Aurora

In Search of the Northern Lights

Dr Melanie Windridge

• Publisher: William Collins
• Year: 2016
• ISBN:
  9780008156107
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

CHAPTER ONE

SWEDEN – AT FIRST SIGHT

THE NIGHT WAS a velvet canvas scattered with stars. Pillars of colour streaked down from above; green fringed with red and yellow disappearing up into the heavens. The lower edge twisted into a bright band against the darkness. Slowly the colours rippled and broke and reappeared elsewhere, until, suddenly, the whole sky exploded into colour like paper bursting into flames. Here were the northern lights in spectacular form.

It seemed this dance of such dynamic energy was stretched above the whole Earth; the individual dancers moving to their own tunes but somehow in harmony. Twirling and writhing, rippling and pulsing, the shapes rose and fell, sometimes exploding across the sky in a delicate shower of pinks and purples within the dominating green. It was as if a thousand people whirled with green silk sashes and scarves, all different shapes and sizes, swooshing and billowing independently. It was as dramatic as a thunderstorm, yet calm. Gentle, yet astonishing.

The northern night sky is not always this active. There are times of glorious intensity and times of pale tranquillity. There are nights when there is colour and motion; others when there is only a vague streak across the sky and any movement is barely discernible. There are places on Earth where this wild sight is commonplace, the calm colours barely remarked upon; yet for the majority of the world’s population the lights are a novelty, a rarity or an irrelevance, all depending on one’s location relative to the polar regions.

The northern lights is just one of a pair of twin phenomena. The aurora, or polar lights as it may be generically called, occurs both in the northern and southern hemispheres in ringed regions around the Earth’s poles – or, more specifically, around the Earth’s magnetic poles. The regions do sometimes expand, giving mid-latitude residents a glimpse of the magic, but, for the most part, to see the lights we need to travel north. So we head to the auroral zone, this narrow band of latitude where the northern lights are a regularity – to northern Scandinavia, Siberia, Canada or Alaska. To seek the southern lights means a trip to Antarctica, unless the night is one of elevated auroral activity when we might see a colourful haze in the south of New Zealand, or in rare, extreme circumstances when the aurora moves equatorwards.

The aurora has a reputation for being elusive, which adds to its mystery and attraction. It is a natural process, completely out of our control. To see it, you must be in the right place at the right time. The conditions must be favourable, too; the skies should be dark and clear. Clouds mask the view, and stray light – even moonlight – can wash out the delicate colours of the aurora. To see it in full splendour is a gift: a coveted, capricious gift.

The need for darkness makes the aurora a winter-time treat. In the summer months in the high-latitude locations where the aurora is created, the top of our planet is tilted directly towards the Sun, always in range of its light. At the Arctic Circle the Sun never sets between mid-June and the beginning of July, and the Earth’s poles see this midnight sun for half the year. Even further south of the Arctic Circle – towards the south of Scandinavia, the northern reaches of Scotland, or central Canada – the sky never darkens sufficiently in summer to see something as diaphanous as the aurora.

This winter-time nature adds to the aurora’s mystique. It is at one with the cold, snow and ice. It becomes part of the landscape and so, for me, this landscape is very much part of the aurora. So, too, are the people who live there; part of a human legacy stretching back for millennia. The sky connects us to our ancestors.

I am a physicist and my fascination with the northern lights grew gradually to a point where I didn’t just want to see it, I wanted to know it. I knew the basic science behind the aurora – that it is an event caused by charged particles that are channelled down magnetic field lines and interact with our atmosphere. But there had to be so much more to it than that. What caused the differences in colour? Why was the aurora predominantly green but then at other times showed flashes of mauves and violets? What caused the various shapes and patterns in the aurora? Why did we sometimes see pillars of light stretching skyward, sometimes twists and ribbons? Why is it sometimes calm and sometimes wild? What causes the sudden eruption in movement and colour? Why does it sometimes move further south? It seemed I still had so many questions.

More than that, I had read accounts of old, heroic polar explorers’ experiences of the aurora and noticed the overwhelming feelings of awe and spirituality. I saw how these feelings mirrored much of the ancient folklore of the northern lights. It is only in the last century that science has come up with a plausible explanation for the aurora and I was curious about the effect of such knowledge. Do modern polar explorers feel the same as those first few men, I wondered, or does science overcome the purity of the experience?

I had seen the pictures, watched the videos, heard the stories. I felt the draw of the Arctic – the light and space, the open wilderness, the long winter darkness and the awe-inspiring aurora borealis. I wanted to stand captivated under a wide sky, watching the heavens move in a graceful, barely choreographed dance. I wanted to feel it.

I imagined looking up and seeing the starburst of an auroral corona above me. I imagined colour and light and form. I imagined the whole length, breadth and depth of the sky dominated by arcing light. I imagined a silken landscape of soft snow, where the snowflakes lap over your skis and pile high on the fir tree branches, weighing them down. I imagined dragging sleeping bags out from the tent so we could lie outside on the snow, warm and marvelling at the dancing sky above us, just as one would the stars in the desert.

I never imagined the cold, or the pain.

* * *

THERE WAS A thin covering of snow on the ground as my plane waited at Stockholm airport. Grass poked through it in tufts and there were small, scraped piles of greying slush at the edge of the runway. From the aeroplane window Stockholm appeared in monochrome: the terminal building white with its black details and greying edges; the sky white; the snow on the dark tarmac the same hue. Men in their fluorescent yellow jackets stood out brightly, as if an old film were slowly merging into Technicolor. As we took to the skies I studied the trees and the small snowy fields below, divided up by narrow roads. An irregular patchwork of whites, greys and pale browns, their textures beneath the snow stood out as much as their colour. Almost immediately we were into the clouds and once again everything was white.

I was flying to Kiruna, in Arctic Sweden, to see the northern lights – the aurora borealis – for the first time. For years I had dreamed of seeing it, and finally there I was, en route to the Arctic, a place that held for me as much fascination as the aurora itself. I am a plasma physicist with a visceral attraction to mountains, ice and snow, whilst my academic research had focused on nuclear fusion as a future energy source. In my research days I was motivated by the youthful optimism that fusion would change the energy domain, and provide energy security and a way to temper the threat of climate change that could devastate the precious landscapes that I loved and the communities within them. I was – and still am – attracted by the practicality of this research, but in the aurora my passions converged. The plasma physics danced above the boundless snowy land that held me in thrall.

My fascination with the aurora wasn’t new; it had, in fact, dawned on me rather slowly. When I was an undergraduate student I spent one summer holiday working at the Rutherford Appleton Laboratory in Oxfordshire. There I worked on a project that looked at the connection between the Sun and the Earth, and it was there that I learned about the aurora – a captivating light show in the upper atmosphere of the polar regions and a product of the intricate interplay between these two celestial bodies. The name ‘aurora’ comes from the Latin for ‘dawn’, and ‘borealis’ from ‘boreas’, meaning the north wind. The term is believed to have been first used as a metaphor by the Italian scientist Galileo Galilei when he referred to the appearance of a bright aurora in the northern parts of the sky as a ‘northern dawn’. Similarly, an aurora seen in the southern hemisphere is referred to as ‘aurora australis’ from the Latin ‘auster’, for south wind.

Whilst at the Rutherford Appleton Laboratory I was also lucky enough to watch the live launch of a spacecraft – not live in Kazakhstan, obviously, but in real-time from the lab’s main lecture theatre. The rocket took two satellites from the Cluster II mission up into orbit that day; two others had been launched a month previously. As I sat and watched the launch it felt extraordinary; I realised that at that very minute that rocket was leaving the Earth and going into space. The realisation of the otherworldliness of it all thrilled me. Scientists were on tenterhooks because this was the mission’s second attempt – the first Cluster spacecraft had been lost in launch in 1996, four years previously. This time the launches were a success and the four identical satellites began their long dance around Earth. They are still flying in formation now, collecting three-dimensional data on near-Earth space and the conditions that cause the aurora. The experiences of that summer stayed with me, imprinting on my mind a latent fascination with our connection with the Sun.

More than ten years later, in northern Sweden on an Arctic Science course, I was about to get my first glimpse of the aurora. This was just the start of my journey, and when I got on that plane to Kiruna I didn’t realise quite how far I would go. It was a journey that would take me back to the north several times, each visit piecing together a story of the aurora borealis that at its core was the science behind the phenomenon, but which was woven with landscape, people and history. This is that story.

That early trip to Kiruna was also to be my first experience of the Arctic. I was excited, of course, but I didn’t really know what to expect. I was not going out into the wilderness, so it would be a gentle introduction, though our course leaders warned us that temperatures could drop to -25ºC and that we should dress appropriately. I didn’t know exactly what they meant by that, never having had to dress ‘appropriately’ for such temperatures before. I had packed mostly ski wear and extra jacket layers, and I was intrigued to know what the Scandinavians wear indoors, when they take off all these layers of outerwear. Did they all walk around in their thermals?

It was a short flight and we were cruising at just 3000 metres (10,000 feet) above sea level. Now, high above the clouds, the sky was a beautiful clear blue. Beneath us the clouds created a thick, woolly covering, stretching on for what seemed like forever. I wondered how much higher I would have to be before I would see the curvature of the Earth, before I would reach the edge of space itself.

However, the divide between Earth and space is blurry rather than sharp. People talk about flying to the edge of space in a fighter jet, or sending a camera up on a weather balloon, but that is in fact only to around 20 kilometres (12 miles) above the Earth’s surface. For aerospace purposes, the edge of space is defined as 100 kilometres (60 miles) above sea level – known as the Kármán line, after the Hungarian-American physicist Theodore von Kármán. That is where the atmosphere becomes so thin that aerodynamic forces can no longer provide lift. But that is not even the edge of our atmosphere.

Go quite a lot higher, up to the International Space Station, and even there the Earth looks large and close; only a small part of her surface is visible but the curvature is very apparent. The International Space Station, or ISS, an artificial satellite which has been continually inhabited by astronauts since November 2000, orbits at an altitude of 400 kilometres (250 miles). Even that is still within the Earth’s atmosphere, albeit a rapidly thinning one. Our planet’s atmosphere gradually peters out into space from about 500 kilometres (310 miles) above the Earth’s surface. Astronauts on the ISS have taken photographs and video of the aurora from above – a view from what we call ‘near-Earth space’.

As we flew on to Kiruna I reflected on Earth and space – the connections, interactions and varying boundaries between the two systems. Both come into the story of the northern lights, and our humble atmosphere plays an important part. The atmosphere is the screen upon which the drama plays out.

The atmosphere has five main layers, defined according to air temperature, and almost every atmospheric phenomenon with which we are familiar is restricted to the lowest of these, the troposphere. Three-quarters of the mass of the atmosphere is contained in this thin layer, which extends up to an average of 12 kilometres (7.5 miles). The air becomes increasingly rarefied the higher we climb. Because the atmospheric layers are demarcated by temperature, the height of the troposphere is slightly variable across the globe. It is thickest towards the tropics, where warm temperatures cause expansion of the air, and thinner at the poles.

The troposphere is named after change, or turning. The air that is warmed at the surface of the Earth rises and mixes with the overlying air, creating convection currents that generate our weather. Fluffy, white, picture-perfect cumulus clouds are found at around an altitude of only 3 kilometres (1.8 miles) or lower. Towering, thunderous cumulonimbus can stretch from below this height up to almost 15 kilometres (9.3 miles) in giant, peaky columns. Wispy cirrus are blown around at high speed at altitudes between 6 and 12 kilometres (3.5 and 7 miles), about the same altitude at which a commercial jet airliner cruises. The average temperature of the air in the troposphere decreases steadily with altitude to around -60ºC. At around 12 kilometres (7.5 miles) up the temperature stabilises and turns, and we enter the stratosphere.

Here, the temperature begins to increase again due to the presence of ultraviolet-absorbing ozone. This ozone layer in the stratosphere restricts the mixing of air, which is why most of the weather we know is confined to the troposphere. The ultraviolet radiation that the ozone layer absorbs increases the temperature back up to around freezing again – a warm zero degrees!

As I squinted out of the window into the bright blue sky and looked down on the clouds, I mused over our sense of scale. Above the clouds it felt as though we were flying so high and yet we were actually so low, in relative terms. I had climbed mountains higher than this before – up to 6000 metres (20,000 feet) – and I planned to go higher. Commercial tandem skydivers would jump from this aeroplane’s altitude, and the world record for a parachute jump was thirteen times higher. In October 2012 the Austrian skydiver Felix Baumgartner had jumped from the stratosphere (from 39 kilometres (24 miles) above sea level), having been taken up in a capsule suspended below a large helium balloon. He wore a pressure suit that allowed him to survive in the rarefied air and his descent lasted around ten minutes, over four of those minutes spent in freefall. Yet in the stratosphere we still haven’t reached the height of the aurora.

At about 50 kilometres (31 miles) the temperature starts to drop again. From here to 80 kilometres (50 miles) up is called the mesosphere. It is the layer of shooting stars, where meteors coming in towards Earth from space burn up due to the pressure and heat created from collisions with air particles. Yet the air is already remarkably thin, and the top of the mesosphere is the coldest part of the Earth’s atmosphere.

Beyond about 80 kilometres (50 miles) altitude is the thermosphere, a layer that is several times thicker than all the others put together. It transitions into the exosphere somewhere around 500 kilometres (300 miles) altitude, but this level can vary wildly depending on the activity of the Sun. If the thermosphere gets hotter it can puff up to as much as 1000 kilometres (600 miles) from Earth. Beyond the thermosphere, the exosphere is where the Earth’s atmosphere finally blends out into space over thousands of kilometres. It is a murky transition. The thermosphere, seen as the fourth layer of our atmosphere from one perspective, may also be seen as space from another perspective – it is in this layer of the atmosphere that the International Space Station orbits.

Up here, the number of air particles is rapidly dwindling. Even at the mesosphere–thermosphere transition at 80 kilometres (50 miles) the density is a hundred-thousandth of the density at sea level. A gas particle here can travel about one metre on average before hitting another particle. At sea level, that same gas particle would travel less than the width of a human hair before colliding with another. Up at 200 kilometres (120 miles) altitude, in the thermosphere, the air is so thin that a particle could travel 4 to 5 kilometres (2.5 to 3 miles) without hitting another one. These extreme conditions, the likes of which have never been achieved on Earth, allow ordinary atoms and molecules to behave in unusual ways. This rarefied zone is the three-dimensional canvas for the aurora.

In the thin air of the thermosphere, particles are ionised by solar radiation. Incoming photons of light from the Sun dislodge electrons from their atoms so they move around freely, creating a charged gas of negatively charged electrons and positively charged ions, the remnant atoms. During daytime, some ionisation occurs towards the top of the mesosphere too, and it also occurs beyond the thermosphere, in the exosphere. This multi-layered shell of ionised gas, reaching from about 85 kilometres to 600 kilometres (53 to 370 miles) above the Earth’s surface, is known as the ionosphere, and it is here that the aurora is created, predominantly between 100 and 250 kilometres (60 and 155 miles). Pictures of aurorae taken from the International Space Station show the clear curtains of light stretched out in a bright green that fades up into red, and then fades out altogether.

The Earth’s atmosphere has various layers, defined according to temperature. Most of the weather we experience is confined to the troposphere, the first 20 km (12 miles), but the aurora occurs much higher up – predominantly around 100 km (60 miles) high but stretching up hundreds of kilometres more.

Seen from this vantage point in near-Earth space, these curtains of colour clothe our planet. From some reports of the aurora, the lights may even seem close enough to touch, yet in reality they are far, far away from us. The aurora may bridge the boundary between Earth and space, straddling the human-imposed divide of the Kármán line.

I was still daydreaming about whether the aurora was an Earthly or space phenomenon when we started to drop closer to the woolly mass of clouds beneath us. The captain came onto the loudspeaker to declare landing in twenty minutes. It was time to stow away the tray tables and turn off all electronic devices. My first glimpse of the Arctic awaited me just on the other side of those clouds.

* * *

KIRUNA IS A relatively new town in European terms, and it is also the least-populated area in Europe. It sprang up in around 1900, when mining began in these parts. The resource here is iron ore, and the town ...