May 18, 2024

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What causes the different colors of aurora? An expert explains the electric rainbow

What causes the different colors of aurora?  An expert explains the electric rainbow

Last week, a massive solar flare sent a wave of energetic particles from the Sun hurtling through space. Over the weekend, the wave reached land, and people around the world enjoyed seeing unusually bright aurora borealis in both hemispheres.

While the aurora borealis are usually only visible near the poles, they were spotted this weekend. South to Hawaii In the Northern Hemisphere, and As far north as Mackay in the south.

This spectacular surge in auroral activity appears to be over, but don’t worry if you missed it. The sun is approaching its zenith The sunspot cycle is 11 years longPeriods of intense twilight are likely to return within the next year or so.

If you’ve seen the aurora borealis, or any of the pictures, you’re probably wondering what exactly was going on. What makes the glow and different colors? The answer is all about atoms, how they are excited, and how they relax.

When electrons meet the atmosphere

Auroras are caused by charged subatomic particles (mostly electrons) colliding with the Earth’s atmosphere. These elements are emitted by the sun all the time, but there are more of them during times of greater solar activity.

Most of our atmosphere is protected from the flow of charged particles by the Earth’s magnetic field. But near the poles, they can sneak in and wreak havoc.

Earth’s atmosphere is made up of about 20% oxygen and 80% nitrogen, with trace amounts of other things like water, carbon dioxide (0.04%), and argon.

The aurora borealis in May 2024 was also visible in the Emilia-Romagna region in northern Italy.
Luca Argaglia/Flickr, CC BY-NC-SA

When high-speed electrons collide with oxygen molecules in the upper atmosphere, they split the oxygen molecules (O₂) into individual atoms. Ultraviolet rays from the sun do this too, and the oxygen atoms generated can react with O₂ molecules to produce ozone (O₃), the molecule that protects us from harmful UV rays.

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But in the case of the aurora, the oxygen atoms generated are in an excited state. This means that the electrons of the atoms are arranged in an unstable manner and can “relax” by releasing energy in the form of light.

What makes a green light?

As you can see in fireworks, atoms of different elements produce different colors of light when they are energized.

Copper atoms give off blue light, barium atoms give off green light, and sodium atoms produce a yellow-orange color that you may have also seen in old street lamps. These emissions are “permissible” under the rules of quantum mechanics, meaning they happen very quickly.

When a sodium atom is in an excited state it stays there for only 17 billionths of a second before emitting a yellow-orange photon.

However, in the aurora, many oxygen atoms are created in excited states with no “permitted” ways to relax by emitting light. However, nature finds a way.

A dappled night sky with bright green lights and pink streaks above it.
The aurora is visible from Oatlands, Tasmania on May 11, 2024.
AAP Image/Ethan James

The green light that dominates the aurora is emitted by oxygen atoms relaxing from a state called “¹S” to a state called “¹D.” This is a relatively slow process, taking on average about a full second.

In fact, this transition is so slow that it would not normally occur at the air pressure we see at ground level, because the excited atom would lose energy by colliding with another atom before it had a chance to send out a beautiful green photon. But in the upper layers of the atmosphere, where there is less air pressure and therefore fewer oxygen molecules, they have more time before they collide with each other and thus have a chance to release a photon.

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For this reason, it took scientists a long time to figure out that the green light of aurora comes from oxygen atoms. The yellow-orange glow of sodium was known in the 1860s, but was not recognized until the 1920s. Canadian scientists I discovered that the crepuscular green color was due to oxygen.

What makes a light red?

The green light comes from a so-called “forbidden” transition, which occurs when an electron in an oxygen atom makes an unexpected jump from one orbital pattern to another. (Prohibited transitions are much less likely than permitted transitions, which means they take longer to occur.)

However, even after emitting that green photon, the oxygen atom finds itself in another excited state without allowing relaxation. The only escape is through another blocked transition, from the ¹D state to the ³P state – which emits red light.

This transition is further blocked, so to speak, and the ¹D state must remain for about two minutes before it can finally break the rules and give the red light. Because it takes so long, red light only appears at high altitudes, where collisions with atoms and other molecules are rare.

Also, because there is a small amount of oxygen there, red light tends to only appear in intense auroras, like the ones we just saw.

This is why red light appears above green. While both arise from forbidden relaxation of oxygen atoms, red light is emitted much more slowly and has a greater chance of being extinguished by collisions with other atoms at lower altitudes.

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Other colors, and why do cameras see them better?

While green is the most common color seen in the aurora borealis, and red is the second most common, there are other colors as well. In particular, ionized nitrogen molecules (N₂⁺, which are missing one electron and have a positive electrical charge), can emit blue and red light. This can produce a purple color at lower elevations.

All of these colors are visible to the naked eye if the aurora is bright enough. However, they appear more strongly in the viewfinder.

There are two reasons for this. First, cameras have the advantage of long exposure, which means they can spend more time collecting light to produce an image than our eyes. As a result, they can take a photo in dim conditions.

The second reason is that the color sensors in our eyes don’t work well in the dark, so we tend to see in black and white in low-light conditions. Cameras do not have this limitation.

No need to worry, though. When the aurora is bright enough, the colors are clearly visible to the naked eye.



Read more: What is the aurora borealis, and why do they come in different shapes and colors? Two experts explain