15 Million Degrees

by Lucie Green

  The space age showed that to really see the true nature of a solar flare, observations must be made from above the Earth’s atmosphere as well as from the ground. There is simply no avoiding this. But working with rockets and spacecraft meant that suddenly scientists had to deal with the world politics that came with what was still very much a military technology.

  I’ve asked my more senior colleagues many times about the early days of space exploration. One story illustrates well the rush to utilize all that space technology could offer, for military reasons as well as scientific. It turns out that the primary aim of the SOLRAD spacecraft we met before was not to look at solar flares: each SOLRAD launch concealed a second, secret satellite, which was sneaked into orbit at the same time – a spy satellite called GRAB. Designed to locate Soviet defence radar sites from 800 kilometres above the Earth, the GRAB spacecraft were the first military intelligence satellites.

  And military operations were not just limited to being stowaways on scientific missions: sometimes they actively disrupted them. The space age was born during the Cold War and there were clashes as both scientists and the military planned to use the space above the atmosphere. Things came to a head in 1962, when the American military programme delivered a destructive blow to the recently launched Orbiting Solar Observatory and Ariel 1 satellites which, due to the secrecy surrounding the military work, the solar scientists didn’t see coming. The US military set off an atomic bomb in space.

  In July 1962 the US Air Force launched a nuclear bomb from Johnston Island in the Pacific, around 1500 kilometres south-west of Hawaii. This was the Starfish Prime nuclear test: a 1.4 megaton thermonuclear bomb. It was actually one of four related tests, but the other three all failed. Two were aborted and blown up (in conventional, not nuclear, explosions) after launch but before they reached space, whereas the last one failed to launch and was destroyed on the launch pad, contaminating the island with plutonium.

  Starfish Prime, however, was successfully launched, and the nuclear bomb detonated 400 kilometres above the Earth, well into what we consider ‘space’. As well as generating heat and light, the explosion also created X-rays and gamma rays, and emitted a swarm of high-energy electrically charged particles. This was the purpose of the test. If enough high-energy electrons could be generated, they could form a belt around the Earth that might knock out the space technology of America’s opponents. For the test though, they knocked out the technology of America and its allies – as the high-energy electrons rushed out they reached the OSO-1 and Ariel 1 satellites. The particles destroyed the detectors on Ariel 1, rendering the satellite useless. This was the end of the first British satellite. The Orbiting Solar Observatory fared better: it struggled through the electron bombardment, survived and went on to become a successful mission, spawning a new series, which finally ended in 1975. Just over a year after Starfish Prime, the US and the Soviet Union negotiated a treaty that banned nuclear-weapons testing in the atmosphere, in space or under water.

  As a slightly related side note, a remarkable clash of military interests with solar scientists was the unfortunate end to the Solwind satellite. Launched in 1979 to study the solar wind, by 1985 it had aged but was still semi-functional and providing data to scientists. That is, until President Reagan chose it for the US Air Force to use to test its anti-satellite technology. The scientists could do nothing as their spacecraft was used as target practice and blown into pieces (285 pieces, as far as could be counted).

  THE MODERN SOLAR FLARE

  The space age revealed flares to be multi-wavelength animals as they produce emissions across the electromagnetic spectrum, from radio waves with wavelengths of metres to gamma rays with wavelengths of less than one thousandth of a billionth of a metre. It also showed that flares reached much higher in the Sun’s atmosphere than anyone had expected.

  Flares begin with intense and short-lived bursts of radiation, lasting from seconds to minutes. Long-wavelength microwave and radio photons are seen coming from the corona, while lower down in the atmosphere X-rays appear in patches. All X-rays are by definition of a very short wavelength, but these are the shortest of the short: so-called ‘hard’ X-rays. Gamma rays are also sometimes produced at about the same time. Then bright ribbons of plasma are seen to glow at hydrogen alpha frequencies and, on rare occasions, there are bursts of visible light flashes coming from the photosphere. These visible light flashes had been the part of the flare emission that the Victorian amateur astronomers Carrington and Hodgson had seen. The series of events in a solar flare occur in almost exactly the reverse order to that in which they were discovered!

  Actually, all of these first bits happen almost simultaneously and are over in a instant, in what is called the ‘impulsive phase’ of the flare. Then, structures that shine brightly in X-rays and extreme ultraviolet start to form in the corona above the locations where the hard X-rays, gamma rays and visible flashes were produced. In the most impressive flares these structures look like a row of massive arches, lined up side by side: like a slinky that has been half submerged in the surface of the Sun, only it is a slinky big enough for the Earth to fit inside many times over! (See plates 13 and 14.)

  These massive solar slinky structures emit X-rays and extreme ultraviolet radiation which slowly fades. This is called the ‘gradual’ phase of the flare as it can take hours for everything to return to normal. Initially the plasma in the slinky must have a temperature of millions of degrees to produce the X-rays, but as it cools the frequency of the radiation goes down as well, ending with the slinky emitting light at hydrogen alpha frequencies. Then they disappear. They simply fade away.

  Thanks to the space age, scientists had gone from just looking at the footprints of solar flares to seeing the body, the tail and the head. Finally, the anatomy of the beast was revealed. And they knew that all of the radiation, no matter which wavelength, is emitted from particles making up the plasma in the solar flare. So by looking at what particle processes could produce the radiation – and in the order in which they are observed – a full description of what is happening inside the flare can be deduced.

  The initial radio emissions can be explained by electrons moving through a magnetic field. If electrons are accelerated in a magnetic field they start to spiral around the field lines and this produces radio waves. So a flare seems to start with a burst of fast-moving electrons, many of which are accelerated downwards from the corona. But things really get exciting when those electrons get to the bottom of the corona and something gets in their way.

  Initially it is easy sailing for the electrons: the plasma is rather sparse in the corona – but it starts to become more and more dense the further down they get. The hard X-rays that appear in patches tell us that some of the electrons are colliding with protons in the chromosphere and that this is producing the X-rays. Patches of hard X-rays show us where this is happening, revealing where the magnetic legs of the flare have their feet in the lower atmosphere.

  The sudden influx of electrons into the chromosphere also gives energy to the hydrogen atoms there. They then release this energy as photons at the wavelength of the hydrogen alpha line, creating the ribbons of hydrogen alpha emission that can be seen with any back-yard telescope that has a hydrogen alpha filter. In this deluge of electrons there may be some that have enough energy to make it all the way through the chromosphere to the photosphere. Those electrons can deposit their energy there instead, and if this happens they heat the plasma and create the visible light emission that Carrington and Hodgson so famously saw.

  Meanwhile, the protons and ions in the coronal plasma are also being accelerated, ju
st as the electrons were. They too spiral down the legs of the magnetic field but their relatively large mass and kinetic energy mean they manage to travel all the way down to the solar photosphere without being stopped. They’re not as easily braked as the electrons were! But, even for them, the dense plasma of the photosphere is the end of the line.

  When the accelerated protons and ions from the corona are finally stopped by the brick-wall-like photosphere, it is quite a show. They collide with the heavy atomic nuclei, such as carbon, nitrogen and oxygen, and these atoms then get rid of this sudden additional energy by admitting gamma rays. But some of the collisions with nuclei actually knock a few neutrons right off them. These liberated neutrons race off at high speeds, causing all sorts of new problems. When they finally slow down after a few collisions, they’ll cause a proton to become deuterium (heavy hydrogen that has a proton and neutron in the nucleus), emitting more gamma rays. In all the high-energy excitement, even some antimatter can be produced! Antimatter particles have the same mass as their regular particle counterpart, but they have opposite properties, like an electric charge. Flares can produce anti-electrons (‘positrons’) from radioactive nuclei. With so much matter around they do not last long, quickly being annihilated in a collision with an electron and producing – you guessed it – more gamma rays.

  All these collisions transfer energy from the fast-moving particles of the solar flare into the material they slam into and this means that the plasma has been greatly heated. The chromospheric plasma is suddenly heated from tens of thousands Kelvin to tens of millions Kelvin. And this rapid heating increases the plasma pressure in the chromosphere so that the second phase of the flare begins. Unable to shed the energy via radiation as quickly as it is being deposited by the charged particles, the heated plasma in the chromosphere expands and rises back up the magnetic field that the charged particles have just flown down.

  This is what forms the slinky. The magnetic field lines the particles came down have now formed a row of massive arches. The super-heated plasma rises up and fills these arches and we can watch this happening with our space telescopes. Initially the plasma shines very brightly in soft X-rays and extreme ultraviolet light. But the plasma trapped by the magnetic field then cools as it radiates photons, and the X-ray and extreme ultraviolet emissions fade away, sometimes taking many hours to do so.

  All the commotion with collisions in the chromosphere and photosphere has also done more than just cause the plasma to heat and rise up. It can also set off sudden bursts of sound like a hammer striking a drum. This burst of sound is called a ‘sunquake’! (See plate 15.)

  We’ve seen from helioseismology that the Sun is always ringing like a bell from the constant movement of plasma causing sound waves inside it. A flare on some occasions is like hitting a softly ringing bell with a sledgehammer. These sunquakes were first discovered in 1998, when a very unusual pattern was reported in the plasma oscillations at the photosphere: a series of concentric circles. It was realized that these sound waves start in or close to the photosphere. They race into the Sun but are then bent upwards and appear at the photosphere in giant concentric circles about the point where the hammer first hit.

  But let’s not get distracted by the drama of a flare. There is a mystery now to solve: how did so many electrons in the corona suddenly manage to be accelerated to such crazy speeds?

  THE ELEPHANT IN THE ROOM

  The fact that I find most staggering about solar flares is that to explain the amount of radiation we see, there must be one billion billion billion billion electrons accelerated, every second, to speeds approaching the speed of light. Something in the Sun is acting as an amazing particle accelerator. And it must be a very high-powered one.

  The electrons carry in total one million billion billion joules of energy as they go crashing down from the corona. To put it in perspective, the 1.4 megaton Starfish Prime nuclear bomb released around 6 million billion joules of energy. This means a solar flare is millions of times more powerful! A single solar flare on the Sun is the same as 170 million Starfish Prime nuclear bombs all going off at once.

  As we saw earlier with the work of Mayer and Joule, one of the big scientific advances of the 1800s was the realization that energy cannot be created or destroyed. Energy can only be transformed from one form to another. So the energy that drives a solar flare must already be in the corona and somehow changes form to accelerate the electrons. But where is it hiding?

  Both the Sun’s core and the Starfish Prime nuclear bomb hide their energy in the same place: inside atoms. One of the big scientific advances of the twentieth century was realizing that energy could be released from within an atomic nucleus, in the case of the Sun, through the fusion process, fusing hydrogen into helium. In the case of Starfish Prime, the process is initiated through fission – the breaking apart of plutonium into smaller atoms. But fusion requires high densities and temperatures and fission requires very large radioactive atoms, none of which the corona has at the start of a flare. The energy must be hiding somewhere else.

  There is one thing the corona does have in abundance though: magnetic field. And it turns out this is where the energy has been hiding all along. I’ve said before, I like to think of magnetic field lines like elastic bands. This is because it helps to visualize the magnetic field being twisted and distorted, forming the Sun’s incredibly complicated magnetic field. Well, just like an elastic band, all that twisting and stretching causes energy to become stored in the magnetic field. And sometimes the field lines ‘snap’, releasing it all at once.

  ‘Snap’ is of course a gross over-simplification of a very complicated process. But it is not a bad analogy. We’ve all been pinged by a snapping elastic band releasing energy into us suddenly. Actually, an even better analogy would be if there were millions of stretched and tangled elastic bands that all snapped at once and then joined back together in a way that was much less stretched and tangled. Allow me to explain …

  The normal situation in the corona is that the plasma is very thin, and so each charged particle can do whatever it wants, uninterrupted. And what charged particles like to do in a magnetic field is flow along magnetic field lines. So we generally assume that, in the corona, protons, electrons and ions can happily move about without getting in each other’s way. Another way to think of this is that the corona is a very good conductor of electricity. It also means that charged particles in the corona flow along their own magnetic field lines and have no reason to jump across from one field line to another. They are loyal to their neighbourhood.

  This is also the situation of the plasma and magnetic fields being ‘frozen together’ that we keep coming across. If the magnetic fields are made to move, they bring the plasma particles with them, and vice versa: if you force the plasma to move it will drag the magnetic field along with it.

  However, under certain plasma conditions, particle collisions will disrupt the flow of the electric currents. Collisions within the plasma will send the particles in all directions and they get decoupled from their original magnetic field lines. This separating of particles and magnetic field lines means they can move independently of each other – they are no longer frozen together. As soon as the magnetic field lines have the freedom to start moving about independently of the charged particles, they can have the freedom to reconfigure themselves.

  We conceptualize the reconfiguration as magnetic field lines being broken and rejoined as they diffuse through the plasma. But in reality they do not actually snap and break: it is just a great visual aid to portray what happens. The theory of this process has become known as magnetic reconnection an
d it is now in common use within the scientific community. I actually imagine magnetic reconnection as being like snipping two elastic bands and gluing them back together in a way that makes a new connection between them.

  11.1 Conceptual reconnection: take two separate loops, break them apart at their top crossing and then join them together.

  Despite our centuries of investigating magnetic fields on Earth, it was trying to understand solar flares which led to the development of the theory of magnetic reconnection. The reason magnetic reconnection is so important for solar flares is that the process converts energy that was stored in the magnetic field into energy of the motions of the particles. A vast amount of energy is liberated from the magnetic fields as they reconfigure and any electrically charged particles in the vicinity of the reconnection region are kicked into action and accelerated down from the corona.

  Understanding that magnetic reconnection is the process which is at the heart of all solar flares means that every observation that has ever been recorded is actually of the secondary effects of the energy release. Really, we are inferring that reconnection is taking place by seeing the consequences of energy being transformed from magnetic into a form that we can observe which is the consequence of charged particles, particle interactions, nuclear process and plasma heating.

  This theory that explains solar flares has been in development for decades. Indeed, it is still in development today although the foundations of an explanation are in place. Even though we have an overall understanding that magnetic reconnection is responsible for solar flares, the theory is not yet complete.