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Northern lights colors: what glows, how high, and why it looks red from afar
Every color of the aurora is a signature of one gas at one height. Distance then decides which of those layers you can see.
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The green of the northern lights is atomic oxygen glowing at 557.7 nanometers, typically between 120 and 400 km above the ground, according to SWPC's aurora tutorial. Red comes from oxygen too, but in a state that lives so long it can only emit in the thin air above about 300 km. The purple or blue lower fringe comes from molecular nitrogen, which glows instantly when struck. Height explains why storms seen from far south often look red or grey rather than green: from a distance, Earth's curve hides the lower part of the display. On a spherical Earth, the 100 km lower border sinks below a flat horizon about 1,120 km away, while the red layer at 300 km stays visible to about 1,920 km. Faint light is also seen by the eye's rods, which register no color, so a dim aurora looks like a pale cloud. Cameras record the colors the eye misses.
How high will it look from where you stand?
Green lower border (100 km up)
9.0° above the horizon
| Red top (300 km up) | 28.1° |
| Top of the tallest rays (400 km) | 35.5° |
| Distance in miles | 311 |
Spherical Earth, no refraction. Heights: SWPC Aurora Tutorial.
One gas, one height, one color
The aurora is made by electrons accelerated in Earth's magnetosphere and guided down its magnetic field into the upper atmosphere. They hit oxygen atoms and nitrogen molecules and lift them into excited states; each relaxes by emitting light at wavelengths fixed by its atomic structure. SWPC's aurora page compares it to a neon sign, where a gas glows in its own color when electrons pass through it. What makes the aurora layered is that the different excited states do not live equally long, and the air gets thinner with height.
The green line, at 557.7 nanometers, comes from oxygen in a state that lasts about a second before emitting. That is short enough for the atom to release its light before another collision takes the energy away, across a broad band of height: SWPC's aurora tutorial puts it between 120 and 400 km. The red emission comes from another state of oxygen that lasts more than 150 seconds. Below about 300 km, collisions are so frequent that the atom is nearly always knocked out of that state before it can shine, so red aurora is restricted to the thin air above. Molecular nitrogen behaves the other way: it emits almost instantly, so it can glow even lower, and it gives the purplish fringe at the bottom of bright curtains.
| Color | Source | Typical height | When it is seen |
|---|---|---|---|
| Green | Atomic oxygen, 557.7 nm | 120 to 400 km | Almost every display; the most common color |
| Red | Atomic oxygen, long-lived state | above 300 km | Tops of tall rays, high storm aurora seen from far away |
| Purple, blue | Molecular nitrogen, prompt emission | lower border of bright curtains | Active displays with fast electrons |
| Purple at the top | Nitrogen lit by sunlight | highest rays | Near sunset or sunrise |
The energy of the incoming electrons decides how deep they go. Faster electrons penetrate further into denser air, which favors the green and the purple; slower ones stop higher and favor the red. That is why the color mix changes from one display to the next and even along a single curtain.
Why storms look red from the south
The aurora is high, but Earth is round. When the display sits far to your north, the curve of the planet hides its lower part, and only the upper layers stay above your horizon. The engine of this site computes that geometry for a spherical Earth without refraction, the same formula as the mini-simulator above. The table shows how high above a flat horizon three layers appear, by their distance along the ground.
| Distance to the aurora | Lower border, 100 km | Red layer, 300 km | Top of the green, 400 km |
|---|---|---|---|
| 200 km (120 mi) | 25.5° | 54.8° | 61.8° |
| 500 km (310 mi) | 9.0° | 28.1° | 35.5° |
| 800 km (500 mi) | 3.5° | 16.5° | 22.2° |
| 1,000 km (620 mi) | 1.2° | 11.8° | 16.7° |
| 1,500 km (930 mi) | below the horizon | 4.3° | 7.7° |
At 1,120 km, about 700 miles, the 100 km lower border reaches the horizon; beyond it, the green curtain is hidden. The red layer at 300 km stays visible to about 1,920 km, and the highest emission at 400 km to about 2,200 km. So an observer in the central states during a G4 storm, with the bright part of the oval over Canada, often sees a glow low in the north that is mostly red: the top of a display whose green base is below the horizon. The same storm seen from under the oval is overwhelmingly green.
The numbers explain two practical points. SWPC's viewing tips say that from a good vantage point aurora can be seen about 1,000 km to the north, which is about where the lower border sits just 1.2 degrees above a flat horizon while the red layer still stands about 12 degrees up. And a degree or two is not much: trees, buildings or a ridge to the north can hide a storm that is perfectly visible from a lakeshore. The forecast on this site counts aurora within 500 km as reaching your sky, where the lower border stands about 9 degrees high; the method page gives that rule.
Distance along the ground converts easily into latitude: one degree of latitude spans about 111 km along a meridian. The lower border therefore slips below the horizon for an observer about 10 degrees of latitude south of the display, and the red layer for one about 17 degrees south. That is the size of the band, several states wide, in which a storm can look red and low while it is green and overhead farther north. Use the mini-simulator to test any distance.
What the eye sees, and what the camera adds
Seeing color needs light. In the dark, the retina's rod cells take over; they are far more sensitive than the color cones but do not separate colors at all. SWPC's tutorial is explicit: people who report a white aurora are seeing it with night vision, and the true green and red appear only when the display is fairly bright. Red loses the most, because rods are almost insensitive to it, which is another reason the red glow of a distant storm can go unseen while a phone picks it up.
Cameras do not have that limit. Their pixels sit under red, green and blue filters and add up light for seconds, so even a faint display comes out in color, often more vivid than anyone on the spot saw it. SWPC notes that digital cameras can detect aurora and its colors when it is too dim for the eye. The phone photography guide explains how to use that. The Park Service at Denali gives visitors the same warning in the other direction: photos and time-lapse videos set expectations higher than what eyes alone will see.
Shapes and heights together
Color and shape are linked through height. A quiet arc is a thin ribbon of green stretched east to west at the bottom of the emission layer. Rays are tall columns along the magnetic field lines, green at the base and sometimes red at the top, which is why a bright curtain overhead can show both colors at once. During a substorm, SWPC's aurora page describes arcs that brighten, twist and spread across the sky, then break into pulsing patches before dawn; those patches are usually faint and pale to the eye.
Two more details round out the picture. SWPC notes that protons also cause aurora, but a faint, diffuse one that is usually hard to see with the eye. And the northern and southern auroras are mirror images, in the words of its tutorial: when the aurora becomes active in one hemisphere, it becomes active in the other at the same time, because the magnetic field lines that guide the electrons link the two polar regions.
None of this changes the forecast. The colors you can hope for depend on how far the display is from you, which depends on the storm, the Kp, and your magnetic latitude. The ranking by town gives the Kp each place needs, the storm scale guide explains the levels, and the aurora map shows where the oval is right now.