Northern Lights: they light up the sky, let's find out why!
- Emily
- Feb 14, 2022
- 6 min read
You’ve probably seen the photos of or even been lucky enough to see those beautiful natural light displays in the sky, the aurora borealis/australis, known colloquially as the northern/southern lights. But why do they occur and why are they concentrated at the north and south poles of the Earth?
First Auroral Sightings
The first recorded sighting of an auroral display on Earth was on 12/13th March 567 BC under King Nebuchanezza II’s rule of Babylonia. The official astronomers recorded their celestial sightings by etching onto clay tablets. One of the clay tablets contains records of a red glow in the sky on the 12/13th March 567 BC, which has now been interpreted as an auroral display. It wasn’t until the early 17th century that these displays of cascading light were named. The aurora borealis was given its name by Galileo Galilei, an Italian astronomer and scientist. ‘Aurora’ deriving from the Roman goddess of dawn and ‘boreas’ from the Greek word for the north wind. Despite the name still being used today, Galileo’s theories that they were caused by sunlight reflecting off the Earth’s atmosphere has since been disproven. The aurora australis was first proposed to exist by de Mairan (Europe) in 1733, and later confirmed and named by Captain James Cook from his expedition to Antarctica in 1770. The word ‘australis’ coming from the Latin word for south.
The auroral displays are concentrated round the magnetic poles of the Earth, meaning the chances of seeing them increase as you get closer to each pole. The aurora australis, southern lights, are less popular than the aurora borealis as there is less land surrounding the south pole, so people are less likely to witness them. They can still be witnessed from high southern latitudes in Australia, New Zealand, Chile, and Argentina. The northern lights are the most commonly photographed as their auroral zone stretches across Greenland, Iceland, North Canada, Norway, and Russia, with occasional sightings as far down as north Scotland, UK.
Solar Activity
The aurorae are caused by explosions occurring on the sun’s surface, known as solar flares. Our sun is a massive ball of burning gases, mainly hydrogen and helium, which is continuously active. The corona is the outermost layer of the sun and is made up of plasma, hot ionised gases, that averages temperatures as high as a few million degrees Celsius. British astronomer, Richard C. Carrington, first proposed the ejection of high energy particles (ions and electrons) from the surface of the sun towards Earth, known as a coronal mass ejection (CME). He witnessed a super solar flare back in 1859, which flung a highly charged CME towards Earth. This event is now referred to as the Carrington event. When this CME reached the Earth’s magnetic field it started to interact with it, leading to an increase in magnetic energy and electrical current – a geomagnetic storm. Intense auroral displays are caused by geomagnetic storms, where “a billion tons of material are lifted off the sun’s surface and accelerated to speeds of a million miles per hour” towards Earth. [C. Crockett] Auroral displays frequently occur during a geomagnetic storm or substorm but can also be caused by energised particles being carried by the solar wind. In fact, auroral displays don’t just happen on Earth. Any planet, namely Jupiter and Saturn, with a dense enough atmosphere that lies within the path of a solar wind can exhibit auroral activity.
“A billion tons of material are lifted off the sun’s surface and accelerated to speeds of a million miles per hour [towards Earth]." - Christopher Crockett
The interaction that a geomagnetic storm has with the Earth’s magnetic field and the increase in atmospheric density has recently caused issues with satellites. Elon Musk’s latest launch of Starlink satellites has seen the loss of 40 satellites caused by a geomagnetic storm that occurred on the 4th February this year. The 49 satellites were released into a low orbit (~ 200 -240 km above Earth) and were due to steadily increase to a higher orbit (~ 550 km) over several weeks. The geomagnetic storm increased the density of atmospheric particles which increased the atmospheric drag the satellites experienced. This caused the satellites to change their orbital paths and 40 of them have started to head back into the Earth’s atmosphere where they will not survive. While Elon Musk is paying the price for underestimating a geomagnetic storm, let’s discover the science behind why these storms cause such beautiful light displays.
The Colourful Science
CMEs frequently travel towards the Earth’s atmosphere. If the particles in the CME gain enough energy they can fall into the Earth’s magnetic field, where they get accelerated to the north and south poles. These energetic particles collide with the atoms and molecules, mainly oxygen and nitrogen, in the Earth’s atmosphere. Upon collision the oxygen and nitrogen molecules become excited. Excitation is where an atom or molecule absorbs enough energy so that one of their ground state electrons (lowest energy) gets promoted to a higher energy state. This excited electron then relaxes back to the original ground state releasing energy in the form of photons (light energy), so light is emitted. This explains the intense colours that we see during an auroral display. Green is the most common colour in the aurora borealis, coming from collisions with oxygen in the atmosphere. Despite nitrogen being the more abundant atmospheric element, oxygen requires less energy to excite so the green light is produced more frequently. Violet can occasionally be seen during a very active auroral display, this colour is caused by collisions with nitrogen. Very rarely streaks of scarlet red can be seen, these are caused by collisions with high altitude oxygen atoms which emit lower energy light (red). Auroral displays can come down as close as just 80 miles above Earth, but often stretch thousands of miles above that. The intensity of the display is dependent on how close the collisions are occurring to Earth and the power of the solar activity.
The wave-like patterns that intense aurorae form are down to the particles following the lines of the magnetic force field in the Earth’s atmosphere. Jim Schroeder, a plasma physicist, performed experiments using a Large Plasma Device (LPD) that has confirmed that Alfvén waves are responsible for the acceleration of electrons during in auroral display. Alfvén waves are disturbances that occur along a magnetic field line caused by the solar activity, they oscillate perpendicular to the field lines – “like waves on a string, launched by shaking the string”. So, when electrons produced from a CME enter the Earth’s atmosphere, they start to “surf” the Alfvén waves, they collide with atmospheric atoms along these waves and produce the incredible moving waves of light we see above us.
Frequency of Aurorae
There are two types of geomagnetic storm: recurrent and non-recurrent, these storms line up with the sun. The sun’s cycle is approximately 11 years in length, starting at the minimum solar activity, peaking at the maximum then returning to the minimum. Recurrent geomagnetic storms tend to occur every 27 days at the sun’s minimum, whereas non-recurrent geomagnetic storms happen frequently in the sun’s maximum. Auroral activity appears when these geomagnetic storms arise, the frequency of which increases with increasing solar activity, and can sometimes be seen much lower than the typical auroral zones. Substorms also induce auroral activity, taking place on average six times a day (much more common than geomagnetic storms) but only visible in the auroral zones surrounding the Earth’s poles. Auroral displays are often visible between November and February as the sky is at its darkest and the evenings are longer. So, for optimum aurorae viewing, you want to be in the auroral zone (as far north as you can get), within the winter months between 9pm and 2am and with cloudless skies.
2025 is expected to be the next sun’s maximum, so we should expect to see more frequent auroral activity during that year. The geomagnetic storms are expected to be some of the biggest we’ve seen during the next maximum, and there are concerns about the intensity and how it may affect the space weather. AKA book your holidays to Iceland and Northern Canada and Elon don’t try to release any satellites in 2025!
Written by Emily Cuffin-Munday
Blog references:
F. R. Stephenson, D. M. Willis, T. J. Hallinan, [https://doi.org/10.1046/j.1468-4004.2003.45615.x]
C. Crockett, Earth Sky [https://earthsky.org/space/what-are-coronal-mass-ejections/]
J. O’Callaghan, MIT Technology Review, [https://www.technologyreview.com/2022/02/10/1045202/spacex-just-lost-40-satellites-to-a-geomagnetic-storm-there-could-be-worse-to-come/#:~:text=An%20eruption%20of%20plasma%20from,the%20density%20of%20its%20atmosphere.&text=This%20phenomenon%2C%20known%20as%20atmospheric,out%20of%20their%20orbital%20paths]
T. E. Bell, T. Phillips, NASA Science [https://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare]
NASA, Space Technology 5 [https://www.jpl.nasa.gov/nmp/st5/SCIENCE/storms.html]
J. Schroeder, Nature: Astronomy Community [https://astronomycommunity.nature.com/posts/electrons-surfing-on-alfven-waves]
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