Canada has a front-row seat to the aurora borealis

The sight of a display of aurora borealis against the stars, over a quiet lake surrounded by forested hills is about as Canadian as it gets.

Over the last few weeks there have been several of these beautiful displays. Some of them have even been visible in England, which is extremely unusual.

The story of the aurora starts at the Sun. Our star emits a continuous blast of particles and magnetic fields moving at hundreds of kilometres a second, called the solar wind. Sometimes it is a breeze, other times it is a gale. Great loops and streamers of extremely hot plasma (gas containing atoms that have had one of more of their electrons stripped away) arc high above the surface.

Over time, the continuing movement of the footpoints of these loops and the encroachment of newly emerging loops generate stresses. These can build up until some nearby disturbance can make the stresses too much. The magnetic fields snap and the loop catapults off into space at speeds that can be as high as thousands of kilometres a second.

These high-speed blobs of magnetized plasma are called coronal mass ejections, or "solar storms.” On occasion one gets shot in our direction. The Sun goes through an activity cycle taking between 10 and 13 years. We are currently at a cycle maximum, when coronal mass ejections can be particularly frequent.

The next ingredient in the story of the aurora is the Earth's magnetic field. Older science textbooks show our planet's magnetic field as a series of lines of magnetic force emerging near one magnetic pole, arcing outwards into space and then down to the other magnetic pole.

In the diagrams our planet is shown sitting in the middle of a huge, magnetic doughnut. This was before the solar wind was discovered. Now we know the solar wind blows our magnetic field outwards, making it look like a huge teardrop, pointing outwards.

In 1958, the first U.S. satellite, Explorer 1, discovered the next ingredient in the auroral recipe, the Van Allen radio belts. The spacecraft was launched into a highly elliptical orbit, ranging between a closest approach of 354 kilometres and then outwards to a distance of 2514 kilometres. As it moved in and out its path took it through zones of high-energy particles, trapped in our planet's magnetic field. The radiation level was so high it saturated the radiation detector on the spacecraft.

When a coronal mass ejection hits the Earth's magnetic field, it deforms it and interacts with it in such a way that particles from the radiation belts are accelerated along the lines of the magnetic field, down towards the magnetic poles. In addition, the tail of the magnetic teardrop is stretched to the point where the magnetic fields snap and reconnect, launching disturbance along the magnetic fields back towards the Earth. These result in the acceleration of more particles.

Left undisturbed, these particles can remain in the radiation belts for years. However, now jets of them are moving along the magnetic field towards the magnetic poles. As they move downwards they encounter our increasingly dense atmosphere. Between 100 and 300 kilometres above the ground they collide with the oxygen and nitrogen atoms, energizing them.

The atoms then unload this energy as pulses of light, usually green, but in particularly high-energy cases, red. The complicated interaction between the particle jets, the magnetic field and the atmosphere can give us glowing curtains, arcs, rays and sometimes just great areas of glow.

If the display is faint, our eyes cannot see the colour and we just see white.

In Canada we have a front-row seat for auroral displays because the North Magnetic Pole, which all those high-energy particles are heading for, is located in the Canadian Arctic.

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• Venus is very low in the sunset glow.

• Around midnight, Saturn lies low in the south, with brilliant Jupiter and Mars high in the southeast.

• The Moon will be new on Nov. 1.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.