What causes the northern lights?

The northern lights are the sky itself glowing. The Sun is constantly blowing a stream of charged particles out into space. When that stream reaches the Earth, our planet's magnetic field catches it, funnels it down toward the poles, and slams it into the thin air high overhead. The air lights up where it is hit. That light is the aurora.

Where the particles come from

The Sun never stops. It blows a steady wind of charged particles, the solar wind, in every direction. When the Sun is active it also fires off sudden eruptions, solar flares and great clouds of plasma called coronal mass ejections, that send a far denser, faster blast toward us. A quiet aurora rides the steady wind. A great aurora, the kind that reaches far south, rides one of those eruptions.

Why it rings the poles

The Earth behaves like a giant bar magnet, with field lines that curve down into the planet near the magnetic poles. Incoming particles spiral along those field lines and are steered toward the north and south magnetic poles. That is why the aurora forms a ring, the auroral oval, centred on each pole rather than appearing evenly across the sky. The stronger the storm, the wider that ring swells, and the farther from the pole the lights come down.

Why the northern lights are different colors

The colors are not random. They tell you which gas is glowing and at what altitude:

• Green is the most common. It comes from oxygen at around 100 to 300 kilometres up, the altitude the aurora favours most.
• Red comes from oxygen much higher, above 300 kilometres. A deep red aurora is the signature of a powerful storm, because only a strong event excites the air that high, and it is the red crown that people far south tend to see.
• Purple and blue come from nitrogen, usually along the lower edge of a bright, active display.

How far south can it reach?

On a quiet night the oval hugs the far north, over places like Alaska, northern Canada and Scandinavia. During a severe geomagnetic storm it swells south over the continental United States, and in the rarest extreme storms it has reached the tropics. The record events, like Carrington in 1859 and the 1872 storm, put red aurora almost at the equator.