Science guide
What causes the northern lights?
The aurora is the visible result of the Sun's breath hitting our planet. Charged particles stream out from the Sun, Earth's magnetic field funnels some of them toward the poles, and when they slam into the thin air high above us, the sky glows. Here's the whole chain, from star to sky.
The short answer
The northern lights happen when charged particles from the Sun collide with gases in Earth's upper atmosphere. Earth's magnetic field steers those particles toward the magnetic poles, so the collisions — and the glow — form a ring around each pole. Oxygen and nitrogen atoms, energised by the impact, release that energy as light: most often the familiar green, sometimes red, pink or purple.
It starts at the Sun
The Sun constantly throws off a stream of electrically charged particles — mostly electrons and protons — called the solar wind. It blows past Earth all the time at around 400 km/s. Two things make it gustier, and the aurora stronger:
- Coronal holes — gaps in the Sun's atmosphere that let out a faster, denser wind, often in a repeating ~27-day pattern as the Sun rotates.
- Coronal mass ejections (CMEs) — enormous eruptions that hurl a billion tonnes of charged gas into space. If one is aimed at Earth, it arrives a day or two later and can trigger a major geomagnetic storm.
How active the Sun is rises and falls over an ~11-year cycle. We are in a strong stretch of Solar Cycle 25, which is why the mid-2020s have brought so many big displays.
Earth's magnetic field catches it
Earth is a giant magnet, wrapped in a magnetic bubble called the magnetosphere. It deflects most of the solar wind around the planet — which is just as well, because that shield protects life from the radiation. But the field isn't a perfect wall.
When the Sun's embedded magnetic field points south (the “Bz” component that aurora-watchers obsess over), it links up with Earth's field in a process called magnetic reconnection. That opens a door: energy and particles from the solar wind pour into the magnetosphere and are stored in its stretched-out night side. When that stored energy is suddenly released, particles are accelerated down Earth's magnetic field lines — which converge on the magnetic poles. That's why the aurora lives at high latitudes.
The glow: particles hitting the air
- An electron spirals down a magnetic field line into the upper atmosphere.
- It collides with an oxygen or nitrogen atom, kicking it into a higher-energy state.
- The atom drops back down and releases the difference as a photon — a tiny flash of light.
Those accelerated particles — mainly electrons — spiral down the field lines and crash into atoms and molecules of oxygen and nitrogen between roughly 100 and 300 km up. Each collision kicks an electron in the atom into a higher-energy state. A moment later it drops back down, and the atom releases the difference as a tiny flash of light — a photon. Multiply that by countless collisions and you get a curtain of light. The vertical rays and folds you see trace the invisible magnetic field lines the particles are riding.
Why the colours?
The colour depends on which gas is hit and how high up — because the air's density, and which atoms can emit before they bump into a neighbour, changes with altitude.
| Colour | Gas & altitude | When you see it |
|---|---|---|
| Green | Oxygen, ~100–250 km | By far the most common — the classic glowing curtains. |
| Red | Oxygen, above ~200 km | The high tops of tall displays, and deep-red glows in big storms. |
| Pink / magenta | Nitrogen, lower edge | Fast-moving fringes along the bottom of an active aurora. |
| Blue / purple | Nitrogen, below ~100 km | The most energetic displays, and aurora still catching sunlight. |
Your eye sees green most easily and struggles with faint colour in the dark, so a display that looks grey-green to you can blaze with pink and purple on a camera. For the full palette — and what STEVE is — see aurora colours explained.
Why an oval around the poles?
Because the particles follow magnetic field lines, the aurora forms a ring — the auroral oval — centred on each magnetic pole, not the geographic one. The stronger the storm, the wider that oval grows and the further it pushes toward the equator, which is exactly what the Kp index measures. It's also why your geomagnetic latitude — not the one on the map — decides your odds; see where the aurora reaches.
Why does it move and dance?
The quiet aurora is a steady arc, but during a substorm — the sudden release of energy stored in the magnetotail — the curtains brighten, ripple and surge across the sky in minutes. That dancing is the magnetosphere unloading, and it's the moment every chaser waits for.
North and south: borealis and australis
The same field lines reach both poles, so the show plays out in both hemispheres at once: the aurora borealis in the north and the near-mirror aurora australis in the south. They are one phenomenon with two names — which is why we cover both hemispheres.
From the science to tonight's sky
Knowing the cause is half of it — seeing it also needs a dark, clear sky at your latitude. Our live forecast turns all of this into a straight answer for your exact location, and the best-time guide helps you plan a trip when the odds are highest.