Where it sits
The colour is an altimeter. You're reading height off the sky.
Tell it apart
| Aspect | Borealis | Australis |
|---|---|---|
| Hemisphere | North | South |
| Named from | Boreas, north wind | Auster, south wind |
| Mechanism | Identical | Identical |
| Perfect mirror | Unverified | Unverified |
The northern and southern counterparts, produced by the same magnetospheric process at opposite magnetic poles — one of two ovals. The popular claim that they are exact mirror images is not asserted by NASA or NOAA in the pages checked, so treat perfect mirroring as unverified.
What it doesn't mean
Colour tracks gas and altitude, though the bands overlap: red from atomic oxygen high up, roughly 200–400 km; green from atomic oxygen lower down, around 100–300 km; and at the bottom edge, below about 100 km, molecular and ionized nitrogen giving blue and the pink-crimson fringe. Green dominates because that is where density and excitation conditions favour it.
The solar wind doesn't reach the atmosphere. It deposits energy in Earth's magnetosphere; electrons are then accelerated in the magnetotail on the NIGHT side and rain down along magnetic field lines. The aurora is powered by the Sun but triggered by Earth's own magnetic system.
What it really means
The aurora is named after the Roman goddess of the dawn, because a glow low on the northern horizon looks like a sunrise arriving in the wrong place, at the wrong hour.
It is a resemblance, not an identification — and it is preserved in the word forever.
What's actually happening starts 93 million miles away and finishes somewhere between 80 and 500 kilometres up. The solar wind reaches Earth and — this is the part usually skipped — does not hit the atmosphere. It deposits energy into the magnetosphere, the magnetic bubble around the planet. Electrons are then accelerated in the magnetotail, on the night side, and funnelled down along magnetic field lines into the polar atmosphere, where they collide with oxygen and nitrogen and knock them into excited states. The atoms drop back down and release the energy as light. Physically it's a neon lamp, at planetary scale, powered by the Sun and triggered by Earth's own magnetism.
The most useful thing to know is that the colour is an altimeter. Atomic oxygen high up, roughly 200–400 km, glows red. Atomic oxygen lower down, around 100–300 km, glows green — the dominant colour, because that's where the conditions suit it. At the bottom edge, below about 100 km, molecular and ionized nitrogen give the blue and the pink-crimson fringe.
So when you look at an aurora you aren't just seeing colours. You're reading height off the sky, in a band roughly 80 to 500 kilometres up.
Where it comes from
Latin — dawn (also the goddess Aurora)
Latin — northern (from Boreas, the north wind)
It was named for dawn because that is what the glow resembles — a light low in the sky, in the wrong place and at the wrong hour. Physically the two have nothing in common: the aurora is atmospheric gas excited by charged particles, not scattered sunlight.
- mechanism
Electrons accelerated in the magnetosphere's night-side tail follow Earth's magnetic field into the polar atmosphere, exciting oxygen and nitrogen.
- 1963
Y. I. Feldstein and, independently, O. V. Khorosheva establish the auroral oval: the aurora forms two ovals approximately centred on the magnetic poles rather than fixed rings. Emission sits typically 80 to 500 km above the surface.
In real life
- On a clear winter night in northern Norway, a pale green arc forms low in the northern sky, then breaks into rippling curtains overhead.
- During the strong geomagnetic storms of May 2024, the auroral ovals spread so far south that the lights were seen from Portugal, Spain and Sardinia.
- The older sense is simply dawn: a poet's 'rosy aurora' is the sunrise, not the polar lights.
Questions
Q01What causes the aurora?
Energetic particles from the magnetosphere collide with atoms and molecules in the upper atmosphere, exciting them so they release energy as light — the same principle as a neon lamp. The solar wind powers the system but doesn't itself reach the atmosphere.
Q02Why is the aurora usually green?
Green comes from atomic oxygen at roughly 100–300 km, where density and excitation conditions favour that emission. Red comes from atomic oxygen higher still, around 200–400 km, and the blue and pink at the bottom edge come from molecular and ionized nitrogen below about 100 km. The colour is roughly telling you the altitude, though the bands overlap.
Q03Where can you see it?
Best between about 60 and 75 degrees north or south latitude, where aurora can be seen on more than half the nights of a given year. During major geomagnetic storms the ovals expand toward the equator and it becomes visible much further south or north.
Sources
- Auroras — NASA Science.
- Aurora — NOAA Space Weather Prediction Center.
- aurora (etymology) — Wiktionary.