15 Million Degrees

by Lucie Green

  The skill of the engineers at the Goddard Space Flight Center in Maryland was tested as they wrestled to get control of the satellite again. Meanwhile, the astronauts were making plans to catch the satellite with the shuttle’s Remote Manipulator Arm. With limited fuel on board, there were only two opportunities to use this approach. The first attempt was unsuccessful but the astronauts held their nerve, and no doubt the engineers and the scientists back at Goddard held their breath, and finally the Solar Maximum Mission satellite was captured. The satellite was successfully repaired and released back to duty on 10 April 1984. It went on to gather observations for five years until it fell back to Earth over the Indian Ocean.

  ATMOSPHERIC PHYSICS

  Thanks to all of these space missions, over only a couple of decades our view of the Sun was transformed. We knew that gamma rays were produced in the heart of the Sun and these photons gradually lost their energy on the way out to eventually be released as ultraviolet light, visible light and infrared radiation from the photosphere. But we are now confronted with X-rays and ultraviolet light being emitted from the plasma in the corona, above the photosphere. Something new was going on in the corona to produce this high-energy radiation. The emphasis in solar physics shifted – from the Sun itself to its atmosphere.

  The corona is not producing this high-energy radiation through the same process taking place at the Sun’s core: it does not have the density for nuclear fusion. Instead, the corona is a tenuous plasma: a wispy, thin plasma of electrons, hydrogen nuclei (protons) and helium nuclei with a smaller number of ions from the more massive elements such as calcium and iron. For this thin ion and electron soup to produce X-rays though, it must be extremely hot.

  The X-ray glow of the corona reveals its temperature. The X-ray emission is being created by extremely fast-moving electrons, only possible in an extremely hot plasma. There are two ways that fast-moving electrons can produce X-rays: as they get deflected by positively charged particles and change direction releasing a photon, or when the electrons are captured by ions and then lose energy, radiating X-ray photons in the process. All of this points to the X-rays we see coming from the corona being created by a plasma that is exceedingly hot – reaching temperatures of around 2–4 million Kelvin. (See plate 12.) The space age revealed our Sun to have an atmosphere that is 300 times hotter than the photosphere below.

  This had been suspected before the first rockets were launched, but it was a controversial speculation. Logically, the Sun should continue getting colder as you go out into the atmosphere: not suddenly get much hotter. But now there was concrete evidence that this was exactly what was happening. It was also happening very quickly in what became known as the ‘transition region’, where the temperature of the plasma rapidly rises from around 10,000 to almost 1 million Kelvin. This discovery presented a puzzle to understand, which physicists rushed to try and solve. And the key to the puzzle seemed to lie with the magnetic field that threads through the corona.

  The space age forced us to shift our thinking about the Sun as merely a ball of plasma – well, everything up to and including the photosphere is a glorified ball of plasma: gravity keeps these regions fairly ball-shaped and the regions have fairly well-defined edges. But things are much more complicated above the photosphere – forces other than gravity appear to be at play. In the chromosphere jets of plasma shoot up into the atmosphere and there is no sharp or well-defined boundary with the transition region. The same is true for the transition region’s interface with the corona: the atmospheric layers are interwoven. And here we turn to the magnetic field.

  10.4 The temperature in the solar atmosphere initially decreases as you go up through the photosphere. But then the temperature starts to increase and this continues through the chromosphere and the corona. The rapid rise between the chromosphere and the corona is known as the transition region.

  Over the solar cycle the corona experiences an influx of magnetic field as more and more sunspots emerge into the photosphere. But the plasma of the corona is much thinner than that in the photosphere. In the photosphere there is enough plasma that it is able to push the magnetic fields around – not so in the corona. The amount of pressure exerted by a plasma is based on its temperature multiplied by its density, and the coronal plasma simply does not have enough density. Suddenly the balance of power switches and it is the magnetic field which can push the plasma about.

  The photosphere in a sense acts as a boundary in two ways. As we have already seen, it is a visible boundary because it is where the light can suddenly escape from the Sun, giving it the defined edge we see in the sky. Secondly, it is the boundary where the magnetic field is rooted in and controlled by the plasma. Above the photosphere the plasma loses this control.

  And there is a second major change in the relationship between plasma and magnetic field going from photosphere to corona. We have already met the magnetic field as the source of sunspots in the photosphere. There the magnetic field traps plasma and it cools to become dark. But in the corona the plasma trapped along the magnetic field structures is heated and appears bright in X-rays and extreme ultraviolet light. If you look at the photos of X-rays coming from the Sun, the bright areas match the massive loops of magnetic field.

  The glowing super-hot plasma is trapped in the magnetic field and traces out its shapes. Magnetic fields that arch up from a positive polarity in the photosphere and back to the photosphere as a negative polarity form the arches seen in the space telescopes. But some magnetic fields pass through the photosphere and extend to such great heights that we don’t see where they bend and turn back to the photosphere. The field lines continue straight out into the Solar System. These are the magnetic field structures we met before where the plasma rushes out to form the fast solar wind, leaving behind dark ‘holes’ in the corona. The magnetic field shapes everything.

  Unsurprisingly, the current theory is that the magnetic fields are responsible for heating the corona, making it much hotter than the photosphere. Understanding how this happens has been a major question in solar physics in the intervening decades. Either the energy is directly extracted from the magnetic fields that thread the corona, or the energy is deposited by waves that propagate along the magnetic field. Either way, the magnetic field is responsible for the beautiful array of shapes seen in the X-ray glow of the plasma. And, either way, the transfer of energy from magnetic and wave to plasma heating is taking place in the chromosphere and the transition region.

  One of the most recent missions to have been launched is NASA’s Interface Region Imaging Spectrograph, most commonly referred to as IRIS. The satellite carries onboard instruments to study the chromosphere and the transition region with the aim of understanding how energy flows through these layers on its way from the photosphere to the corona. All the motions can be studied and the forms that the energy takes will be investigated.

  Sending up spacecraft to study the Sun has been continuing ever since those first explorations. My work uses several different spacecraft, some carrying equipment and detectors designed and built in my department at UCL. Many people lament that NASA continued to launch rockets whereas the proto-UK space agency did not do the same. What people do not realize, though, is that the UK rocket programme lives on. Our Blue Streak missile became the first stage of a European rocket, which in later designs became the Ariane that is used today. And the UK was a founding member of what became the European Space Agency (ESA). Locally we played to our strengths within the funding landscape and specialized in the actual equipment that ends up in space. Our engineers built many instruments and our scientists analysed the data and made some
incredible discoveries.

  This is what we are going to look at next. From space we see the most dramatic phenomena in the Solar System. They are the result of the constant movement of the magnetic field in the photosphere and the emergence of new magnetic flux into the atmosphere which stores up colossal amounts of energy. But studying these phenomena has involved more than just rockets and spacecraft: there are balloons and atomic bombs as well.

  11. The Flare Necessities

  To catch a flare

  Solar flares are the most powerful explosions in the Solar System. We saw a hint of their power through the story of Carrington and Hodgson, who in 1859 became the first humans to witness a flare. Any light source that can outshine the dazzling photosphere must be impressive in magnitude. And after the flare there was a major auroral display and disruption to telegraph lines, leading some to speculate that somehow this energy was able to propagate across the Solar System to us. Carrington and Hodgson saw an intense burst of white light coming from the Sun’s photosphere. But now we know that what they saw was only the very base of the flare, merely its footprint, and that much more was going on above it.

  In the century following Carrington and Hodgson’s observation, the Sun could only be glimpsed in the visible light coming from the photosphere and chromosphere. When looking at the photosphere, seeing a flare was a rare event. On average, one ‘white light’ flare was seen every two years. But we started to work our way up the legs of solar flares when just the light of hydrogen alpha was used, which had been so successful for Hale and his work on sunspots. Looking through this narrow red wavelength of light, many more flares were revealed and an intriguing structure was seen above the bursts of white light.

  Seen in this wavelength, flares show a bewildering array of shapes and sizes in plasma that is around 10,000 Kelvin, hotter than the plasma in the photosphere. Some flares are accompanied by sprays of material that shoot hundreds of thousands of kilometres up into the Sun’s atmosphere before falling back again. Others are just small, compact flashes of light. There are even some spectacular events that are accompanied by a sudden upward eruption of a vast amount of plasma into the corona. Two things are clear from these observations: flares involve a tremendous quantity of energy and there is more to flares than the white-light flash. Before the space age, scientists were like the metaphorical blind people, feeling the feet and legs of an elephant and trying to guess what it looked like further up.

  It was time to look up at the rest of a solar flare. And that involved not only a rocket, but a rocket suspended from a balloon. A ‘rockoon’.*

  GUARDIANS OF THE ROCKOON

  In the summer of 1956, just before the peak of solar cycle 19, the USS Colonial and the USS Perkins went out to sea on an unusual voyage. The Colonial had been designed to transport vehicles and launch them during amphibious assaults. The Perkins was a destroyer, whose purpose was to escort and defend larger ships. But that summer they set out on a peaceful mission. The USS Colonial was being used as a floating lab by scientists at the Naval Research Laboratory and the cargo it was carrying included a set of rockets and balloons. The USS Perkins had been enlisted to help. Together, they headed out into the Pacific to gather information about the most energetic explosions in the Solar System. They were going to use their cargo to try and catch a solar flare.

  There were several things coming together by the time the USS Colonial and USS Perkins sailed. The rocket technology that was being used to study the Sun from above the atmosphere was becoming established and there was a change in attitude about the radiation that might be emitted during a solar flare. The view that flares were a chromospheric phenomenon, shining only in visible light and confined to the lower atmosphere, was changing. A new view was emerging that flares might be a phenomenon that emits very high-energy radiation – like X-rays. It had been noticed that there were breakdowns in radio communications occurring during solar flares. This implied that solar flares were giving off enough X-rays to have a substantial impact on the Earth’s ionosphere so that radio waves used for communication were no longer predictably bouncing off it, and instead were passing straight through.

  But using a rocket to look at a solar flare is very difficult as it involves launching the rocket right as a flare is going off. Flares are unpredictable and fleeting events. And rockets only have a few minutes to make their observations. To get the brief observation window of a rocket to overlap with a short-lived solar flare means the rocket must be ready and able to fly at a moment’s notice. The Naval Research Laboratory scientists solved this problem by using rockoons.

  The word rockoon is not just a hybrid of the two words rocket and balloon, but also a blending of those two technologies. A balloon was inflated until it had around the same volume as a couple of double-decker buses, and was attached to a rocket, lifting it to around 20–25 kilometres. The rocket then waited, bobbing beneath the balloon until a flare went off. With very little delay the rocket was launched straight up through the balloon (this burst the balloon, and while not much thanks for its help, it did remove the problem of what to do with it afterwards). It was a novel approach. And there was something poetic about using helium balloons to help study the Sun.

  Launching from a balloon meant the rocket could reach a much higher altitude than if it was launched from the ground. And it was simple and cheap and meant that launch towers weren’t tied up with rockets that may or may not be launched – depending on whether a solar flare would happen. The balloon, with the rocket dangling underneath, would lift off from the USS Colonial and it was the job of the swifter USS Perkins to track the balloon as it drifted in the wind. Then, when word came through from a ground-based telescope that a flare was in progress, a radio signal was sent up to the rockoon and the rocket fired to head off and make observations with the detectors it carried on board.

  The Colonial carried enough rockoons to launch one a day for ten days to try and catch a flare. Even though the Sun was close to solar maximum, when flares are most frequent, there were no flare sightings for the first three days. On the fourth day, two flares! But no rockoon had been lofted. On the fifth day there was success. A flare occurred, the rocket launched and, crucially, X-rays created by the flare were detected. This was the first step in answering the question about what was causing temporary changes to the ionosphere, but it opened a whole new kettle of fish. How were solar flares getting enough energy to create a brief but intense X-ray flash that could outshine the whole sun?

  The problem was not even as simple as explaining how flares were producing such high-energy radiation: they were also producing unexpectedly low-energy radiation! It was discovered in 1942 that flares were producing long-wavelength radio waves, which are much lower energy than the visible spectrum or even infrared radiation. This was actually a serendipitous discovery, when a radio source was detected that overwhelmed signals received at a British radar station. It was soon realized that the radio waves swamping their receiver were not terrestrial: they had come from a flare on the Sun!

  Any explanation scientists could come up with for flares had to include how they were such multi-wavelength events. They somehow produced all types of radiation seemingly at once.

  POLITICS AND PLUTONIUM

  By the end of the 1950s enough data had been collected to show that bursts of X-rays were a fundamental part of flares. And since X-rays are emitted by incredibly hot plasma they showed that the solar flare had heated regions of the Sun’s atmosphere to temperatures as high as 10 million Kelvin; this is several times the temperature of the ambient corona and a good percentage of the tempera
ture in the Sun’s core. Each observation using a rocket unveiled another part of the overall picture of what a solar flare is and it drove the early space pioneers to want to see more and for longer. They needed to observe the Sun for longer than a rocket or rockoon could achieve. They needed to use rockets to put solar observatories into orbit around the Earth.

  The first solar observatories to be launched came once again from the Naval Research Laboratory. They began launching solar satellites in 1960 with the SOLRAD series (SOLar RADiation). They were small satellites, around half a metre across. NASA followed in the NRL’s footsteps when it launched its Orbiting Solar Observatory on 7 March 1962. This satellite provided images that could pinpoint the location of the flare and took very detailed information about the ultraviolet and the X-ray spectra of flares.

  The Soviets were working hard on understanding solar flares too and launched their missions Cosmos 166 and Cosmos 230 in 1967 and 1968, carrying telescopes that could see more detail in the corona. What was seen led to a change in perception of where to look to find out more about solar flares. The Soviet observations revealed that even though scientists had been looking in the right wavelengths to study solar flares, they had been thinking about the wrong part of the atmosphere.

  The Soviet satellites showed that the X-ray emission from solar flares could be seen as high up as 20,000 kilometres above the chromosphere. And not only that: they hinted that the high-altitude flash of the flare came first – flares initially burst into view at high altitude and the chromosphere only lights up as a secondary effect. Were the observations made in visible light during the previous 100 years something of a red herring?