15 Million Degrees

by Lucie Green

  Parker predicted that as the solar wind blows out from the Sun it creates a bubble we call the ‘heliosphere’. For the Voyagers to find the edge of the heliosphere they need to sense and measure the conditions in the surrounding space so that they can identify when the particles around them stop being those of the solar wind and start being those of the material between the stars. Inside the heliosphere is the domain of the solar wind and outside is interstellar space. But there is one more way to characterize where the solar wind ends and where interstellar space starts.

  The Sun’s magnetic field needs to be considered too. Parker’s description of the expanding corona didn’t include the effects of the magnetic field. For a complete description and a thorough understanding of the solar wind it must be included. And when it is included, it solves a problem that we met in chapter 3: that the Sun, which formed at the centre of a collapsing and spinning nebula, is revolving 400 times more slowly than we expect. We expect the Sun to be rotating rapidly as a consequence of the same physics that speeds up a spinning ice-skater as they bring in their arms. On the Sun, the solar wind plasma flows out, guided by the magnetic fields, which are spinning with it. This effectively increases the size of the Sun. The outward flow of the solar wind is equivalent to the ice-skater opening out their arms, and it slows the Sun’s rotation down. But while the magnetic field might have solved this ‘angular momentum’ problem, it presents another one for our understanding of how the solar wind escapes in the first place.

  MAGNETIC HIGHWAYS

  As we already know, the whole corona is threaded by magnetic fields that emanate from the Sun’s interior and rise through the photosphere. And the magnetic field in the corona creates a complex web of structures – some large and some small. Since the magnetic field and coronal plasma are frozen together – particles can move along the lines of magnetic force, but not across them – the escape of the solar wind is much more complex than initially thought. And almost sixty years after Parker predicted the existence of the solar wind, there is still an awful lot that we want to find out.

  The fast solar wind is easier to explain. We are getting slightly ahead of ourselves, and I have already said that the solar wind causes some of the Sun’s magnetic field to reach out into the Solar System to Earth and beyond. Amazingly, there are magnetic field line-links between the Sun and the Earth! We call them ‘open’ field lines (in that they do not immediately come back down into the Sun to ‘close’ the loop) and they act as magnetic super-highways along which the plasma can flow. (See plate 7.)

  We know these are the cause of the fast solar wind because we can track the plasma as it moves. The same spacecraft that measure the speed of the solar wind as it flows over them can also measure what particles the wind is made of. And the make-up of elements changes between the fast and slow streams, giving each bit of the solar wind a signature composition. When we use spectroscopy to look at what elements are in the plasma at the base of these open field lines in the corona, it matches the plasma which later washes over our spacecraft as the fast solar wind.

  The problem comes with the magnetic fields above sunspots on the Sun’s surface (called active regions). We saw before that these are loops of magnetic field emerging out of the photosphere, bending back over and then going back down into the Sun. Any plasma in these magnetic fields is doomed to remain trapped – unable to escape the magnetic field because it is channelled back down to the Sun.

  But some must get out. The reason I confidently state that the plasma escapes, despite the apparent paradox, is because we have the measurements to prove it. It becomes the slow solar wind. Measurements of the slow solar wind made by spacecraft sitting in the flow exactly match the elemental signature of plasma that was last seen trapped in the magnetic fields of active regions. By studying the spectrum emitted by this plasma we can work out what it is made of. This is the plasma’s great escape.

  Some light was shed on this paradox in 2012, when one of my colleagues at UCL, Lidia van Driel-Gesztelyi, led a team that created a series of models of the complex web of magnetic field in the corona. And, just like Parker, she took a dynamic approach and used the magnetic structures revealed by mathematical models to understand where and how the plasma trapped in the active regions might be re-routed and be able to escape.

  The team found that the magnetic web around the equator can include ‘null points’. Null points are interesting features and even though they are effectively regions that are devoid of magnetic field, magnetic fields can approach them and pass through. But as they pass through, they experience a change in where they are connected. This happens under special conditions in a process known as ‘magnetic reconnection’ and it has important consequences for the plasma. As the magnetic field reconnects, the plasma can move from being trapped in the magnetic field of an active region to being on ‘open’ magnetic field lines, which means the plasma is released into the slow solar wind flow, highlighting once again the dynamic nature of the solar corona.

  This can be happening at several locations on the Sun at the same time. If you create a map of the composition of the plasma across the corona, as another of my colleagues, Dave Brooks, has done, you can look for the regions which show the same composition as the slow solar wind. In the images of the solar corona in ultraviolet light (see plates 8 and 9), you will see several sites in the corona which simultaneously show the right composition and upward-moving plasma which is able to get onto ‘open’ magnetic field lines to flow out and form the slow solar wind.

  We are equipped with enough knowledge of the solar wind now to get back to the Voyagers on their epic mission. How far have they got? What have they found? But, first of all, what are our ideas about the outermost regions of the heliosphere? We need to think about this so that we can identify when the Voyagers are approaching their interstellar destination.

  First, the solar wind will begin to slow down as it gets held back from expanding further by the interstellar medium: the tenuous collection of remnants of other stars’ wind, nebulae and bits of dust. This sudden decrease in speed as the solar wind approaches the interstellar medium will be accompanied by an increase in density as the plasma is forced to pile up. After the turn of the millennium the position of this region went from a theoretical prediction to a measured quantity as the Voyager spacecraft finally reached it.

  Voyager 1 reached the pile-up in 2004 when it was at a distance of 94 times the distance from the Sun to the Earth (94 ‘astronomical units’, around 14 billion kilometres) and Voyager 2 was the runner-up in 2007 at a distance of 84 astronomical units (just over 12 billion kilometres). The two spacecraft were sent off out of the Solar System in different directions. This marked the first milestone on their way to interstellar space. The next milestone is to leave the flow of the solar wind altogether. NASA expected that this would happen ten to twenty years later.

  Signs that Voyager 1 was coming tantalizingly close to passing out of the solar wind came fairly quickly though, in 2012. The excitement and anticipation led one American science writer and blogger to go online in October 2012, claiming that what he had seen in the data showed that the crossing had already happened.* From the outside, NASA looked like it had been beaten to this epochal announcement. The UK media responded with excitement and I was called for an interview on BBC Radio 4’s Today programme. It was a Monday, so a quiet day for news, but it was a good opportunity to discuss how we would decide that the boundary had been passed: just how would we know that the crossing had been made?

  The evidence needs to show that the material surrounding the spacecraft is no longer that of the solar wind but
instead is that of the interstellar medium. The American journalist had pointed out in his online article that there had been a significant and persistent drop in the number of low-energy positively charged ions – particles that are found in the solar wind. He also pointed out that there had been a significant increase in high-energy atomic nuclei, or cosmic rays, from outside our Solar System that don’t normally easily penetrate into the solar wind stream because the magnetic fields deflect them. He was making a good case that the crossing had indeed been made.

  There is another measurement that Voyager 1 had been taking though – the magnetic field – and this indicated that the spacecraft could still be inside the heliosphere and detecting the magnetic field of the solar wind. Mixed messages like this show that defining the edge isn’t easy. No wonder NASA were more hesitant to commit than the science writer was.

  Even though predictions about the edge of the heliosphere have been made in the past, you always need data to find out what the real Universe is like. No one knew for certain if the ‘edge’ of the solar wind is a thin region or a thick one, for instance. Or whether particles are able to diffuse across the boundary, making it look like you are outside when in fact you are still really on the inside. The edge could be permeable to cosmic rays, for example. This is why NASA were waiting to make an announcement – they wanted to see what the magnetic field measurements were going to show in the days, months and possibly years ahead.

  As we ponder what the heliosphere is like at the edge there are teams of scientists and engineers working on projects that will investigate the conditions close to the source. There is still so much more to learn about how the Sun creates and controls the heliosphere, which is a dynamic place. To do this, two spacecraft are currently being built. In America Solar Probe Plus is due for launch in 2018 on a trajectory that will take it closer to the Sun than any previous spacecraft. Solar Probe Plus will eventually spiral its way into an orbit just 6 million kilometres above the photosphere – 8.5 times the radius of the Sun and the closest we have ever been.

  In Europe we are building a spacecraft called Solar Orbiter. Solar Orbiter is a European Space Agency mission that will work its way into an orbit that takes it closer to the Sun than the planet Mercury. It’s an exciting mission – it will be able to hover over regions of the Sun for a few weeks at a time and in the later years of the mission its orbit will change and it will rise up so that it can start to look at the poles of the Sun. For many years now, scientists and engineers have been discussing how to take images of the Sun from up close as well as measure the solar wind directly around the spacecraft. We will be able to look at where the solar wind is coming from and then measure it when it gets to the spacecraft en route to the edge of the heliosphere.

  I get very excited about Solar Orbiter but I’m a bit biased: my space lab at UCL is helping to build it. I can leave my office, walk down some stairs, through the common room, out to the workshops and see parts of the spacecraft being worked on right now. We are currently building sections of the Solar Wind Analyser, which will detect the elemental signature of the wind, and our electronics in the Extreme Ultraviolet Imager will mean this telescope can photograph the Sun in ultraviolet frequencies. It is hard not to get excited about a new spacecraft when bits are being put together in the building where you work!

  It’s also a technically challenging mission. All of the ten instruments it carries must work in the demanding environment so close to the Sun. The side of Solar Orbiter that faces the Sun will heat up to 600 degrees Celsius and will face the full onslaught of the charged particles coming from the Sun. But when Solar Orbiter and Solar Probe Plus are in place we will have two spacecraft that are exploring the creation of the solar wind and two exploring the edge, just in time before the latter’s power sources run out. For the first time the beginning and end of the heliosphere will be studied simultaneously. I cannot imagine that this will ever happen again.

  AT THE EDGE

  A year after the excitement of August 2012, NASA scientists working on the Voyager 1 data had collected enough, and had had enough time to look for a consistent picture in the information coming back from the spacecraft, to confidently answer the question of whether it had indeed left the heliosphere. The particle and the magnetic-field measurements had been studied and interpreted. The scientists concluded that Voyager 1 had indeed left the heliosphere and entered a new, transition region, outside the heliosphere.

  Upon reflection, the team saw that Voyager 1 had entered a new region of space on 25 August 2012. The strong increase in galactic cosmic rays suggested that it was in a region accessible by the interstellar medium. This day marks one of the major achievements of the twenty-first century as the first human-made object left the heliosphere, demonstrating our phenomenal drive to explore, no matter how hard the challenges are or how extreme the environment is.

  Voyager 1 is now over 20 billion kilometres from the Sun* with this distance increasing by 17 kilometres every second. To send a signal to the spacecraft using radio waves that travel at the speed of light takes almost seventeen hours. A round trip is thirty-four hours. If you send a command to Voyager 1 at 9 a.m. in the morning, by the time it reaches the spacecraft and it responds you will not receive the reply until 7 p.m. the following night! A very drawn-out conversation. But, luckily, despite the astronomical distances, this conversation is still being had.

  Voyager 1’s leaving the heliosphere may not be the end of its story. In the back of the minds of some people was an even more significant event. In fact, a precedent had been set by two previous NASA missions, called Pioneer 10 and 11, which had been sent into the Solar System. They only visited the planets Jupiter and Saturn but they carried plaques on board that marked the time and place of their launch, just in case an advanced civilization should chance across them in the millennia ahead because they will also exit the heliosphere – although we won’t know when as contact with these spacecraft is no longer possible. The two Voyagers carry a more sophisticated message. Engraved onto a 12-inch gold-plated copper record are images and sounds that act as a cosmic message in a bottle. The records contain information not just about where they were launched from but also about the humans who achieved this phenomenal feat and the world they inhabit. Even a needle is included so that the record, in theory, can be played.

  Should extraterrestrials ever encounter the spacecraft and decipher images that are encoded on the record, among them they will find a set of images of the Sun, taken by the telescopes at one of Hale’s observatories. Carl Sagan, who chaired the NASA committee that selected the contents of the disc, commented: ‘The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this bottle into the cosmic ocean says something very hopeful about life on this planet.’ So, do the Voyagers have any hope of being found by another civilization?

  Voyager 1 will not come across a star until around 40,000 years’ time. And even then it’s not a close pass. It will only get within 1.6 light years (15 trillion kilometres) of the star (named ‘AC+79 3888’) in the constellation of Camelopardalis. Once Voyager 2 has left the heliosphere it too will take 40,000 years before it comes within a couple of light years of a star. They probably have a very lonely future ahead.

  The Voyagers are perhaps the most iconic of those early electronic explorers of the Solar System, for their longevity, for their scientific investigations and also because they represent a genuinely peaceful quest to learn more about the Universe – humans literally reaching out into the cosmos. The Voyagers weren’t the first, though, and they cert
ainly won’t be the last. Many spacecraft have joined them. A small section of this fleet are a number that diligently and constantly observe the Sun – watching its every detail. These are the spacecraft that I use: the European/American SOHO mission, the Japanese/American/British Hinode mission (the name translates as ‘sunrise’), the twin NASA STEREO craft and NASA’s Solar Dynamics Observatory. The space age opened our eyes to the real nature of our Sun and accelerated our scientific understanding of it. And all this ultimately came from technology that emerged from the Second World War and small groups of scientists and engineers who suspected that the Sun would look very different indeed when viewed from space.

  10. Space Age

  The sky is not the limit

  Astronomers will go to great lengths for photons. The Mount Wilson Observatory, founded by Hale, was, as the name suggests, up a mountain, and even today is reached by a long and winding road. And modern telescopes in Hawaii and Chile are in places which, while great for a holiday, are certainly not easily accessible. This is all because putting a telescope up a mountain means that the photons coming from space have less atmosphere to pass through.

  Photons carry valuable information and each and every one of them is important. But as they pass through the Earth’s atmosphere, some of them are absorbed.* And the atmosphere is more damaging for some frequencies than others. It is not a coincidence (but rather an evolutionary convenience) that the frequencies which make it through our atmosphere, and are therefore the most abundant on the ground, are also the ones we use for vision. Telescopes on the ground are actually looking at the Sun through white-rose-tinted glasses; they are exploring the Sun by collecting visible light photons.