The bright green, red and blue lights of the aurora borealis are still one of the great natural wonders of the world. People travel from all over the world to the polar regions with an express desire to catch a glimpse of the magical phenomenon.

Through history, many different explanations have been proposed for how they form, but it took several centuries for us to piece together the full story. Hippocrates suggested it was caused by reflected sunlight in the fifth century BC, and Bishop Gregory of Tours was the first to describe it as a natural (rather than supernatural) phenomena in 585AD.

Galileo proposed the name “aurora borealis” and “aurora australis” (referring respectively to the northern and southern lights) around 1620, while Benjamin Franklin was the first to theorise in 1752 that they were caused by a concentration of electrical charges in the polar regions, which he believed were intensified by the snow.

Johan Wilcke in 1777 noticed that aurorae converged on magnetic field lines and Henry Cavendish successfully measured the height of the phenomenon in 1790. Then, around 1900, Kristian Birkeland — known as the father of modern auroral science — used electron beams and magnetised spheres in a vacuum chamber to show that the energy in the process comes from beams emitted from the sunpots on the surface of the Sun.

Kristian Birkeland used magnetised spheres to show that the energy emitted by the aurora comes from the Sun

Eventually, we figured out how it works, mainly thanks to the work of Carl Størmer — a Norwegian mathematician who dedicated his life to the phenomenon, pumping out more than forty-eight scientific papers on the topic. These papers were collected together into a book called The Polar Aurora, published two years before his death in 1957, which remains a standard reference book on the topic today.

It all starts with the Sun — the sheer power of the natural nuclear fusion reactor at its core means that our star constantly spews out charged particles (called ‘ions’) that carry lots of energy — a phenomenon known as the ‘solar wind’. Occasionally the Sun belches out a lot of particles at once, known as a ‘coronal mass ejection’, which creates a ‘solar storm’ when it arrives at the Earth.

The Earth’s core, on the other hand, contains a lot of molten iron. The motions of this iron in the outer core (known as the geodynamo) turns the entire planet into a huge magnet. As well as making compasses point north, this also traps the charged particles from the Sun that pass nearby, accelerating them towards the polar regions of our planet.

When they meet our atmosphere, the aurora is born. The charged particles crash into gases about 100km above the surface of the Earth, transferring their energy and putting them into a unstable state that physicists call ‘excited’. As they calm down again, they release photons of light in different colours depending on the gas and its height in the atmosphere. It’s worth noting that this is the exact same process that happens in neon lights.

Jan Curtis

At the highest altitudes, red dominates. However, there is a low concentration of atoms at this height and human eyes aren’t as sensitive to the colour red, so we only tend to see it during very intense solar activity.

Green light, from oxygen at lower altitudes, is the most common aurora for two reasons. The first is that there’s a lot of atomic oxygen in the atmosphere at this height. The second is that green lies in the very centre of the visible spectrum, making human eyes highly sensitive to it.

The third colour we see is blue — lower in the atmosphere still. Here, atomic oxygen isn’t so common and molecular nitrogen takes over instead. This emits both red and blue light, but more weakly than oxygen — meaning that blue and purple aurorae are only seen in the very strongest solar storms.

The area where the aurora can be seen, known as the “auroral oval”, fluctuates on a daily basis depending on the strength of the solar wind. It typically sits in a band between three and six degrees latitude wide, about ten to twenty degrees from the magnetic pole. During solar storm events, however, it can expand to lower latitudes.

An auroral oval surrounds both of the Earth’s magnetic poles

While not being as well-known as the classifications for different types of cloud, there are actually classifications for different types of aurorae.

The Roman Stoic philosopher Seneca the Younger wrote widely on aurorae in the first book of his Naturales Quaestiones. According to his classification system, when the aurorae is circular and “rims a large hole in the sky” it’s known as putei. When it looks like a cask it’s called pithaei. When it’s bearded it’s classified as pogoniae. When it looks like a cypress tree it’s called cyparissae, and when it appears as a yawning chasm in the night sky it’s classified as chasmata.

In 1930, Carl Størmer developed his own classification, referring to arcs, bands, patches, veils and rays as the basic forms of aurora, which can also be split into curls, folds, spirals, drapes, curtains, coronas and black auroras. Phew! It’s not entirely clear what process causes the different shapes to form, but it’s thought to be due to fluctuations in both the solar wind and the Earth’s magnetic field.