Polar lights and colorful physics
Polar lights are one of my favorite phenomena. They are colorful, hard to catch, and still not fully understood! But let’s dive into the science behind them!
We need to start our journey somewhere very hot: the sun. Here convections of charged particles (plasma) in the outer parts of the sun cause strong magnetic fields. Sometimes these fields move outwards creating rings that act like rubber bands that snap, blowing out great amounts of plasma. These events are called solar flares, or when they’re really big: coronal mass ejections. The frequency of such events is linked to the sun's 11-year activity cycle in which our star flips its whole magnetic alignment — basically every 11 years the north pole goes south and vice versa.
As you can see here, scientists have been measuring the sun's activity for over 400 now. Since the intensity of the cycles has been falling over the last four decades, some worry that we may be heading into another Maunder-minimum. This would be unfortunate for the science community and everyone who wants to see some auroras.
As you probably have guessed, these solar flares are part of the so-called solar wind which ultimately causes the polar lights. This constant stream of mostly electrons, protons and some nuclei such as helium moves towards earth at speeds between 400 and 700 km/s (that’s New York to London in 10 seconds!). Now a complicated process of deflecting the particles, magnetic recombination, acceleration, and other crazy stuff starts. The whole process hasn’t been understood fully and many effects (or theories) have been shown to play a role but how they all interact with each other is mostly unknown! I will try to map out the clearest elements of this complicated process.
Coming from the sun the solar wind encounters the bow shock about 12.000 km away from earth. Most low energy particles are diverted by the earth's outermost magnetic fields and move around to the magnetotail. Some particles move into the polar cusp, creating the so-called day-light aurora which we can obviously not see with our eyes.
Other particles will move into one of the two Van Allen radiation belts which are regions where the plasma gets stuck. These belts hold great amounts of plasma and act as a reservoir for the aroura but are also constantly washed away and then refilled by stronger solar flares. The Van Allen belt region can be thought of as a magnetic bottle. These bottles consist of two magnetic mirrors placed together to create a trap for charged particles (see image on the left). When an electron moves in a helical (corkscrew) path along the magnetic field he will eventually approach a pole (the magnetic mirror). Here the magnetic fields become denser thus creating a backward force on the particles (by the Lorenz Force, that thing with the three fingers).
Sometimes particles have enough energy to come spiraling as close as 80km to the surface. This creates an auroral zone as seen in the image below. This auroral zone will move towards the equator as long as the particles have enough energy and then move to the pole again. So once you’ve experienced some strong polar lights moving southward (if you’re on the north hemisphere), then wait for them to come back!
Here comes the physics
Now apart from being wonderful to watch, there is a lot of science behind the colors, shape, and height of the aurora. To understand these aspects, we need to get back to the electrons currently swirling around the poles: when they come down into the atmosphere they start colliding with the Oxygen and Nitrogen molecules which excites them. Exciting molecules means that they are moved to a state of higher energy, think a ball moved to a higher step on the stairs. When these molecules de-excite (fall back to the ground floor), they emit light which is then ultimately what we see as aurora.
The colors of the aurora come from the fact that, in the quantum world, energy levels are discrete. Imagine you could only have a discretized size, i.e. only be 175cm or 180cm but not 177.43cm large, or imagine you could only run integers of km/h — that would be a discretized world. The oxygen molecules, for example, have a “distance” between the main step and the ground floor that corresponds to a light green aurora (557.7 nm). These green curtains which appear at 100–150km (high concentration of Oxygen up there) are the most iconic and frequently occurring aurora. These curtains have a sharp cut at 100km due to a fast concentration drop of oxygen, see on the right. Slightly below the curtains, one may spot some blue due to Nitrogen molecules being the dominant light source there.
Those two colors are considered the discrete aurora while the most common diffuse aurora is a red emission line of Oxygen at high levels of altitude. These can be hard to spot by eye because of the dominant green curtains, but can often be seen on the horizon or in pictures due to the camera's better sensitivity (in comparison to our eyes) to red.
Why do height and density matter? Most of these excitation steps, called spectral lines, are highly improbable because of some complicated quantum mechanics stuff. “Normal” light emissions — which happen all the time — occur on a nanosecond timescale. The red emission of oxygen, however, is very slow (107s) therefore this de-excitation will only occur in very high altitudes above 150 km where the probability of colliding with another molecule is low enough. Otherwise, if excited molecules collide, they will move to a lower energy state without emitting any red light.
To sum things up, there are a handful of variables that go into the aurora: the energy of the electrons from the solar wind determines how far they will move downwards to earth and how far they will move south; the density of Oxygen and Nitrogen in the atmosphere determines how bright that color appears at a height but also determines which colors can occur without colliding with others; and finally, the curious rules of quantum mechanics only allows for a de-excitation at certain wavelengths resulting in the typical light green, red and purple aurora colors.
The effect of solar winds on the earth’s magnetosphere and human digital communication is an important and very active field of study, so check it out!
Useful takeaways:
The next peak of solar activity should be in 2023, so that’s definitely a year to plan some polar vacation. The process of particles like electrons moving from the sun down to our atmosphere is not fully understood. There are indicators and forecasts for polar lights that you can look up online to plan your trip. If you happen to experience strong polar lights moving towards the equator don’t leave! They are probably going to come back to the poles in the next hours.
Hopefully, you are now fully equipped to show off some science knowledge when you standing in the freezing cold looking up to a dark sky. I can only recommend spending some time in the far north and experiencing this breathtaking dance of nature.
And as always, stay curious!
