by Lucie Green
DELIVERING A BLOW
In short: a CME is an ejection of solar magnetic field, and if it hits Earth in just the right way, it can really mess with our magnetic field. And that means that we feel its effects all the way down to the surface of the Earth. This has really made the study of the Sun far more practical than physicists could have ever expected. So far we have seen our understanding of the Sun being driven by curiosity, through serendipitous discoveries and deliberate attempts to answer questions. Piece by piece, we have put together a jigsaw that links the physical processes which explain our Sun and its control over the Solar System. Now we need to re-evaluate this jigsaw to understand possible threats from the Sun, using everything we have learnt from 400 years of science, over 100 years of studying the magnetic nature of our Sun and almost sixty years of observing from space.
First off: the impact that we feel at the Earth’s surface is not the whole story. Above our atmosphere but within our magnetic field are ring-doughnut-shaped regions where electrically charged particles are trapped. These are the Van Allen belts, named after James Van Allen at the University of Iowa. Van Allen was an early pioneer of the space age and had used the V-2 rockets captured by the Americans after the Second World War for cosmic ray studies. He even worked with the creator of the V-2, von Braun, on Explorer I – America’s first satellite: von Braun worked on the rocket and Van Allen on the scientific instrumentation. We have met other projects that Van Allen worked on already in this book as he was also involved in the rockoons. The detectors that he designed and which were launched on Explorer I in 1958 discovered the trapped particles. It was a game changer for the way we viewed the space above our atmosphere – it is not empty – and led Ernie Ray, a colleague of Van Allen, to exclaim: ‘My God, space is radioactive!’
The radiation belts are an area of study today because many of our satellites orbit through them. The satellites that you use for communications and TV broadcasts all risk being damaged by the particles trapped in the belts. The particles can degrade solar panels and cause electrical sparks in the spacecraft that can damage the electronics. This is the same effect that we saw the US military trying to exploit before.
It was the Van Allen belts that the 1962 United States Air Force Starfish nuclear explosion was designed to influence. The US military were testing whether enough high-energy electrons could be injected into the radiation belts, making them more deadly and scuppering Soviet surveillance and missile technology. We know that at a minimum the electrons damaged research satellites. And by 1969, when the first Apollo landing occurred and Buzz Aldrin saw the flashes of light in his eyes, the electrons had diffused away and their levels had dropped to less than 10 per cent of the 1962 value. NASA were concerned about the Van Allen radiation belts but it turned out that the astronauts passed through with no problems.
So, it’s not a perfect vacuum above our atmosphere (although it is better than any vacuum we can make on the Earth). The particles trapped in the belts come from two places: again local and far away. Some particles are stripped off from the top of our atmosphere and others leak in from the solar wind. It is unknown what the actual ratio of Earth-origin to Sun-origin particles is in the Van Allen belt. What we do know is that changes to the Earth’s magnetic field that are the result of space weather can cause these particles to be accelerated down into the atmosphere and make the gases there glow. This forms the aurora, and the more particles that are accelerated, and the faster they move, the more dazzling the aurora displays. If that sounds familiar, it is a very similar process to what causes solar flares on the Sun – fast-moving electrically charged particles giving up their energy to gases, which makes the gas glow.
The global effect that a CME has on our magnetic field actually depends on the details of the CME’s magnetic field. When the orientation of the magnetic field in the coronal mass ejection is opposite to the orientation of the field of the Earth’s magnetic field, it creates an opening in our normally protective magnetic bubble. This changes the strength and shape of the magnetic field – affecting it all the way down to the surface of the Earth. The physical process behind this is also the same as the process that is at the heart of solar flares: magnetic reconnection.
As always, magnetic reconnection is best visualized as being the breaking and rejoining of magnetic field lines. In this case it joins up the magnetic field within the coronal mass ejection to the magnetic field lines in the Earth’s magnetic field’s outer layer, creating field lines that can be traced from the coronal mass ejection all the way down to regions close to the magnetic poles of the Earth. But the CME is still on the move, sweeping over the Earth. By the time a CME hits our planet it has expanded to be many times the size of the Earth. It does not so much hit the Earth as flood around it.
This means that as a CME flows around the Earth, it connects to the magnetic field on the ‘front’ of the planet (the Sun-facing side) and drags that magnetic field around the Earth to the far side. Even without a CME, the solar wind causes the Earth’s magnetic field to deform like a kind of solar windsock on the night side. But the CME exaggerates this to a huge extent, so much so that magnetic field and energy are stored up in this region and magnetic reconnection can then occur in what’s called the tail. This reconfigures the magnetic field and actually allows it to snap back towards the Earth and move back to the dayside, where it can reconnect once more with the magnetic field of the same CME and the whole process starts again.
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This circulation is known as the Dungey cycle (see figures 13.1–13.6), after the British physicist James Dungey who developed this theory whilst working at Imperial College London. Apparently the idea came to him during a eureka moment as he stirred his coffee at a street café in Paris – a humble origin for a theory that went on to be the backbone of our understanding of the way the Sun interacts with the Earth. The huge changes to the Earth’s magnetic field create geomagnetic storms.
The impact on the Earth’s magnetic field is strongest when a fast coronal mass ejection comes our way – such ejections deliver more of a punch. Our modern view of the Sun has shown us that all three types of activity – flares, coronal mass ejections and energetic particles – can be initiated at roughly the same time, but only in the worst cases (events which are fairly rare but which can happen at any time) do all three occur together. Which is what happened in 2012, when a CME similar to the Carrington one was actually launched by the Sun but, thankfully, we were able to watch as it missed the Earth and sailed harmlessly past us. In the coming decades we may not be so lucky. We had better be prepared.
SPACE WEATHER
Today, monitoring of the Sun in the US is carried out by the Space Weather Prediction Center, in Boulder, Colorado. This is the latest incarnation of the Space Disturbance Forecast Center that was set up during the Apollo era, and which itself had a heritage that dated back to 1903, when interest in space weather was first aroused because of its effects on radio propagation.
In the UK, space weather forecasting is the responsibility of the Met Office: you can get your weather and your space weather information from the same place – very convenient. But to be able to make accurate space weather forecasts we need to collect the right data. Just as weather forecasts require weather satellites to make observations of wind patterns and the temperature of the oceans, we need space weather satellites to measure the patterns in the solar wind and the formation of sunspots that might produce flares and coronal mass ejections. If we have enough warning that a storm is
coming, we can batten down the hatches.
So we are looking at what a space weather satellite should be like in order to forecast space weather in an accurate and reliable way. Up until now the vast majority of our data has been coming from satellites that have been designed for scientific research. The satellites I use are designed to understand the physics underlying phenomena like coronal mass ejections. This is important for understanding space weather, but they aren’t designed to forecast it. What should a space weather monitor be like?
Well, it would need to be able to measure the strength of the solar wind, including coronal mass ejections, and the direction of the magnetic field before it arrives at the Earth. Actually, we already have some instruments that do this at an incredibly useful point in the Solar System to put a space weather monitor: 1.6 million kilometres closer to the Sun than the Earth is, known scientifically as the ‘first Lagrange point’. The first Lagrange point is the location where the gravitational pull of the Sun combined with the gravitational pull of the Earth means it’s possible to place spacecraft into orbit at that location. The spacecraft will always stay between the Sun and the Earth. And from this position it tells us what is coming before it arrives, allowing us to make a forecast of whether a geomagnetic storm might form. The drawback is we only have a warning time of about an hour for the solar wind and twenty minutes for a fast coronal mass ejection. But it doesn’t have to be this way.
If we can get our satellites closer to the Sun, and make our measurements from there, we will have more time to make our forecast. One idea that I like has become known as the ‘space weather diamond’, so called because it makes use of four satellites that form a diamond pattern. These satellites are in orbit around the Sun, but in a way that from our perspective makes them seem to dance around the Earth, ten times further away from us than our current solar spacecraft, at the first Lagrange point, giving us ten times the warning time. Four satellites are needed so that, as they move in relation to the Earth, there is always one on the sunward side. This could be a costly mission to launch though, so solutions with fewer spacecraft are also being considered.
Another option is to watch the Sun and look for times when a coronal mass ejection is launched and heading towards us. Then we have around one to four days to make our forecast about whether it will produce stormy space weather. And if we want to look for coronal mass ejections that are headed our way, a good approach is to step to one side and look back at the space between the Sun and us. This approach has already been shown to be successful by NASA’s STEREO spacecraft – twins that were put into orbit around the Sun.
13.7 A schematic showing how four satellites forming a space weather ‘diamond’ could provide an early warning of emissions from the Sun that are headed our way. ‘L1’ indicates the location of the first Lagrange point.
One STEREO spacecraft is slightly closer in to the Sun than us so that it moves more quickly and drifts ahead of the Earth orbiting around the Sun; the other is in an orbit slightly further out so that it moves more slowly, causing it to lag behind. Gradually, the position of these spacecraft changed and they could both look back at the Earth and the Sun and the space in between. Both STEREO spacecraft carry a cleverly designed telescope that blocks the light of the Sun to enable us to track the very faint coronal mass ejections far out into the Solar System and predict which ones will hit us and when. Whilst the STEREO spacecraft have been useful to demonstrate a proof of concept, a drifting spacecraft is no good for space weather forecasting (they are currently behind the Sun), but luckily there is another option that we can make use of.
It is possible to place a spacecraft in orbit at a point 150 million kilometres behind the Earth in its journey about the Sun – the same distance away that the Sun is from us. This is the fifth Lagrange point. Looking at the Sun from this position allows us to watch the CMEs headed towards us but also peek over the edge of the Sun and see regions that are hidden from us on Earth. We can see what is about to rotate to face us. If we watch the surface and the atmosphere of the Sun from this viewpoint we can monitor the Sun’s magnetic field and possibly get several days’ more warning of stormy weather ahead. A longer forecasting time, giving us enough time to make our preparations, is the holy grail of space weather forecasting – like predicting the strength and motion of a hurricane as it is developing over the ocean. It’s also an important place to take measurements of the solar wind directly, which is absolutely vital for making an accurate space weather forecast. The forecasts are made using models, and models need real data to drive them. We have to know what the real Solar System is doing.
But it’s not only observations from space that matter. Novel space weather stations are being set up around the globe. And they can be much, much cheaper. One of my colleagues in Ireland, Peter Gallagher, has taken this approach. He has set up a space weather ground station on the site of what was once the world’s largest astronomical telescope, a telescope so large that the locals named it the Leviathan. The Leviathan was built by the third Earl of Rosse. When it was completed in 1845 it became the largest telescope in the world, a title it kept for over seventy years. Now Peter is reviving this site by setting up a space weather monitoring station. He has persuaded the current Earl of Rosse to donate a disused building on the site and Peter’s team has transformed it into the Rosse Solar-Terrestrial Observatory. From there they monitor the Sun’s flare activity and look for changes in the Earth’s magnetic field caused by the flares and CMEs.
It’s not just forecasting that is important though. The reason why the Royal Academy of Engineering undertook the report is so that we can find engineering solutions to prevent the problems in the first place. We can improve our technology and make it more resilient to space weather, such as designing buildings that are to be constructed along fault lines to be able to withstand earthquakes. In the space weather context this might include changes to how power grids are designed and operated, or designing satellites with enough shielding to protect them from high-energy particles.
For me, one of the most important outcomes of the space age is that we have redefined our relationship with our local star. Space weather has brought a new relevance to understanding the Sun and an imperative to watching its behaviour. Behind the scenes there are teams of scientists developing the theories to explain what is happening, and forecasters use this knowledge to make their predictions. They are working to help keep the lights on. And across the country engineers are working to develop new technologies that won’t be so susceptible to space weather.
LEVITATION
We started the chapter with the Apollo astronauts because they were some of the early adopters of space weather forecasts. From that point on it may have been felt that space weather is a rather sombre subject because of the problems it can cause for the technology that we rely on. A gloomy subject except for the beautiful aurorae that space weather creates. I want to end on a more positive note, one which reminds us that space weather is really about us understanding our place in the Solar System and the scientific processes that are happening all around us. If you allow me a final trip back to the Moon, we’ll see that the science of space weather solves the mystery of why the Apollo astronauts saw something at sunrise that no one was expecting.
When the Apollo 17 command module went into orbit around the Moon in 1972 the astronauts had a clear view towards the horizon as they flew through the lunar night. Ahead of them, the Sun was just about to rise. Module Commander Eugene Cernan was ready to draw the view of the lunar sunrise and, as he watched, a faint bulging glow began to appear over the edge of the Moon. The glow wasn’t the appearance of t
he Sun itself, but sunlight being scattered over the horizon by dust, both dust in the Sun’s atmosphere and dust between the planets that has been laid down by comets and asteroids over the millennia. This glow was expected.
But there were some aspects to the glow that were very odd. There was a glow that spread out along the horizon and shafts of light were seen shooting upwards. The shafts looked similar to the shafts of light we sometimes see coming down through holes in layers of cloud on a sunny day. On the Earth these shafts are easy to understand because we have an atmosphere, and sunlight scatters off particles in it. On the airless Moon, the shafts of light were a complete puzzle.
The Moon simply isn’t massive enough to hold on to any gas to form an atmosphere of its own and with no atmosphere there should be no suspended dust particles to scatter the sunlight. Yet somehow the Moon has dust fountains that give it a very tenuous atmosphere. These dust fountains need a way to be lifted up from the lunar surface. And with no wind on the Moon, the answer was in the charged particles trapped within the Earth’s magnetic field.
The Apollo astronauts saw the dust fountains as they flew along their orbit and crossed over from lunar night to lunar day. Just as on the Earth, on the Moon the line that divides day and night is called the terminator. It is along the terminator that the dust was seen to be lifted up.