How the Auroras Form: An Explanation in Plain English

Ancient folklore from China and Europe describes auroras as great dragons or serpents in the skies. The Cree called them “dance of the spirits.” In Scandinavia, Iceland, and Greenland, an aurora was often seen as the great bridge Bifrost, the burning archway by which the gods traveled from heaven to Earth (NASA). The aurora borealis and aurora australis, popularly known as the northern and southern lights, are a strikingly beautiful spectacle that occurs mostly at the northern and southern poles. In fact, they are so intense that many would think that images of the phenomenon are computer-generated or photoshopped images (Wikipedia has some extraordinary videos of the auroras which must be seen to be believed). The next question is, inevitably, how do they form?

The Northern Lights. Photo taken by Yuichi Takasaka.
The Cree called the northern lights “dance of the spirits.” Photo taken by Yuichi Takasaka.

As with most processes on earth, it all starts with the sun. The sun is so hot that no atoms can survive the heat; inside the sun, there are only charged subparticles. In the outer layer of the sun, called the corona, collisions between many of these charged particles take place. Some of these collisions occur at such high speeds and temperatures that the particles are actually ejected from the sun. This stream of emanating charged particles is called the solar wind, and it is constantly blowing in space. The particles speed at velocities as high as 900 kilometers per second – they would zip past a region of space the size of the earth in 14 seconds! And when solar wind plasma approaches earth, its temperature approaches 150,000° K. All objects that surround the sun are unceasingly bombarded with protons and electrons from the solar wind.

If this deadly solar wind were to reach earth’s surface, the outcome would be detrimental. We would be exposed to 1000 times the radiation we are exposed to on earth: besides swiftly killing out all life, this could even damage our DNA.  There are also some strange phenomena that occur when dealing with these particles. According to Futurism, “While on lunar missions, astronauts often reported seeing flashes of light when they closed their eyes – the direct result of cosmic rays passing through their retinas.  A few even developed cataracts years later.”

Earth’s magnetic field. Photo from Wikipedia Commons.

However, the solar wind does not reach the earth because the earth is protected by a magnetic field. Where does the magnetic field come from? The earth’s interior is composed of different metals. Although scientists are not certain about the exact composition, is it probably a mixture of mostly iron and nickel, with some sulfur. As a result, the earth is essentially a giant magnet; the North Pole is the magnetic south pole of the magnet, and the South Pole is the magnetic north pole. (This is why a compass always points north. The needle is a magnetized piece of metal, which aligns itself with the local magnetic field.) Because the earth constantly rotates, this causes a magnetic field to form around earth.

The earth’s magnetic field deflects the flow of the solar wind, and solar particles are diverted along the magnetic field lines. However, because the magnetic field bends towards and meets at the poles (see the above picture), the solar particles bend along the magnetic field and follow it towards the earth at the poles. When the particles come in contact with the outer layer of the atmosphere, called the thermosphere, interesting results occur.

The electrons and protons of the solar wind (mostly hydrogen and helium), which are charged, come in contact with the atmospheric molecules. These molecules absorb the solar wind subparticles and become energized. The particles are then released as light, creating the striking aurora lights.

The earth’s magnetic field diverts the solar wind to the poles. Image obviously not to scale. Photo from KJMagnetics.

The above image, from KJMagnetics, colorfully illustrates this process. The solar wind comes from the sun and is diverted along the earth’s magnetic field. It heads towards the earth at the poles, where it is converted into light as it comes in contact with atmospheric particles.

The auroras are a striking byproduct of the chemical processes that ensure our survival on earth. Albert Laighton wrote that they are “The flaming banner of the North unfurled / The mystery that dares a boasting world!” It is the beauty of these magnificent lights that once again proves that no matter how much we humans test the frontiers of “beauty,” nature beat us to it – and wins spectacularly every time.

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