The aurora borealis - also known as the northern lights - is a vivid demonstration of the Earth's magnetic field interacting with charged particles from the sun. It's also beautiful, and worth braving a cold night out when visiting the high northern (or southern) latitudes.
What are the Northern Lights?
The northern lights are one of several astronomical phenomena called polar lights (Aurora Polaris), are shafts or curtains of coloured light visible on occasion in the night sky. The light show we see may appear for a staggering three hours, or a disappointing three minutes. They may be green, purple, red and gold, or merely a dim shade of pine. There is no rule governing the northern lights. They come and go as they please.
Those blessed to see the lights in all their majesty will be in a for a breathtaking experience, quite unlike anything else on earth. It is as if the night sky comes alive, as though it is communicating with you, personally.
What Causes the Northern Lights?
The light show we see from the ground is caused by electrically charged particles from the Sun entering the Earth's upper atmosphere at a very high speed. Auroras are centred on the Earth's magnetic poles, visible in a roughly circular region around them. Since the magnetic and geographic poles aren't the same, sometimes the auroras are visible farther south than one might expect, while in other places it's farther north.
The origin of the aurora begins on the surface of the sun when solar activity ejects a cloud of gas. Scientists call this a coronal mass ejection (CME). If one of these reaches earth, taking about 2 to 3 days, it collides with the Earth's magnetic field. This field is invisible, and if you could see its shape, it would make Earth look like a comet with a long magnetic 'tail' stretching a million miles behind Earth in the opposite direction of the sun.
When a coronal mass ejection collides with the magnetic field, it causes complex changes to happen to the magnetic tail region. These changes generate currents of charged particles, which then flow along lines of magnetic force into the Polar Regions. These particles are boosted in energy in Earth's upper atmosphere, and when they collide with oxygen and nitrogen atoms, they produce dazzling auroral light.
Aurora are beautiful, but the invisible flows of particles and magnetism that go on at the same time can damage our electrical power grid and satellites operating in space. This is why scientists are so keen to understand the physics of aurora and solar storms, so we can predict when our technologies may be affected.
Where is the best place to see the Northern Lights?
In the Northern Hemisphere, the auroral zone runs along the northern coast of Siberia, Scandinavia, Iceland, the southern tip of Greenland and northern Canada and Alaska. Auroras are visible south of the zone, but they are less likely to occur the farther away you go.
The Southern Hemisphere auroral zone is mostly over Antarctica or the Southern Ocean. To see the southern lights (or aurora australis), you have to go to Tasmania, and there are occasional sightings in southern Argentina or the Falklands - but those are rare.
When is the best time to see the Northern Lights?
For the best chance of seeing the lights, you need to be under or close to one of the auroral ovals.
The northern lights are similar to other weather patterns, in that they can be forecasted and need certain conditions to occur. They can happen anytime you have dark skies, anytime between dawn and dusk, and are least common in the summer months. They can occur all year round but are best seen between September and April.
Photographing the Northern Lights
A modern SLR is ideal, as usable ISO of around 3200 will be extremely useful. It's also advisable to stock up on memory cards so you can fire at will without having to worry about running out of space.
Fast optics are a necessity for shots like this, with an aperture of f/2.8 being the absolute minimum and f/1.4 an advantage. Wide angle zooms are perfect for this and fisheye lenses are remarkably good as well.
A cable release can be useful to take multiple exposures without touching the camera. This not only ensures sharp shots at slow shutter speeds but also leaves you free to view the spectacle as it unfolds in the sky above.
You'll need to keep your camera super-steady to avoid unwanted motion blur during long exposures, so mount your camera on a sturdy tripod and on a flat stable surface.
Set up your camera with an ISO of around 800 at your lenses widest aperture. In manual mode, set the shutter speed to 5 seconds.
To avoid disappointing shots, make sure that your lens is focused correctly before you commit to a shot. Set it to infinity, take a test shot, then zoom into the image on the LCD and check the stars for sharpness. It can be a good idea to tape the focus ring in place to avoid accidentally moving it.
Check and recheck your images for sharpness. It is easy to nudge the focus ring by accident and ruin all your remaining exposures.
Depending on your camera's capabilities, I would suggest starting with an ISO of 800 to keep the noise down and shoot with the lens wide open. Take a test shot in aperture priority mode and view the image on the LCD. A dark image where the aurora is barely visible is underexposed so you need to dial in +1 or +2 stops exposure compensation. Once you have the basic exposure setting you can make the picture more interesting by including some foreground subjects. This provides scale and greatly improves the overall composition. Moonlit nights may provide enough light, or you can "paint" your subject with the light from a torch.
What Causes Different Colours in the Northern Lights
Different ions in the ionosphere make different colours when they are energised by the solar winds.
The most common colour seen in the Northern Lights is green. When the solar wind excites millions of oxygen atoms in the Earth's atmosphere at the same time, it excites the oxygen atoms briefly. They then return to their original state and emit the green hue we can see from the ground.
The red light we sometimes see is also caused by oxygen atoms. These particles are higher up in the atmosphere and are subject to a lower energy red light emission. The red colour is always there, but our eyes are five times less sensitive to red light than green, so we can't always see it.
On rare occasions, the solar wind is strong enough to excite nitrogen ions. These nitrogen ions emit a purple hue. This is quite a rare colour to see, and usually only happens during a particularly active display.
The Northern Lights in Mythology
Many cultural groups have legends about the lights. In Roman myths, Aurora was the goddess of the dawn. In medieval times, the occurrences of auroral displays were seen as harbingers of war or famine.
The Scottish, Irish, English and French all considered the lights an omen of coming strife, whilst alternatively, the Scandinavians associated them with bountiful fishing, painless childbirth and warmth.
Interestingly Old Norse and Icelandic literature don't mention auroras much. The Vikings thought the displays might be fires that surrounded the edge of the world, an emanation of flame from the northern ice, or reflections from the sun as it went around the other side of the Earth. All three ideas were considered rational, non-supernatural explanations in the Medieval Period.
The Maori of New Zealand shared a belief with many northern people of Europe and North America that the lights were reflections from torches or campfires.