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Aurora Borealis and Aurora Australis—those mesmerizing, flickering ribbons of color dancing across the night sky—have captivated humanity for centuries. But what exactly is going on up there? Why does the aurora glow with such vivid green, red, and purple hues? More importantly, can we blame it all on aliens or mystical forces, or is science offering us a more grounded (and far more fascinating) explanation?
This captivating light show is not only beautiful, but it’s also a mesmerizing intersection of physics, chemistry, and atmospheric science. But first:
What Are the Auroras?
Before we go into the details of their colors, let’s establish what we’re talking about. The auroras, or northern and southern lights, are natural light displays predominantly seen near the Earth’s poles. The Aurora Borealis lights up the northern hemisphere, while its southern counterpart, the Aurora Australis, can be observed in the southern hemisphere.
These lights appear when charged particles from the Sun, mostly electrons and protons, interact with Earth’s magnetic field. As these particles travel along magnetic field lines, they collide with gases in the Earth’s upper atmosphere. This collision results in a release of energy in the form of light—a phenomenon we know as the aurora.
But here’s the thing: not all auroras are created equal. The colors we see depend largely on the type of gas the particles interact with, and how high up in the atmosphere these collisions occur.
The Green Glow: The Most Common Color of the Aurora
If you’ve ever seen an aurora, chances are you’ve spotted a lot of green. Why green? Well, this color is the result of the most common interaction between charged particles and oxygen molecules in Earth’s atmosphere.
When high-energy electrons collide with oxygen molecules about 60 to 150 miles (100 to 240 km) above the Earth’s surface, they excite the oxygen atoms. This excitement causes the oxygen to release energy in the form of light, specifically a brilliant green hue. The wavelength of the green light emitted falls in the range of 557.7 nanometers—just within the visible spectrum, making it the most eye-catching color of all.
This green glow is so prevalent that it’s often the first color people associate with the aurora. It’s also the result of the type of oxygen in the atmosphere that interacts most readily with the charged particles from the Sun. These oxygen atoms are also a bit more “stable” at lower altitudes, which makes it easier for them to get excited and emit green light when the particles come barreling in from space.
The Red Aurora: The Rarer, But Stunning Color
Now, for those who are lucky enough to witness a more elusive phenomenon, the red aurora is a sight to behold. Unlike the green glow, the red aurora doesn’t occur nearly as frequently, and it also requires a bit more atmospheric drama to make it happen.
So, what’s the deal with the red hue? Red auroras are also a result of oxygen interactions, but here’s the twist: they happen at much higher altitudes, upward of 200 miles (320 km) above the Earth’s surface. In these upper atmospheric layers, the oxygen molecules behave differently than they do closer to the ground. They require more energy to get excited, and when they do, they emit red light, which has a longer wavelength than green light (around 630 nanometers).
This phenomenon occurs more often during strong solar activity, such as solar storms. These storms send an intense shower of energetic particles toward Earth that increases the likelihood of encountering those high-altitude oxygen molecules and causing them to light up in glorious red.
One of the reasons why red auroras are more infrequent has to do with how much energy is required for the oxygen molecules to emit light at such high altitudes. Simply put, the particles need to be supercharged to excite the oxygen enough to create the red glow. It’s like trying to start a fire with wet wood: the conditions need to be just right for it to happen.
The Purple and Violet Hues: The High-Altitude Drama
If you’ve ever witnessed a spectacular aurora that’s shifted between green and purple tones, you’re likely seeing nitrogen at work. While oxygen gets most of the credit for the green and red hues, nitrogen molecules can also be involved in the aurora’s color spectrum.
In fact, nitrogen can emit both purples and reds in different atmospheric conditions, though the purple tones are generally the result of interactions at lower altitudes.
When charged particles collide with nitrogen molecules at altitudes below 60 miles (100 km), they can excite the nitrogen in such a way that it emits purple or violet light. The wavelengths of this light fall between 400 and 450 nanometers—right on the edge of the visible spectrum, giving these auroras a more subtle, ethereal appearance compared to their green and red counterparts.
What makes purple auroras even more captivating is that they can occur in regions of the atmosphere where the density of gases is lower, meaning these lights tend to be seen during times of lower solar activity or in areas closer to the auroral oval (the zone around the poles where auroras are most frequently observed).
How Does Solar Activity Affect Aurora Colors?
The intensity and variety of colors we see in an aurora are heavily influenced by solar activity. Solar winds, which are streams of charged particles emitted by the Sun, are typically steady, but during periods of heightened solar activity, such as solar flares or coronal mass ejections (CMEs), the Sun releases a massive burst of particles. These charged particles can bombard Earth’s magnetic field with more energy than usual, leading to more spectacular auroras, and, often, more varied colors.
During these solar storms, the charged particles travel faster and deeper into the Earth’s atmosphere, allowing for higher-altitude collisions that produce the rarer red and purple auroras.
If you’re hoping to catch a glimpse of these colors, timing is everything. The best chances for observing vibrant reds and purples are during solar maximum, the peak of the 11-year solar cycle when solar activity is at its highest.
The Science Behind the Science: Why Does Light Emit These Colors?
At the heart of all this color-changing magic is a concept in physics called atomic excitation. When energetic particles collide with atoms in the atmosphere, they essentially bump those atoms into a higher energy state. Once the atoms settle back into their usual energy state, they release the extra energy as light. This is the same principle that powers light bulbs or neon signs. Except in this case, the energy comes from the Sun’s particles rather than electricity.
The different colors emitted—green, red, purple—correspond to the amount of energy released during this process, as well as the type of atom involved. Oxygen atoms, for example, emit green and red light, while nitrogen molecules can emit purples and reds depending on the conditions.
So whether you’ve experienced the aurora borealis yourself or simply dreamed of seeing it in person, there’s no denying the awe and wonder these lights evoke. The interplay of solar particles and Earth’s atmosphere creates a light show that’s not only stunning to behold but also an extraordinary demonstration of the forces that shape our planet and its atmosphere.
