How Aurora Lights Are Formed

BOMBARDED FROM SPACE, the air glows; shimmering curtains of light drift across the sky to form aurorae. In centuries gone by, the Vikings believed they represented the beauty of dead maidens. Other cultures thought the greens and reds were gods either at play or war.

Despite having been transfixed by aurorae for centuries, there are some surprising gaps in our knowledge about how they develop and exactly what’s going on inside them. But this year will be an Important one for aurora researchers, because the chances of seeing these celestial firework displays are the best they have been for a decade.

An aurora is the result of electrically charged particles thrown off by the Sun. If those particles happen to get caught In the Earth’s natural cloak of magnetism, they can be funnelled down Into the atmosphere, where they collide with oxygen atoms and set them glowing (see ‘How It works: the aurora’, on p77).

And this May, the Sun is expected to reach the peak of its 11-year activity cycle. Although this ‘solar maximum’ is unlikely to be a particularly eventful one, there will still be more solar flares than at other times, releasing more charged particles.

It means this year is likely to be a busy one for researchers at the Kjell Henriksen Observatory in Norway – the largest observatory of its kind dedicated to studying aurorae. Run by the University Centre in Svalbard, an institution that specialises in Arctic studies, it is located on a group of islands between mainland Norway and the North Pole.

Its northerly location means the six scientists who work there endure some pretty inclement weather – the average summer temperature is little more than 4“C. But it’s also one of the few places on Earth where you can see aurorae during the day – for a 10-week period around the winter solstice it is sufficiently dark.

Beneath domes on the observatory’s roof sit an array of optical instruments – cameras, photometers, which measure the intensity of light, and spectrometers, which split light into different wavelengths and measure their intensity. A key instrument at the observatory is a 32m-diameter radar. It emits radio waves into the atmosphere that are reflected back. The returned signal enables researchers to study particles in the upper atmosphere -electrons and ions – creating the aurorae. “Combining the measurements from the optical instruments and the radar we can learn more about what’s going on inside aurorae through the different layers of the atmosphere,” says Dr Marglt Dyrland, one of the scientists at the observatory.

Among the more recent additions to the observatory’s equipment is NORUSCA II – the ultimate aurora camera. Whereas your average camera is effectively a ‘light bucket’, bringing together all the wavelengths of visible light into one image, NORUSCA II is a ‘hyperspectral’ camera that can simultaneously capture 41 wavelengths so each can be analysed separately. “It can detect specific atmospheric constituents by their unique fingerprint – the wavelengths of light they emit,” says Prof Fred Sigernes, who runs the observatory. These spectral signatures can reveal subtle changes In atmospheric behaviour, such as the ionisation of gases, during aurorae.

Even during the camera’s first stint of research, back in January 2012, it proved its worth. It revealed something unexpected – a faint wave pattern in the lower atmosphere that resembles airglow, a weak emission of light by Earth’s atmosphere. It’s produced by several sources, including cosmic rays striking the upper atmosphere. And its appearance at the same time as an aurora suggests it may also be caused by a previously unrecognised source.

RECORD BREAKER

The largest aurora we know of hit our planet on 2 September 1859. Two-thirds of the atmosphere was bathed in celestial glows after a giant solar flare triggered a massive eruption of gases. The appearance of this gigantic aurora coincided with the widespread disruption of magnetic and electrical systems.

In the 19th Century, these mainly took the form of compass needles, which spun uselessly, and the telegraph. The long cables needed to relay messages were perfect conduits for the aurora.

The buffeting of the Earth’s magnetic field by the solar particles, known as space weather, induced electrical currents far larger than normal in the wires. These not only swamped messages but caused sparks to leap from the equipment, in some cases stunning operators unconscious and setting offices aflame.

Today, we are almost totally reliant on electrical technology. Satellites and power grids are vulnerable to space weather and this is an area where the observatory at Svalbard could help. “As well as learning about the particles through different layers of the atmosphere, we want to understand how aurorae disturb GPS signals, high frequency . communications and the like,” says Dyrland.

“We also want to be able to provide . warnings about solar storms that might harm vital infrastructure on Earth.”

THE AURORA MACHINE

But aurora researchers aren’t just peering into the skies, they’re creating thei rown. At the University of Leicester is the Planeterrella – an aurora simulator. It features a miniature Earth made out of metal with a magnet inside to simulate our planet’s magnetic field, as well as an electron gun that acts as the Sun, firing particles at the metallic world. The whole thing sits in a vacuum chamber and when the gun is switched on, a beautiful glow – an aurora -appears around the miniature Earth. “It’s pretty authentic,” says Dr Gabby Provan of Leicester University. “To get an auroral glow you have to leave one-ten thousandths of atmospheric pressure in the chamber, since air is much thinner at altitudes where aurorae are observed.”

Although mainly a teaching aid, the Planeterrella is used for research. “I can simulate the magnetic fields of other planets,” says Provan. Uranus’, for example, is unique in the Solar . System as there is a significant misalignment between its magnetic poles and its rotation axis. Whereas Earth’s magnetic north is just 11“ away from the North Pole, on Uranus the angle is almost 60“; follow a compass on Uranus and you would end up close to the planet’s equator rather than its pole. It also means aurorae appear at equatorial latitudes more often than the polar regions, something that can be simulated by moving the magnets inside the Planeterrella,

“I often see professors standing around the Planeterrella discussing what is happening to the electrons in there,” says Provan, who studies aurorae on other planets, Jupiter is a favourite. Its magnetic field is so strong it forms an almost impenetrable barrier to the Sun’s particles. You may think that this makes the planet aurora-less – but not so. Its volcanic moon, lo, spews charged gas, some of which is guided by the magnetic field to the poles of Jupiter, where it sparks aurorae.

A SMOKING GUN

But there are still some fundamental questions to be answered about aurorae here on Earth. “You’d imagine that we would know it all by now,” says Dr Jim Wild, an aurora researcher at Lancaster University. “But were not entirely sure of the mechanism behind the gun.”

The gun in question is the magnetic field close to Earth. Somehow, at altitudes of about 5,000-10,000km, it configures itself into a natural particle accelerator that fires particles into the atmosphere, producing the aurorae. The European Space Agency’s Cluster mission has been in space investigating this effect. It maps the ever-changing magnetic landscape and, on 5 June 2009, It passed through one of these natural particle accelerators for the first time. The data revealed that the correct configuration is only stable for about five minutes. So auroral displays that last for hours must be powered by many of these forming spontaneously out of the ever-shifting magnetic landscape. But to test this more data is needed and Cluster cannot always be in the right place at the right time.

We need to keep our gaze fixed on aurorae using facilities like the Svalbard observatory as well as eyes in space. “We’re just beginning to understand how closely our planet is connected to space,” says Wild. Aurorae are a clear demonstration of that link, so understanding more about them will be vital in coming years.