Auroras occur when charged solar particles flying from space collide with Earth’s magnetic field and atmosphere. Depending on what chemical elements the star plasma interacts with, a specific color of brightness appears.

Colors

Last week, a massive solar flare sent a wave of energetic particles from the Sun to Earth. Over the weekend, this wave reached the planet and people around the world enjoyed the sight of an extremely bright aurora, even though it does not normally occur. The aurora is most often only visible near the poles, but due to its powerful X-class flares it can also be seen as far away as Ukraine, although not in all regions.

Most of our atmosphere is protected from the flow of charged particles by the Earth’s magnetic field. However, near the poles they can penetrate and cause damage. Earth’s atmosphere consists of approximately 20% oxygen and 80% nitrogen, with small amounts of water, carbon dioxide (0.04%), and argon.

When the high-speed electrons of the ordinary solar wind hit oxygen molecules (O₂) broke them into individual atoms in the upper layers of the atmosphere. Ultraviolet light from the Sun also does this, and the oxygen atoms formed can then react with O molecules.₂ozone forming (O₃) is a molecule with a dense layer that protects us from harmful ultraviolet radiation.

But in the case of an aurora, when the flow of particles increases hundreds of times following an explosion on the Sun, the resulting oxygen atoms are excited. This means that the electrons of atoms are unstable and can emit energy in the form of light.

How is green light produced?

Fireworks are a good example of how atoms of different elements emit different colors of light when energized. Likewise, we can see that the flame can have a different color when different substances burn. The same principle works here. Copper atoms emit blue light, barium emits green light, and sodium atoms emit the yellow-orange light you might see in old street lamps. These emissions are part of quantum mechanics and occur very quickly.

When a sodium atom is excited, it remains excited for only 17 billionths of a second before emitting a yellow-orange photon. But oxygen is another matter. It doesn’t react that quickly, and as scientists say, there are no “allowed ways” in quantum mechanics to react in a way that emits light. But nature found a way.

The green light that dominates the aurora borealis is emitted by oxygen atoms that move from one state (1S) to another, called “relaxed” (1D). This process involves the transition of an electron to another orbital, which is an unexpected event, and therefore scientists introduced the terminology of “forbidden” and “permitted” (more likely) processes.

This is a relatively slow process, taking almost a second on average. This transition is so slow that it does not occur at the air pressure we usually see at ground level because the excited atom loses energy by colliding with another atom before it has a chance to emit a green photon. But in the upper atmosphere, where the air pressure is lower and therefore there are fewer oxygen molecules, they have more time before colliding with each other and therefore have a chance to emit photons.

That’s why it took scientists a long time to figure out what happened. The green light of the aurora comes from oxygen atoms. The yellow-orange glow of sodium was known in the 1860s, but it was not until the 1920s that Canadian scientists discovered that the green color of the aurora borealis was caused by oxygen.

What causes red light?

Red light is no less interesting than green. After the emission of a green photon, the oxygen atom finds itself in another excited state, without the possibility of transitioning to the “relaxed” state. The only way out is another “forbidden” transition (to the 3P state), which emits red light.

This transition is even slower. For this to happen, oxygen must exist in the 1D state for approximately two minutes to emit red light. Because it lasts so long, the red light only appears at high altitudes; here collisions with other atoms and molecules are negligible due to the low oxygen concentration.

Also, because there is so little oxygen there, red light tends to only appear during intense auroras like the ones we saw over the weekend.

Therefore red light appears above green. Although both result from the forbidden “relaxation” of oxygen atoms, red light propagates much more slowly and is more likely to be dimmed by collisions with other atoms at lower altitudes.

Here is an example of a red light appearing above a green light:

Other colors and why cameras see them better

Although green is the most common aurora color and red is the second most common, there are other colors as well. In particular, ionized nitrogen molecules, which lack an electron and have a positive electrical charge, can emit blue, violet and red light. This can create a purple tint at lower altitudes.

All these colors are visible to the naked eye if the aurora is bright enough. But they appear with greater intensity in the camera lens.

There are two reasons for this:

• First, cameras have the advantage of long exposure; This means they may spend more time collecting light to form an image than our eyes. As a result, they can produce images in dim conditions.
• The second reason is that the color sensors in our eyes do not work well in the dark. That’s why we usually see a black and white image in low light conditions. Cameras do not have this limitation.

But don’t worry. When the aurora borealis is bright enough the colors are clearly visible to the naked eye.