Secrets of the Aurorae

What are Aurorae?

Named after the Roman goddess of dawn, the aurorae are spectacular formations of light that fill the night sky near the north and south poles (sometimes even visible from Timperley). And some spectacular physics is at play here to bring us these wonders of the night sky [1].

It all starts with the Sun, which ejects floods of plasma (charged gas), composed of mostly hydrogen gas which has been torn apart into its constituent protons (positive) and electrons (negative). Known as solar wind, it travels at hundreds of kilometres a second (500 million particles passing a fingertip sized point per second) until some of it reaches Earth’s magnetosphere (its magnetic field).

However, this plasma has a secret up its sleeve: the Sun’s magnetic field. This field is then the key into the Earth’s protective magnetosphere: if it has the opposite charge to the magnetic field of the plasma at that point, magnetic reconnection sometimes happens. This is where the two magnetic fields join forces, funnelling the solar wind down a narrow channel.

The electrons within the plasma are then shot down along the magnetic field lines towards the poles (known as precipitation). During their transit, they collide with various atmospheric molecules such as oxygen and nitrogen (the two most abundant gases in the atmosphere), exciting some electrons within the molecules (this is where electrons jump further away from their atom, since they have more energy).

Then, these electrons shed this excess energy to get back to a more stable position in the atoms which make up the nitrogen and oxygen molecules. As a result, light of characteristic wavelengths is emitted. The most common, green, is due to oxygen; whilst violet, blue and pink aurorae are due to nitrogen (found closer to sea level). However, vibrant red colours are the rarest, produced by oxygen high up in the atmosphere (only emitted when the aurorae are particularly strong, because of increased solar activity) [2][3][4].

Aurora Borealis in Lapland

Aurora Borealis in Lapland. Credit: Well Lucio, CC BY-ND 2.0

A Clap of… Aurorae?

The complex process has many quirks, not least of which is its sound. Research at Aalto University in Finland in 2012 looked into the indigenous Sami people’s claim that the aurorae say ‘klip-klap’.

Although they couldn’t prove their claim, when microphones were placed in areas where aurorae are regularly seen, a faint ‘clap-crackle’ was audible (although the sound had to be amplified a lot) and found to be 70 metres from the ground.

The cause was found to be large bursts of solar wind which hit charged particles that were trapped in a part of the atmosphere created when it’s a cold night. These particles then discharge and create the ‘clap’ sound. That said, there may be other things at play, since there are a wide variety of sounds created by the aurorae [5][6][7][8][9].

Aurora Borealis from Finland. Credit: Paul Williams, CC BY-ND 2.0

Different Forms

Aurorae come in many shapes and sizes, but fit (mostly) into two distinct categories: discrete (an arcing aurora) and diffuse (a pulsating and patchy aurora); both caused by different processes.

Discrete aurorae are mostly due to electrons accelerated into the atmosphere by the morphing of Earth’s magnetosphere by the solar wind, whilst their discrete counterparts are formed because of the scattering of electrons due interactions with waves of charged particles from the solar wind.

Discrete Aurora Arc over Lake McDonald, north west USA. Credit: NPS/Jacob W. Frank

However, these two types can take on many different forms due to the different angles the viewer can look at the aurora from: bursts, arcs, rays, patches, twists, curtains, bands and coronae can all be viewed from just two main types of aurorae [10].