path of totality’ and there will be a location along this path where the total eclipse has the longest duration. I was waiting to see the eclipse at this point on the Earth where the total eclipse would last for 6 minutes and 39.4 seconds. The position and bearing of our ship, which was chasing the Moon’s shadow, were to give us an extra three seconds of totality. Totally worth it.

  ECLIPSES THROUGH THE CENTURIES

  I find it remarkable that we can talk about eclipse positions, timings and durations down to a fraction of a second. It shows just how detailed our knowledge of the motion of the Moon around us is, and our motion around the Sun. We can predict exactly when an eclipse will occur and how long it will last and how fast the shadow will be moving. I had an eclipse map based on these predictions and it showed me exactly where and when the Moon would cast its shadow. These maps are vital for allowing people to get to the right place at exactly the right time. Remarkably, eclipse maps are not a new thing.

  The first time an eclipse map was used to tell the public about the location of the Moon’s shadow was in 1715. The map was created by Edmond Halley, the British mathematician and astronomer who is famous for his work on the orbits of comets. He even had one named after him. Halley wanted to know how fast the shadow would sweep over the Earth, but to work this out he needed observations from various points along the path of totality. Since he couldn’t be at more than one place at once, he was going to need some help.

  Thankfully the 1715 eclipse was going to be visible across England and Wales, meaning there would be plenty of people who could help. It was the first eclipse to be observed in England for 500 years and Halley realized it would generate excitement that he could capitalize on to get some science done. He devised what was probably the first citizen science project and requested that the ‘curious’ of the country who were along the path of totality observe ‘what they could’ and make a note of the time and duration of totality from their location.

  Halley had to ask the public for help because there were only two universities in England at that time, so there was no large team of professional researchers spread across the country to call upon. And it was just as well that he did call on the public because the astronomy professors at the two universities had no luck in seeing the eclipse. Clouds obscured the view at one site and the Reverend Cotes at Cambridge ‘had the misfortune to be oppressed by too much company’! – a common problem for astronomers during eclipses. Enough observations came in, though, and Halley was able to calculate that the shadow swept over the Earth at a staggering 2800 kilometres per hour.

  Eclipses are incredibly versatile and have contributed to many areas of science. Just over 200 years after Halley’s experiment, the total eclipse of May 1919 was used to investigate Einstein’s new ideas about how gravity was the result of massive objects distorting the shape of space-time. Once again, it was Eddington who observed this eclipse so that he could test Einstein’s ideas, and when he gave a lecture at the University of Cambridge about what had happened he inadvertently inspired Cecilia Payne to become an astronomer.

  In short, Einstein’s theory of general relativity made predictions about how massive objects, like the Sun, should bend the path of light passing by. This means that the stars we see near the Sun should actually appear in slightly the wrong place as the light coming from them is bent by the Sun’s mass. But this was impossible to check normally, because the Sun is so bright we can’t see stars near it in the sky. Only during a total solar eclipse would it be possible to check the locations of the stars right next to the Sun and see if they were ever so slightly displaced, as predicted by Einstein. This was exactly what Eddington planned to do.

  Eddington did not get to go on a cruise in the Pacific though; the 1919 eclipse was best viewed on the African island of Principe. But he did get to see an even longer eclipse than the so-called ‘eclipse of the century’ I got to see. The 1919 eclipse totality lasted a full 6 minutes 51 seconds, beating mine by around 9 seconds. And thankfully it was a very long eclipse as the weather on the day was terrible. Only briefly during the almost seven minutes did the skies clear enough for Eddington to take a group photo of the Sun and nearby stars. Before 1919, Einstein’s ideas were unverified and seemed too strange to be true. But that one photo was all it took to change this: the stars were indeed in slightly different places. It looked like Einstein’s theories had been confirmed.

  TOTALITY

  At 26 minutes and 40 seconds past eleven on the morning of 22 July the Moon slid perfectly into place and completely covered the Sun. Up until then, everyone had been looking at the Sun through ‘eclipse glasses’. These are glasses made from the solar filters you can put on telescopes (the ones which are darker than sunglasses or even welding masks). The Sun is so bright that even if it is partially covered by the Moon it can still cause damage to your eyes if you look directly at it. Only when the Moon is completely blocking the Sun is it safe to do this.

  So the moment had arrived: I could finally look without my eclipse glasses. Around me the temperature had fallen noticeably without the direct heat from the Sun. The colours of the darkened sky had changed and taken on an unfamiliar purple hue and part of the horizon had the appearance of sunrise. But I didn’t care about any of this. I was looking up at the Sun. And there it was, a wispy halo that surrounded the occulted Sun: the
Sun’s atmosphere.

  I was struck by how extended it is – 99 per cent of the Earth’s atmosphere is held, by gravity, below 55 kilometres from the Earth’s surface. That distance is less than 1 per cent of the Earth’s radius. For the Sun, I could see its atmosphere extending out to a distance of around 700,000 kilometres, which is roughly equal to the Sun’s radius. The physical extent of the Sun’s atmosphere is much greater than you might expect.

  The light I was looking at coming from the Sun’s atmosphere is a million times fainter than the photosphere, giving it a pearly appearance. And in the temporarily dark sky the planets Mercury and Venus were suddenly revealed. And, just like Eddington, I could see a small number of stars too. As I stood there marvelling at the eclipse, I was following a long line of astronomers who had done the same. For centuries, the only time that humans could see the Sun’s atmosphere was during a total solar eclipse.

  But the fragile wispy halo I could see was only one part of the Sun’s atmosphere, a part called the ‘corona’, so named because it sits like a crown around the Sun. Staring up at it, I could see exactly why it was given that name: it did look like a majestic crown. The other layer of the Sun’s atmosphere that I wanted to see wasn’t as obvious as the corona, as the Moon was still hiding some of it.