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Northern Lights

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Ken Tapping, November 17, 2004

In the sky this week...

> Saturn rises around 9 p.m
> Jupiter, Venus (brightest) and Mars (faintest) are grouped quite close together in the dawn sky, rising at roughly 4 a.m., 5 a.m., 6 a.m. respectively.
> The Moon will reach First Quarter on Nov.18.


It was one of those funny coincidences. A few nights ago, I was standing on our back deck looking at a really nice display of the Northern Lights (also called the aurora borealis). At one point a particularly bright bit of shimmering green curtain appeared, and, as I opened my mouth to say "Wow", exactly that word came from a nearby backyard. At least one other person was enjoying the spectacle. We have had quite a few good displays of the Northern Lights over the last few evenings. This has been the result of a period of high solar activity, which seems to be continuing, and the nightly show might enjoy a few more re-runs.

If you pick up an encyclopaedia or popular astronomy book, you are very likely to see the statement that the Sun is a big ball of gas, with a nuclear fusion reactor in the middle. This is only part of the story; the Sun is actually more complicated than that. Just think about the way the Sun looks in the sky. Even though it is not safe to stare at the Sun, we have all had enough glimpses to see that it is not a gassy-looking blob; it looks more like a yellow ball, with edges so sharp that it looks solid. Getting hot gas to behave this way requires another important ingredient – magnetic fields.

Somewhere inside the Sun there is a process that we call the solar dynamo. Powered by the circulation of material inside the Sun, the dynamo makes electric currents, which in turn generate magnetic fields. Although these magnetic fields are the same as we get with a toy bar magnet, the environment in which these magnetic fields form makes things rather different. The hot gas and the magnetic fields stick together, resulting in things we call flux tubes or flux ropes. Imagine a long string of sausages being made. The tube contains the meat and herbs, but it has not yet been twisted to form the individual sausages. Now imagine this tube being coated with oil so that it is almost unbelievably slippery. Magnetic flux ropes are like this. Once formed, they slither around in the Sun's interior, sometimes carried along by flows of material. On occasion they rise to the surface, where they cause patches of strong magnetic field we call active regions. When these ropes erupt through the surface into the solar atmosphere, which is much more rarified, they form complicated systems of magnetic loops. As more loops come up, and the "surface" churns around, the loops get stressed and twisted. Occasionally one gets overstressed, snaps and catapults upwards into space at up to a thousand kilometers a second, taking a large amount of solar material with it. These events are called "coronal mass ejections". A CME takes between 36 and 48 hours to cover the 150 million kilometers to Earth's orbit. If it hits the Earth's magnetic field, instabilities develop and electrons are accelerated downward along the magnetic field towards the poles, where they collide with atoms of oxygen and nitrogen, making them glow. This is what produces a display of aurora.

We'll continue with the Christmas shopping next time.

Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,