by Lucie Green
Halley managed a glimpse of this other layer when he viewed the eclipse of 1715. Just as totality was ending, Halley, who had been using his six-feet-long telescope, saw that as the photosphere began to reappear he didn’t see the bright white light that he was expecting. Instead, he had a fleeting glimpse of a long but narrow crescent of deep red light that arced round the edge of the Moon. In seconds it was gone and the photospheric light lit up the daytime sky once again. What on the Sun had he just seen?
It turns out that what Halley saw is a layer of the Sun’s atmosphere which is the stepping stone between the photosphere and the corona. Its distinct rosiness is in stark contrast to the white light of the photosphere and the corona and has earned it the name of the ‘chromosphere’, meaning sphere of colour. It spans several thousand kilometres above the photosphere but can only be seen with the naked eye just as totality begins and ends during a total eclipse. The Moon is big enough to cover the chromosphere, and so you only see it when the Moon is not quite in position (but close enough to being in position for it still to block the light from the photosphere).
Eclipse observations showed that the chromosphere is an irregular and jagged layer of the atmosphere. It has a shape resembling the spines on a hedgehog, vertical tubes that have become known as ‘spicules’ that extend upwards for 9000 kilometres, a distance that is not far off the diameter of the Earth but only a tiny fraction of the size of the Sun. Jets of hot plasma shoot up the spicules and they live for no more than fifteen minutes or so before fading away, only to be replaced by new ones. And then there are clouds of chromospheric plasma that are lofted up to great heights, rising 150,000 kilometres into the corona above and visible when the Sun is fully eclipsed. The Victorian eclipse observers described them as ‘red mountains’. Today we call them ‘prominences’.
Thankfully the brief flash of the chromosphere at the beginning or end of an eclipse is all that is needed to capture an image of the chromospheric spectrum. As always, we learn about the plasma in different parts of the Sun by studying the spectra that they emit. At first glance there are some similarities between the spectra of the chromosphere and of the photosphere, in that a very faint rainbow of continuous colours is seen in both. But there are two major differences.
Firstly, the spectrum of colours is much dimmer in the light coming from the chromosphere. The whole rainbow is there, but it has been muted. Secondly, there are no dark Fraunhofer lines in the spectrum of the chromosphere. In place of the dark lines there are, bizarrely, extra-bright lines! When the light coming from the chromosphere is split into its spectrum the cosmic barcode is reversed: what was dark becomes bright and what was bright is diminished. So why does the chromosphere produce a spectrum that looks so different?
We know the dark Fraunhofer lines appear because light from the photosphere has to pass through the rest of the photospheric plasma as it travels out and some of its photons are stolen along the way. The photosphere gets cooler with height and some of the atoms and ions in the relatively cool gas are able to absorb photospheric photons, but only those photons with just the right wavelength (frequency) to make an electron jump up to a higher energy level. This gives us the sharp absorption lines in the otherwise continuous rainbow.
Importantly, though, when these atoms and ions absorb a photon, that energy is not destroyed or otherwise disposed of. The electron can drop back down from this higher-energy orbital and the same frequency photon can be re-emitted. But that released photon can go in any direction, not necessarily travelling on towards us like the light it was originally stolen from was. So while most frequencies can stream out of the photosphere, those few frequencies that can be absorbed and re-emitted by atoms and ions in the chromosphere have a much harder time, constantly being bounced away from us, and perhaps even being sent back into the photosphere, where they can be absorbed by a negative hydrogen ion so that the whole process has to start again.
But in the chromosphere at the start and end of a total solar eclipse the exact frequencies that struggle to get out of the photosphere suddenly become the frequencies we see the most of! But this is a kind of optical illusion, a trick of contrast.
Normal Fraunhofer lines are not actually completely dark. The photons at those frequencies are absorbed and re-emitted in random directions which still allow them to eventually escape the Sun. It’s just in comparison with the other frequencies from the photosphere, which are so much brighter, that these lines are made to look dark. When we view the chromosphere during an eclipse, we are looking high enough in the Sun’s atmosphere for, from our point of view, the photosphere not to be behind it. So when the photosphere is completely blocked by the Moon, we only see these selected frequencies being scattered sideways out of the chromosphere and towards us. So they seem relatively bright.
Now we can see what these lines in the spectra tell us about the chromosphere. The most prominent line in the chromospheric spectrum is the hydrogen alpha line. This line is in the red part of the spectrum and it’s what gives the chromosphere its trademark red colour. This was the wavelength in which Hale chose to image the Sun in 1908 and it allowed him to see the plasma in the chromosphere just above the sunspots outside of the time of a total solar eclipse and not pick up the whole spectrum of colours of the light coming from the photosphere. This is why the spectroheliograph that he used was so important: it filtered out all of the light from the photosphere, leaving only the faint chromosphere. It was the same effect seen during an eclipse but, instead of only being at the very edge, it worked right across the surface of the Sun.
If you do look at the Sun using just the light of hydrogen alpha it is incredibly beautiful, the swirling prominences the Victorians had seen around the edge of the Sun actually extending right across it. Prominences appear as dark features known as filaments and can be tracked across the Sun as it rotates. They are actually clouds of relatively dense and cool chromospheric plasma suspended in the corona. In hindsight, the name ‘red mountains’, which the Victorians used for prominences, was surprisingly appropriate: they contain around the same mass as a mountain on Earth – 100 billion kilograms. They are vast structures that are somehow held aloft by an invisible force. Gravity isn’t able to pull these plasma mountains inwards.
THE NEW ELEMENTS
The chromospheric spectrum shows more than just the inverted Fraunhofer lines: there are extra lines – other wavelengths of light are emitted that are not seen in a normal solar spectrum. In the nineteenth century a spectral line was spotted in the yellow part of the visible spectrum that doesn’t appear as a Fraunhofer line and didn’t match anything that had been produced by a gas studied in the laboratory. Scientists couldn’t work out what element was responsible for this new line.
The same thing happened in the corona. When the technique of spectroscopy was used to study the faint light coming from the corona it wasn’t long before another bright emission line, which hadn’t been seen before in the laboratory, was observed. By the end of the nineteenth century, three unidentified coronal emission lines had been discovered: a green line at a wavelength of 530.29 nanometres, a yellow line at 569.45 nanometres and a red line at 637.45 nanometres.
As all of these lines had never been seen using elements on Earth, there was only one conclusion: there must be new elements on the Sun! Thirty new elements had already been discovered during the nineteenth century so it must have seemed entirely plausible that this was the explanation. The mysterious new element in the chromosphere was called ‘helium’ after the Greek personification of the Sun, Helios, and the new element in the corona was called ‘coronium’. You cannot help but notice that
one of those elements is more familiar-sounding than the other …
In our modern world of helium party balloons, it is hard to imagine a world where helium was completely unknown. But that was the case in the 1800s. The discovery of the chromospheric spectral line assigned to helium came independently to the French astronomer Pierre Janssen and the British scientist and astronomer Norman Lockyer in 1868. Both were very adept at astronomy and spectroscopy and both had developed techniques to study the chromosphere’s light.
Janssen was also an early eclipse chaser. After the eclipse of 1868 he went on to travel to see eclipses in 1870, 1875, 1883 and 1905. They took him all over the world. To get to Algeria for the eclipse of 1870 he had to find a way out of his hometown of Paris, which was surrounded by Germans at the height of the Franco-Prussian war. He made his escape in a hot-air balloon (yet another situation when helium would have helped).
Lockyer in his later studies stayed closer to home, his pioneering work in the area of spectroscopy and astronomy being reflected in his appointment as the first ever university professor in ‘Astronomical Physics’ at what is now Imperial College, London. It was Lockyer who suggested the name ‘helium’, and for the next thirty years the Sun was the only place where it had ever been found.
It wasn’t until 1895 that helium was finally discovered on Earth, when an experiment with a radioactive uranium mineral was carried out at my university, UCL, showing once again that the material that makes up the Sun is also the material that constitutes the Earth. The experiment was carried out by William Ramsay, who was head of chemistry at the time and one of the most famous scientists of his day. He discovered five new elements, which have become collectively known as the ‘noble’ gases, was knighted in 1902 and was awarded the Nobel Prize for Chemistry in 1904.
We still have at UCL today the original sample that Ramsay used to make his discovery, its having been rediscovered after many decades in storage. From this discovery and the work of Cecilia Payne-Gaposchkin we know today that helium makes up around 25 per cent of the mass of the Sun, and in fact it is the second most abundant element in the Universe, having been formed along with hydrogen after the Big Bang. So why was the second most abundant element so hard for us to find that it was discovered on the Sun before we found it right on our doorstep?
Following the solar nebula model for the formation of the Solar System, the Earth should have had its fair share of helium. But that original stock has since been lost. The reason for this, and the reason why helium is used in party balloons, lies in the small mass of the helium atom: the gas particles are simply too light and move too fast for the Earth’s gravity to be able to hold on to them. Like many an accidentally released party balloon, the early helium on the Earth simply drifted up and away.
What we have now is ‘new’ helium extracted from natural gas. All the helium you see in balloons has actually come from deep underground.
The Sun kept its original helium but it is hard to see and the reason for this is also a consequence of the very small size of the atom. In order for helium to reveal its presence on the Sun it must either absorb a photon and contribute to an absorption spectral line or emit a photon and contribute to an emission line. But the smallness of the helium atom, where the two electrons are held tightly to the nucleus, means that absorption lines are hard to produce. There is actually very little radiation emitted by the Sun with enough energy to do this. The result is that there are no helium absorption lines in the photospheric spectrum for our eyes to see.
For a helium atom to emit a photon the atom must have an electron that has already been promoted to a higher energy level, perhaps through a collision with a freely moving electron that has been liberated from another particle. When the electron falls back to its original energy level it releases the energy gained by emitting a photon of a very particular wavelength and this contributes to an emission line. The problem here is that the plasma needs to be at around 20,000 Kelvin for the freely moving electrons to be moving fast and have enough energy to be able to do this. In the photosphere the plasma simply isn’t hot enough to excite the helium atoms and cause them to radiate light. Yet helium emission lines are seen being emitted by the chromospheric plasma, and this reveals something important – the plasma in the chromosphere must be hotter than that of the photosphere.
The observation of helium in the chromosphere not only meant the discovery of a totally new element but it also showed that going up in altitude from the photosphere, moving further away from the energy source in the centre of the Sun, the temperature of the plasma starts to increase rather than continuing to decrease. This is completely counterintuitive. Up until this layer the temperature of the plasma has been dropping off with distance from the core because energy is being lost through radiation at the photosphere. Now that situation is reversed. There must be another energy source that is heating the chromosphere. (We’ll come back to this later.)
Coronium has a very different story and sadly never found its way into party balloons. Mainly because it does not exist. One of the other great inventions of the nineteenth century was the periodic table, in which all elements could be categorized. It was so powerful that gaps in the table even make it possible to predict the existence of previously undiscovered elements. The hunt was on for coronium to fill one of the gaps.
By the late 1800s the periodic table was filling up, though, and there was less and less room for coronium to fit in. It was becoming apparent that a previously undiscovered element might not be the cause of the new spectral lines in the corona. Seventy-four years after coronium was first discovered it was found that it does not exist – it was ‘anti-discovered’. It turns out that coronium was actually iron and calcium in disguise.
The green coronium line is produced by the emission of photons from an iron ion that has lost thirteen of its electrons (Fe XIV). The yellow line is formed by the emission of photons from a calcium ion that has lost fourteen of its electrons (Ca XV) and the red line from iron with nine lost electrons (Fe X). These particles have all lost a significant number of their electrons.
The lines were difficult to identify because the laboratory conditions in which incandescent gases were being studied were so vastly different to the plasma of the corona. The density of the corona is only 10 million billionths of that of water, or 10,000 millionths of the air around you, and such a thin gas cannot be reproduced easily in the laboratory. This means that the ions in the lab can behave differently to those same ions in the Sun. And conversely it shows why the Sun makes such a fantastic laboratory in its own right – we can study an environment that we can never re-create here on Earth.
In the laboratory, changes to the energy that an ion has are controlled by collisions with electrons – the gas is always dense enough for collisions to be occurring. In the low-density corona the frequency of collisions is low. If a collision between an electron and an ion does occur it will be a long time until the next collision takes place. During this long wait, the ion can spontaneously release the energy it previously gained by emitting a photon. The exact wavelength of the photon depends on the ion and how much energy it has, but the possibilities include the green, yellow and red emission lines. The very low likelihood that these emission lines would be seen on Earth earned the name ‘forbidden’ lines. Actually, they are not forbidden, they are just extremely rare.
This explained coronium but produced a new mystery. For so many electrons to have been knocked out of iron and calcium atoms, the plasma temperature must be very high. If scientists had been shocked to discover that plasma in the chromosphere had a temperature in the 10,000s Kel
vin, they were about to be blown away by the corona. For iron and calcium to produce the coronium lines, the plasma in the corona must be over a million Kelvin. That’s several hundred times hotter than the photosphere.
CROWNING GLORY
Looking up at the corona from the cruise ship, I could not see any of these spectral details. To me the corona looked its normal pearly white. But even that reveals something about the nature of the corona. The thin plasma of the corona shouldn’t be able to produce a continuous spectrum – that’s only something that a much denser gas can do.
The hint to what is going on is that the continuous coronal spectrum has a very similar appearance to the photosphere’s spectrum. This tells us that the photons that we see during an eclipse coming from the corona were created in the photosphere. When I looked at the eclipsed Sun in 2009, it was actually photospheric light that I was seeing. The light originates in the cooler, denser plasma lower in the atmosphere. But as the photons rush outwards from the photosphere they flood into the million-degree coronal plasma where many electrons are flying around that have been stripped away from the parent atoms. In the same way that photons are scattered in the radiation zone by electrons, photons also get scattered in the corona. The ones that are scattered in our direction are the ones that we see.
Then, finally, I looked right out, at heights of about two or three times the radius of the Sun, where a very faint ring of light could still be seen. In this region the photons are still being scattered in our direction, but not by free electrons in the coronal plasma. This light is scattered by tiny dust particles that were left over from the formation of the Solar System from the solar nebula. High above the corona sits a faint souvenir from the birth of the Solar System itself.
