Browse a gallery of aurora photos taken in Norway: green curtains rippling above the fjords. Now look at the photos taken in France in May 2024: towering red, pink and purple pillars, from Calais to the Pyrenees. Same phenomenon, two radically different faces. It's neither chance nor an editing trick: it's atomic physics combined with geometry. This article explains why an aurora seen from France is almost always red, and why your photos will always be more colourful than your eyes.
The observation: green up north, red down here
Under the auroral oval - in Norway, Iceland, Lapland - the observer stands directly beneath the auroral curtain. They see the whole structure, from its bright green base around 100 km altitude up to its reddish, diffuse top several hundred kilometres higher. Green dominates because it's the most intense emission and the base of the curtain is its brightest part.
From France, the situation is reversed. The aurora borealis typically occurs 1,000-2,500 km north of our latitudes, over Scandinavia or the North Sea. You don't look at it overhead: you watch for it low on the northern horizon. And at that distance, the curvature of the Earth changes everything. The night of 10-11 May 2024 was the perfect illustration: red pillars photographed in nearly every French region, with very little green, and almost only near the horizon.
The physics of the colours: a matter of atoms and altitude
An aurora is produced by precipitating electrons guided along Earth's magnetic field lines. As they plunge into the upper atmosphere, they collide with the atoms and molecules in their path and kick them into an excited state. Falling back to their ground state, those atoms emit light at very specific wavelengths - emission "lines". An aurora's colour is therefore not a continuous gradient: it's the spectral signature of the excited chemical species, which depends directly on altitude.
Green: atomic oxygen, low altitude
The classic auroral green is the atomic oxygen line at 557.7 nm, emitted mainly between 100 and 150 km altitude. That's where atomic oxygen density and the energy of the precipitating electrons combine best: the emission is intense, fast, and draws the sharp dancing curtains of the Norwegian postcards.
Red: the same oxygen, but very high up
The deep red is also an atomic oxygen line, but at 630 nm, emitted between 200 and 400 km altitude. Its peculiarity: it comes from a so-called metastable state, with a de-excitation time of roughly 110 seconds. Almost two minutes during which the atom "holds on" to its photon. In the lower atmosphere it would never get the chance to emit it: a collision with another particle would steal its energy first. Only in the extremely rarefied very high atmosphere is the atom left alone long enough to glow red. That's why red only exists at the top of auroras.
Blue and purple: nitrogen
The blue and violet hues come from ionised molecular nitrogen, with an emission band around 427.8 nm. They show up mostly along the lower edge of the curtains, when very energetic electrons make it deep enough, and they readily blend with the red to produce the spectacular pinks and magentas of great storms.
Horizon geometry: why France only sees the top
Let's recap: green sits low (100-150 km), red sits high (200-400 km). Now add Earth's curvature. When an aurora happens 1,500 km north of you, the planet itself gets in the way: everything below a certain altitude is hidden below your horizon.
The numbers speak for themselves. An emission at 100 km altitude (the green base) is geometrically visible only out to about 1,100 km. An emission at 300 km altitude (the heart of the red layer) remains visible out to about 1,900 km. In other words, for an aurora over southern Norway seen from Paris:
- the curtain's green base is below the horizon, hidden by Earth's curvature;
- only the high, red part rises above the horizon and remains reachable by eye and sensor;
- the further away the aurora, the more the visible slice shrinks to the top - hence to the red.
Picture a 300 km tall curtain standing behind a hill: from the valley, you only see its upper third. The hill is Earth's curvature; the upper third is the red 630 nm layer.
Only during extreme storms, when the auroral oval pushes down over the Channel or northern France, does the green base climb back above our horizon. In May 2024, some observers in northern and eastern France did capture low green along the horizon beneath the red pillars - the telltale sign that the oval was exceptionally close.
SAR arcs: the red that isn't an aurora
Another player on mid-latitude storm nights: the SAR arc (Stable Auroral Red). Despite its name, it isn't an aurora in the strict sense. It doesn't come from electrons precipitating out of the magnetotail, but from the interaction between the ring current (a belt of energetic particles that swells during storms) and the plasmasphere, the envelope of cold plasma surrounding Earth. That heat transfer excites oxygen in the very high atmosphere, which radiates at 630 nm - the same red as the top of auroras.
The result: a diffuse, broad and remarkably stable red arc, often higher in the sky than the auroral glow itself, which can persist for hours during major storms. SAR arcs are typical of mid-latitudes like France, and many "aurora" photos taken well away from the northern horizon are actually SAR arcs. The distinction matters in the field: a SAR arc barely moves and never "dances", whereas a dynamic aurora pulses and restructures within minutes.
Your eyes versus your camera sensor
Which leaves the awkward question: why does the photo show gorgeous colours where your eyes only saw a faint grey glow? The answer lies in the physiology of night vision.
In darkness, your retina switches to rods, photoreceptors that are highly sensitive but colour-blind - and, on top of that, practically blind to red, their sensitivity collapsing beyond 620 nm. The cones, the only cells able to perceive colour, need light levels that most auroras seen from France never reach. So you end up observing a mostly red phenomenon with the one visual system that... cannot see red.
A camera sensor has none of those limits. It accumulates light over several seconds of exposure, with no colour prejudice, and amplifies the signal through ISO. A 10-second exposure gathers hundreds of times more photons than your retina integrates. The vivid red pillars in your photos are entirely real - your eye simply lacks the instrument to perceive them at that brightness level.
So you need to manage your expectations in the field: most of the time, an aurora from France is experienced as a pale glow or an oddly bright patch to the north, then revealed on the camera screen. That takes nothing away from the magic - knowing the whole sky is glowing red 300 km above Scandinavia while your sensor records it remains a rare experience. And during major storms, the red genuinely becomes visible to the naked eye: no one who looked up in May 2024 has forgotten it.
In practice: what to expect at each intensity
- Kp 6-7: greyish or faintly pinkish glow on the northern horizon, often only detectable on camera. This is the most common scenario in France. Get the camera out, long exposure facing north, and check the screen before concluding there's nothing.
- Kp 8-9: the red becomes visible to the naked eye - pillars, tall draperies, sometimes pink and purple hues, with possible green near the horizon in the north of the country. This is the regime of historic nights like May 2024.
Either way, the recipe stays the same: a clear northern horizon, as little light pollution as possible, dark-adapted eyes, and a camera configured before you leave - our guide to photographing the aurora covers the settings. And to know whether tonight is worth the drive, the tonight forecast by city combines Kp, weather and lunar phase for your exact position.
Read more
- Understanding the aurora borealis - the phenomenon explained from A to Z.
- The May 2024 solar storm - the red night seen from France, told and analysed.
- Photographing the aurora: settings and gear - capturing what your eyes cannot see.
- Tonight's forecast by city - your aurora odds tonight, adapted to your latitude.