Christy's weather: The science behind the northern lights

If you live in Canada’s North, look up this week.

Conditions are favourable for increased aurora activity, which should mean another spectacular show, provided the clouds clear away.

But what’s the hard science behind these softly dancing lights? If you’ve ever wondered, I’ve tried to answer some of the most common questions below.

So, why do they happen?

The northern lights are caused by the collision of charged particles with molecules in our atmosphere. These collisions release energy in the form of light, which we see in the sky.

The particles largely come from coronal mass ejections (or CMEs) from flares and spots on the sun. They are guided toward the earth by the planet’s magnetic field, which penetrates the atmosphere at the poles. That’s why we see the lights most prominently in those regions.

What’s with all the colours?

The dancing colours are a result of the types of gases in our atmosphere. When the stream of energetic particles react with molecular nitrogen — the most common gas in our atmosphere — we see green. Red aurora are caused by a reaction with oxygen.

The aurora colour can also be affected by the altitude in the atmosphere where the lights originate. Red aurora typically occur at higher levels and green auroras happen in lower levels of the atmosphere. The northern lights "dance" or move like a ribbon due to the motion of the Earth’s magnetic field.

Why are the lights so much brighter some days?

Usually a sun with many sunspots and flares facing Earth, means more active aurora,  while a quiet sun means dull lights. Last month, for example, one of the strongest solar storms in years brought extremely active aurora that were visible as far south as the United States.

The sun goes through solar maximums and minimums, roughly every 11 years (although irregular cycles can happen). Right now, we are in what’s known as a "mini-max." This means that although the number of solar flares and sunspots are high, this is a relatively weak solar maximum.

Dean Pesnell of the Goddard Space Flight Centre said in a recent conversation with NASA’s Tony Phillips that “we’re not out of the woods yet.” Even this weak solar maximum could bring strong flares meaning active space weather and bright aurora.

How accurately can we predict aurora activity?

Like the weather in our atmosphere, space weather can be predicted. Usually solar flares and spots that release CMEs occur two to three days before we see the effect on Earth. This means aurora activity can be predicted with some accuracy within that time.

These forecasts can still be difficult though. Jim Spann, a NASA scientist and aurora expert, explained this difficulty in a recent online chat session. "The state of the art of space weather prediction is about as good as our terrestrial weather prediction was 30 years ago," he said.

So for now, the aurora remain an elusive but beautiful part of our atmosphere.