Aurora Borealis Colors Explained: The colors of the aurora are probably what make most of its charm. When people think about the aurora, they usually imagine green curtains dancing across the sky. In reality the aurora harbors many more colors!
The colors of the aurora are created when charged particles (electrons) from the solar wind bump into the different entities of our upper atmosphere. These collisions transfer a certain amount of energy to the entities that gets them to an excited state. They radiate back out this energy in the form of a photon of light to come back to a more stable energy state.
The color of the aurora is determined by several factors:
The entity present:much like a neon tube glows red when its gas atoms are excited by electricity, each atom or molecule of the upper atmosphere produces a glow with a very characteristic color (wavelength) when they’re hit by solar particles. These specific colors can be viewed in an emission band spectrum measured by a spectrometer. Our eyes interpret the wavelengths as different colors.
The altitude:Firstly the altitude determines what sort of entities are present in the air mixture. Secondly the altitude plays a key role in allowing the entities of the atmosphere to glow. It’s all related to itsdensity(number of entities per unit volume of atmosphere). As a rule the density decreases as we get further away from the Earth’s surface. This also applies for the part of the atmosphere where the aurora is created from ~80 km (mesosphere) to ~500km (exosphere). Even at 80km of altitude the air is already very thin and you wouldn’t even be able to breathe. Above 300km the density of the air is so low that the entities are too far apart from one another and rarely even bump into each other anymore! Believe it or not this has an impact on the colors. Certain entities take more time to emit the photon of light once hit by an electron. If another entity collides with the former before it has time to glow, it steals its energy and no glow is created! So the denser it is, shorter time the entities have to produce the glow.
The solar particles themselves. Electrons are channeled around magnetic field lines creating a current. The more energy and speed they have, the deeper they penetrate down into the atmosphere and the brighter the resulting aurora will be. Consequently the brighter the glow is the more detectable it is by our eyes! By reaching other layers the particles also meet other entities that will produce different colors. However these more energetic currents tend to hold fewer particles than less energetic ones (small torrent versus big river analogy). Less energetic currents usually remain at high altitudes and cannot punch through very deep but harbor many more particles, allowing certain entities to glow.
What are the entities at play?
The main entities responsible for the aurora are monoatomic oxygen (O), diatomic nitrogen (N2) and ionized diatomic nitrogen (N2+). Oxygen emits either a red-orange or a lemon-green glow. N2emits a deep red glow while its ionized form emits a deep blue/purple glow. As a rule the colors are set and never change but the three parameters stated above make the colors vary. Sometimes the color limits are quite defined but most of the time the entities mix with currents at different altitudes and create other colors like pink, yellow, emerald-blue, magenta and purple. During periods of intense geomagnetic storms you can potentially get a lot of different colors at once.
It’s also to be noted that there are other entities than O, N2 and N2+ at these altitudes: Hydrogen, NO, Helium…etc. Nevertheless their interaction with solar particles doesn’t produce an auroral glow.
In the figure above you can see how the main colors(pink, green, red) are created. To make it simpler we just showed the collision of one particle with an oxygen atom at high altitude producing a red glow.
Pink is usually the brightest and most vibrant color overall found at the base of the aurora. It occurs only when the auroral activity is very high, typically during substorms displaying very bright bands at high geomagnetic latitudes. When the solar particles have great speed and energy, they can penetrate the furthest down into the atmosphere up to the mesosphere (~80km of altitude). There they find bigger molecules to collide with like N2andN2+. At this altitude the air density is the highest in comparison to higher altitudes. Therefore the particles are closer to one another and collide much more. Consequently only the entities with an extremely short photon-emitting time, namely nitrogen, can glow before other entities ‘steal’ this surplus energy.
Only a few number of particles make it past 100km of altitude but they do it with great energy and speed. This usually results in a brighter and more vibrant glow than any other color.
Once hit by energetic electrons, ionized nitrogen molecules (N2+) emit photons which wavelengths are in thedeep-blue/purple(450-400nm).
Since N2+is ionized there are lower energy electrons being shot out as well, allowing stable gaseous N2to get excited and emit photons, which wavelengths are in the active red/deep-red(650-680nm).Our eyes and cameras do the additive color synthesis and interpret this mix of colors as a brightPINK.
Between ~250 and ~100 km altitude above the Earth’s surface, only solar particles with moderate speed and energy reach down. It’s a zone where lots of entities mix but the particular density generally allows only one species to dominate: monoatomic oxygen.
Why not diatomic oxygen (O2)? At these altitudes strong UV radiations from the Sun tend to split O2into two atoms of oxygen during the day. Therefore oxygen exists in its monoatomic form as well.
Solar particles firstly hit diatomic nitrogen. The high energy level of the particles does two things. It not only ionizes N2into N2+but also causes a lower wavelength light bluephoton as a side effect. The ionization makes N2lose an electron! This electron shoots out and bump into a nearby oxygen atom with an energy level enough to excite them. To de-excite, the oxygen atom dissipates out a photon of visible light with a wavelength of about 550nm, which corresponds to lemon green. In turn our eyes and cameras do the additive color synthesis and interpret this mix of colors as a lime green.
This chain reaction is very ubiquitous. It happens even during low solar input. This alone makes green the most frequent color. It’s extremely rare to have an aurora without green. Green is the color people usually have in mind when they think of aurora.
Above ~250km of altitude, the entities are extremely isolated and scares. The general density of the atmosphere is so low that particles rarely bump into each other anymore. We mostly find monoatomic entities including monoatomic oxygen. Nitrogen is generally too heavy to exist by itself further up. Because of that, oxygen does not get bombarded by nitrogen electrons but rather by solar wind ones. In turn that produces another color.
Solar particles with a bit less energy hit monoatomic oxygen and excite it. To de-excite the atom produces a photon of light in the wavelength oforange–red(~630nm). During normal auroral activity the red glow is much fainter than green or pink. It’s oftentimes very hard to observe the canopy if the observer stands right under the aurora. The bright green and pink have a tendency to outshine it. The best way to observe it is to be located away from the display or above it! As this photo taken from the ISS demonstrates, this is what the red canopy looks like during moderate auroral activity, as seen here on the left. Notice how the red glow is much dimmer and in much smaller proportion compared to the green glow below. Because of this property and our eyes being bad at picking up colors in the red channel, the color of the canopy can almost never be seen with the naked eye.
During exceptionally intense geomagnetic storms, the number of particles bumping into high oxygen atoms can allow a much more intense red glow. Sometimes the glow is so intense and diffuse that the whole sky turns red. This is generally intense enough for your eyes to pick up the color. When this happens, many people who witness the phenomenon think something evil is happening or even call the emergencies claiming they’re witnessing something large burning on the horizon!
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