a great cloud, and a fire infolding itself, and a brightness
was about it, and out of the midst thereof as the colour of
amber, out of the midst of the sky.
Ezekiel I. iv. (KJV)
According to Norwegian folklore, the flashing curtains of pale green light seen across the night sky were the spirits of old maids dancing and waving, or the reflections of the shields of the Valkyries as they made their way to Valhalla. The Finns thought they were caused by a magical fox sweeping his tail across the snow spraying it up into the sky. Across Greenland, Canada and Alaska, native tribes believed the lights represented the spirits of the dead. Further south, where the lights are typically shades of pink, the Greeks took them to be portents of war and sickness. Cave paintings in Rouffinac, France show lines and arcs drawn with the fingers of stone-age hunters who lived about 10,000 years ago. For thousands of years, fantastical stories, steeped in spirituality and deep meaning, have been told of the aurora borealis.
Video produced by Dr. Claus & Anneliese Possberg
It wasn’t until the turn of the nineteenth century, and the pioneering work of the great Norwegian physicist, Kristian Birkeland, that we began to develop a scientific explanation of what are commonly known as the Northen Lights. Interested in electromagnetic phenomena, between 1896 and 1913, Birkeland pursued a series of gas-discharge experiments in his laboratory at the University of Christiania (modern-day Oslo) which involved firing electrons at a terella, a small magnetised ball suspended in a vacuum tank, representing the Earth. He found that the electrons produced a glow in the regions around the poles of the terella, which mapped closely onto measurements of global patterns of electric currents and the magnetic field patterns he had observed in the arctic.
He identified the Northern Lights as electromagnetic phenomena by basically recreating them on a small scale in the lab. He proposed that the source of the electrons which caused the aurora borealis were bursts of solar activity as these were commonly associated with auroral activity. Despite how well Birkeland’s theory seemed to explain the origin of the lights, his ideas were dismissed for nearly 70 years as ground measurements alone were not considered strong enough evidence by the rest of the geophysics community. Birkeland was finally vindicated in 1967 when a US navy satellite collected data showing magnetic field disturbances in locations and patterns just as he had predicted.
We now have a detailed explanation of the origins of the Northern Lights. Solar winds constantly batter the earth with ions (charged particles) at around 400 metres per second (or faster during coronal mass ejections, when the sun spews massive quanities of matter and electromagnetic radiation into space). These particles get trapped by the earth’s magnetic fields, where they travel along a set of currents, named Birkeland currents in honour of the maligned Norwegian, which flow along geomagnetic field lines connecting the Earth’s magnetosphere to the Earth’s high latitude ionosphere. As they accelerate towards the Earth, the ions collide with atmospheric gasses; nitrogen atoms become ionized (gain an electron) and both nitrogen and oxygen turn from what’s ‘excited’ state into a ‘ground’ one. These collisions results in the release of photons (particles which transmit light), causing the beautiful lights that we see.
Variations in the colour of the aurora are due to the type of gas particles that the ions collide with. The common pale yellow-green colour is produced by oxygen molecules located about 90km above the earth. The oxygen at higher altitudes (around 300km) is responsible for the rare all-red auroras, whilst it’s nitrogen that causes the blue-violet lights. In most instances, the aurora borealis, and its southern counterpart, the aurora australis, are mirror-like images that occur at the same time, with similar shapes and colours, but southern auroras are a rarer sight as they are concentrated in a ring around Antarctica and the southern Indian Ocean.
|The red “ring of fire” around the Earth’s polar regions represents the contribution of the electrically charged oxygen atoms to the aurora borealis and australis. (Source: Wikimedia Commons)|
On extremely rare occasions, and only if conditions are perfect, it might be possible to not just see but hear the aurora.
There have been numerous anecdotal reports of sounds accompanying the lights but it wasn’t until last year that researchers at Aalto University in Finland managed to record the extremely rare crackling and clapping sounds, and definitively correlate it with the presence of the lights. How exactly the lights produce sound, and how they are able to be heard by people on the ground given they are produced as far as 600km away, remains a mystery.
Soaring spirits and dancing maidens have been replaced by the crashing of atoms and the flow magnetic fields; the scientific explanation for this awe-inspiring phenomenon is no less beautiful, and all the more powerful for being true.