Aurorae are spectacular light shows in the sky – nature’s equivalent of a fireworks display. If conditions are correct, billowing curtains of green and red can be seen playing across the sky around the arctic and antarctic circles. The aurora borealis is the phenomenon as it is observed in the northern hemisphere, and is more colloquially known as the northern lights. The southern lights, aurora australis are created in an identical manner on the southern hemisphere. In fact, all planets with a magnetic field display some kind of aurora behaviour, and probes have recorded spectacular lights around the polar areas of both Jupiter and Saturn.
The first written record of the aurora borealis are possibly from ancient China, around 2600 B.C. Documents state that the mother of the Yellow Emperor observed ‘big lightning’ circling around the constellation of Bei Don – which we now know as Ursa Major. In the western world, the first reference comes from the Greek trader and explorer Pytheas, in the 4th century BC. It’s said he circumnavigated Britain while exploring routes for tin trading, and also became the first Greek to see the midnight sun.
The name aurora borealis wasn’t coined until the 17th century. Sources are divided as to the originator of the name. Historically, it’s been attributed to the French scientist Pierre Gassendi in 1621. However, research in the last 30 years suggests that Galileo may have used the term a couple of years before. Aurora was the Roman goddess of the dawn, and Boreas was Greek god of the north wind. Although classicists most likely frown upon this combination of two different ancient languages, the name aurora borealis roughly translates as ‘dawn of the north’.
Three physical phenomenon combine to create the aurora borealis:
In space, the solar wind provides the raw energy which drives the northern lights. The solar wind is a high-speed stream of charged particles emanating from the sun in all directions. The strength of these particles would, ordinarily, be sufficient to put all life on earth at significant risk of radiation damage. However, the earth’s magnetic field protects us from harm by deflecting the particles around the planet.
That said, it is also the earth’s magnetic field which, through its interaction with the solar wind, creates the correct conditions to form aurorae. Not all of the particles of the solar wind are deflected away from the earth. Due to the shape of the magnetic field, a small proportion are deflected away from the equator and around the earth towards the poles. At a latitude of between 60 and 72 degrees, the magnetic field funnels and accelerates a small proportion of the particles down towards the earth’s surface, and it is these particles interacting with the atmosphere that creates the lights.
When the charged particles strike oxygen and nitrogen in the atmosphere, these atoms can lose an electron, or enter an excited state. As the oxygen and nitrogen return to their normal state, they emit photons of visible light, forming the patterns we observe from the surface. The exact frequency of the light is dependent not only on whether oxygen or nitrogen is emitting, but also on the energy of the incoming solar particles, and the amount of time before the atoms emit.
Oxygen can emit either green or red light. Green emissions come from lower in the atmosphere. Red emissions take longer for the oxygen to emit. Since the atoms can often lose their ability to emit through collision with other atoms, only oxygen at higher altitude, where the atmosphere is thinner, is able to retain the required energy for long enough to emit red light. Nitrogen emits blue or red light. Blue is the result of an ionized nitrogen particle recapturing an electron, and red is the result of an atom moving from an excited state to the normal state.
Because of the way the earth’s magnetic field funnels particles down into the atmosphere, the lights can generally only be seen in a ring that circles the magnetic north pole. Closer to the pole, the lights are as absent as they are south of the arctic circle. The lights are at their strongest in the northern regions of Russia, Canada and Alaska. However, when solar storms flare up, they can bee seen in much of northern Europe, and the northern states of the USA. In contrast, the aurora australis is rarely seen outside of Antarctica.
Evidence for the electromagnetic nature of the lights was first discovered in the 18th century, when recordings of magnetic fluctuations were correlated with increases in auroral activity. It was discovered later that large electric currents flow through the aurora, and it is this behaviour that explained a remarkable event in 1859. Two telegraph operators in Boston and Portland were able to have a conversation lasting two hours without any external power supply. The electric current they used to talk was generated entirely by the effect of the aurora borealis on the telegraph wires.
Given the spectacular nature of the northern lights, it comes as no surprise that there are numerous mythological explanations for the phenomenon. Many cultures saw them as dancing figures, either human or animal. While some cultures saw the lights as the spirits of the dead, the Sioux turned this concept upside-down – to them, the lights were the spirits of future, yet to be born.
Regardless of the origins of aurorae, they are simply one of the most beautiful natural marvels on the planet. As long as hardy folks are prepared to endure the weather of the arctic circle the aurora borealis will remain an inspiration to travellers, scientists and artists alike.
