Aurora Colors and What They Mean

Green is the default aurora color. Charged particles from the solar wind hit atomic oxygen, and the oxygen releases light at 557.7 nanometers. That is the yellow-green part of the spectrum, which is also where human eyes are most sensitive. Both facts explain why green dominates photos and eyewitness reports.

Most green glows between about 100 and 250 km up. The bottom edge of a curtain often looks sharp and bright, because oxygen thins out quickly below roughly 100 km. The top fades more gradually.

Green is also the color that survives weak activity. On a quiet night at high latitude, a thin green arc along the northern horizon is all you get, and that is still a real aurora. Brightness changes within seconds, so a faint band can brighten into moving rays and then fade again before you finish a cup of tea.

If you see a pale gray-green smudge in the north and wonder whether it is cloud, watch it for ten minutes. Aurora changes shape. Cloud lit by a city drifts and keeps its outline.

Red aurora is oxygen at 630 nm. It needs low-density air, so it forms higher than the green, and it needs strong energy input to show up well. Your eyes are poor at picking up deep red in the dark, so it usually looks like a dull brown-red glow, or you miss it entirely.

Red tends to dominate during very strong geomagnetic storms, when energetic particles reach the thin upper layers in large numbers. That is why red is the color of the great low-latitude sightings in history, while ordinary displays near the Arctic Circle are mostly green. If you only see red, you are probably far from the auroral zone.

The upper part of a big display can turn red above the green. Red also appears as a broad, diffuse glow with no curtain structure at all.

Blue and violet come from nitrogen molecules that have lost an electron. They glow at the bottom of the curtain, below the green, where atomic oxygen is scarce. Blue is most visible in strong displays, and it can appear in sunlit auroras near twilight, when the upper atmosphere is already lit by the Sun.

Purple on a photo is often a mix of this blue with the red above it. It is real, but a camera records it much more strongly than your eyes do.

Blue is also the color most affected by moonlight and light pollution. A bright moon or a city dome turns the lower edge of the curtain white, and the blue disappears. Dark ground and a moonless night give the blue and violet fringe its best chance.

Pink appears when red and green overlap, usually along the lower fringe of an active curtain. Yellow is the same idea with more green than red. Both are less common than plain green and tend to show up in more energetic moments.

A pink edge can flash for a minute during a substorm, then vanish. If your forecast shows a rapid rise in activity, keep watching the north rather than leaving after the first quiet spell.

You will see the most variety in color when activity jumps quickly. During a substorm the aurora brightens, moves and changes shape, and different heights light up in turn. In the quiet stretches between, you may see only a faint green band. This is why you should stay out for at least an hour on a promising night.

In dim light your eyes switch to rod vision, which is nearly colorblind. A faint aurora can look gray or pale white-green even when it is green in a photo. A phone or camera collects light for several seconds, so it adds up color your eyes cannot.

That is normal and does not mean the display was weak. Brighter displays push into cone vision, and then you do see green, and sometimes pink or red, directly. For camera settings, see the beginner guide.

A simple test: take a photo with a few seconds of exposure on your phone night mode, then compare it with what you see. If the photo is far more vivid, that gap is exposure, not exaggeration. Treat the photo as a record and your eyes as the real measurement of how strong the aurora is.

At mid-latitudes you often see only the top of an aurora far to the north, and that top is the red part. A red or pink glow low on the northern horizon, with no green, usually means a strong geomagnetic storm lit the sky at high altitude while the lower, green part sits below your horizon.

This is how storms reach cities like Edinburgh, and in the southern hemisphere how Hobart sees the glow in the south. Check the live Kp index and the map to see whether a storm is under way, and point a camera at the pole-ward horizon.