In brief
An aurora borealis is a coloured glow produced in the upper atmosphere when solar-wind particles, channelled by Earth's magnetic field, strike oxygen and nitrogen molecules between 100 and 300 km altitude. It forms a ring around the poles and descends to mid-latitudes during geomagnetic storms.
The phenomenon, in one sentence
An aurora borealis is the light signature of atomic excitation in Earth's upper atmosphere. When charged particles from the solar wind enter the magnetosphere, they are accelerated along magnetic field lines towards the poles. Striking thin nitrogen and oxygen molecules between 100 and 300 km altitude, they transfer energy to these atoms, which re-emit it as photons. Colour depends on the species excited and the altitude.
Why this colour?
An aurora's hue physically encodes its altitude and composition.
| Colour | Emitter | Altitude |
|---|---|---|
| Green | Atomic oxygen (557.7 nm line) | 100-150 km |
| Red | Atomic oxygen (630 nm line) | 200-300 km |
| Purple / pink | Ionised molecular nitrogen | 90-100 km (lower edge) |
| Blue | Ionised molecular nitrogen (rare) | 95-120 km |
Auroras seen from high latitudes (Norway, Iceland) are mostly green because observers look at the main auroral ring between 100 and 150 km. From mid-latitudes like France, auroras appear red or purple, because only the upper part of the aurora, at higher altitude, reaches above the northern horizon.
Where do the particles come from?
The Sun continuously expels a stream of charged particles called the solar wind. Typical speeds range from 300 to 800 km/s. During solar flares and coronal mass ejections (CMEs), much denser bubbles of magnetised plasma are hurled into interplanetary space. When they reach Earth (one to three days later), they compress and distort the magnetosphere: this is a geomagnetic storm.
Aurora intensity depends on both particle flux and the orientation of the magnetic field carried by the CME. A southward field (negative Bz) enables efficient magnetic reconnection with Earth's field, injecting particles massively towards the polar cusps. Bz is the parameter forecasters watch first.
From where can you see one?
The auroral ring (or auroral oval) is normally centred on the magnetic pole at 65-70° geomagnetic latitude. On the European continent that means Norway, Iceland, northern Finland. In France, geomagnetic latitude runs from 51° (Lille) to 42° (Perpignan): a significant storm is required for the oval to drop down to us.
The Kp index quantifies this drop. In practice:
- Kp 4 or less: aurora above the Arctic Circle only.
- Kp 5-6: aurora north of Scotland and Scandinavia.
- Kp 7: aurora potentially visible across northern France.
- Kp 8-9: aurora visible across almost all of France, even from major cities.
Storms of G4-G5 intensity occur a few times per solar cycle. The 10-11 May 2024 storm (Kp 9) made auroras visible to the naked eye from Bayonne, Marseille and even points in Corsica - a historic event that will be the reference for a generation of observers.
Southern hemisphere: aurora australis
The phenomenon is symmetrical: the southern hemisphere has its own auroral oval centred on the southern magnetic pole. Auroras australis are observable from Tasmania, New Zealand, southern Chile and Argentina. Hemispheric power (AHP) measures the total energy deposited in each oval, in gigawatts. In calm conditions it runs at 5 to 20 GW; during a major storm it exceeds 200 GW.
Forms and dynamics
An aurora is never static. Forms include:
- Homogeneous arcs: calm, regular bands on the horizon - the most common form.
- Rays and folds: vertical animated columns, sign of sustained activity.
- Coronas: rays converging at the zenith, visible only inside the oval.
- Pulsations: brightness variations over seconds, typical of a storm's late phase.
During a substorm, a stable arc can suddenly break up and transform into moving rays within minutes: the most spectacular moment to witness.
Myths and history
Before being physically understood, auroras fed mythologies. For the Sami, these are the souls of the dead dancing in the sky. For the Inuit, spirits playing football with a walrus skull. Jean-Jacques d'Ortous de Mairan, in 1733, first proposed a link between auroras and the Sun. But it was Kristian Birkeland who, in the early 20th century, demonstrated in the laboratory that auroras are caused by solar particles guided by Earth's magnetic field. The electric current loops that power the oval are still called Birkeland currents.