Northern Lights Colors — What Causes Them?

Colors in Brief

The colour of the northern lights depends on two factors: which gas (oxygen or nitrogen) and at what altitude the collision occurs. Different gases and altitudes produce different wavelengths of light — and different wavelengths appear to us as different colours.

This is the same physics principle as neon lights: different gases (neon, argon, helium) glow different colours when electricity passes through them. In aurora, the "electricity" is solar wind electrons and the "gas" is atmospheric oxygen and nitrogen.

Green — The Most Common Color

Green aurora forms when solar wind electrons collide with oxygen atoms at approximately 100–200 kilometres altitude. The oxygen atom becomes excited and releases energy as green light at a wavelength of 557.7 nanometres.

This is the most common aurora colour for two reasons: oxygen is abundant at this altitude, and the excited state releases quickly (about 0.7 seconds). Faster release means more efficient light production.

Green is also the colour the human eye is most sensitive to — so it stands out well against a dark sky. Green aurora is visible practically whenever northern lights are present.

Red — Rarer and Higher Up

Red aurora also comes from oxygen, but at higher altitude — typically 200–300 km. At this height, the atmosphere is so thin that the oxygen atom excites differently. The excited state takes much longer to release (about 110 seconds) and emits energy as red light at 630 nanometres.

Red aurora is seen less frequently because:
• It requires stronger solar wind (more energy)
• The long release time means collisions with other atoms can "steal" the energy before light is emitted
• Red light is harder for the human eye to detect in darkness

During very strong storms (KP 5+), however, red aurora can be spectacular — the entire upper sky glows deep red above the green curtain. This is one of aurora's finest sights.

Purple and Blue — Nitrogen

Purple and blue aurora form when electrons collide with nitrogen molecules (N₂) and nitrogen atoms (N). Nitrogen produces purple and blue light at wavelengths 391–470 nanometres.

These colours are often visible at the lower edge of aurora curtains — where aurora energy is strongest and electrons penetrate deeper into the atmosphere. Purple and blue are more common during strong storms.

Purple is often a combination of red and blue light — the eye interprets this mix as purple. Pure blue aurora is rarer and requires very energetic electrons.

White and Pink

Sometimes aurora appears white or pink to the naked eye. This is usually due to two reasons:

White aurora is often a combination of multiple colours (green + red + purple) that the eye can't distinguish separately in dim conditions. The human eye's colour vision weakens significantly in low light — so faint multi-coloured aurora appears grey or white. A camera with long exposure reveals the true colours.

Pink results from a nitrogen and oxygen combination — especially at the lower edge of aurora curtains where nitrogen's blue/purple mixes with oxygen's green/red. Pink is often a sign of strong aurora where electrons penetrate deep into the atmosphere.

Why Does the Camera See Differently Than the Eye?

This is one of the most common questions. The answer relates to human eye structure:

The eye has two types of photoreceptors: cone cells (colour vision, work in bright light) and rod cells (dim light vision, don't distinguish colours). In darkness, the eye primarily uses rod cells — so faint aurora appears grey or pale green even though it may be bright green or multi-coloured.

A camera, however, collects light during a long exposure (10–25 seconds) and uses its sensor's entire colour filter array. It "sees" all light hitting the sensor during exposure — revealing bright greens, reds, and purples.

This doesn't mean the camera lies. It simply collects more light and separates colours more efficiently. During strong storms (KP 5+), colours are clearly visible to the naked eye too — green, red, and purple stand out brightly without a camera.

Predicting Colors

You can partially predict aurora colours based on the KP index:

• KP 2–3: Likely green — faint or moderate green arc on the northern sky
• KP 4–5: Green + possibly purple — stronger display, multiple colours begin to appear
• KP 5–6: Green, purple + red — multi-coloured display, colours visible to the naked eye
• KP 7+: Full spectrum — green, red, purple, blue, pink. Geomagnetic storm, spectacular experience

Remember: colours also depend on solar wind particle energy and the local structure of the magnetic field. The KP index gives guidance, but surprises happen — sometimes KP 3 shows stunning colours, sometimes KP 5 produces only green.

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