Does Collisional Quenching Explain Red Auroras During Storms?

If you saw photographs of the May 2024 aurora over Spain, you may have wondered why the sky looked so red. Aren’t the northern lights usually green? And does collisional quenching—the process that can stop an excited atom from emitting light—explain why red auroras seem to accompany the biggest storms?

The answer is partly, but two different questions need separating: why red auroral light survives at certain heights, and why you can see it from somewhere as far south as Spain.

Collisional quenching helps determine where red light can shine. A geomagnetic storm helps determine how bright that light becomes and how far south it is visible.

How an Oxygen Atom Makes Auroral Light

Auroras begin when energy carried by charged particles—especially electrons—reaches Earth’s upper atmosphere. Through collisions, those particles can transfer energy to atoms and molecules.

An atom that has absorbed extra energy is said to be excited. You can picture it as briefly occupying a higher rung on an energy ladder. When it returns to a lower rung, it can release the difference as a photon, a packet of light.

The photon’s colour depends on the energy difference between those rungs. Atomic oxygen produces two particularly important auroral colours:

• Green light, at a wavelength of about 557.7 nanometres.
• Red light, most prominently at about 630.0 nanometres.

A wavelength is the spacing between successive peaks of a light wave; visible red light has a longer wavelength than visible green light.

But an excited atom does not always release its energy as light. Sometimes another collision gets there first.

What Collisional Quenching Actually Means

Collisional quenching happens when an excited atom or molecule loses its excitation through a collision rather than through the emission of the photon you would otherwise expect.

The energy is not destroyed. It is transferred into other forms—for example, motion or internal energy in the particles involved—but the intended light emission is suppressed.

A race between light emission and collision

Imagine an excited oxygen atom waiting to ring a bell. If it remains undisturbed long enough, the bell rings and a photon escapes. If another atmospheric particle bumps into it first and removes its excitation, that particular bell never rings.

The crucial point is that oxygen’s red-emitting state has a much longer wait than its green-emitting state:

These are characteristic times, not fixed countdowns for individual atoms. Still, the contrast explains why the atmosphere’s density matters so much.

Why altitude changes the outcome

Lower in the atmosphere, particles are more closely packed, so collisions are frequent. Higher up, the air is thinner, and an excited atom has a better chance of remaining undisturbed.

That makes oxygen’s long-lived red-emitting state particularly susceptible to quenching at lower altitudes.

As a rough guide:

• Much of the familiar green aurora is produced around 100–150 kilometres above Earth.
• Much of the red oxygen emission comes from higher regions, often around 200–400 kilometres or above.

These are broad ranges, not rigid boundaries. Both the delivery of energy and the atmospheric conditions affect the height of an aurora.

Quenching therefore explains an important part of the vertical colour pattern: red light tends to survive higher up, where collisions are less frequent. Green emission can also be quenched; it is simply less vulnerable because its excited state usually emits much sooner.

Does Red Aurora Require a Big Storm?

No. Red emission can occur above green aurora during less extreme activity too.

Quenching is not a storm-strength switch. It does not suddenly turn off when a storm becomes powerful. Even during a major storm, dense air remains an unfavourable place for oxygen’s long-lived red-emitting state to produce light.

What a large geomagnetic storm—a major disturbance of Earth’s magnetic environment—can change is the extent, intensity and energy supply of the auroral display.

The auroral oval, the broad region around each magnetic pole where auroras commonly occur, can expand toward lower latitudes. Storm conditions can also strengthen high-altitude red emissions.

For you as an observer, this makes a crucial difference:

• At high latitudes, red auroral light may already be within view during ordinary activity.
• Farther south, an unusually strong event may be needed to bring auroral emission close enough, or make it bright enough, to see.

So the association between red skies and major storms at places such as Spain is largely about access and visibility, not a requirement imposed by quenching.

Why Spain’s May 2024 Aurora Looked So Red

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

1. High-altitude red light was easier to see over the horizon

Earth’s curvature can hide the lower part of a distant aurora while leaving its upper part visible.

Think of a distant mountain: you may see its summit even when the lower slopes are concealed. Likewise, from a southern observing site, high-altitude red emission can remain above the horizon while lower-altitude green emission is hidden.

That does not mean the distant display contained no green. It means your viewing position could favour the red portion.

The Calar Alto report describes the predominantly red appearance seen from that location.

2. The storm also produced unusually strong red emission

Viewing geometry is not the whole explanation. The May storm generated unusually strong red emission at low latitudes.

Researchers have linked some of that emission to storm-driven processes resembling those responsible for stable auroral red arcs, often abbreviated SAR arcs. These are high-altitude red glows associated with heating of the upper atmosphere during geomagnetic disturbances, rather than simply the usual particle bombardment that makes familiar auroral curtains.

Different routes can therefore supply the energy that ultimately makes oxygen glow red. Quenching still helps determine whether that excited oxygen can emit before a collision interrupts it.

A study of the May 2024 event discusses these storm-related red emissions. That does not mean every red feature photographed over Spain was a SAR arc: colour alone cannot establish the mechanism.

The Takeaway: Height, Brightness and Viewpoint

To understand a red aurora, keep three questions separate:

1. Where can the light survive?
   Collisional quenching favours oxygen’s red emission in the thin, high atmosphere.

2. How much energy is available?
   A major storm can intensify auroral activity and other sources of high-altitude red glow.

3. Which part can you see?
   From far south, Earth’s curvature may hide lower green emission while leaving higher red emission visible.

Spain’s red skies in May 2024 were therefore not simply “quenching during a big storm.” They reflected the combination of high-altitude emission, exceptional storm conditions and a southern viewing position.