What are the northern lights?

That supernatural event — once shrouded in mystery, once met with fear — today we not only understand its cause; we can almost forecast it like the weather: when, where and at what magnitude it will occur.

Throughout history, people living near the northern and southern poles — unlike the rest of the world's population — witnessed a veil of light dancing across the sky on cold winter nights, usually green, sometimes pink and red. For those far from space science, this supernatural sight kept its mystery for centuries. Thanks to today's technology and accumulated knowledge, an event people once approached with fear can now be followed with great fascination, and its cause understood.

The northern lights — known scientifically as Aurora Borealis over the North Pole and Aurora Australis over the South Pole — can be observed, depending on your hemisphere, generally from late autumn to early spring. They are most often called the “northern lights” because the northern hemisphere offers far more landmass from which to see them, and that land falls within inhabited areas. This is why “northern lights” became the more popular and widespread term.

The sky's coloured layers.

If you could look down on our planet from very high above, you would see that the sky is made of layers of different colours at different altitudes. Between 80 and 105 km above the ground lies a layer of sodium; by day this layer is chemically excited, and at night, as the atoms cool away from sunlight, it settles into a more stable state and usually takes on a yellow glow. Higher still, oxygen, nitrogen and hydrogen atoms bond with one another to create different optical effects.

For example, when hydroxyl (OH) molecules combine with the sodium layer, a red glow appears at lower altitude. Above these layers sits the bright green airglow, produced by nitric oxide (NO) and monatomic oxygen (O) molecules. Above the green layer a blue layer rarely forms — when two oxygen atoms join to make molecular oxygen (O₂). At the highest altitude, electrons descending along their orbits interact with moving oxygen atoms and hydroxyl radicals (OH⁻) to produce red airglow.

The Sun's part.

All of these routine events arise from the interaction between the Sun's rays and the Earth over the course of a day. By day these rays strike the atoms, molecules and ions above; by night the electrons drop to a lower-energy state and emit the light we see.

The Sun emits not only heat and light but also a stream of particles. These particles are released continuously into space from the Sun's corona by way of the solar wind. But sometimes, owing to explosions large and small on the Sun, the speed of that wind and the amount of particles it carries surge sharply — and it is this surge that sets the stage for a visual show on Earth.