AuroraWatchEvery aurora starts about 150 million kilometres away, at the Sun.
It starts with the solar wind
The Sun constantly blows a stream of charged particles out into space, called the solar wind. On a normal day it travels at around 400 km/s. When the Sun throws off a coronal mass ejection, or a fast stream escapes from a coronal hole, the solar wind gets faster and denser, and those are the nights that light up.
Earth's magnetic field steers it to the poles
Most of the solar wind is deflected by Earth's magnetic field. But when the magnetic field carried by the solar wind points south (a negative Bz), it links up with Earth's own field and lets energy in. Charged particles are funnelled along the field lines towards the North and South Poles, into two rings called the auroral ovals.
That's why the Northern Lights and the Southern Lights usually happen at the same time, and why you need to be closer to the poles, or have a bigger storm, to see them.
Why the sky glows green, red and purple
High in the atmosphere, roughly 100 to 300 km up, those particles slam into oxygen and nitrogen and make them glow:
- Green comes from oxygen, and is the colour you'll see most often.
- Red also comes from oxygen, but higher up, and shows during stronger storms.
- Blue and purple come from nitrogen, usually along the lower edge of the curtains.
The curtain shapes you see trace Earth's magnetic field lines.
Knowing when to look up
Because the aurora depends on the solar wind, you can see a good night coming. Watch for faster solar wind, a strongly negative Bz and a rising Kp index, then check your sky is clear and dark. AuroraWatch puts all of that on one screen, for both hemispheres, and alerts you when conditions line up where you are.
Want to go deeper? Read our guides to what Kp index you need to see the aurora and how Bz and the solar wind trigger it.