Northern Lights Colors Explained: Why Aurora Is Green, Red, Blue, and Purple

Why aurora has colour at all

The colours in aurora are not reflected light, refracted light, or scattered light — they're emitted light, produced when atoms and molecules in the upper atmosphere return to their ground state after being excited by collisions with energetic particles from the solar wind. It's the same physics as a neon sign or a fluorescent tube: pass electrical energy through a gas, the gas glows with a characteristic colour. Different gases produce different colours because the specific energy transitions in each atom correspond to different wavelengths of visible light.

In the aurora, the energy comes from electrons and protons accelerated along Earth's magnetic field lines and crashing into the upper atmosphere. Each collision transfers energy to an atmospheric atom, briefly elevating one of its electrons to a higher energy state. That state is unstable. The electron drops back down, and the energy difference is released as a photon — a single particle of light — at a very specific wavelength determined entirely by the physics of that particular atom and the energy transition involved.

The result is that aurora produces extremely pure, narrow-band light. Unlike sunlight (which spans the full visible spectrum), each aurora emission line is a precise colour at a precise wavelength. Multiply trillions of identical atomic transitions per second across hundreds of thousands of square kilometres, and you get the coloured curtains visible from the ground.

The green aurora: oxygen at 100–150 km

Green is the dominant aurora colour because it comes from the most efficient emission line of one of the most abundant gases at the right altitude. The specific transition is atomic oxygen releasing a photon at 557.7 nanometres — a bright, saturated yellow-green that sits near the peak sensitivity of human night vision.

This emission happens primarily at altitudes between 100 and 150 km above the ground. At these altitudes, the atmosphere is dense enough for frequent collisions between charged particles and oxygen atoms, but thin enough that the excited oxygen atom has a chance to emit its photon before being de-excited by a collision with another molecule.

Why oxygen specifically? Oxygen atoms at these altitudes exist in an intermediate state between molecular oxygen (O₂, which dominates lower altitudes) and fully ionised oxygen (which dominates very high altitudes). Atomic oxygen has a specific metastable excited state — called the ¹S state — that lives for about 0.74 seconds before emitting at 557.7 nm. That lifetime is short enough to emit before the next collision, but the transition is quantum-mechanically forbidden from happening through most pathways, which is why the green line is called a forbidden transition. Forbidden lines only produce strong emission in environments where collisions are rare enough not to interrupt the process — exactly the conditions at 100–150 km altitude.

The result: virtually every clear aurora night in Tromsø produces bright green. It's the aurora's signature colour, visible with the naked eye even at modest activity levels (Kp 2–3).

The red aurora: oxygen at 200–300 km

Red aurora comes from the same atom — oxygen — but a different excited state at much higher altitudes. The emission line is at 630.0 nanometres, a deep crimson-red. The physics difference is significant: this transition involves oxygen's ¹D state, which has a lifetime of about 110 seconds before emitting. For this emission to succeed, the oxygen atom must remain undisturbed for nearly two minutes after excitation.

At 100–150 km, atmospheric density is high enough that an excited oxygen atom will collide with another molecule within a millisecond of excitation — the 110-second ¹D state is immediately quenched, and no red light is produced. But at 200–300 km altitude, the atmosphere is so thin that collisions are rare enough for the ¹D state to survive long enough to emit. This is why red aurora only appears at high altitudes — it's a physical filter: the red transition cannot happen at low altitudes regardless of how much energy is being deposited.

Practically, this means red aurora appears at the tops of aurora curtains during strong geomagnetic storms. During a Kp 6–8 display, the aurora often appears as: bright green at the base (100–150 km), transitioning to yellow-green in the middle, and deep red or crimson at the very top (250–300+ km). During the most extreme events (Kp 8–9), the red emission can dominate the entire display and be seen at lower latitudes even when green is not visible from those locations — because the aurora oval's poleward edge sits higher in altitude and further from the observer.

Red aurora from extreme events has been documented as far south as Spain, Italy, and the United States during Kp 9 events. Observers who don't know the science sometimes report it as an eerie blood-red sky without the characteristic curtains of lower-altitude aurora.

Blue and violet: the nitrogen contribution

While oxygen dominates the most visible aurora emissions, nitrogen produces the blue and violet colours seen at the lower edge of aurora curtains during energetic events. The main emissions are from molecular nitrogen ions (N₂⁺) at wavelengths around 391.4 nm (violet) and 427.8 nm (blue-violet), as well as neutral nitrogen molecules at other wavelengths.

Nitrogen emission happens at lower altitudes than the oxygen green emission — typically below 100 km. This requires more energetic particles that can penetrate deeper into the atmosphere. During calm nights with modest aurora, the particles typically don't have enough energy to reach nitrogen-rich altitudes. During strong storms (Kp 5+), higher-energy electrons penetrate to 80–100 km and nitrogen emission becomes visible.

The blue-violet nitrogen emission is also less efficient at stimulating the human eye than the oxygen green — our night vision is most sensitive around 507 nm (blue-green), and becomes progressively less sensitive at shorter wavelengths. So even when nitrogen emission is occurring, your eyes detect it less easily than the green. Camera sensors, especially those with UV-sensitive sensors and wide-range RAW capture, show the blue and violet much more vividly than the naked eye. This is a major reason why aurora photos often show more blue and purple than you remember seeing in person.

Pink and magenta: the layered colour effect

Pink and magenta aurora are not produced by a single atomic transition — they're mixtures of multiple emission lines viewed simultaneously from the ground. The most common mechanism: during a moderate-to-strong aurora, you have bright green oxygen emission at 100–150 km altitude sitting above lower-altitude blue-violet nitrogen emission at 80–100 km. Visually, the bottom edge of the aurora curtain blends these two colours — green and blue-violet together produce a magenta or pink that can be startlingly vivid.

Another pink mechanism: very strong events where red oxygen emission at 250+ km mixes with green oxygen emission at 150 km, producing a spectrum from green through yellow to pink to red as you look from the base to the top of the curtain. During a Kp 7–8 display in Tromsø, ribbons of aurora can show all of: blue-pink at the very base, bright green through the main body, and crimson at the top — all from the same physical display.

Photographers often capture pink more prominently than naked-eye viewers because cameras record the full emission spectrum without the human eye's wavelength sensitivity variation. If your aurora photos show vivid pink and purple that you don't remember seeing, you were probably seeing it but your eyes were less responsive to those wavelengths in low light.

White aurora: why some displays look colourless

Some aurora displays — particularly faint ones — appear white or slightly grey-green rather than vivid green. This isn't because a different physical process is happening. It's because your eyes switch between two operating modes at different light levels:

• Photopic (colour) vision: Your colour-sensitive cone cells work well above about 3 candela per square metre (normal indoor lighting). Colour perception is accurate.
• Scotopic (monochrome) vision: In near-darkness, your rod cells dominate. Rods are extremely light-sensitive but don't distinguish colour — everything appears in shades of grey.
• Mesopic vision: In between — the transition zone where colour becomes partially visible but washed out.

Faint aurora sits in the mesopic or scotopic range for most viewers. Your eyes know something is there, and you can often tell it's greenish, but the colour saturation is dramatically lower than what a long-exposure camera capture reveals. The aurora hasn't changed — your visual system just doesn't work well enough in the low-light range to saturate the colour cones. During a bright display (active substorm, Kp 4+), the aurora gets bright enough for photopic vision to kick in, and colour becomes vivid and unmistakeable.

What colours you can actually see vs. photograph

A camera with a 10-second exposure at ISO 3200 will always show more colour than your naked eye in the same conditions. This is normal and not deceptive — the camera is integrating 10 seconds of photons while your eye is refreshing about 10 times per second. The colours in the photo are real; your eyes just can't accumulate enough of them in the moment to trigger full colour vision.

What this means practically:

• Faint aurora: Naked eye sees grey-white glow. Camera shows vivid green, sometimes purple. Both are accurate at different integration times.
• Moderate aurora (Kp 3–4): Naked eye sees green clearly, hints of purple at edges. Camera shows full spectrum.
• Bright aurora (Kp 5–6): Naked eye sees vivid green, pink curtain edges, some red at top. Camera matches naked eye closely.
• Extreme aurora (Kp 7–9): Naked eye sees everything — green, pink, vivid red. Display may be bright enough to cast faint shadows. Camera agrees with naked eye almost perfectly.

Colour intensity and the Kp connection

Higher Kp doesn't just push the aurora oval further south — it changes the colour signature of the display. At low activity (Kp 2–3), the vast majority of energy deposition happens at 100–150 km, and the aurora is predominantly green. As Kp rises:

• Kp 4–5: More energetic particles reach lower altitudes, nitrogen blue appears at the base. Red oxygen faintly visible at the top of tall curtains.
• Kp 6–7: Vivid pink and magenta from nitrogen-oxygen mixing at lower altitudes. Red clearly visible at curtain tops. Display develops complex vertical colour gradient.
• Kp 8–9: Full colour spectrum visible to naked eye. Red may dominate the entire display seen from southern latitudes. Curtains move too fast for slow shutter speeds.

Rare aurora colours: yellow, orange, cyan

A few additional colours appear rarely in aurora photography:

• Yellow: Not a distinct emission line but a visual blend of green and red emission layers close together. Appears at the transition zone between green and red in a vertically layered display.
• Orange: Another green-red blend, or occasionally sodium D-line emission from meteoric sodium injected into the upper atmosphere. Very rare.
• Cyan or turquoise: Green-blue blending, or specific emission lines from atomic nitrogen at unusual energy levels. Reported in some intense displays but photographically documented rarely.
• Ultraviolet: Aurora emits strongly in UV, but UV is invisible to human eyes and blocked by standard camera glass. UV aurora is measurable by spacecraft but never seen from the ground.

The canonical aurora colour sequence from ground up during a strong Kp 7 display, viewed from Tromsø: deep pink-violet at the very base (below 100 km, nitrogen), transitioning to bright green through 100–150 km, then a yellow-green middle zone, then crimson-red above 200 km at the curtain tops. The curtain dances and folds in real time as the field lines are pushed by varying solar wind pressure — each fold catching a slightly different altitude and showing a different colour to the observer below.