By Isla Quinn
Cold air. A black sky. Then a pale smear that you take for cloud until it moves.
That is the aurora, and this piece is about what it is made of, in plain terms, and why people go to Yellowknife to find it. It points to the Canadian Space Agency and two tourism sites, so read it as a starting point, not a forecast.
What lights the sky
Wikipedia’s aurora article says charged particles, mainly electrons and protons from the Sun, are funnelled into the upper atmosphere after the solar wind disturbs Earth’s magnetosphere. They collide with atoms and molecules, which are excited or ionized and then give off light.
It happens high up. The article says auroras occur above about 80 km, in the thermosphere and exosphere. It adds that camera measurements by Carl Størmer, starting in 1911, found none below 70 km, only 6.5 per cent above 150 km, and a peak near 100 km.
Why the colours
The colour tells you which gas is glowing and roughly how high.
- Green, at 557.7 nm, is atomic oxygen at lower altitudes. It is the most common colour.
- Red, at 630.0 nm, is atomic oxygen at the highest altitudes.
- Blue or purple, with 428 nm dominant, is molecular nitrogen at the lowest altitudes. The article says it appears most during high solar activity.
- Pink and yellow are mixes of red with green or blue. They are rare.
Oxygen is slow. Wikipedia says it takes about 0.7 seconds to emit green and up to about two minutes for red. Our reading: that is why green is crisp and red tends to look like a soft stain.
Where the lights sit
The article says auroras are most common in an auroral zone about 6 degrees wide, centred near 67 degrees north and south. The oval is pushed by the solar wind: about 15 degrees from the geomagnetic pole toward the Sun, and about 23 degrees away on the night side. Geomagnetic storms expand the oval and bring aurora to lower latitudes.
A scale called Kp measures this. Kp = 3 means relatively low geomagnetic activity; Kp = 9 means high. The article does not tell you which Kp number you need at a given latitude, and we will not guess.
What kind of sources these are
The Canadian Space Agency, Spectacular NWT and Extraordinary Yellowknife. The last two promote the destination.
What we left out
Tour operators, viewing-package details, and anything priced. We also left out cultural stories about the lights, which belong to other people to tell.
Quick answers
What makes the aurora green? According to Wikipedia, atomic oxygen at lower altitudes, glowing at 557.7 nm. It is the most common colour.
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