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A magical fox, running across the Arctic fells, its tail spraying snow and sparks into the air. The dead, playing football in the heavens with a walrus skull. Or the souls of dead children, dancing in the sky.
These are myths surrounding the aurora, stories ancient cultures close to the North Pole told to explain the ethereal lights they saw flickering in the night sky above their heads. Now, we know that these breathtaking displays aren’t of supernatural origin but due to chemical processes high in the atmosphere. This graphic explains how auroras form and what causes the different colours.
While the aurora may not be paranormal, their origins are extraterrestrial. Their genesis is in charged particles ejected from the sun’s surface, a stream known as the solar wind. These particles, mostly electrons and protons, are flung from the sun’s corona at speeds of around three hundred kilometres per second and hurtle through the whole solar system.
When these particles reach Earth, our planet’s magnetic field shields us from them. This is a good thing because, without this shielding, the charged particles would wreak havoc with our electronic and communications devices and networks. Earth’s magnetic field shield deflects the majority of the solar wind around the planet; however, a small number of particles enter the atmosphere at the magnetic field’s weak point, the poles.
Once the charged particles, primarily electrons, have entered the atmosphere, the atoms and molecules in their path are at their mercy. High-speed collisions transfer extra energy to air atoms and molecules, a process called excitation. These excited atoms and molecules don’t remain in this state for long; they return to their original or ground state, losing their excess energy as light. This process generates the aurora.
The different colours of the aurora stem from the atoms and molecules the solar wind electrons collide with. Our planet’s atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. The composition and concentration of gases vary with elevation; for example, high in the atmosphere, ultraviolet light from the sun splits oxygen molecules into two oxygen atoms. These oxygen atoms play a key role in auroral displays.
The greens, most common in aurora, result from the excitation of oxygen atoms between 60 and 150 miles above the Earth’s surface. As these excited atoms decay to a lower, but still excited, energy level, they emit green light. This takes approximately one second, which sounds short, but for these kinds of processes is pretty long. It means it’s only possible high in the atmosphere. Here, concentrations of atmospheric constituents are low enough that the energy isn’t lost to collisions with other atoms or molecules.
Oxygen atoms are also responsible for the appearance of red in aurora. This is even rarer and involves an oxygen atom dropping from an excited state to the ground state. It takes longer than the transition that produces green light (110 seconds to be exact), so can only happen high in the atmosphere, at least 150 miles up. Here, the air is so thin that the energy is unlikely to be lost to collisions with other atoms. Even then, these red colours are only seen during more intense periods of solar activity.
Finally, nitrogen, the main constituent of Earth’s atmosphere, also contributes to the colours produced. Nitrogen molecules are less prone to being split into atoms than oxygen molecules, due to their exceptional stability. However, nitrogen molecules can be both ionised and excited by the electrons from the solar wind. As these molecules lose their excess energy they give off blue or purple light. Again, these colours are less frequently seen.
Solar activity doesn’t just affect the colours you see in the aurora, but also where you can see them from. Aurora are visible near both the north and south magnetic poles; the aurora borealis and the aurora australis respectively. The aurora borealis tends to get more attention as the normal area of visibility for the aurora australis is mostly the Arctic Ocean.
Most of the time the aurora are visible between 10-20 degrees latitude from the poles (or approximately 700-1400 kilometres). The aurora borealis is often visible in Greenland, Iceland, and northern Scandinavia. The sun’s output of charged particles varies with solar activity, and more tempestuous conditions in the sun’s corona translate to more particles racing towards the Earth’s poles. This, in turn, can extend the reach of the aurora, making them visible at lower latitudes.
The sun’s activity waxes and wanes with the solar cycle, and in 2024 we’re approaching a peak or a solar maximum. That’s why, in recent months, there’ve been reports of aurora as far south as northern Europe. The exact point of the peak is hard to predict, but it’s thought that as we head into 2025 the sun’s activity will again start to wane. The next solar peak is not expected for around another decade.
The current solar maximum is nowhere near the most impressive recorded when it comes to the aurora. The Carrington Event in September 1859 was an intense geomagnetic storm on Earth, thought to have been triggered by a mass ejection of particles from the sun: a solar flare. The resulting aurora was visible across the globe, even in countries close to the equator, such as Colombia.
The aurora is the attractive side of this solar activity, but such events don’t come without less welcome consequences. The geomagnetically-induced currents generated knocked out telegraph systems in 1859. Today, a similar event would damage satellites, knock out GPS systems and cause electric outages.
How can you know, during this period of increased solar activity, if the aurora is likely to be visible from where you are? If you’re in the UK, you can use AuroraWatch, a site set up to offer alerts of when aurora might appear. Elsewhere, the Space Weather Prediction Centre offers a handy prediction map for aurora in the coming 30-90 minutes.
References and further reading
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