SKY · FIELD GUIDE
What Is the Aurora — and Why It Glows
Those shifting curtains of green and red light aren't reflections or clouds — they're the upper atmosphere lit up by particles from the Sun. Here's what's actually happening.
The aurora is one of the few genuinely cosmic things you can watch with your own eyes from the ground. Strip away the folklore and it's surprisingly simple: it's the sky glowing because particles from the Sun are slamming into the air, high above the weather.
Where it comes from
The Sun constantly throws off a stream of charged particles called the solar wind. Most of it sweeps harmlessly past Earth, deflected by our planet's magnetic field. But that field isn't a perfect shield — it channels some of those particles down toward the magnetic poles, north and south.
When the particles arrive, they crash into oxygen and nitrogen atoms roughly 100 to 300 kilometres up — far higher than clouds, planes or even most satellites' lowest passes. Each collision nudges an atom into an excited state, and as it settles back down it releases that energy as a tiny flash of light. Multiply that across a vast region of sky and you get the shimmering curtains we call the aurora.
It's the same physics as a neon sign or an old TV screen: energise a gas, and it glows in a colour that's characteristic of that gas.
Why the colours
- Green — by far the most common — comes from oxygen around 100–150 km up. Our eyes are also most sensitive to green, so it dominates what we notice.
- Red is oxygen too, but higher (above ~200 km), where the glow is fainter and slower. Big storms often add a red fringe above the green.
- Blue and purple come from nitrogen lower down, and tend to show on the most active nights, often along the bottom edge of bright curtains.
A camera or modern phone on a long exposure will pull out colours your eyes can barely register — which is why aurora photos almost always look more vivid than the live view.
The oval, and why your latitude matters
The aurora doesn't appear randomly. On any given night it forms a glowing oval centred on each magnetic pole. When the solar wind is calm, that oval is small and stays over the high Arctic and Antarctic. When a geomagnetic storm hits, the oval swells and pushes toward the equator — which is how people in Scotland, the northern US, or even further south occasionally catch it.
That's the whole game for a viewer: is the oval big enough tonight to reach me? The standard way to measure "how disturbed is the field right now" is the Kp index, a 0–9 scale — and the bigger the Kp, the further from the poles the aurora can be seen. That's exactly what a live tracker reads.
How to use a live forecast
A good aurora forecast pairs two numbers: the current Kp (how active things are) and the Kp you need for your location (set by how far north or south you are in geomagnetic terms). If the current value clears your threshold, the skies are clear, and you can escape city lights — it's worth heading out and looking poleward.
Frequently asked questions
What causes the aurora?
Charged particles streaming from the Sun are funnelled by Earth's magnetic field toward the polar regions. When they collide with oxygen and nitrogen high in the atmosphere, those gases briefly glow — the same principle as a neon sign, but 100 kilometres up.
Why is the aurora usually green?
Green comes from oxygen atoms about 100–150 km up, the most common and efficient aurora colour. Red is oxygen even higher up; blues and purples come from nitrogen lower down. The colour you see depends on which gas is glowing and at what altitude.
Can you see the aurora from anywhere?
No — it normally sits in an oval around each magnetic pole. The further you are from the poles, the bigger the geomagnetic storm needs to be to push that oval down to you. During strong storms it can reach mid-latitudes; during quiet spells it stays in the far north and far south.
Is it called the aurora borealis or aurora australis?
Both — 'borealis' is the northern lights and 'australis' is the southern lights. They're the same phenomenon at opposite ends of the planet, often happening at the same moment.
SEE IT LIVE
Everything in this guide is on the live sky map.