by Lucie Green
As part of this work, we studied the magnetic fields in active regions of the Sun (identifiable by their sunspots) from the moment they emerged into the surface until the moment they dispersed completely. We watched as the once concentrated magnetic fields of the sunspots broke into smaller and smaller pieces, month by month, as the photospheric plasma flows spread them over a wider and wider area, until they were so fragmented that we couldn’t track them any more.
This meant we could measure how much magnetic helicity was injected into the active region’s magnetic field because of surface motions that move the magnetic field around and distort it, and how much was ejected because of CMEs, and then we checked whether the difference between these two values matched what was present in the corona.
The first surprising finding was just how many eruptions a decent-sized active region is capable of during its lifetime. One particular active region I studied produced thirty-five coronal mass ejections during the five months over which it was followed. There was a problem though. We can only ever see the side of the Sun facing us; we can never see the horribly misnamed ‘dark side of the Sun’. Nowadays we have NASA’s twin STEREO spacecraft, which are currently positioned to see the side of the Sun facing away from the Earth, but this was before they had been launched. So we estimated. Assuming that the region always produced coronal mass ejections at the same rate, we almost doubled the estimated number of ejections to sixty-five: over only five months – a phenomenal amount of activity. And a phenomenal amount of magnetic helicity shed into the Solar System. To think that before coronal mass ejections were known about, the corona was viewed as a slowly evolving environment where only large-scale magnetic structures varied as the solar cycle progressed from year to year!
This first surprising finding led to the second. The measurements we made showed that the active region’s magnetic field was able to shed much more magnetic helicity in its CMEs than was being put in by the surface motions. There appeared to be a helicity reservoir inside the Sun that was somehow replenishing the stocks in the corona.
The discovery of coronal mass ejections was a milestone in developing our understanding of how the Sun works. But it also solved a long-standing question about what was responsible for causing some very intense changes to our own environment – our magnetic field.
For a long time it had seemed to scientists that the sunspot cycle and observations of flares correlated with magnetic changes here on Earth. But how could the Sun extend out and be responsible for these changes? Aside from electromagnetic radiation and the solar wind, there was no direct link between the Sun and the Earth. Then when coronal mass ejections were discovered, they offered a means to bring the Sun’s dynamic magnetism out to the Earth.
And what began as an observation that these eruptions might be responsible for problems with the electric telegraph in the nineteenth century has become a realization that they pose a much bigger threat to twenty-first-century technology. In fact, they produce a whole new type of weather that we are now monitoring and forecasting: space weather.
13. Living in the Atmosphere of the Sun
Lessons from Apollo
Astronauts are the heroes of the modern age and meeting one is an honour. What they have achieved and what they have seen separates them out as exceptional people with exceptional stories. But there is one group among them who conjure up the deepest sense of awe: the Apollo astronauts who travelled to the Moon and back. They ventured across 384,000 kilometres of space to walk on the surface of the Moon, becoming the only humans to have ever set foot on another body in the Solar System. The Apollo astronauts are a key part of this story about the Sun because cutting-edge solar physics knowledge was needed to keep the astronauts safe. NASA knew that the Sun posed a threat to them at the times when it was active. And this same threat is now relevant to all of us today.
Very sadly, the first human on the Moon, Neil Armstrong, has since passed away. But the second lunar sightseer, Buzz Aldrin, is still alive and I have had the pleasure of meeting him. He was a guest on a BBC programme that I am a presenter on and, of course, both on and off air everyone wanted to talk to him about his experiences on Apollo 11. How did the Earth look from the Moon? What was the Moon landscape like? How does a Saturn V rocket launch feel? But, for me, meeting Buzz was the opportunity to ask something else: what did he see when he closed his eyes?
You see, Buzz and his fellow space passengers were some of the first humans to leave the protection of the Earth’s atmosphere and go to the edge of its magnetic field, which together protect us from the dangers of space. For while space is almost as empty as the name implies, there are high-energy particles flying around: atomic nuclei, mostly individual protons, which race through the Solar System at speeds close to the speed of light. They were some of the first people to ever directly experience ‘space weather’ as the particles rained down on them.
These high-speed, high-energy particles were discovered in 1912 during a balloon flight that ascended to over 5 kilometres in height. Today we know that these ‘cosmic rays’ can be broadly split into two types: ‘alien’ and ‘local’. The ‘alien’ rays have travelled vast distances before reaching the Earth, originally having been accelerated to very high speeds during supernova explosions far beyond our own Solar System. They are travellers from elsewhere in our massive Galaxy, the Milky Way. Other cosmic rays are far more ‘local’, coming as sporadic bursts that the Sun produces as a by-product of flares and CMEs.
Many cosmic ray particles of both types are either deflected by the magnetic field of the Sun or the Earth or else they collide with our atmosphere and disintegrate into smaller particles. The few that do make it down to the Earth’s surface are not a health risk and generally pass by unnoticed. Only if you have a specially prepared tank of super-saturated alcohol vapour (known as a cloud chamber) will a cosmic ray make its presence known. As it moves through the vapour it will cause some droplets to condense out, tracing a ghostly cloud along its path. But mostly what you will see in a cloud chamber are the tracks left by the shower of those smaller particles, created when the cosmic rays reach the atmosphere. It’s long been an ambition of mine to build a cloud chamber into my coffee table so that I am reminded that these particles are all around us.
On the way to the Moon, Buzz will have been exposed to cosmic rays flying in from all over the Galaxy. And there was a chance he could even have seen them directly. I had heard that if cosmic rays reach your eyes they could interact with your retina, causing you to see flashes of light. Thankfully I got a chance to ask Buzz about it, both live on the TV show and in more detail after the filming. My space lab is working on the next lander being sent to Mars (ESA’s Exomars mission due for launch in 2018), a subject very dear to Buzz, so despite the rest of the cast and crew clamouring to talk to him as well, I was able to hold Aldrin’s attention briefly by talking about future plans for the Red Planet.
And he did see cosmic rays with his own eyes! It was fascinating to hear Buzz describe the flashes of light that he had seen and thought were something inside the spacecraft. He saw them during their night and woke the next day to ask his crewmates if they saw the same. Mike Collins said no, Neil Armstrong said he saw a hundred or so. Buzz indicated that Neil Armstrong had a competitive streak! The next Apollo crew were told to look out for these flashes during the night and once their eyes had become dark-adjusted – they saw them too.
But NASA had known about this space radiation – even if they didn’t know the Apollo astronauts would see it. The flashes of light were a novelty and make a good story now, but NASA knew that if the astronauts were to
successfully land on the Moon and be returned in good health to the Earth, they needed to be protected from the particles. Cosmic rays are actually dangerous ionizing radiation and if the dose the astronauts received was high enough they could have been left with problems such as radiation sickness or even cancer.
What Buzz was seeing were probably the cosmic rays coming from the Galaxy. What were more worrying for NASA were the cosmic rays that come from the Sun. We call these local cosmic rays ‘solar energetic particles’. They are the same kind of particle that Buzz experienced zipping through his eyes, but the numbers are very different. Buzz experienced a light drizzle, whereas a solar energetic-particle event is a torrential downpour. And if an Apollo astronaut were to be caught in that downpour the consequences could be fatal.
So during the Apollo missions NASA had a network of telescopes monitoring the Sun and they had radiation experts working in the Mission Control Center Space Environment Console. They looked for solar flares because at that time flares were the only major solar activity that was known about. Coronal mass ejections and their role in creating energetic particles were still to be discovered.
The health risks that the particles posed to the small number of men going to the Moon, though, were weighed up against the political gains that the nation would achieve if they became the first to walk on the lunar surface. And solar energetic particles were just one of the many risks they faced. So despite the Apollo missions being launched at the maximum phase of solar cycle 20 and on into the declining phase, when flares would have potentially been very frequent, the missions went ahead.
In hindsight the dose of high-energy particles received by the Apollo crews was small. And no solar energetic-particle event happened during any of the missions, so the astronauts never had to cope with the worst conditions that are possible. But plenty of particle events happened in between missions, such as between the penultimate mission and the last, the one that took Eugene Cernan to be the last man on the Moon. This is one of the largest particle events recorded during the space age. Looking back and knowing what we do today, it seems this would have given the astronauts moderate radiation sickness. It was sheer luck that there were no missions scheduled to take place then and there was, and still is, no way of forecasting these events.
The impact of solar activity isn’t only of concern for the lucky few who have journeyed above the Earth’s atmosphere. Today we know that the consequences are felt all the way down to the Earth, and even under its surface. And the main drivers of the worst space weather are not solar flares but coronal mass ejections, which hadn’t even been discovered at the time of the Apollo programme. Things have moved on enormously and today making daily forecasts of our space weather has become an important aspect of modern society.
VICTORIAN SPACE WEATHER
The Carrington event of 1859 continues to be our best example of an extreme space weather storm. It happened at a time when technology was advanced enough for some of its impact to be detected on the Earth but not so important to us that society was crippled. Recently, the Royal Academy of Engineering looked into the impact that a modern-day Carrington-sized event might have. It found that the effects could be significant but that above all we should aim to be prepared rather than be alarmed.
Waking up on the morning of a Carrington-esque solar storm, you will see no obvious warning signs of what is about to happen: the sunrise will look completely normal. Following the exact times on the day of the flare that Carrington and Hodgson witnessed, by 11.18 a.m. the flare will be in progress but nothing too severe has happened to the Earth yet. On the Sun, though, energy stored in the magnetic fields of a sunspot group is being released and converted into the kinetic energy of particles being accelerated down to the photosphere. There, the interaction of the particles with the dense plasma is rapidly heating it, causing it to radiate visible light photons. This visible light will take eight minutes to travel from the Sun to the Earth, as Carrington and Hodgson saw. If for some reason you happen to be looking at the Sun in white light, perhaps because you have made a trip to your local astronomical society, you may spot this burst of brightness, but otherwise there is nothing to notice.
Although it wasn’t seen in 1859, the hot plasma rapidly expanded and rose up into the magnetic structure in the atmosphere above, glowing in X-rays and ultraviolet light. Today we would notice this with spacecraft looking at the Sun in these wavelengths. The only tip-off in 1859 was when these high-energy photons reached the Earth’s upper atmosphere, ionizing the gases there and changing the Earth’s electric currents and magnetic field. At the Kew magnetic observatory, a disturbance to the Earth’s magnetic field was detected in their instruments. But this was only a tiny taste of what was to come.
As you now go about your normal day, a massive coronal mass ejection has blasted off the Sun, cutting its magnetic tethers and accelerating out into space. The coronal mass ejection has been launched from the sunspot group that also produced the flare, and is very near the centre of the Sun’s disc. So the Earth is directly in the firing line.
The first blow comes from a cascade of high-energy protons of the kind that NASA was looking out for during the Apollo era. The fastest ejections can be travelling at speeds so high relative to the solar wind that they punch their way through, and that forms a shock wave. And in the shock, particles are accelerated to speeds approaching the speed of light, sending a shower of protons to the Earth. They reach the Earth in around twenty minutes. Not bad when you remember that light from the Sun takes just over eight minutes. There is no direct evidence that the Carrington event produced a shower of energetic particles, but modern data suggest that it would be unusual for an event of this size not to do so.
Following this initial commotion (albeit unnoticed by the general population) there will now be a few hours of relative calm. But then, just 17.5 hours after the flare would have been seen, the coronal mass ejection that has been racing our way at an average speed of 2300 kilometres per second will slam into the Earth’s magnetic field. Now you notice that something is up.
When it arrived in 1859, the magnetometers at the Kew observatory went off the scale as a huge geomagnetic storm commenced – it remains one of the largest geomagnetic storms ever recorded. The storm created electric currents that flowed through the communications system of the era – the telegraph – allowing operators to switch off the batteries and operate using this natural electricity. The Carrington event was widely reported, but society at that time relied very little on technology and so relatively little disruption was caused. Were it to happen today, the story could be very different.
As you sit at home with a Carrington-style CME colliding with the Earth’s magnetic field, what you absolutely don’t want to see happen is for your power to go off. In the early hours of March 1989 a much slower-moving CME crashed into our magnetic field and Canada felt the effects when the Hydro-Québec electricity grid experienced strong voltage fluctuations. The fluctuations were so large that the grid’s protection system was triggered, leading to the whole network shutting down in less than two minutes. The result was that several million people awoke to find that they had no electricity. It was a cold winter’s morning and the power stayed off for more than nine hours. The lack of electricity to power society ended up costing the economy $6 billion.
This 1989 event was a wake-up call for just how much of a threat the Sun can pose to society as a whole. The Royal Academy of Engineering focused very much on the UK, where it found that a modern Carrington event could cause disruption to the electricity distribution in some regions, but probably only in remote areas, on the periph
ery of the network. Thankfully, since the scare of 1989, power grids are being hardened to be able to withstand this level of solar storm. Realistically, the power outage in the UK is likely to be along the same lines as that caused by other extreme weather events such as heavy snowfall. In these cases the electricity is normally back fairly quickly. Any countries that have not fully prepared might not be so fortunate though. Power grids around the world vary and some are better at withstanding space weather than others.
We’ll assume that you are in a house that is on a CME-ready power grid. You may briefly lose power, but nothing too bad. Unfortunately, above your head, satellites are not so lucky. The Royal Academy of Engineering report found that perhaps up to 10 per cent of the entire satellite fleet might be disrupted, meaning that many prosaic aspects of our lives become impossible. On top of this, the changes to the ionosphere through which satellite signals pass mean that even some functioning satellites can no longer be relied on.
Satellite navigation signals will be significantly degraded. Banking activities may go awry as satellite navigation signals are often used to time-stamp money transfers. Airlines may choose to ground their fleets as changes to the ionosphere lead to the loss of radio communications and poor satellite navigation. And, on top of all that, the charged particles threaten the function of the microelectronics at the heart of the plane and may subject humans to a radiation dose as high as three chest CT scans.
There is one silver lining to all of this: the aurorae will be spectacular. The Northern and Southern Lights normally only occur close to the magnetic poles, but during a major geomagnetic storm they move further towards the equator. On the night of 1 September 1859 the northern aurora pulsated with a blood-red colour and was so bright that people in England were able to read their newspapers without any additional lighting. So your satellite TV has stopped working, your flight has been cancelled and you may not even have power at home: but at least you will get to witness one of the greatest astronomical displays ever.