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A dark winter night at high latitude
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THE best chance to see Earth’s aurorae is during the long winter nights at high latitudes, which is why they are also known as the polar, northern or southern lights. It is almost winter in the northern hemisphere, although it is summer in the southern hemisphere, of course – but read on to find the best places to spot the aurorae when winter arrives.
The aurorae are generated by the solar wind – a stream of charged particles travelling from the outer layer of the sun, or corona, and slamming into Earth’s magnetic field. This acts like a shield around the planet that deflects most of the particles. But at its weakest points around the poles, some can penetrate into the upper atmosphere, where they collide with and excite gas molecules. As these molecules de-excite, they release the photons of light that make the aurorae.
The type of excited molecule, along with the altitude of the collisions, determine the colour of the aurorae. The most common colours are pale yellow and green from oxygen molecules around 120 to 180 kilometres up. Less frequent are red aurorae, generated from oxygen around 200 km above the ground, while red-purple aurorae come from nitrogen below 100 km.
Most of us will have to travel closer to the poles to catch a glimpse of the aurorae, with Iceland, northern Scandinavia, Yukon in Canada and Alaska popular locations in the northern hemisphere. For the southern hemisphere, Patagonia and the Falkland Islands are good, along with southern New Zealand, and Tasmania and Victoria in Australia.
The best aurorae happen when solar activity is high. This is hard to predict, although websites and organisations that monitor the sun can give a forecast for the coming day, or even week. The US National Oceanic and Atmospheric Administration website, for example, provides observations for the past three days along with 30-minute predictions of the sun’s activity. This is measured using the planetary K index, or K-p, on a scale from 0 to 9. The greater the K-p value, the higher the activity.
If it is a clear, dark night and the sun looks like it will be active, get yourself to a dark spot as far from any light pollution as possible. Then you need to wait and let your eyes adjust. But don’t expect aurorae in the stunning, bright colours shown in photos. Seen with the naked eye, the aurorae are much subtler, and can be tricky to spot the first time you try. If you are in an area popular for aurora watchers, consider going on a tour with a guide who can show you exactly what to look for and help you capture those all-important photographs.
Abigail Beall is the author of The Art Of Urban Astronomy (Buy from *)
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For next week
Binoculars may be helpful
Next in the series
2 How to watch the Leonid meteor shower
3 Venus and Jupiter in conjunction
4 Mercury at its greatest elongation
5 How to see the Northern Lights
6 Find the Andromeda galaxyThe most distant object visible to the naked eye
7 How to see Santa (the ISS) on Christmas Eve
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