Aurora Colors Explained – Southern and Northern Lights

Have you ever wondered how aurora colors work and why sometimes the aurora is just green or red, while other times it is a whole rainbow of hues? The short answer is that nitrogen and oxygen in the upper atmosphere release light in specific colors in response to solar radiation. Here’s a closer look at the process and the resulting aurora colors.

How the Aurora Works

In order to understand why there are different colors, let’s review the basic process behind the aurora.

• Charged particles in the solar wind strike the ionosphere or thermosphere layer of the Earth’s atmosphere, which contains low density gases, including nitrogen (N₂) and atomic oxygen (O), along with small amounts of hydrogen, helium, and other gases.
• Atoms, molecules, and ions absorb the energy from these charged particles, which kicks electrons into an excited energy state.
• The excited state is not stable, so eventually the electrons return to a lower energy state. When this happens, the atom, molecule, or ion releases this extra energy as a photon.
• Photons are light. The color of the light depends on the energy released by the atom or molecule. The energy that gets absorbed and the color of light released is a characteristic of that atom. For example, molecule nitrogen (N₂) releases blue to violet light, while atomic oxygen releases red or green light, depending on the energy it absorbs.

Colors of the Aurora

The ionosphere mainly contains nitrogen and oxygen, so these are the key players in producing the colors of the aurora. While they only release a few colors, sometimes you see a whole rainbow of hues because of the way the different wavelengths of light mix.

Moving from the highest altitude in the atmosphere to the lowest, here are the sources of the aurora colors:

• Blue and Violet [300-400 km (> 180 miles)]: Hydrogen and helium release blue and violet light at the top of the aurora. However, these colors are relatively faint and are not usually visible except under very dark skies or with strong solar storms. Also, because these gases are relatively rare, the light is diffuse.
• Red [300-400 km (> 180 miles)]: Red light at the top of an aurora comes from monatomic oxygen (O). At lower latitudes, sometimes only the red aurora appears, facing the north (northern hemisphere) or south (southern hemisphere). The other colors are below the horizon line.
• Yellow-Green [100-300 km (60- 180 mi)]: Vivid lime green is the most common color of the aurora. It comes from monatomic oxygen (O), but while the red aurora comes from oxygen absorbing solar radiation, green comes from oxygen atoms colliding with lower-energy electrons released by excited nitrogen molecules (N₂⁺).
• Blue [100-300 km (60- 180 mi)]: Excited nitrogen molecules (N2⁺) release blue light when they return to a more stable energy state. Whether an aurora is blue, blue-green, or yellow-green depends on the interplay of light from nitrogen ions and oxygen atoms.
• Deep Red [100 km (60 mi)]: A deep red color at the base of the aurora comes from diatomic nitrogen (N₂). When the red light overlaps blue light, you see purple or sometimes vivid pink. Similarly, when the red bleeds into the yellowish green from oxygen, you get orange aurora.
• Blue-Violet [100 km (60 mi)]: Near the base of the ionosphere, ionized diatomic nitrogen (N₂⁺) releases blue and violet light, but the released electrons don’t excite as much oxygen at the lower altitude.

Clouds, moonlight, and sunlight from sunrise or sunset also affect the color you see.

Black Aurora

In addition to colorful light bands, sometimes an aurora has black bands that block starlight. The dark regions likely come from electric fields in the upper atmosphere that block electrons from interacting with gases.

Colors Not Due to the Aurora

Colored lights in the sky sometimes have sources other than an aurora.

• Sunrise or Sunset: Sunrise or sunset colors sometimes change the apparent color of the aurora.
• Moonlight: Moonlight also affects the colors seen in the sky at night.
• Light domes: Cities, fires, and other bright sources of light illuminate the sky.
• Airglow: Like the aurora, airglow results from an interaction between the atmosphere and the solar wind. But, it is visible anywhere there is a dark sky (not just the polar regions). Typical airglow colors are green, red, or blue. Airglow can occur with the aurora or without it.
• STEVE: STEVE (strong thermal emission velocity enhancement) is a rare phenomenon with a purple or mauve arc or green “picket fence” of light. STEVE is not an aurora, instead coming from plasma waves in the magnetosphere.

Aurora Colors on Other Planets

Other planets and even moons experience the aurora. For example, images show aurora in the atmospheres of Venus, Mars, Jupiter, Saturn, Jupiter’s moon Io, Uranus, and Neptune. Comets also display auroras. The color of the aurora on other worlds depends on the composition of the atmosphere. Jupiter’s aurora is electric blue, while the Cassini spacecraft imaged an orange-yellow aurora above Saturn.

Bodies with magnetic fields have symmetrical aurora, like Earth’s aurora borealis and aurora australis. But, worlds without magnetic fields experience the phenomenon as diffuse bright patches. Really, the only requirement for the aurora is that a body have some atmosphere. So, the aurora also occurs in the atmospheres of planets outside of our solar system.

References

• Burch, J. L. (1987). Akasofu S-I and Y Kamide (ed.). The Solar Wind and the Earth. D. Reidel. ISBN 978-90-277-2471-7.
• Helling, Christiane; Rimmer, Paul B. (2019). “Lightning and charge processes in brown dwarf and exoplanet atmospheres”. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 377 (2154): 20180398. doi:10.1098/rsta.2018.0398
• Nishimura, Y.; Gallardo‐Lacourt, B.; et al. (2019). “Magnetospheric signatures of STEVE: Implication for the magnetospheric energy source and inter‐hemispheric conjugacy”. Geophysical Research Letters. 46 (11): 5637–5644. doi:10.1029/2019GL082460
• Reiff, P. H.; Collin, H. L.; et al.(1988). “Determination of auroral electrostatic potentials using high- and low-altitude particle distributions”. Journal of Geophysical Research. 93 (A7): 7441. doi:10.1029/JA093iA07p07441
• Sandholt, Even; Carlson, Herbert C.; Egeland, Alv (2002). “Optical Aurora”. Dayside and Polar Cap Aurora. Netherlands: Springer Netherlands. ISBN 978-0-306-47969-4. doi:10.1007/0-306-47969-9_3