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Stars and solar physics

Stars and solar physics

Adventures in search of auroras

22 Sep 2016
Taken from the September 2016 issue of Physics World

Aurora: In Search of the Northern Lights
Melanie Windridge
2016 William Collins £18.99hb 320pp

Adventures in search of auroras

Tell people that you study the northern lights for a living and you generally get the response “Really? Have you seen them? I’d love to see them!” There is something about these collisions between electrons and other particles 100 km up in the atmosphere (and the resultant light show) that captivates people. Maybe it’s the sense of adventure, of seeing something that happens far, far away in a cold, inhospitable land – but not so far away or inhospitable as to be completely out of reach. Maybe it’s seeing a beautiful natural spectacle light up the whole night sky. Maybe it’s just awesome, in the literal meaning of the word. Whatever it is, the wonder of the northern lights is a draw for many people around the world.

Melanie Windridge is one such person. Captivated by these fantastic displays and inspired to learn more, in her book Aurora: In Search of the Northern Lights, she describes travelling around the Arctic Circle on a quest to see the biggest and best auroral displays and to understand the physics that drives them. Each stopping point provides the backdrop for understanding the science, which covers everything from the basic physics of plasma in space to the potential consequences of a massive space weather event. Peppered throughout are nuggets of history, putting into context the struggles of earlier aurora explorers to use emerging techniques – such as photography, spectroscopy and quantum mechanics – to understand what they were seeing. Throughout, Windridge manages to convey a sense that although we know a lot about the aurora, we don’t necessarily understand a lot about the aurora.

The basic principles behind the northern lights are well established. A flow of energy, electromagnetic fields and charged particles (plasma) from the Sun strikes the magnetic field of the Earth. Some of this is captured by the Earth’s magnetic field through a process known as reconnection, resulting in a build-up of energy and plasma. When that energy is released, some of the plasma is accelerated along the Earth’s magnetic field and into the atmosphere, where the charged particles lose energy by exciting electrons in neutral atmospheric atoms, causing them to glow. The remaining particles can be trapped in the Earth’s magnetic field, getting up to energies well above those on the Sun and creating the radiation belts. What isn’t so clear, though, is exactly when this will happen or how big any particular auroral event will be – as Windridge discovered on her quest.

“Blue skies” research into the aurora has brought us new information on the physical processes taking place beyond the surface of our planet. The fact that most of the auroral light comes from so-called “forbidden” atomic transitions provided insights into the composition and structure of the upper atmosphere long before any in situ measurements could be made. But it is the space weather aspect of the aurora that really has the potential to affect our day-to-day lives. Processes linked to the aurora can cause power and communications outages, damage to satellites and disruption of air travel. Due to the interconnectedness of much of the technology in our lives, disturbances that would once have been local or insignificant can now cascade into something far greater. As Windridge notes, the eruption of Iceland’s Eyjafjallajökull volcano, a relatively minor event, caused a seven-day disruption to air travel costing over $1bn and affecting more than a million air passengers each day.

Since the start of the space age there have only been a handful of aurora-related events with significant impacts on infrastructure. Of these, the 1989 event that knocked out the electricity supply in the Canadian province of Quebec, and the 2003 Halloween storm that had similar effects on Sweden, are the most notable. Most engineers working in the power sector have not experienced a truly major event, and it is sobering to imagine the impact that the 1859 Carrington event, in which aurora were seen as far south as the Caribbean, would have if it occurred today. When discussing space weather, it can be very easy to verge into scare-mongering, but Windridge tackles this subject in a very sensible and grounded way. The book examines the different ways space weather can interact with ground and space-based technology, the steps that can and are being taken to mitigate these impacts, and how we can improve this resilience in future.

If I have one criticism of Aurora, it is that I found its style somewhat inconsistent. At times, the book reads like the commentary to a documentary; at others a diary of adventures; and at others an in-depth (but accessible) discussion of the science. None of these, on their own, are bad styles, but the way the book jumps between them can be a little jarring. This is particularly noticeable in the early chapters, where the reader is necessarily taken through the basic science behind the aurora as a springboard to understanding the rest of the book. But I would urge people to get beyond that, since Windridge manages to explain the history and science of the aurora in a way that should be understandable to most people who pick up this book.

I don’t study the northern lights for a living; instead, I use them as a way of studying the interaction between the Sun and the Earth and the processes of energy storage and release. But even as a space scientist, I found I learned things from this book. While Aurora is no replacement for the standard textbooks (and it doesn’t pretend to be), it provides a concise and accessible overview of auroral science, and in particular space weather, that puts this research in a useful context. It discusses the past, present and potential future of this research and should be of interest to anyone wishing to know what the aurora is all about.

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