What Causes the Aurora Borealis' Colors?

The Aurora is the name given to the bands of colored lights seen in the sky at the higher latitudes. The aurora borealis or Northern Lights are seen mainly near the Arctic Circle. The aurora australis or Southern Lights are seen in the southern hemisphere. The light you see comes from photons released by oxygen and nitrogen in the upper atmosphere. Energetic particles from the solar wind strike the layer of the atmosphere called the ionosphere, ionizing the atoms and molecules. When the ions return to the ground state, energy released as light produces the aurora. Each element releases specific wavelengths, so the colors you see depend on the type of atom that is excited, how much energy it received, and how the wavelengths of light blend with each other. Scattered light from the sun and moon may affect the colors, too.

Aurora Colored From Top to Bottom

You can see a solid-colored aurora, but it's possible to get a rainbow-like effect through the bands. Scattered light from the sun can impart a violet or purple to the top of an aurora. Next, there may be red light atop a green or yellow-green band. There may be blue with green or below it. The base of the aurora may be pink.

Solid Colored Aurora

Solid green and solid red auroras have been seen. Green is common at the upper latitudes, while red is rare. On the other hand, Aurora viewed from the lower latitudes tend to be red.

Element Emission Colors

• Oxygen: The big player in the aurora is oxygen. Oxygen is responsible for the vivid green (wavelength of 557.7 nm) and also for a deep brownish-red (wavelength of 630.0 nm). Pure green and greenish-yellow aurorae result from the excitation of oxygen.
• Nitrogen: Nitrogen emits blue (multiple wavelengths) and red light.
• Other Gases: Other gases in the atmosphere become excited and emit light, although the wavelengths may be outside of the range of human vision or else too faint to see. Hydrogen and helium, for example, emit blue and purple. Although our eyes can't see all of these colors, photographic film and digital cameras often record a broader range of hues.

Aurora Colors According to Altitude

• Above 150 miles: red, oxygen
• Up to 150 miles: green, oxygen
• Above 60 miles: purple or violet, nitrogen
• Up to 60 miles: blue, nitrogen

Black Aurora

Sometimes there are black bands in an aurora. The black region can have structure and block out starlight, so they appear to have substance. The black aurora most likely results from electric fields in the upper atmosphere that prevent electrons from interacting with gases.

Aurora on Other Planets

The Earth is not the only planet that has aurorae. Astronomers have photographed the aurora on Jupiter, Saturn, and Io, for example. However, the colors of the aurora are different on different planets because the atmosphere is different. The only requirement for a planet or moon to have an aurora is that it have an atmosphere that is bombarded by energetic particles. The Aurora will have an oval shape at both poles if the planet has a magnetic field. Planets without magnetic fields still have an Aurora, but it will be irregularly shaped.