by Lucie Green
The Sun is far more complicated. As a giant ball of plasma it is a fluid object and does not all have to rotate in unison: different parts of the Sun can be spinning at different speeds. In addition, it can be hard to observe how fast bits of it are moving because, with the exception of sunspots, it just looks like a bright blank disc to us. But, as always, scientists have come up with some pretty ingenious ways to find the answers they are after, but there remains a question: why do we care?
I am a career scientist myself, so like generations of people before me the innate human curiosity to understand the world around us is more than motivation enough. All the people I’ve covered in this book have striven to understand the Universe around them to sate their inquisitive minds. Sometimes it was out of curiosity about the Sun, but mostly it was just out of a curiosity that then found an application in understanding the Sun. But in 1859 the Sun gave a motivation to hurry along the investigation for pragmatic reasons: it took a shot at us. It started innocently enough. During the night of 1 September 1859 there was an intense display of the aurorae that erupted all over the globe. Normally a beautiful but rare natural occurrence unless you live at high latitudes, the aurorae were seen by the masses that night because they moved from their usual position around the poles and, in the northern hemisphere, reached as far south as Florida. Some people panicked, thinking that great fires had started. But most people just gathered on street corners to admire the spectacle.
Telegraph operators were not having such a great time though. The telegraph was the internet of the nineteenth century and was vital for sending information all around the globe. Suddenly the operators were struggling to send their messages and many lines had to be closed. Some telegraph offices caught fire as the equipment started to spark uncontrollably. Others disconnected their equipment from its batteries in an attempt to shut it down but found that the lines continued to operate with no battery attached at all.
And, at the same time, the magnetic observatories at Kew and Greenwich in London recorded something astonishing. They had magnetometers monitoring the Earth’s magnetic field and they suddenly detected large changes. At one point the recordings even went off the scale. The Earth’s magnetic field had suddenly and drastically changed. Then, over a number of days, things gradually returned to normal: the telegraph lines started working normally again and the aurorae receded back to their oval round the poles.
There had been some warning of this spectacle though. Seventeen and a half hours earlier a British brewer and amateur astronomer, Richard Carrington, had been projecting an image of the Sun onto a screen so that he could study its appearance that day. He had developed a very good method of filtering and projecting the Sun in order to measure and record the positions of sunspots. But going about his routine on that day he suddenly became, in his own words, an ‘unprepared witness of a very different affair’ when, within a large sunspot group, two patches of intense white light flared into sight. At the same moment, thirty kilometres to the north at Kew, the magnetometers twitched slightly.
The light that Carrington saw in the sunspot group was so bright he thought it must have been a hole in his equipment. But this wasn’t the case – everything was in order. Realizing that something very unusual was happening, he ran to find someone to share the brilliant sight with, and even though he returned hastily, within just a few minutes the light had moved across the sunspot group and was quickly vanishing – leaving no sign of what had just happened. As Carrington looked on, the appearance of the sunspots was just as it had been before the flash of light. Richard Hodgson, another amateur astronomer, who was observing sunspots from London, also saw the fleeting intense light and these corroborating observations were to prove crucial in getting the astronomical community to take what had happened seriously. Nothing like this had been witnessed before.
Carrington and Hodgson had been the first to witness what we now call a ‘solar flare’. What ravaged the Earth later that day was an intense geomagnetic storm, the magnitude of which we thankfully haven’t seen since. At the time, because there was no physical understanding to link the geomagnetic storm to the flare on the Sun, Carrington was not prepared to rule out its being a coincidence. We now know that the Sun was to blame and, more than that, it poses a serious threat to our way of life on Earth. Understanding and predicting these events is directly linked with what Carrington was trying to do all along, study the rotation of the Sun.
IN A SPIN
The Sun rotates about an axis like the Earth does and this had been realized from the early sunspot observations. For observers in the northern hemisphere it rotates in a right-handed sense. Close your right hand into a fist, with your thumb pointing up, and your fingers point in the direction that the Sun rotates and your thumb represents the axis of rotation and points north. Sunspots then move from left to right across our view.
Carrington used the movement of sunspots to track the rotation of the Sun’s photosphere. We see the legacy of his work today in our use of the ‘Carrington rotation’. This is a twenty-seven-day rotation rate, seen from the Earth, that Carrington worked out from sunspots near the Sun’s equator. Carrington rotations are an extremely useful way to track the Sun’s rotation given that there are no fixed features on the photosphere which would enable lines of longitude to be tracked. The first Carrington rotation was designated as beginning on 9 November 1853 and we have been counting them ever since. As I type we are in Carrington rotation number 2162.
But it is not as simple as just looking at the sunspots near the equator. It had been noted, even as early as 1630 by Christoph Scheiner, that sunspots near the equator move faster than those up towards the poles. So Carrington was able to use sunspot motions to quantify speed variations in the latitudes where spots formed. And by the time of his measurements it had been discovered that there were intriguing regularities and patterns in the formation of sunspots.
The first pattern spotted in the behaviour of sunspots was a serendipitous discovery. The Sun was actually being used as a backdrop to try and find unknown planets that are orbiting closer to the Sun than the Earth. Anything that passes between the Sun and us will be silhouetted against the luminous disc and will be seen scuttling across it. Venus does this twice (eight years apart) roughly every 115 years. Mercury, which is much closer to the Sun and orbiting once every eighty-eight days, crosses much more frequently. This is why Kepler’s original interpretation for the spot he saw was that it was Mercury transiting the Sun.
In the 1800s there was a popular idea that there might be a planet orbiting the Sun even closer than Mercury. Like all the planets in the Solar System, Mercury makes an elliptical orbit around the Sun rather than following a perfect circle; its distance varies from 69.8 million to 46.0 million kilometres. The point at which the planet is the furthest from the Sun is known as the ‘aphelion’ and the closest approach known as the ‘perihelion’. Over time the elliptical orbit of a planet drifts around the Sun like a hula-hoop – we say it ‘precesses’ – and astronomers could see Mercury’s perihelion and aphelion drifting around accordingly.
Only they were not moving the way people expected. The perihelion and aphelion points were moving faster than the calculations predicted they should. Factoring in all of the effects from the Sun and other planets predicted a precession in Mercury’s orbit far slower than what was actually happening. These calculations had been done meticulously using Newton’s equations. It was reasoned that either Newton’s equations were wrong, or there must be an unseen planet that had not been included in the calculations.
A precedent had already been set in this area after William Hersche
l had discovered Uranus. Its orbit also was not exactly like that predicted by Newton’s theory, so another planet was posited to exist that would have the right gravitational pull on Uranus to explain its motion. A French mathematician, Urbain Le Verrier, was one of the people who calculated exactly what additional planet could be added to the Newtonian model to give the observed orbit of Uranus. Astronomers turned their telescopes to that location and sure enough it wasn’t long before the planet we now know as Neptune was found in 1846.
Flushed with his success, Le Verrier turned his maths to the orbit of Mercury. He predicted that there was another planet orbiting even closer to the Sun and he named this hypothetical planet ‘Vulcan’ after the Roman god of fire. Astronomers went to great lengths to be the first to spot Vulcan.* The difficulty was in working out what was a sunspot and what was a potential planet passing in front of the Sun. Both cases would look like a black dot.
One person who was on the hunt for the new planet was Samuel Heinrich Schwabe, a German pharmacist turned astronomer. He patiently observed the Sun on every day that he had clear skies, year after year from 1826 to 1843. He duly recorded all the sunspots. He knew that a planet would stand out as a spot that repeatedly came back, again and again, whereas sunspots would randomly appear and disappear. But, alas, his decades of work produced no evidence for the hypothetical planet that promised to keep Newton’s theory on its scientific pedestal. However, Schwabe’s perseverance paid off in a way that he couldn’t have expected when he made a major discovery about sunspots.
As Schwabe looked through his vast record of sunspots a pattern started to emerge. Their number seemed to change over time and did so in a regular way. Schwabe realized that over roughly a ten-year period the number of sunspots rose and fell as if it were following a cycle. His careful and long-term observations also showed that sunspots didn’t appear in a random way on the Sun – there seemed to be a repetitive process at work. Schwabe had discovered, and published in 1843, what is now known as the ‘sunspot cycle’, or ‘solar cycle’, and it represents the very slow heartbeat of the Sun.
Although the length of the cycle can vary between eight and fifteen years, the average cycle length is found to be almost exactly eleven years. The very early sunspot observations were used to reconstruct the sunspot number back to 1755 and this revealed a cycle that ran between 1755 and 1766. This is known as ‘cycle 1’ and the following cycles have been numbered consecutively. I started my career in solar physics in cycle 23 and we are currently enjoying cycle 24.
In the decades following the discovery of the sunspot cycle, in Germany the astronomer Gustav Spörer became interested in understanding the distribution of the latitudes at which sunspots formed. At the start of a solar cycle, the first spots form at mid-latitudes around 30–40 degrees above and below the equator. As the cycle progresses, spots start to appear more frequently but the latitude at which they form moves closer and closer to the equator. This is now known as ‘Spörer’s law’.
As for the planet Vulcan, sadly it was never found. But it turns out that the Sun’s spinning had more to do with Mercury’s orbit than anyone expected.
WORKING IN SHIFTS
We had better fill in the gaps for how the rest of the Sun is spinning. The regions between 40 degrees either side of the equator were revealed by the movement of sunspots, but outside these bands sunspots are absent. A new method is needed to work out how the featureless plasma is moving in those regions of the photosphere.
In 1842, an Austrian mathematician and physicist, Christian Doppler, developed an area of physics that solved the problem. He realized that the wavelength of a spectral line would be shifted if the material that was emitting or absorbing the light was in motion. If the material was travelling towards us, the line would be shifted to a shorter wavelength and higher frequency, such as towards the blue end of the visible spectrum. If the source was moving away from us, the spectral line would shift into the red, or long-wavelength and low-frequency, end of the spectrum.
This change in frequency of a wave because of motion now has the generic name of ‘Doppler shift’. It is an effect we are used to in sound: a siren on a vehicle approaching us sounds higher in pitch than when it recedes from us; racing cars produce the classic high then low engine noise as they race past. Detecting this shift in the wavelength of spectral lines turns telescopes into interplanetary speed cameras.
This Doppler shift could be tested out on the Sun. Firstly, the shift in spectral absorption lines around the equator exactly matched the rotation speed calculated independently by Carrington using the movement of sunspots. Secondly, looking at the left and right sides of the Sun indicated that the plasma was coming towards and going away, respectively, exactly as you would expect from a rotating sphere.
When this technique was used on the plasma up towards the poles on the Sun it was shown that those regions were moving far slower than at the equator. What was first noticed using sunspot observations continues at higher latitudes. The Sun rotates fastest at the equator and then that speed drops off towards the poles. Imagine the Earth if countries near the equator raced around the globe faster, and had a shorter day, than those at higher latitudes! The question now was, if there is differential rotation on the surface of the Sun, what could possibly be going on inside?
Our ability to see inside the Sun began with observations made at the Mount Wilson Solar Observatory, but some time after the death of George Ellery Hale in 1938. In the post-Hale years, solar physics had become rather repetitive at Mount Wilson. Observations had become routine and there was perhaps little innovation, until a physicist called Robert Leighton arrived. An experimental physicist who did work across all aspects of astronomy and even in early particle physics, he was a friend of and collaborator with the Nobel laureate Richard Feynman. Leighton built and used incredibly accurate Doppler shift solar cameras to reveal that the Sun had another kind of motion. It was constantly vibrating, like the surface of a bell that is being hit repeatedly by a hammer.
The granulations in the photosphere moved around in the chaotic manner you would expect from convection cells, but layered over this was a much more regular movement. The surface was oscillating, with patches rising and falling continuously and regularly. Curiously, everywhere, the oscillations took on average five minutes to vibrate once. It was these oscillations at the visible surface that became the stepping-stone for probing the rotation inside the Sun.
Almost ten years after their serendipitous discovery, it was suggested that the photospheric oscillations were caused by sound waves trapped inside the Sun – the hammer that strikes the Sun is inside – so that the Sun constantly rings like a very low-frequency bell – so low in frequency that it’s well below the range of our hearing. And the theory that the Sun was full of sound waves held up to scrutiny – the observations matched the details of the theory – which meant that astronomers now had another way to investigate the Sun: as well as using light they could use sound. And the sound waves are a product of the constant motion of the plasma inside the Sun.
Inside the convection zone, the plasma is always moving and expanding and contracting, and that means that the Sun is acting like a bell which is constantly being struck with tiny blows. Imagine a bell that has sand constantly poured on it rather than being given one large hammer blow. But the Sun has its own natural filter that removes much of this noise and leaves series of sound waves at what are called the ‘resonant frequencies’. It is these sound waves that cause the photosphere to vibrate at specific frequencies, and there are millions of them.
Much of what I have learnt about the sounds
of the Sun has come from the group that does research in this area at the University of Birmingham. They have been studying the Sun’s sounds for many years, and to collect their data they operate a series of telescopes around the world that form a network to continuously monitor the Sun twenty-four hours a day. The network is called BiSON, for Birmingham Solar-Oscillations Network. And using sound to study the Sun is an area of solar physics called ‘helioseismology’.
The first person I met in the group was Bill Chaplin and I was pleased when I was able to interview him for the iconic BBC programme The Sky at Night. For the interview he took me to the university’s Great Hall. As soon as we walked in, I was faced with a vast pipe organ that stands either side of the hall. It has an impressive set of pipes, ranging in size from long fat ones several times as tall as I am, to shorter and skinnier ones. He asked the organist to play a note and a low-pitched, or low-frequency, boom came out. Instinctively I knew that the note was coming from one of the larger pipes.
Bill was making the point that the frequency of the sound that a pipe creates tells us something about the size of the pipe. They too have their own resonant frequencies. The air entering my ear from the sound that the pipe made was probably vibrating at around 100 times a second, which gives it a frequency of 100 hertz. The vibrations of the plasma at the surface of the Sun are much slower, taking place on a timescale of about five minutes (or about 0.003 hertz). But these so-called ‘five-minute oscillations’ are just part of a much larger family of vibrations and some take up to an hour to make one oscillation.
