by Lucie Green
These frequencies are far below the range that the human ear can detect, which goes from roughly 20 to 20,000 hertz. The sounds of the Sun would need to be speeded up tens of thousands of times if we wanted to audibly enjoy the solar symphony. But it’s not only the size of the pipe that matters – the gas inside is important too. The organ pipes that I was standing next to with Bill were full of air at the same temperature and pressure as the air around us in the hall. Inside the Sun the gas is in the form of plasma and its pressure and temperature increase with depth into the Sun. This has a very useful effect on the sound waves: as they travel into the Sun, the increasing plasma temperature speeds up the deeper part of the wave more than the part in the shallower plasma; this bends the wave back up to the photosphere, where we can observe it.
The millions of tones that ring inside the Sun produce a complex pattern of oscillations at the photosphere that are detected by the Doppler shift of the light emitted from the moving plasma. So we do not listen to the Sun in a literal sense: rather, the signatures of the trapped sound waves are coded into the Doppler shifts, and by measuring these shifts we get access to the information carried by the sound.
Measuring these delicate shifts is not an easy task – it requires meticulous observations and the careful removal of the signature of motions caused by convection. But, once extracted, this web of oscillations is used to reconstruct the sound waves that produced them. All this effort is worth it because the trapped sound waves carry information about the interior layers that they have travelled through. This is how they become useful as a probe for the inside of the Sun.
This is not that different from how we know what is going on inside the Earth. Seismic tomography uses the sound waves produced by earthquakes to deduce what is going on inside our own planet. This is how we know that the Earth is more complicated internally than I implied at the start of this chapter – with various fluid shells moving differently.
Helioseismology allowed the internal plasma flows to be probed directly for the very first time when it was realized that sound waves travelling in the direction of the Sun’s rotation (with the moving plasma) would have a slightly higher frequency than similar sound waves travelling against the direction of rotation (swimming upstream). Bill Chaplin told me that what they found was not what the models had predicted.
In the outer parts of the core and the radiation zone the solar plasma is rotating almost like a rigid sphere. This shell of plasma extends out to over 550,000 kilometres from the centre and is spinning about once every twenty-five days – more slowly than the photospheric plasma near the equator, but faster than the photospheric plasma towards the poles. So around the equator, the convection zone is sliding over the radiation zone, constantly overtaking it. However, at the poles, the convection zone lags behind the radiation zone spinning below it. (See plate 5.)
You may have noticed that above I used the phrase ‘outer parts of the core’. This was very deliberate. In the innermost part of the core, it becomes very hard to use the technique of helioseismology to find out about the rotation rate. Very few of the sound waves make it this far into the Sun and it’s hard to pick out the information about the rotation rate of the core and distinguish this from the rotation rate of the higher layers of plasma that the sound wave has travelled through. So for now we can only say that the plasma at the very centre may rotate as a rigid sphere at the same rate as the plasma in the outer part of the core and the radiation zone, but it may not. The movement of the innermost region of the Sun’s energy-generating core remains a mystery and an important challenge for helioseismology to solve in the future.
The successes of helioseismology created a new view of the solar interior and showed a plasma sphere that has two sections which rotate in very different ways – a rigidly rotating interior on top of which sits a layer with a differential rotation. The outermost layer is constantly slipping and sliding over the innermost one. This finding was once again totally unexpected. And the region where the rotation transitions from varying across latitudes to being that of a rigid sphere, just below the convection zone, has been given its own name – the ‘tachocline’.
This name is inspired by the ocean’s thermoclines. In an ocean, the thermocline lies below the top layer of water, which is heated by the Sun and mixed by waves so that it has a fairly constant temperature with depth. Below this lies the main body of cold water. At the interface between the warm upper layer of water and the cold region is the thermocline and the temperature varies greatly across this relatively thin layer. In the tachocline, it is the rotation rate of the plasma that is varying very rapidly across a narrow region.
Helioseismology quickly showed itself to be a very powerful technique in probing the interior plasma flows of the Sun. It came to be used not only to see inside the Sun, but also to see all the way to the other side so that images of the locations of sunspots could be created before that side of the Sun rotated into view. The magnetic fields of sunspots affect the propagation of sound waves. Sound waves that converge on the distant sunspot are modified slightly, causing a change in the frequency of the wave. In this way, sound waves which have bounced from the near side to the far side to the near side again will carry with them information about sunspots that are hidden from our view. I find it amazing that daily maps of the sunspot activity on the far side of the Sun can be created by using sound waves to look right through the Sun.
PROVING EINSTEIN RIGHT
With the planet Vulcan never having been discovered, a new solution to Mercury’s orbit was proposed. In the early twentieth century, Einstein was changing the face of physics with his new ideas, including an overhaul of Newton’s theory of gravity. Using his upgrade of Newton’s equations predicted the orbit of Mercury to be exactly as it was observed. Einstein’s theory was that the perihelion position of Mercury should advance over time because of the strong curvature of space-time close to the Sun.
Except it wasn’t the only non-Vulcan solution to the Mercury problem. The Sun was also in the running.
The Sun is formed into a sphere by its own gravity, which is constantly pulling the plasma towards the centre. As gravity pulls, a sphere forms, but it might not be a perfect sphere. The rotation of the Sun brings into play another force that can change its shape: centrifugal force. You feel this force when cornering in a car, and we saw earlier how it gave us the flat disc of the Solar System when it collapsed from a cloud of dust. If the Sun spins fast enough it will bulge around its equator, and if it bulges sufficiently this will also affect the Sun’s gravitational field and contribute to Mercury’s peculiar perihelion movement. This would mean that the contribution proposed by Einstein’s theory was less than predicted and the near-perfect match between theory and observation would be lost. If a bulge in the Sun could be detected, it would throw Einstein’s theory into question.
In short: Einstein’s theories matched the observed orbit of Mercury, which Newton’s didn’t. But Einstein knew about Mercury’s peculiar orbit in advance. A cynic would say it’s easy to make a theory fit data you already have. The upshot of Einstein’s work was that it would only work perfectly if we had a bulge-free Sun, and that was something we didn’t know in advance. No one had ever measured the Sun’s bulge.
However, this is an extremely challenging task. Precise and accurate measurements of the width of the Sun versus the distance from pole to pole need to be made to see whether or not the visible disc of the Sun makes a perfect circle in the sky. This was simply not possible with telescopes. By then it was known the bulge scientists were looking for would only be a few tens of kilometres. This is tiny compared to the
696,000-kilometre radius of the Sun.
To this day, the bulge of the Sun has not been confirmed or denied by telescope observations – and it seems that the exact shape of the Sun varies during the solar cycle. But helioseismology has come up with an answer because it can probe how fast the Sun is spinning on the inside. For a start, the Doppler shift of the photosphere of the Sun had shown that the outer layer was not spinning fast enough for it to bulge out. Any bulge must come from rotation deeper within the Sun. Using sound waves to probe the Sun’s interior has revealed that the radiation zone – the rigid ball of plasma which contains the majority of the Sun’s mass – is not spinning enough to bulge either. Einstein was right after all, adding weight to a new description of gravity that has been ushered in.
So, in this way, the study of the Sun across the ages, up to and including the modern helioseismology work at the University of Birmingham, is not just for esoteric reasons. It has had direct impacts on other areas of physics. Few modern theories have been put to as much practical use in our modern society as the work done by Einstein. But I started this chapter talking about a practical motivation for understanding the Sun that is a bit more frightening: its ability to disrupt our way of life on Earth.
That also comes down to the movement of plasma within the Sun. Now we know how the parts of the Sun rotate, and we understand that magnetic fields form sunspots, we can combine the two to explain the dramatic event that happened in September 1859, when the skies lit up with aurorae and some mysterious effect paralysed the electric telegraph. What we’ll see next is that the rotating plasma inside the Sun is powering a magnetic dynamo, which amplifies the Sun’s magnetic field and stores energy in it. This energy can be released in the Sun’s atmosphere to drive the biggest explosions in the Solar System. And they have grave implications for humankind’s future. Our deadline to understand the Sun is becoming more urgent.
7. The Dynamic Sun
If the Sun had no magnetic field, it would be as uninteresting as most astronomers think it is.
Attributed to Robert Leighton, c.1965
This quotation attributed to Robert Leighton, father of the five-minute oscillations, often comes to my mind when I am talking about the Sun. I love it because I can see that the Sun probably does appear unremarkable when it is pitted against other astronomical objects like super-massive black holes, distant galaxies or the mystery of dark matter. With cosmic wonders like these, how can a seemingly bright circle with a few occasional dots compete?
Well, the Sun comes to life when you realize it has a magnetic field. It is the presence of this field that turns an otherwise bland ball of hydrogen into a star which has spots and flares and which changes from day to day and month to month. And as we saw with the Carrington event, the Sun is an electromagnetic body that can release enough energy for it to directly interact with our own magnetic field here at home, on Earth. To understand these solar attacks, we need to answer three questions:
Why does the Sun have a magnetic field?
How strong is the magnetic field?
Why is its magnetic field so complicated and variable?
Incredibly, the answers to all these questions come down to the thin convection zone on the outside of the Sun, not the massive interior below, where the fusion is happening and which actually powers the Sun.
ORIGIN OF THE MAGNETIC FIELD
Despite only being able to see less then 5 per cent of the Universe directly, observations reveal that magnetic fields are everywhere. Stars have them, galaxies have them and even clusters of galaxies that are bound together by gravity have magnetic fields threading between them. One scenario for these cosmic magnetic fields is that they were created during the first 400,000 years after the Big Bang, by the freely moving electrically charged particles – electrons and protons – which the Universe was made of at that time. We see that magnetic fields pervade the Universe and we think that they may have been there for a very, very long time. For this reason, it is possible that these primordial magnetic fields were still there when the Sun formed.
Magnetic fields don’t last for ever but cosmic magnetic fields can have very long lives. If the electric current that is creating them dissipates and disappears, perhaps because collisions between the charged particles interrupt its flow, so too does the magnetic field. Size matters too. When you turn off your kettle, the electric current stops and the magnetic field goes to zero. For a kettle, this is almost instantly. For plasma structures on an astronomical scale, it can take much, much longer. And this could explain the origin of the Sun’s magnetic field.
It is conceivable that the cloud of gas and dust out of which the Sun formed had its own weak magnetic field threaded through it. If some of the gas that makes up the nebula was ionized – electrons having been stripped away from the atoms – as the nebula collapsed, the ionized gases would have dragged the magnetic field with them. Not only did matter coalesce and clump together to form the Sun, but magnetic field could have been piled up as well.
From the size and electrical conductivity of the Sun it’s expected that any magnetic field that was there when the Sun formed from the solar nebula might take billions of years to decay. So there could be some remnant still trapped inside the Sun today. But that would be far too weak to match what we see, and definitely not enough to explain the strong magnetic fields inside sunspots. And the remnant of the magnetic field could be expected to form a simple field with magnetic poles close to the rotation poles – as we have on the Earth. But even though the Sun and the Earth share some similarities when it comes to their magnetic fields, there are many differences to what we see at their respective surfaces. We need to explain a magnetic field on the Sun that is very complex as well as being very strong, forming sunspots that vary in number and position at the photosphere.
It’s worth remembering that the presence of a magnetic field in the Sun isn’t surprising since the Sun is made of plasma. After all, moving electrically charged particles are the origin of all magnetic fields, and the spinning Sun has an abundant supply of these. The plasma of the solar sphere makes the Sun a great conductor of electricity on a colossal scale. A plasma is one of the best electrical conductors there is. So when electric currents start flowing inside the Sun, they don’t meet any resistance and it’s very hard to stop them. As long as the electric currents keep flowing, the magnetic field will remain.
But there is an added complication. A magnetic field embedded in a plasma is nothing like the magnetic fields we are used to at home. When you attach a magnet to your fridge, or turn on the kettle, you are using a permanent field locked in a bar magnet or inducing a new one with a flowing current. A plasma falls somewhere in between.
A plasma is not a solid object which has moving electrons within it (either moving within atoms or electrons flowing down a wire) but rather a mobile mist of its own charged particles. And it is vast. In your home the magnetic field created by your fridge magnet or the current in your kettle’s wire emanates out into the air in the room. In the Sun, the magnetic field spreads through the plasma, which is moving all the time in a multitude of directions. A branch of physics has been developed to describe the interaction of magnetic field and a plasma: magnetohydrodynamics.
This branch of physics allows us to probe how a magnetic field changes over time when it threads through a mobile plasma. As we might expect, this area of physics has a long history and is the outcome of people’s curiosity about electric and magnetic fields. To understand how magnetic fields change over time requires a combination of discoveries that have been applied to electrically conducting fluids like the
Sun. They are named after the people who worked on them. Combining Ampère’s law to describe the magnetic field around the electric current that is flowing, Ohm’s law to describe the electric fields associated with a mobile plasma and a magnetic field, and Faraday’s law which describes the relationship between magnetic field and electric fields, gives rise to a curious consequence.
As well as the mobile particles in a plasma moving and creating a magnetic field, the magnetic field becomes ‘frozen in’ to the plasma. This is the result of how the forces applied to a charged particle in a magnetic field strangely always act orthogonally to the field lines (called the ‘Lorentz force’, after the Dutch mega-physicist Hendrik Lorentz). The field can have as much control of the plasma as the plasma has of the field.
This exclusively sideways force means the particles are never pushed further away, or drawn closer to the magnetic field, but rather are made to spiral around the field lines, forever trapped like a planet around a star. And either one can take the other with it. The plasma cannot move without shifting the magnetic field as well and the magnetic field will never let the plasma escape. This is why the intense magnetic fields of sunspots trap plasma in place and allow it to cool.
The task now is to use this knowledge of magnetohydrodynamics to link the original simple magnetic field the Sun was born with, and that probably lives on in the radiation zone, with the magnetic mess we see coming out of the plasma-filled convection zone. The leftover primordial magnetic field by itself would simply be too weak to form the surface magnetic features of sunspots. But if the Sun formed with a weak magnetic field already present, it could have acted like a seed that has grown and become the magnetic field that we see today. Somehow the plasma takes the magnetic field the Sun was born with and mutates it into something much more interesting.
