Myths abound about the Aurora Borealis or Northern Lights and have done for centuries. The recent opportunity to view them from the United Kingdom is a reminder that we are not immune to a few of our own. Professor Brian J Ford separates fact from received opinion.
Did you go out to see the aurora the other week? Neither did I. A glass of wine and a warm hearth are a more welcoming option on these chilly nights. Many people did – in Britain it was photographed as far south as Dorset.
Yet the popular picture of the northern lights is wrong. They don’t look like the photographs, and the dancing beams portrayed in films aren’t the way they appear in the real world. Neither is it true that you need to be as far north as possible to witness this spectacle. At the North Pole you are as likely to see the recent displays as you would be in Spain.
The aurora is fainter than you expect. It is photographed with a time exposure which exaggerates the image, so published pictures have deep colours that aren’t seen in life. Until your eyes are accustomed to the darkness, you won’t observe anything. Tourists can stay the weekend in Reykjavík and never see a thing, while out-of-town viewing sites – well away from urban light pollution – have skies filled with slowly moving shards of light.
Filming the aurora is worse; a film camera is run slowly (perhaps at five frames per second) so that each exposure is increased from 1/25 second to one-fifth. The resulting movie, when played back, has the contrast and vividness of the aurora greatly increased, while the speed at which it moves is five times too fast.
The aurora is fainter than you expect. It is photographed with a time exposure which exaggerates the image, so published pictures have deep colours that aren’t seen in life
The same applies to video. The public are regularly shown the lights of the aurora seeming to be brightly coloured and intensive, dancing across the heavens like flickering candles. In life the colours are more muted, and their alluring movement is slower and more subtle than you see on the screen.
They say you can sometimes hear the aurora, sounding like faint crackling or rustling, similar to silk sliding on paper. So many legends surround this most beautiful of natural spectacles; both the indigenous North Americans and the Norse communities believed them to be the spirits of the deceased, waving back at them from the heavens.
The Laplanders, the Sámi people, tend to stay indoors when the lights are slowly playing across the sky. To them, the aurora is a bad omen. In Iceland it was believed that the sight of the aurora eased the pain of labour, though pregnant women who saw it believed they’d have a child who was cross-eyed. And all northern communities insist that you should never whistle at the aurora.
This phenomenon is born as the sun emits beams of ions and electrons which, three days later, reach the Earth and interact with gas atoms in our magnetosphere. Atoms of nitrogen and oxygen become excited and radiate light that usually appears as slow-moving diaphanous curtains of faint green or (rarely) red; sometimes blue is glimpsed. They hang in the sky, typically about 30 degrees from the poles so you can see them most nights in the sky above Iceland, and the northerly parts of North America, Scandinavia, and Russia. The aurora Australis is seen most frequently in a ring above the oceans that encircle the Antarctic continent.
Occasionally the surface of the sun erupts, and high concentrations of particles are emitted, which is when the aurora can sometimes be seen in Britain. On those nights the aurora Australis can also be observed to the south of New Zealand and Tasmania.
Most often the slowly shifting lights are green, the colour to which our eyes are particularly sensitive. This hue is emitted when ions collide with atoms of nitrogen; they excite oxygen, radiating light at 558 nanometres (nm). Excited atoms of oxygen in the upper fringes of the atmosphere emit a red glow at 630 nm, more often perceived at latitudes further from the poles. Rarely are ions of nitrogen excited; they emit blue light at 428 nm.
When confronted with the sky covered with a faint, undulating curtain of ghostly lights it is so easy to see why the aurora acquired spiritual significance. The spectacle is alluring, overwhelming, and utterly unforgettable. Next time, see it if you can. That wine can wait.
Pics: Northern Lights viewed from the Alaskan coast (Brian J Ford)