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Why Are Auroras Green at the Bottom and Red at the Top?

Category: Space & Astronomy

Search for photos of the northern lights (aurora borealis) or the southern lights (aurora australis), and a pattern keeps turning up. A green curtain hangs in the middle. Sometimes a reddish glow floats above it, and sometimes a pink or purple hem trims the bottom edge.

Here is the odd part. The green and the red can come from the very same kind of atom. So what sorts one color low and the other high? The answer is a waiting game, played in air that gets thinner the higher you go.

1. The Colors Really Are Stacked by Height

An aurora glows far above where airplanes fly, more than 100 km (60 miles) up. The most common color is a pale green. Red is rarer and appears higher than the green. When the display is strong, the very bottom of a curtain can look pink or reddish purple[1][4].

So the usual order, from the bottom up, is pink or purple, then green, then red. The exact heights depend on who you ask. NOAA gives roughly 120 to 400 km for green[1]. Atmospheric Optics says 100 to about 150 km[2]. NASA puts green at roughly 100 to 200 km and red above 200 km[4]. Because of that spread, this note treats heights as rough guides and sticks to the order.

2. Oxygen Can Glow Green or Red, and the Difference Is a Wait

An aurora starts when charged particles from space slam into atoms and molecules in the upper atmosphere and give them extra energy. The atoms release that energy as light[3]. Atomic oxygen is behind both of the main colors: green (wavelength 557.7 nm) and red (630.0 nm)[3].

Here is the surprise. An oxygen atom excited into the "green" state glows within about one second (that is how long the state lasts). An oxygen atom excited into the "red" state does not glow for a long time. Atmospheric Optics gives about 110 seconds[2], and NOAA says more than 150 seconds[1]. Either way, the red state lasts far longer, roughly two minutes or more. Nothing in the atom is different except which energy state it is in.

3. Why Red Is More Likely to Glow in Thin Air

The air gets thinner as you go up. Low down, atoms and molecules are packed close together and bump into each other constantly. High up, they are far apart.

A red-waiting atom that gets bumped before it glows loses its stored energy without making any light. Scientists call this quenching. NOAA explains that at lower altitudes oxygen in the red state collides with other atoms or molecules before it can give off a photon, and that red is limited to the thinner air above about 300 km[1]. Atmospheric Optics describes the same idea: even where the air is very thin, any excitation is quickly removed by collisions, so the red state needs the emptier space higher up[2]. WebExhibits adds that the colors at different heights relate to the changing makeup of the air and its falling density[3].

A simple way to picture it is carrying a full cup of water for two minutes. Do that down a packed school hallway and someone will bump you and spill it. Do it across an empty field and you can arrive with the cup still full. The green atom only has to hold on for about a second, so it can glow in air that is a bit more crowded. That is why green sits lower and red sits above it. (This is a picture, not a measurement: what spills in the real case is energy, not water.)

4. What Makes the Pink or Purple Hem at the Bottom?

The lowest edge of a curtain often looks different because a different gas is glowing there. NOAA says the purplish lower border comes from molecular nitrogen[1]. Atmospheric Optics also describes a deep red-violet border beneath the usual green in very intense displays, and attributes it to excited molecular nitrogen[2]. NASA says nitrogen can give off pink and blue light[4].

Now the order makes sense from the bottom up. Nitrogen gives the pink or purple edge. Oxygen with the one-second wait gives green. Oxygen with the long wait gives the red, and it only gets to finish glowing where the air is thinnest. One caution: the sources do not agree on the exact heights of the hem, so this note gives none.

5. Why Red Shows Up in Displays Seen Far from the Poles

Auroras are usually seen closest to the polar regions. NASA explains that during strong geomagnetic storms, energy stored in the magnetosphere pours down Earth's magnetic field lines into the atmosphere and can produce auroral displays far from the poles[4].

Many photos of such displays from lower latitudes show a lot of red. The sources I checked do not give one settled explanation for this, so treat the next idea as a picture rather than a finding. If the aurora is far away over the horizon, the low green part may be hidden by the curve of the Earth, while light that glows very high up could still be seen above the horizon. High-up light is exactly where red is allowed to glow.

Check the Color Order Yourself in Aurora Photos

  • Find a few aurora photos and sketch the layers. Look at three or four photos in a library book or from NASA's image galleries, with an adult if you are online. Using crayons or colored pencils, shade the colors from the bottom of the curtain to the top. Does the order pink or purple, green, red hold in every photo? Where does it break?
  • Compare photos taken near the poles with photos from farther away. Does red show up more in one group than the other? Write down what you notice before you read any explanations.
  • Read an original page. NOAA's Space Weather Prediction Center has a tutorial on auroras that explains the colors in plain language. It is linked below.

Sources

This article is a personal summary based on the public sources listed below. Heights and waiting times differ between sources, so the text treats them as rough guides.

  1. NOAA Space Weather Prediction Center, "Aurora Tutorial." https://www.spaceweather.gov/content/aurora-tutorial (Source for green and red from atomic oxygen, the red state lasting more than 150 seconds and surviving only in thin air above about 300 km, collisions removing the energy at lower heights, and the purplish lower border from molecular nitrogen. A public agency page. I read it through a page-text tool that returns extracts, not as a full line-by-line reading.)
  2. Atmospheric Optics, "Glowing Gases - Aurorae." https://atoptics.co.uk/blog/glowing-gases-aurorae/ (Source for the green state lasting about one second, the red state about 110 seconds, collisions quickly removing excitation in thin air, and the red-violet border from molecular nitrogen. Its heights, 100 to 150 km for green and 150 to 250 km for red, differ from NOAA's. Read through the same extract tool.)
  3. WebExhibits, "What causes the colors of the aurora?" https://www.webexhibits.org/causesofcolor/4D.html (Source for charged particles exciting atoms, oxygen giving green at 557.7 nm and red at 630.0 nm, and colors relating to air composition and falling density. Read through the same extract tool.)
  4. NASA Science, "Auroras." https://science.nasa.gov/?p=407651 (Source for the heights of green and red, nitrogen's pink and blue light, and storms driving displays far from the poles. Read through the same extract tool. The idea in section 5 about the horizon hiding the low green part is my own geometric reasoning and is not stated by any source.)

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