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. 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].
Aurora Corona. Credit: Ronnie Robertson, CC BY-SA 2.0
Although, that’s not the whole story! There are two more special types of aurorae, and the first is called the Strong Thermal Emissions Velocity Enhancement, but we’ll call him STEVE. As the name suggests, STEVE is pretty hot, at 3,000ºC. He’s also visible from lower latitudes than the typical aurorae, emitting spectacular purple, and sometimes green, arcs across the sky.
STEVE isn’t all that he seems though, because he’s not one but two phenomena: some sky glow and an aurora. The sky glow side is the striking purple streak which is caused by friction of low-energy charged particles bumping into neutral particles (like a lightbulb heating up); whilst the aurora side is the less common green tinge, caused by the solar wind exciting atoms in the atmosphere. Also, even though STEVE has only recently been discovered, he is more common than you’d think, so if you know what you’re looking for you might even be in with a chance of seeing him! [11][12][13][14].
STEVE at Childs Lake, southern Canada. Credit: NASA Goddard Space Flight Centre
And then there’s the dune aurora. Most aurorae are vertical, pointing towards the ground below; dune aurorae, on the other hand, are horizontal, moving south towards the equator.
The cause, recently confirmed by comparing photography on the ground to satellite data, is an increased density of oxygen due to mesospheric bores: atmospheric waves which become sandwiched in a gap between the mesopause (the boundary between the thermosphere and mesosphere, with the coldest temperatures in Earths atmosphere: -90ºC) and an inversion layer (a place where temperature increases with height, in areas of high pressure), found between 80 and 100km up. This creates the perfect conditions for certain wavelength mesospheric bores to travel horizontally without tapering off, allowing charged particles to interact with the abundant oxygen to produce aurorae.
However, these events are pretty rare, but if you’re in the right place at the right time with the right knowledge, they are certainly a natural wonder to behold [15][16][17][18][19][20].
Images of dune aurorae taken by citizen scientists on 20th January 2016: (a) Aurora, Finland, 17:23UT, (b) Engerdal, Norway, 20:13 UT, (c) Karmøy, Norway, around 20:30 UT, (d) the Isle of Mull, Scotland, at 20:57 UT, (e) Lendalfoot, Scotland, 21:15 UT, (f) Rattray, Scotland, at 21:15 UT (g) shows the directions the aurorae were going (a to f, left to right).
Credit: Jukka Hilska (a), Knut Holmseth (b), Kjetil Vinorum (c), Graeme Whipps (d), Mark Ferrier (e), Barry Whenman (f), Grandin, Palmroth, Kalliokoski, Paxton, Mlynczak
The North-South Divide
Charge particles from the Sun are not only funnelled to the north pole, creating the Aurora Borealis (the Northern Lights), but they’re also funnelled into the south pole, creating the Aurora Australis (the Southern Lights).
These two events, however, aren’t entirely the mirror image we’d expect. This is because the magnetic field produced by the solar wind doesn’t always line up with Earth’s, thereby sometimes favouring either the north or south pole to travel towards [21].
The aurorae don’t just vary between the north and south though; they vary, as you’d expect, with distance from the poles. The further away you are, the higher the Kp index needs to be for you to have a good chance of seeing them.
The Kp index isn’t a bag of nuts, but an index of the disturbances in our magnetosphere by solar wind, helping to predict the aurorae. Kp 0 is little activity (aurorae visible north of Iceland), whilst: Kp 3 is unsettled (visible near Allesund in southern Norway), Kp 5 gives a good chance of aurorae (visible in Orkney), and Kp 9 is an especially large storm, producing strong aurorae (visible in Berlin). However, not everything at the same latitude as Berlin, for example, will get aurorae only at Kp 9, since the aurorae are formed around the geomagnetic poles, which are 10º different to the geographic poles [22].
A great example of when the Aurora Borealis was particularly strong was back in 1859, where they could be observed as far south as Honolulu in Hawaii, at just 21º north of the equator; whilst the most northerly Aurora Australis seen was possibly in 1909, where it was observed in Singapore, just 8º south of the equator (although this is disputed) [23]
And, as the Kp index (and history) shows, the further towards those geomagnetic poles you go, the displays get more common, but only to a point. Instead of aurorae occurring directly at the poles (though they do sometimes), they occur in rings known as auroral zones. The boundaries are dictated by a region between an open field line (which stretches off into Space instead of connecting back at the opposite pole) and a closed field line (which connects back at the opposite pole). This gap is where electrons can be fired down to produce the aurorae we know and love [24].
Aurora from Space. Credit: ESA, CC BY-SA 3.0 IGO
Hopefully, in this new solar cycle, with increasing amounts of solar wind coming to Earth, some more spectacular events will be visible in the coming years, maybe even from Timperley!
By George Abraham, ADAS member
Click here for the previous news article
Click here for the next news article
Click here to see the map of what Kp index is visible from which places
Click here to see images of different types of aurorae which won prizes in the Insight Investment Astronomy Photographer of the Year 2020.
Click here to help out in some citizen science, logging any sightings of aurorae to help build a better picture of the science behind them.
Click here to listen to the aurorae recorded by the Aalto University.
Click here to watch a video of dune aurorae, created by the University of Helsinki
Click here to use widgets to find out if the aurorae have a chance of being visible
Click here to view aurorae live
References
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"Confirmation of an auroral phenomenon discovered by Finns". University of Helsinki. Archived from the original on 8th May 2021.
"Citizen science discovers a new form of the Northern Lights". University of Helsinki. Archived from the original on 8th May 2021.
"Glowing green 'dunes' in the sky mesmerised sky gazers. They turned out to be a new kind of aurora". Space.com. Archived from the original on 8th May 2021.
"The Mesosphere". UCAR Centre for Science Education. Archived from the original on 8th May 2021.
"What is a temperature inversion?" Met Office. Archived from the original on 8th May 2021.
"Here's Why Auroras on Earth Are Different in the North and South". Space.com. Archived from the original on 8th May 2021.
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"What's the Furthest south the Northern Lights have been seen?" Science Focus. Archived from the original on 8th May 2021.
"Why do aurorae appear strongest within rings centred on Earth's poles instead of filled-in circles?" Astronomy Magazine. Archived from the original on 8th May 2021.
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