Auroras: The Science of the Northern and Southern Lights
How auroras glow: solar particles racing down Earth's magnetic field lines excite oxygen and nitrogen high above the poles — the colour reveals which gas and altitude.
Auroras — the northern lights (aurora borealis) and their southern twin, aurora australis — are curtains, arcs, and coronas of light that dance in the night sky near the poles. They look like magic, but they are a plain physics event: an electric light show driven by the Sun, staged roughly 100 km above your head. The name aurora borealis, "northern dawn", dates from the early 1600s; scientists now describe the whole phenomenon as space weather made visible.
The chain from the Sun to the sky
Every second the Sun releases a stream of charged particles — mostly electrons and protons — called the solar wind, travelling at hundreds of kilometres per second. Earth's magnetic field, the magnetosphere, deflects most of it, which is one reason our atmosphere survives. But the field is not a perfect shield: near the magnetic poles its lines loop down into the atmosphere, and along those lines energy from the solar wind is funnelled inward. Electrons are accelerated downward and slam into oxygen and nitrogen atoms and molecules between about 100 and 300 km up. The collisions knock the atoms into excited states; when they relax back down, they release the extra energy as photons of light. Each element emits its own signature wavelengths, exactly as different gases glow different colours inside a neon tube — the colours of an aurora are fingerprints written across the electromagnetic spectrum.
Why the colours differ
Two things decide an aurora's colour: which gas is hit, and how high the collision happens.
| Colour | Emitting gas | Typical altitude |
|---|---|---|
| Green | atomic oxygen | about 100 km |
| Red | atomic oxygen | above roughly 200 km |
| Blue and violet | molecular nitrogen | around 100 km and lower |
Green is the classic aurora colour, produced by atomic oxygen's 557.7 nm emission line. Red oxygen light comes from higher up, where the air is so thin that the excited state survives long enough to radiate instead of being quenched by collisions. Nitrogen adds blue and violet at the lower edge of a curtain, and a magenta fringe appears where its emission mixes with oxygen red. A single tall display can show several colours at once because one curtain spans many altitudes.
Storms, the solar cycle, and how far south auroras reach
Ordinary auroras hug the polar ovals — rings centred on the magnetic poles, where field lines dive into the atmosphere. During a geomagnetic storm, triggered when a burst of solar wind from a coronal hole or a coronal mass ejection reaches Earth a few days after leaving the Sun, the ovals swell and migrate toward the equator. Forecasters rank these storms on the Kp index and the G-scale (G1 to G5). In May 2024 a G5 storm, the strongest in two decades, pushed auroras to latitudes where they are almost never seen, including much of the southern United States and central Europe.
Solar activity runs on an approximately 11-year cycle, and the current cycle peaked around late 2024. The result is a golden era for aurora watchers: displays that were rare in the quiet years around 2020 now appear every few weeks at high latitudes, and mid-latitude sightings are no longer a once-a-decade event. The same physics operates on other planets — Jupiter and Saturn have auroras of their own, driven partly by their moons — and spacecraft such as Juno photograph them in ultraviolet.
Auroras are harmless to observers: the incoming particles are steered away long before they reach the ground, and the glow itself is about 100 km up. The same storms that paint the sky can, however, disturb satellites, GPS, radio, and power grids — the aurora is the visible symptom of a magnetic storm, not the storm itself. To see one, find dark, clear skies far from city lights and watch forecasts built on the Kp index. When the Sun is restless, the sky writes its own weather report overhead.
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aurora light magnetosphere solar wind