by Lucie Green
And it was an instrument of Hale’s making which gave us the required insight into sunspots, an instrument that I use to this day to continue this study of sunspot regions. Called the ‘spectroheliograph’, Hale’s equipment was a steam punk-esque creation made out of glass and mirrors, wood and metal. I use the modern equivalents today, which need electronics and are built into spacecraft, but the basic principle of how they look at the Sun is exactly the same.
When Hale was young, the Sun was typically studied using telescopes that made an image using the entirety of the visible radiation, just as our eyes do. Photographic plates replaced the astronomer’s eye so that a permanent record of the Sun and its spots could be created – this kind of instrument is known as a ‘heliograph’. It was effectively pointing a camera with a zoom lens at the Sun: all the light coming from the Sun was allowed to reach the photographic plate.
The developments in spectroscopy had led to the use of a second instrument, where the Sun’s light was split into its spectrum so that the individual spectral lines could be analysed. These instruments are called ‘spectroscopes’ and they are vital because they allow the chemical composition of the plasma to be probed without the need to visit the Sun directly. But they are not fundamentally different to Newton letting a beam of sunlight hit a glass prism and cast a spectrum on the wall.
Hale devised a third way to view the Sun. The spectroheliograph is a photographic telescope combined with a selective spectroscope. A telescope is still used to magnify the Sun and project its image onto a photographic plate, but along the way all frequencies of light but one are removed.
The business end of the spectroheliograph pointing at the Sun was a thin slit. It only allowed the light from one vertical line across the Sun’s surface to enter the device. You can imagine the reverse of this set-up, looking out through the slit towards the Sun: you won’t see the full Sun – just a narrow strip from top to bottom.
Once inside the telescope this light was cast onto a prism, splitting the light into a full spectrum (or a grating could be used instead of a prism to produce the same result). This spectrum was cast onto an obstructing panel. This stopped all of the light from going any further, except for where one tiny slit in the panel allowed the light from just one spectral line to pass through onto a photographic plate behind and expose a vertical stripe.
So far this probably doesn’t sound like a particularly useful device, or even significantly different from a spectroscope. But when the entrance slit is gradually moved across the entire Sun and the photographic plate is moved in unison, you progressively expose an image of the entire Sun in only one wavelength of light. Importantly, that one narrow wavelength range you allow through corresponds to a specific movement between orbitals in one given element. Using a spectroheliograph you can take a photo of the Sun and only see the magnesium emission, completely removing the dazzling light from all other elements. Or you can choose any element you please (as long as it is emitting light).
Building this kit was no easy task – any vibrations from the moving telescope will blur the photograph. But by 1891 Hale, who was a final-year undergraduate student at the Massachusetts Institute of Technology at the time, had successfully crafted one.* Hale went to work at the University of Chicago and from there founded the Mount Wilson Observatory, about a fifty-kilometre journey north-east of Los Angeles in the US. He resigned from the University of Chicago to work full time at Mount Wilson in 1905.
Hale had almost exclusive use of the spectroheliograph at Mount Wilson and he used it to study the Sun’s lower atmosphere, first of all using a spectral line formed by calcium atoms that have lost one of their electrons. Calcium ions are then formed in the plasma in the photosphere and the region of plasma just above it. By imaging the Sun in this one specific frequency only produced by calcium in certain situations, Hale could selectively take a photo of just the one aspect of the Sun. He was imaging the Sun in ways that would transform solar research.
Hale also used light emitted by hydrogen atoms. After all, this is the most abundant element in the Sun and there are several spectral lines that hydrogen produces. In the visible part of the spectrum are four prominent lines, ‘hydrogen alpha’, ‘hydrogen beta’, ‘hydrogen gamma’ and ‘hydrogen delta’. These form a sequence by wavelength, going from hydrogen alpha, which has the longest wavelength (in the red part of the spectrum), to hydrogen delta, which has the shortest wavelength (and is in the violet part of the spectrum), each one formed by electrons moving between different energy levels (orbitals) leading to the emission of light in the visible part of the spectrum. Hydrogen beta is in the blue part of the spectrum and hydrogen gamma at the blue/violet interface.
Out of these hydrogen spectral lines it was those in the blue and violet parts of the spectrum that were first selected for use in the spectroheliograph, and for purely practical reasons. Images of the Sun were being recorded on photographic plates, and in the early days of using this technique no commercial plates were available that were sufficiently sensitive to red light. But Hale was curious to know what the Sun would look like in the light of hydrogen alpha so he was in luck when, at the end of 1907, developments in the chemical preparation of photographic plates made it possible for images to be created in light at the red end of the spectrum.
The hydrogen alpha line has a wavelength of 656.3 billionths of a metre and is formed when an electron falls from the third to the second energy level in a hydrogen atom, most likely when a free electron recombines with a hydrogen nucleus and cascades inwards. The light we see at this wavelength is coming from a height of around 5000 kilometres above the photosphere, from glowing plasma directly above sunspots. In my research into the conditions above sunspots I still use hydrogen alpha images.
What Hale saw at Mount Wilson on 28 March 1908 changed solar physics for ever. He loaded in a new-fangled photographic plate sensitive to red light, set the spectroheliograph to the hydrogen alpha wavelength and scanned it across the Sun. The image it produced revolutionized our understanding of the Sun. Although the image was faint, it showed signs of structure above sunspots. And by 30 April Hale had mastered taking images in this new wavelength and saw magnificent whirling vortices of plasma. There was structure and detail in the plasma above sunspots that was clear to see. But the image taken on 28 March 1908 is, for me, the image that changed the direction of solar physics from counting and recording the positions of sunspots to understanding the physics of them.
In hindsight, there had been hints before of the whirling structure seen in the light of hydrogen alpha. But these images were so clear that they triggered Hale’s interest and got him thinking. As Hale considered these whirling structures he began to see connections with other developments in physics.
In 1876 experiments that would eventually lead to our understanding of electricity and magnetism were just taking shape. At the time people were marvelling that if an electrically charged rubber disc was set spinning, it produced a deflection of a compass needle. They did not realize that in this movement of electrical charges causing a magnetic field they were opening a Pandora’s box of modern physics. Only in 1897 did the scientist J. J. Thomson discover something he called the ‘electron’. In 1908 there was still not complete agreement as to what atoms were or even if they existed.
5.2 First image of the Sun taken in the light of hydrogen alpha (Carnegie Observatories).
5.3 Image of the Sun in the light of hydrogen alpha, taken on 30 April 1908 at the Mount Wilson Observatory (Carnegie Observatories).
5.4 Zooming in on the sunspots shows the radial features around the spots that were called ‘v
ortices’ by Hale (Carnegie Observatories).
Hale took these latest findings and speculative results and used them to explain what was going on above sunspots. He reasoned that the high temperature of the solar atmosphere would mean that the particles there are torn apart so that electrons would be separated from their parent atom and moving freely in the solar atmosphere. Hale saw the newly confirmed vortices, and the sunspots that they connected to, as regions of swirling electrons where magnetic fields are being generated. Which was a bold judgement; at that point the only magnetic field that was known to exist in the entire Universe was the Earth’s. Was the Sun going to be the second? Hale knew just the way to find out.
The last piece of information that Hale drew upon was an 1896 experiment by a Dutch physicist, Pieter Zeeman, who had shown that a strong magnetic field could affect the light given off by a ‘luminous vapour’. If a gas was in a magnetic field when it produced its emission spectrum, the magnetic field had an effect on the spectral lines. A weak magnetic field would cause the lines to get wider and a strong magnetic field would cause some of them to split entirely into two or more separate lines.
Hale wasn’t the only one to realize the astronomical potential of this discovery. Indeed, Zeeman’s paper, published in 1897, already speculated that this discovery might be used to detect cosmic magnetic fields, including any present in the Sun. The effect of a magnetic field on spectral lines is now known as the ‘Zeeman effect’. Zeeman was awarded the Nobel Prize in 1902 jointly with Hendrik Lorentz, someone whom we will meet again shortly, for their work in this area.
Hale knew that if magnetic fields existed in sunspot regions, and if they were strong enough, they could be detected by telescopes on the Earth by means of a detailed study of spectral lines as Zeeman suggested. And Hale had both the scientific knowledge and the technical ability to test this. Sure enough, when the spectral lines of light produced in sunspot regions were carefully observed, they showed the broadening and splitting which is characteristic of the presence of strong magnetic fields. When Hale studied the spectral lines of light coming from the surface of the Sun where there were no sunspots, the spectral lines looked normal.
Hale had done it. Sunspots, which were often many times the size of the Earth, had been found to be the source of a magnetic field many thousands of times stronger than the magnetic field at the surface of the Earth. The paper that Hale published on this in 1908 (within three months of taking the 28 March image) is now regarded as the birth of modern solar physics. In fact Hale had already started thinking that sunspots might be regions of magnetic field in 1905. But his observation – direct proof that he was right – allowed the study of the sunspots to move beyond simply recording and classifying them, to beginning to understand their physical origin. From this point on we could finally start to comprehend not only what the Sun is but also why it behaves the way it does. And it gave the first evidence that magnetic fields existed beyond the Earth.
ALL HALE
It may seem like a failure of science that the true nature of sunspots wasn’t understood until 1908 – almost three centuries after the first telescopic observations. In reality, though, understanding sunspots wasn’t possible until spectroscopy had been developed, along with an understanding of how changes to spectral lines could be used to infer the presence of a magnetic field at the light source. But this discovery of the magnetic fields associated with sunspots suddenly explained all the mysterious observations.
This was why sunspots came in pairs: they are a pair of north and south magnetic poles. The names of ‘north’ and ‘south’ for magnetic poles are a throwback to Earth’s magnetic field remaining (relatively) fixed from pole to pole. On the Sun, we tend to call the two magnetic ends of a pair of sunspots the ‘positive’ and ‘negative’ magnetic fields. This helps convey the sense that in the north polarity the magnetic field is pointing out of the Sun (positive) and in the south polarity it is pointing into the Sun (negative).
If you think back to the bar magnet diagrams you saw at school, with the field lines looping around from north pole to south pole, this is almost exactly the same as what is happening on the Sun. The magnetic field lines come straight up out of a positive-magnetic-field sunspot, loop around high above the photosphere and come back down through the other sunspot in the pair, the negative-magnetic-field sunspot. Instead of the iron filings used in the classroom to see this magnetic structure, in the Sun Hale was seeing plasma guided by the magnetic field (delightfully, containing iron ions).
In the first chapter I described a magnetic field as being a kind of field of influence, conceptualized using field lines that are like elastic bands that loop back around on themselves. This convention of field lines started with Michael Faraday, the great British scientist and popularizer of science, when he needed a visual image to help describe this invisible force. Faraday introduced the concept of ‘lines’ of magnetic force. These are purely imaginary but Faraday was a great communicator and this conceptual approach to magnetic fields is still invaluable today.
Faraday’s concept of magnetic lines of force helps us visualize how this force varies across space, using lines that connect between the north pole and south pole of a magnet. And just as contours on a map indicate the steepness of a slope, magnetic field lines are more concentrated in regions where the force is strongest and more spread out where the force is weak. I think of them as elastic bands because, as we can already start to see from Hale’s image, these field lines can become twisted and stretched. This was later to become more important in understanding the Sun than even Hale realized.
The discovery that sunspots are regions of intense magnetic field is also able to solve the riddle of why they are cooler and darker than the surrounding photosphere. On average, the plasma in the photosphere has a temperature of around 5700 Kelvin, whereas in the sunspots the temperature can drop to 3700 Kelvin. This is because the strong magnetic field disrupts the convection that resupplies hot plasma to the photosphere. Normally as plasma loses its energy into space and cools, it would sink, only to be replaced with hot plasma, rising up fresh from within the convection zone. The intense magnetic fields trap plasma though, stopping hot plasma from coming in and letting it continue cooling.
5.5 and 5.6 Modern magnetic maps of the Sun. The positive (north) polarity is always colour-coded in white and the negative (south) in black. The image on the left is the observation, the image on the right has had the magnetic field lines added, along with the numbers assigned to the sunspot groups (NASA/SDO, HMI science team and LMSAL).
The picture which has since emerged from Hale’s work is that sunspots do not produce this magnetic field – it is the magnetic field itself which produces the sunspots. A sunspot is just the visual manifestation of what happens when a massive arch of magnetic field penetrates the photosphere in two places. When we look at a sunspot it is like taking a horizontal slice through that intense bundle of magnetic field.
Magnetic fields cannot ever simply end, either; they always loop back to where they started. The fields are three-dimensional, which means there is much more to them than the cross-section that forms a sunspot. The bundle of intense magnetic field that comes out of a positive sunspot is like a ‘tube’ of intense magnetic field that extends high above the photosphere. It then bends over to go back down through the photosphere in a negative sunspot, but the same thing must be happening below the surface of the Sun. Somewhere deep in the Sun the magnetic field must complete its loop. But it was a few decades after Hale before we could follow the magnetic fields on their journey back into the Sun.
As always, no physic
s theory is perfect straight away. There were a few observations that did not completely match this magnetic description of sunspots. Occasionally there would be a lone sunspot which did not seem to have a partner. These ‘alpha’ sunspots are a problem as a magnetic field never exists with only one pole. We now know that the magnetic field of the other pole is so spread out it is too weak to interrupt the convective flows in those places and no visible change in the photosphere temperature occurs. Slightly harder to explain is how the temperature of the plasma in a sunspot cools to become around 2000 Kelvin less than the surrounding photosphere but then somehow maintains that temperature without cooling any further, as we would expect it to. It’s still not well understood how this is happening – some hot plasma must be being delivered still. Even 100 years on from Hale’s discovery we still have questions about sunspots that need to be answered.
Hale died in 1938 but he left a huge legacy to solar physics and astrophysics in general. He saw the Sun as a typical star and never became exclusively a solar physicist; there were the observatories that he founded (four of them in total), the young scientists he supported at Mount Wilson, who include Edwin Hubble and Harlow Shapley, the journal that he founded and edited for most of his life (the Astrophysical Journal), which is one of the premier journals for the research community today, and the work he did in building and supporting solar physics and astrophysics. I am hard pushed to think of someone who had a greater impact across so many areas.
6. The Spinning Sun: The Day the Sun Fought Back
This chapter is about how fast the Sun is spinning. If this were a book about the Earth, the chapter on how long it takes to rotate would be a very short one indeed. Actually, it would just be a page that says: ‘Twenty-four hours’. Or maybe ‘86,400 seconds’. There could be some confusion about whether we measure the rotation relative to the Sun or the stars (a difference of less than four minutes), and the rotation does vary ever so slightly (the day has increased by about 1.7 milliseconds over the past 100 years). But these are minor details. For the most part, because the outer layer of the Earth is a solid lump of rock, it all rotates in unison.
