Why Oxygen Makes Green Auroras—and Red Ones Higher Up

An aurora can look like a green curtain with a red glow floating above it. You might expect those colors to come from different gases. Often, though, they come from the same kind of atom: oxygen.

The difference is not that oxygen changes its identity with height. It is that oxygen has more than one way to release energy—and some of those ways need much more uninterrupted time than others.

How an oxygen atom makes light

Earth’s upper atmosphere contains atomic oxygen: individual oxygen atoms, rather than the two-atom oxygen molecules that make up much of the oxygen you breathe.

During an aurora, energetic particles—especially electrons—transfer energy to atoms and molecules in the atmosphere. An oxygen atom can absorb some of that energy and enter an excited state, meaning its electrons occupy a higher-energy arrangement than usual.

Think of these arrangements as rungs on a ladder. An atom can occupy particular rungs, not any arbitrary position between them. When it moves to a lower-energy rung, it can release the difference as a photon, a packet of light.

The energy of that photon determines its color:

• A larger energy drop produces a higher-energy photon, toward the blue end of visible light.
• A smaller energy drop produces a lower-energy photon, toward the red end.

For the familiar oxygen aurora, two transitions are especially important:

A wavelength is the distance between successive peaks of a light wave; a nanometer is one-billionth of a meter. The ground state is an atom’s lowest-energy arrangement.

There is a useful connection between these two colors: an oxygen atom that emits a green photon can be left in the excited state capable of emitting a red photon afterward. It does not necessarily complete that second step, however. What happens next depends on its surroundings.

The crucial difference: how long oxygen has to wait

Excited atoms do not all release light at the same speed. Some states shed their energy almost immediately; others can hold it for surprisingly long periods.

For oxygen’s auroral emissions:

• The green-emitting state has a characteristic lifetime of roughly a second.
• The red-emitting state has a characteristic lifetime of roughly two minutes.

These are typical timescales, not fixed countdowns for every atom. But the contrast is enormous: an oxygen atom trying to emit red light must remain undisturbed much longer than one trying to emit green.

Auroral color depends not only on which excited states are created, but also on whether those states survive long enough to produce light.

That survival problem explains why altitude matters.

Lower down: collisions interrupt the red glow

Even the upper atmosphere is not empty. Its atoms and molecules move around and collide.

At lower altitudes, the air is denser, so collisions happen more frequently. If an excited oxygen atom collides with another particle before emitting its photon, it can lose its excitation energy through the collision instead.

This is called collisional quenching: an excited state loses energy without producing the light you would otherwise expect.

Imagine trying to finish a sentence in a crowded room where someone keeps interrupting you. A short sentence has a reasonable chance of getting through. A two-minute speech is much more likely to be cut off.

Oxygen’s green emission is the short sentence. Its red emission is the long speech.

The green-emitting state can often release its photon before a collision interrupts it. The much longer-lived red-emitting state is more easily quenched.

Higher up: red has time to appear

Higher in the atmosphere, particles are farther apart. Collisions become less frequent, giving the red-emitting state a better chance of lasting long enough to release a photon.

As a broad guide:

• Much of the familiar green aurora occurs around 100–150 kilometers above Earth.
• Red oxygen emission is commonly associated with heights above roughly 200 kilometers.

These are not rigid boundaries. Auroral height and brightness also depend on the incoming particles and atmospheric conditions.

The central point is simpler:

Altitude does not change oxygen into a different substance. It changes the competition between light emission and collisions.

This timing-and-collision explanation is described in NOAA’s aurora tutorial.

Does red aurora require a major storm?

No. Collisional quenching explains why red oxygen light survives most readily high in the atmosphere. It does not mean red aurora can occur only during a powerful geomagnetic storm.

A geomagnetic storm is a substantial disturbance of Earth’s magnetic environment, driven by activity from the Sun. Strong storms can make auroras brighter and extend their reach much farther toward the equator. But red emission can also appear above green aurora during less extreme activity.

To understand a red display, you need to separate two questions:

1. Why can the oxygen emit red there?
   The thin upper atmosphere allows its long-lived excited state to survive.

2. Why can you see that red light from your location?
   That depends on the display’s brightness, position, height, and your viewing geometry.

Why Spain saw so much red in May 2024

During the May 10–11, 2024 storm, observers at Spain’s Calar Alto Observatory reported a predominantly red glow toward the northern horizon.

Height helps explain that view. From far south of an auroral display, you may see its high-altitude red portion while its lower green portion remains hidden below the horizon—much as you can see the top of a distant building without seeing its base.

The storm also produced unusually strong red emission at low latitudes. Researchers have linked some of that emission to storm-driven processes related to stable auroral red arcs: red upper-atmospheric glows powered by heating associated with geomagnetic activity, rather than solely by the direct particle impacts that produce ordinary auroral curtains.

So the explanation has several parts: thin air lets red light survive, the storm makes it bright and widespread, and viewing geometry can favor red over green. The Calar Alto Observatory report describes the display seen from southern Europe.

Where blue and purple fit into the picture

Oxygen explains the familiar green and high red glow, but not every auroral color.

Blue and purple auroral light usually involves nitrogen molecules or nitrogen ions. An ion is an atom or molecule with an electrical charge because it has gained or lost electrons.

Nitrogen emissions can become especially noticeable near an aurora’s lower edge, where sufficiently energetic incoming electrons penetrate deeper into the atmosphere. In a bright display, they may appear as a blue, purple, or pink fringe beneath a green curtain. Those apparent colors can reflect mixtures of emissions, rather than one perfectly pure color.

Why do you notice them less often than green?

• Green oxygen emission is often prominent across much of a typical display.
• Nitrogen colors may be concentrated in narrower regions, such as the lower fringe.
• Your eyes struggle to distinguish colors in dim light. A camera can record faint colors that look pale or nearly colorless to you.

“Rarer,” then, often means less often conspicuous, not that nitrogen is missing from ordinary auroras.

The takeaway: auroral color is a race against interruption

When you look at a green aurora capped with red, you are seeing more than different colors. You are seeing the same species of atom releasing energy through different transitions, each with its own waiting time.

Green can appear after roughly a second. Red needs much longer—and therefore benefits from the quieter, thinner atmosphere higher up.

The sky’s color is partly a record of which atoms were excited, and partly a record of which ones were left alone long enough to shine.