Why Do Auroras Change Color With Place and Height?
― Atmospheric Gases That Can Only Glow in Set Colors
Look at photos of auroras and you will see mostly green, but also red, purple and even pinkish shades. The basic process is the same: particles from the Sun hit the atmosphere and make it glow. So why do the colors differ so much?
In a science class, did you ever see a flame that had been dipped in a solution containing a metal? Copper gave green, sodium gave yellow, potassium gave purple. The flame's color was clearly fixed by the substance in it (the flame test).
Neon signs glow red or orange for the same reason: the gas inside can only give off certain colors. The colors of the aurora are thought to be set for much the same reason.
Electrons in an atom can only give off light of certain fixed amounts of energy. So each gas is thought to have its own set of colors it can glow.
The type and thickness of the gas differ with height in the sky. So even for the same gas, oxygen, the color that appears most easily changes with height.
Let's look at these two ideas in turn: "fixed colors for each gas" and "color differences with height."
Why each gas can give off only certain colors
When particles in the solar wind hit gas in the atmosphere, electrons inside the gas's atoms or molecules are briefly pushed into a higher-energy state. When an electron drops back to its lower-energy state, it is thought to release the difference in energy as light of a fixed color (wavelength). An electron can only take certain energy states, with jumps between them, and these are fixed for each gas. So a different gas means a different set of colors it can give off. Fireworks, neon signs and flame tests always glow the same color for the same substance, and the reason is the same.
Why the color that glows easily changes with height
Oxygen atoms can give off both green light and red light. But the two colors take very different times to appear. Green comes out quickly, while red takes a relatively long time.
That delay is the problem. Low down, the air is thick, and an oxygen atom is constantly bumping into its neighbors. If it collides while waiting to give off red, it loses its energy without producing any light. Only high up, where the gas is thin, is there time to give off red before a collision. Lower down, the air is too thick for oxygen to wait, so its light almost disappears. The blue and purple of quick-glowing nitrogen molecules stand out instead.
Stronger solar wind particles are thought to penetrate deeper into the atmosphere. In an especially active aurora, particles reach the nitrogen layer near 100 km, and you may see a blue or purple fringe below the green aurora.
The kind of gas, and the "waiting time" before it glows, decide the color.
What you can check for yourself
- Look for chances to see colored flames, such as a campfire or a fireworks show (please do not try flame tests at home)
- Watch the colors of neon signs and street lights
- Notice that even among things that "glow," the color is clearly fixed by the gas or substance used
It helps to picture the aurora this way: the atmosphere is a giant "neon tube," where set gases glow in set colors.
Summary
Auroras look different in color from place to place and height to height because of two things working together. First, each gas can give off only certain colors of light. Second, the thickness of the gas changes with height, so colors that take time to appear can only come from high up. Green is seen most often because oxygen atoms can give it off relatively quickly.
An aurora is not a single light.
It is a gradient of color played by different gases at different heights.
How solar wind particles come to gather near the North and South Poles is explained in this article.
For those who want to know more ― terms, numbers and links to textbooksFrom middle school science to topics under research, each part is labeled with its level
- Middle schoolCovered in Japanese middle school science
- High schoolCovered in high school "Basic Chemistry"
- High school+High school physics or chemistry, or advanced or sidebar content in textbooks
- UniversityUniversity-level specialist content (atmospheric physics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: words about aurora colors
- Flame test: A reaction in which a metal or other substance gives off light of a characteristic color when heated.
- Wavelength: The length of one repeat of a wave, which sets the color of light.
- Electron energy state: One of the fixed, separate energy values an electron in an atom can have.
High schoolChecking with a formula: which has more energy per light particle (photon), green or red?
We use a commonly used approximate relation that estimates light's energy from its wavelength, and compare green with red.
| In symbols | E ≒ 1240 ÷ λ |
| In words | Energy of one light particle (electronvolts) ≒ 1240 ÷ wavelength (nanometers) |
| Where it comes from | It comes from Planck's relation linking a light's energy to its frequency (energy = Planck's constant × frequency). If you use electronvolts and nanometers, the constants combine into 1240. |
What the symbols mean: E is the energy of one light particle (in electronvolts), and λ is the wavelength (in nanometers).
| Green wavelength | About 560nm (a guide to the color oxygen atoms often give off) |
| Red wavelength | About 630nm (a guide to the longer-wavelength color oxygen atoms give off) |
| Green energy (eV) | 1240 ÷ 560 ≒ 2.21 |
| Red energy (eV) | 1240 ÷ 630 ≒ 1.97 |
| Result | Green has more energy than red |
The shorter the wavelength, the more energy each light particle (photon) carries. You can see this clearly in green (2.21) being a larger number than red (1.97). The bigger the drop from a high-energy state to a low one, the shorter the wavelength (the higher the energy) of the light given off, and this difference in numbers shows it.
High school+How different "waiting times" sort colors by height
Oxygen's green glow happens fairly quickly. Its red glow is a "forbidden transition," a kind of change that is naturally unlikely, and it is thought to take a long time before the light is released. At low height, an atom collides with neighbors during this long wait and loses its energy (this is called quenching). So red is thought to be seen as light in practice only at great height, where the air is thin.
UniversityModeling glow height in atmospheric physics
In atmospheric and upper-atmosphere physics, researchers study models that calculate which color glows at which height, and how brightly, from the energy of solar wind particles and the density of the air. Work continues to compare these models with direct measurements from satellites and rockets.
The calculation combines the electron transport equation, which follows a particle from entering the atmosphere until it stops, the Einstein coefficient, which gives the rate at which atoms and molecules emit light, and the rate at which collisions wipe out the glow (quenching). The Planck relation you used in high school is one part of it.
📖 Derivations and further reading: Aurora ／ Photoelectric effect (light particles and energy)
ResearchWhat is still unclear
- The exact conditions that produce rare colors (deep pink, or unusual purples) seen in very active auroras are still a topic of observation and research.
- Research is under way to predict more accurately how changes in aurora color and shape relate to effects on power grids and communication equipment on the ground (space weather).
- Comparative study also continues on how the colors of auroras seen on other planets (Jupiter, Saturn and others) vary with the makeup of each planet's atmosphere.
Even one aurora color holds a rich topic, where atomic physics meets atmospheric science and research still goes on.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: flame tests | The basic idea that each gas has fixed colors |
| High school | Basic Chemistry: atomic structure | Calculating photon energy |
| High school+ | Physics / Chemistry (advanced) | Forbidden transitions and quenching |
| University | Atmospheric physics | Modeling glow height |
| Research | Upper-atmosphere physics (ongoing research) | Conditions for rare colors, space weather forecasting, comparing planets |
- Explanations of aurora emission heights and colors in textbooks on atmospheric and upper-atmosphere physics.
- Explanations of forbidden transitions and emission lifetimes in atomic physics materials.
- Explanations of flame tests and the principle of light emission in chemistry education materials.
※ The wavelength and height figures are rough guides to help you understand the mechanism. Actual values are said to vary with the strength of the solar wind and the state of the atmosphere.
※ This article is a general-audience science explainer. The numbers shown are rough estimates to help you understand the mechanism. Actual values are said to vary with the strength of the solar wind and observing conditions.