What Makes The Different Colors Of The Northern (And Southern) Lights?

By Debra Kelly on Thursday, March 10, 2016
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“For night’s swift dragons cut the clouds full fast, / And yonder shines Aurora’s harbinger; / At whose approach ghosts, wandering here and there, / Troop home to churchyards.” —William Shakespeare, “A Midsummer Night’s Dream”

In A Nutshell

While we’re not sure why auroras have so many different shapes, we do know what causes the different colors. When the Sun’s particles interact with oxygen, the greenish-yellow lights occur. Nitrogen causes the red, blue, and purple lights, and height has something to do with it, too. The lowest lights are the blue ones, while red lights are the highest. And there’s a fascinating variety of myths associated with the lights, from the idea that they’re the spirits of the unborn to harbingers of tragedy or good fishing.

The Whole Bushel

The aurora light show is one of the most brilliant natural phenomena we know, and it’s been fascinating mankind for thousands of years. It’s believed that the first depiction of the auroras came in the form of a Cro-Magnon cave painting that’s more than 30,000 years old, and the first written mention of the lights comes from China in 2600 BC. It was written, “Fu-Pao, the mother of the Yellow Empire Shuan-Yuan, saw strong lightning moving around the star Su, which belongs to the constellation of Bei-Dou, and the light illuminated the whole area.”

It wasn’t until 1619 that the northern light show was named “aurora borealis,” a term that came from Galileo. At the time, it was thought that the lights came from the morning sun reflecting off the Earth’s atmosphere, so he named them after the goddess of the morning and the Greek north wind.

In 1749, we started keeping track of the solar cycle. We’ve learned that when there are sunspots and solar storms, the auroras are at their brightest. There have been just 22 solar cycles since we started counting back then.

Today, we know that they’re not caused by the morning light, but we’ve kept the name. It’s only the northern lights that are called the aurora borealis; the southern version is the aurora australis.

Both are caused by the same thing—charged particles given off by the Sun, which is why they’re stronger and brighter during peak times for solar storms.

When those charged particles hit the molecules in our own atmosphere, they interact to produce photons and, in turn, lights we can see. It’s sort of what happens inside the tubes of a neon sign, and as in a neon sign, different molecules cause the different colors.

The most common color we see is a sort of greenish-yellow, which is caused by the charged particles of the Sun interacting with oxygen. These green lights are also in the middle range as far as height goes, and usually happen somewhere around 240 kilometers (150 mi) above the planet’s surface.

When the Sun’s particles interact with nitrogen, the lights are either red, blue, or purple. The color depends on the height, with the red lights produced at heights above 240 kilometers, blue lights below 95 kilometers (60 mi), and purple lights at the distance in between the two.

There are also the colors we can’t see, and it was only when we got a good look at the lights through special satellite-based cameras that we could prove there was ultraviolet light there.

One thing we’re still not sure about is what causes the different shapes of the auroras. They can appear in clouds, waves, spirals, and straight lines along the horizons, and all the different shapes and patterns can occur on the same night.

There are even more myths and folklore tales than there are shapes and colors to the northern lights. In Greenland, it’s said that anyone who whistles under a sky filled with the lights will attract the lights, which will come down and chop off the offender’s head. Other cultures believe they’re the spirits of the ancestors, that they’re guiding hunters toward their prey, or that they’re the spirits of unborn children.

Some cultures believe they’re more evil than good and that they herald a coming battle or tragedy. They’re “fire foxes” in Finland, fairies in Scotland, and signs of good fishing in Scandinavia. For the Inuit, they were the souls of animals, dancing in the night sky.

Show Me The Proof

NASA: The History of Auroras (Aurora flyer)
Space: What Causes the Northern Lights & Where to See Them
EarthSky: What causes the aurora borealis or northern lights?
Polar Bears International: The Aurora and Its Fantastic Indigenous Interpretations