A massive solar flare last week caused a surge of energetic particles from the Sun to travel through space. Over the weekend, this wave reached Earth, resulting in unusually vivid aurora sightings in both the northern and southern hemispheres. Typically, auroras are only visible near the poles, but this weekend they were spotted as far south as Hawaii and as far north as Mackay. Although this surge in auroral activity has ended, there is no need to worry if you missed out, as periods of intense aurora are expected to return over the next year due to the Sun approaching the peak of its 11-year sunspot cycle.
If you witnessed the aurora or saw any photos, you may be curious about what causes the glow and the different colors. The answer lies in atoms and how they become excited and relax. The auroras are created when charged subatomic particles, mostly electrons, collide with Earth’s atmosphere. These particles are constantly emitted from the Sun, but their numbers increase during times of heightened solar activity.
Earth’s magnetic field protects most of our atmosphere from these charged particles, but they can penetrate near the poles and cause disruptions. The composition of Earth’s atmosphere is approximately 20% oxygen, 80% nitrogen, and trace amounts of other substances like water, carbon dioxide, and argon.
When high-speed electrons collide with oxygen molecules in the upper atmosphere, they split the molecules into individual atoms. This process also occurs due to ultraviolet light from the Sun, and the resulting oxygen atoms can react with other oxygen molecules to create ozone, which shields us from harmful UV radiation. However, in the case of auroras, these oxygen atoms are in an excited state where their electrons are arranged unstably. As a result, they release energy in the form of light to relax.
Different elements produce various colors of light when energized, as seen in fireworks. For example, copper atoms emit blue light, barium emits green light, and sodium atoms produce a yellow-orange color. These emissions occur quickly due to the rules of quantum mechanics.
In the aurora, many oxygen atoms are created in excited states without any allowed ways to relax by emitting light. Nevertheless, nature finds a way. The dominant green light in the aurora is emitted when oxygen atoms relax from a state called “¹S” to a state called “¹D.” This relaxation process is relatively slow, taking nearly a whole second on average. It occurs more readily in the upper reaches of the atmosphere where there is lower air pressure and fewer oxygen molecules, allowing the atoms more time before colliding with each other and releasing a photon.
The red light in the aurora comes from another forbidden transition in oxygen atoms. After emitting the green photon, the oxygen atom finds itself in another excited state with no allowed relaxation. The only way to escape is through another forbidden transition, emitting red light. This transition takes even longer, around two minutes, and only occurs at high altitudes where collisions with other atoms and molecules are rare.
In addition to green and red, there are other colors visible in the aurora. Ionized nitrogen molecules can emit blue and red light, resulting in a magenta hue at lower altitudes. While these colors are visible to the naked eye when the aurora is bright enough, they appear more intense in camera lenses. Cameras have the advantage of longer exposure times, allowing them to capture more light and produce images in dimmer conditions. Additionally, our eyes’ color sensors do not work as well in the dark, so we tend to see in black and white under low light conditions. However, when the aurora is sufficiently bright, the colors are clearly visible without the aid of a camera.