Why Are Auroras Different Colors? Green, Red and Purple • The Science 360

During the great storm of 10 and 11 May 2024, people in southern Europe, Florida and northern India posted photos of a sky glowing crimson and pink. In Norway, under the main oval, the same storm was a frenzy of green. Most explainers will tell you that green means oxygen and red means "higher up" or "nitrogen". Half of that is wrong, and the half that is right skips the actual reason.

The green and the red both come from the same atom, atomic oxygen. What separates them is a stopwatch. An oxygen atom excited into the state that makes green light waits about three quarters of a second before it glows. An atom in the state that makes red light waits, on average, more than two minutes. Whether it ever gets to glow depends on how often it is bumped by its neighbours in the meantime, and that depends on altitude. That one idea explains the layering of colours in a curtain, why red auroras show up so far south, and why your phone sees colours your eyes do not.

Quick answerAuroral colours come from atoms and molecules excited by electrons from space. Oxygen emits green (557.7 nm) at about 100 to 200 km and red (630.0 nm) above about 200 km, because the red state lives about 134 seconds and is destroyed by collisions in denser air. Nitrogen gives blue, violet and pink, mostly at the lower edge.

Infographic: Why are auroras different colors? An aurora curtain rises over hills, pink at its fringe, green in the middle and deep red at the top. Green and red both come from oxygen: the green state glows after about 0.75 s, the red state waits about 134 s, so in denser air a collision usually steals its energy first. At 200 km only 6% of red-state atoms glow, and to night vision red looks 110 times fainter. A chart of horizon distance against layer height shows a green layer at 110 km visible to 1,175 km and a red layer at 400 km to 2,200 km. Tiles: 557.7 nm green and 630.0 nm red. Myth: red does not mean nitrogen. — Tap the image to view it full size. View the accessible, editable SVG.

What makes an aurora glow at all?

Auroras are powered by electrons, mostly with energies of a few hundred to a few tens of thousands of electronvolts, accelerated down along Earth's magnetic field lines into the upper atmosphere. Each fast electron makes hundreds of collisions on its way down. Some of those collisions knock bound electrons in oxygen and nitrogen into higher energy levels or rip them off entirely. When an excited atom or molecule drops back down, it emits a photon whose energy is fixed by the gap between the levels. That fixed energy is a fixed colour, which is why an auroral spectrum is a set of sharp lines rather than a smooth rainbow.

The three emissions that matter for what you see are in Figure 1. Two of them are oxygen: the green line at 557.7 nm and the red doublet at 630.0 and 636.4 nm. The third is ionised molecular nitrogen, N₂⁺, with bands at 391.4 nm (violet) and 427.8 nm (blue). Neutral N₂ adds a group of red bands between about 650 and 690 nm.

Energy level diagram. Atomic oxygen has a ground state, a 1D level at 1.97 eV and a 1S level at 4.19 eV. The 1S to 1D drop gives 557.7 nm green light with a lifetime of 0.75 seconds; the 1D to ground drop gives 630.0 nm red light with a lifetime of about 130 seconds. The nitrogen ion N2+ drops from an excited B state at 3.17 eV to its ground state emitting 391.4 and 427.8 nm, with a lifetime of about 60 nanoseconds. — Figure 1. The transitions behind the main auroral colours. Oxygen's green and red lines are "forbidden" transitions with very long lifetimes; the nitrogen ion's blue and violet bands are allowed transitions that happen almost instantly. Lifetimes from NIST transition probabilities (oxygen) and standard references (N₂⁺, approximate).

You can check the diagram with one line of arithmetic. A photon's energy in electronvolts is 1,239.8 divided by its wavelength in nanometres.

Photon energies of the oxygen lines

E = \frac{hc}{\lambda} = \frac{1{,}239.8\ \mathrm{eV\,nm}}{\lambda}

\begin{aligned}\text{Green: } \frac{1{,}239.8}{557.7} &= \mathbf{2.22\ eV} \\ \text{Red: } \frac{1{,}239.8}{630.0} &= \mathbf{1.97\ eV}\end{aligned}

Sum: $2.22 + 1.97 = 4.19\ \mathrm{eV}$ , the energy of the $^1S$ level above the ground state.

So the green line is the upper step of a two-step staircase and the red line is the lower step. An oxygen atom that emits green lands in the ¹D state and could then emit red too. Same atom, two colours.

Why does red aurora appear higher than green?

Both oxygen transitions are what physicists call forbidden: the quantum selection rules for ordinary dipole radiation do not allow them, so they happen only through much weaker processes. The NIST Atomic Spectra Database gives the transition probabilities. For the green line, $A = 1.26\ \mathrm{s^{-1}}$ , and with a small extra channel at 297.2 nm the ¹S state lives about 0.75 s. For the red lines, $A = 0.00565\ \mathrm{s^{-1}}$ at 630.0 nm and 0.00182 per second at 636.4 nm, so the ¹D state lives about 134 s. (Older textbooks often say 110 s; the difference comes from which set of calculated transition probabilities you use.) For comparison, a normal allowed transition like the N₂⁺ blue band takes tens of nanoseconds.

A long lifetime only matters if something can interrupt it. In the upper atmosphere that something is a collision with N₂, O₂ or another O atom, which can carry off the excitation energy as motion instead of light. This is called collisional quenching. The rate of collisions that quench ¹D oxygen is the gas density times a rate coefficient: for N₂ the measured value is $2.1\times10^{-11}\,e^{115/T}\ \mathrm{cm^3\,s^{-1}}$ (Ravishankara and colleagues, 2002).

Will an excited red-line atom get to glow? (U.S. Standard Atmosphere 1976)

At 200 km ( $T = 855\ \mathrm{K}$ )

$\mathrm{N_2} = 2.9\times10^{9},\ \mathrm{O_2} = 1.9\times10^{8},\ \mathrm{O} = 4.3\times10^{9}\ \mathrm{cm^{-3}}$

Quenching rate:

\begin{aligned}q &\approx 2.4\times10^{-11} \times 2.9\times10^{9} \\ &\quad + (\mathrm{O_2},\ \mathrm{O}\ \text{terms}) \\ &\approx 0.11\ \mathrm{s^{-1}}\end{aligned}

Radiative rate:

A = \frac{1}{134\ \mathrm{s}} = 0.0075\ \mathrm{s^{-1}}

Fraction that emits:

\frac{A}{A + q} = \frac{0.0075}{0.0075 + 0.11} = \mathbf{6\%}

At 120 km

\begin{aligned}q &\approx 13\ \mathrm{s^{-1}} \\ \Rightarrow\ \frac{0.0075}{13} &= \mathbf{0.06\%}\end{aligned}

Green line at 120 km

\begin{aligned}A &= 1.34\ \mathrm{s^{-1}} \\ q &\approx 0.02\ \mathrm{s^{-1}} \\ \frac{A}{A+q} &\approx \mathbf{99\%}\end{aligned}

That is the whole story in one box. At 120 km a ¹D oxygen atom is hit about 13 times a second, so the chance that it survives the 134 seconds it needs is negligible: the red light is quenched. The green state, with its 0.75 s clock, barely notices. Go up to 300 km, where the air is several hundred times thinner than at 120 km, and about half of the red-state atoms get to emit. Figure 2 shows the full curves.

Plot of the fraction of excited oxygen atoms that emit a photon versus altitude from 80 to 400 km. The green 557.7 nm curve is near 100 percent above about 100 km and falls to 50 percent at about 86 km. The red 630 nm curve is near zero below 150 km, reaches 6 percent at 200 km and 50 percent at about 300 km. — Figure 2. Fraction of excited O(¹S) and O(¹D) atoms that radiate rather than lose their energy in a collision, computed from U.S. Standard Atmosphere 1976 densities, NIST lifetimes and laboratory quenching coefficients (the O(¹S) and atomic-oxygen terms are less certain). The red line needs air roughly a hundred thousand times thinner than the green line does.

The emission you actually see also depends on how many atoms are excited, which falls with height as the air thins. Multiply a falling density by the rising curve in Figure 2 and red oxygen emission peaks broadly at roughly 200 to 300 km, while green peaks where the electrons deposit most of their energy, usually 100 to 150 km. That is why a tall auroral ray often runs green at the bottom and fades into dull red at the top.

The misconception: “red means nitrogen” or “red means more energetic”

Two errors come up constantly. The first is that red aurora comes from nitrogen. Nitrogen does emit some red (the N₂ bands around 650 to 690 nm that colour the pink lower fringe), but the deep, high red glow that photographers chase is the oxygen 630.0 nm line.

The second is that red signals more powerful particles. It is roughly the reverse. To reach 100 km an electron needs about 10 keV. To stop at 200 km and above it must be soft, around 0.1 to 0.5 keV. Figure 3 shows the relationship, using the standard electron range in air and the 1976 standard atmosphere.

Plot of altitude reached versus incoming electron energy on a log scale from 0.1 to 30 keV. A 0.1 keV electron stops near 222 km in the mostly red zone, a 1 keV electron near 146 km in the green zone and a 10 keV electron near 102 km in the zone where green mixes with nitrogen pink and purple. — Figure 3. Penetration depth of auroral electrons, from the Rees range-energy fit R = 4.3 × 10⁻⁷ + 5.36 × 10⁻⁶ E^1.67 g cm⁻² (E in keV) converted to altitude with the 1976 standard atmosphere. Peak energy deposition sits a few kilometres above the stopping altitude. Colour bands are approximate.

What big storms do is push the whole auroral oval toward the equator and pour in large fluxes of soft electrons, which light up the high red layer over a huge area. Storms also heat the upper atmosphere, which produces stable red arcs (often called SAR arcs) that are pure 630 nm, with almost no green. So a red sky over Italy is a sign of a big storm, but the individual electrons making the red are the gentle ones.

Where do blue, purple and pink come from?

Blue and violet come from N₂⁺. An electron with enough energy can ionise an N₂ molecule and leave the ion excited at the same time; the ion then emits at 391.4 or 427.8 nm within about 60 nanoseconds. No lifetime problem here, so these bands appear wherever energetic electrons hit nitrogen, which is everywhere along the path but most strongly low down, 90 to 110 km, where hard electrons dump their energy and nitrogen dominates.

Mix the blue-violet nitrogen ion light with red N₂ bands and you get the magenta or pink fringe along the bottom of a fast-moving curtain. That fringe is a sign of unusually energetic electrons reaching below 100 km. Green oxygen still contributes there (Figure 2 shows the green state is not quenched until about 86 km), but atomic oxygen becomes scarcer than N₂ as you go down, so nitrogen colours take over the lower edge.

There is one more source of blue. When an aurora extends high enough to be in sunlight while the ground is dark, N₂⁺ ions at the top scatter sunlight resonantly at 391.4 and 427.8 nm. The result is a purple or blue top on tall rays, seen around twilight.

Why does an aurora look grey or pale green to the eye but colourful on a phone?

This is the question most people end up asking after a storm, and the answer is in your retina. In dim light, vision runs on rod cells, which do not distinguish colour at all and whose sensitivity peaks at 507 nm. The CIE scotopic (night vision) sensitivity curve, plotted in Figure 4, makes the problem obvious.

Plot of relative eye sensitivity from 380 to 700 nm. The night-vision rod curve peaks at 507 nm and is almost zero at 630 nm; the daylight cone curve peaks at 555 nm. Vertical lines mark aurora emissions at 391.4, 427.8, 557.7, 630.0 and 636.4 nm, and a shaded band marks nitrogen bands at 650 to 690 nm. — Figure 4. Main auroral emissions against the CIE 1951 scotopic (rods, dashed) and photopic (cones, solid) luminous efficiency curves, from the Colour and Vision Research Laboratory tables. The green line sits near the peak of both; the red line sits almost off the end of the night curve.

How much dimmer the red line looks at night

Scotopic sensitivity at 557.7 nm about 0.36 (interpolated between 0.402 at 555 nm and 0.329 at 560 nm); at 630 nm it is 0.0033.

\frac{0.36}{0.0033} \approx \mathbf{110}

In daylight vision (cones): $1.00/0.265 \approx 3.8$ .

To a dark-adapted eye, a red glow with the same power as a green one looks about 110 times fainter, and colourless. Bright aurora pushes your vision into the mesopic range where cones start working, which is why strong displays do look green and occasionally red. A phone camera has none of these limits. Its sensor integrates for seconds, keeps a colour filter on every pixel, and is quite sensitive in the red. It records the 630 nm glow your eye turned into a faint grey haze. Night-mode processing then boosts saturation, which is why social media aurora often looks more vivid than anything anyone saw.

Why do people far from the poles see red auroras?

Geometry. A glowing layer high above the ground stays above your horizon from farther away. Ignoring atmospheric refraction, an emission at height h is on the horizon for an observer at ground distance d:

\begin{aligned}d &= R\arccos\!\left(\frac{R}{R + h}\right) \\ R &= 6{,}371\ \mathrm{km}\end{aligned}

Green layer, $h = 110\ \mathrm{km}$ :

\begin{aligned}\arccos\!\left(\frac{6{,}371}{6{,}481}\right) &= 0.1845\ \mathrm{rad} \\ \Rightarrow\ d &= \mathbf{1{,}175\ km}\end{aligned}

Red layer, $h = 400\ \mathrm{km}$ :

\begin{aligned}\arccos\!\left(\frac{6{,}371}{6{,}771}\right) &= 0.3454\ \mathrm{rad} \\ \Rightarrow\ d &= \mathbf{2{,}200\ km}\end{aligned}

At 111 km per degree of latitude: about 10.6° for green versus 19.8° for red.

Plot of ground distance from which an aurora is visible versus height of the glowing layer. On the horizon, a green layer at 110 km is visible from 1,175 km, a red layer at 300 km from 1,918 km and at 400 km from 2,201 km. A dashed curve for 10 degrees above the horizon lies lower, about 500 km for 110 km height. — Figure 5. Maximum ground distance at which an auroral layer is visible on the horizon (solid) or 10° above it (dashed), for a spherical Earth with no refraction. The red top of an aurora can be seen almost twice as far away as its green base.

If the oval moves down to, say, 55° latitude during a storm, an observer at 40° to 45° sees nothing of the green base, which is below the horizon, but can see the red tops low in the north. That is why low-latitude aurora sightings are so often red, and why they hug the horizon.

Is now still a good time to see colourful auroras?

Solar Cycle 25 reached its maximum in late 2024: the 13-month smoothed sunspot number peaked at about 160.8 in October 2024, well above the official forecast of 115. As of late 2026 the cycle is in its declining phase, but that is not the end of the show. Some of the most intense storms of recent cycles came in the declining years, such as the Halloween storms of 2003, two to three years after the cycle 23 maximum. Large, fast eruptions and recurring high-speed solar wind streams both stay common for a few years after maximum, so the next couple of winters should still deliver red skies at middle latitudes now and then.

Reading an aurora by colour: green at 100 to 200 km is the default; red on top or alone, often seen from far away, means emission at 200 to 400 km from soft electrons; a pink or purple lower fringe means energetic electrons reaching below 100 km; a blue-violet top near twilight means sunlit nitrogen ions.

FAQ

What is the rarest aurora color?

Pure, bright red overhead and blue or violet are the least common to see with the naked eye. Red needs very high-altitude emission from low-energy electrons, which is common faintly but rarely bright, and the eye is poor at seeing red in the dark. Blue and violet from nitrogen ions are usually mixed with green and look purple or pink at the bottom edge of active curtains.

Why are most auroras green?

Most auroral energy is deposited by electrons of a few keV that stop at 100 to 150 km. There, excited oxygen atoms in the state that emits 557.7 nm green light live only about 0.75 seconds, short enough to radiate before a collision steals the energy. Your night vision is also far more sensitive at 557.7 nm than at the 630 nm red line.

Why do auroras look more colorful on a phone camera?

In dim light your eye switches to rod cells, which see no colour and are about 110 times less sensitive to the 630 nm red line than to the 557.7 nm green line. A camera sensor integrates light for seconds and keeps its colour response, so it records the red and purple that your eye renders as grey or faint green.

Why do people far from the poles see red auroras?

Red oxygen emission comes from 200 to 400 km up, so it stays above the horizon from much farther away than the green layer at about 110 km. An aurora at 400 km is visible on the horizon from about 2,200 km away, roughly 20 degrees of latitude, while green at 110 km is visible from about 1,200 km. Distant observers see only the red tops.

Sources

NIST Atomic Spectra Database, O I forbidden lines (A = 1.26 s⁻¹ at 557.7 nm; 5.63 × 10⁻³ and 1.82 × 10⁻³ s⁻¹ at 630.0 and 636.4 nm): physics.nist.gov/PhysRefData/ASD/lines_form.html
A. R. Ravishankara et al., "Redetermination of the rate coefficient for the reaction of O(¹D) with N₂", Geophysical Research Letters 29 (2002): esdpubs.nasa.gov/node/29956
U.S. Standard Atmosphere 1976 (NASA-TM-X-74335), number densities and temperatures computed with the open-source ussa1976 Python package: ntrs.nasa.gov/citations/19770009539
Colour and Vision Research Laboratory, CIE 1951 scotopic and Judd-Vos photopic luminous efficiency tables: cvrl.org/lumindex.htm
Live Science, "Why are auroras different colors?": livescience.com
UC Berkeley Space Sciences Laboratory, "Auroras: Paintings in the Sky, why are they different colors?": cse.ssl.berkeley.edu
Wikipedia, "Solar cycle 25" (smoothed maximum about 160.8 in October 2024; 2019 panel forecast of 115): en.wikipedia.org/wiki/Solar_cycle_25
NOAA Space Weather Prediction Center, solar cycle progression and news: spaceweather.gov
M. H. Rees, Physics and Chemistry of the Upper Atmosphere, Cambridge University Press (1989), electron range-energy relation.

Method note: quenching fractions use NIST radiative rates, O(¹D) quenching by N₂ (Ravishankara 2002), by O₂ ( $3.3\times10^{-11}\,e^{55/T}$ ) and by O (8 × 10⁻¹², uncertain), and approximate O(¹S) quenching by O₂ and O. Real thermospheric densities vary by a factor of several with solar activity, which moves the curves in Figure 2 up or down by tens of kilometres.

Header image: NASA (image iss072e159172), regraded for this article.

What makes an aurora glow at all?

Why does red aurora appear higher than green?

The misconception: “red means nitrogen” or “red means more energetic”

Where do blue, purple and pink come from?

Why does an aurora look grey or pale green to the eye but colourful on a phone?

Why do people far from the poles see red auroras?

Is now still a good time to see colourful auroras?

FAQ

What is the rarest aurora color?

Why are most auroras green?

Why do auroras look more colorful on a phone camera?

Why do people far from the poles see red auroras?

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Sources