Rumi, a 13th century Persian poet, once said, ‘behind every atom of this world, hides an infinite universe’, and it is perhaps this phrase that captures ever so romantically the true essence of the auroras. A grandiose phenomenon that ultimately reduces to the behaviour of individual particles, the auroras start far away at the sun and travel all the way to the earth and into its atmosphere. The physics behind the auroras can be likened to the farm to fork journey of food, our sun the farm and our sky the plate on which we ultimately experience the lights. Humans have been observing and studying both the aurora borealis and the aurora australis for hundreds of years, and while the simplest explanation of their mechanics remains unchanged, new knowledge about the auroras has surfaced in recent decades, such as that of the Alfven waves. Through solar winds and storms, the earth’s magnetosphere, Alfvén waves, and plasma properties, this article will navigate and describe the physics of the Northern Lights, so the next time you look up at the auroras you see that universe that hides behind them.
The auroras start 147 million kilometres away, at our sun, from where a solar wind blows. A solar wind is created by the outward expansion of plasma. Plasma is usually thought of as a subset of gases but has strong electrostatic interactions, unlike gases. This is because the electrons of a large number of atoms, in plasma, have been stripped away, leaving positively charged nuclei or ions. This plasma, as well as other particles like protons and electrons continue to heat up until they reach about 2 million °F, a point where the Sun’s gravity can no longer hold them in place. They then stream out of the star and into space as a solar wind. This cloud of gas that the Sun ejects is also known as Coronal Mass Ejection (CME), since it usually comes from the Sun’s outer layer, the corona. The contents and radiation levels of the solar wind change over time through its 11-year cycle, a cycle in which its magnetic field completely flips, i.e. in 11 years the sun’s north pole and south pole switch places; it takes another 11 years for them to switch back. While the contents may vary, it is important to note that since plasma can trap magnetic fields, solar winds also carry the sun’s magnetism.
It is when this solar wind encounters the Earth’s magnetic field that particles are able to penetrate our planet’s atmosphere and so interact with atoms there to create the lights. Due to its iron and nickel centre, the earth has a magnetic field surrounding it. The solar winds first encounter the field on the sunward side, where there is an initial shock wave, the bow shock, that slows the stream down considerably before the particles are guided around the magnetic field to the opposite side of the Earth (the night side). Here, an interaction between the Sun’s magnetism in the plasma and the Earth’s own field creates a tail that extends millions of kilometres away. Visually, it is similar to the tail of a comet, and the magnetosphere is the region of this tail that the Earth’s magnetism dominates. The magnetotail is largely unstable and soon is stretched enough that it breaks like an elastic band, producing two regions: one controlled by the Earth’s magnetism and one that recollects back to form solar wind and moves away from the earth. The particles in Earth’s dominance now accelerate toward the earth and spiral along the field lines toward the poles where they penetrate the upper atmosphere and are now primed to form the aurora.
An important question about the auroras that remained answered until June 7th 2021 was how the particles gained enough velocity to hit the Earth’s atmosphere with a large force. The theory of Alfvén waves explains this phenomenon by proving that the particles ‘surf’ on waves whose energy is transferred to accelerated particles. Quiescent until activity in the magnetosphere with the magnetic reconnection, Alfven waves play a major role only after the unstable magnetotail breaks. These waves are produced by shaking magnetic fields such as during the magnetic reconnection when the solar wind encounters the Earth’s magnetic field, and they carry electrons surfing on them downward into Earth’s atmosphere. As of 2021, research groups in the University of Iowa and University of California, Los Angeles (UCLA) have been able to simulate and model the phenomenon to produce direct proof that Alfven waves can indeed produce the accelerated particles that lead to the aurora, a breakthrough in the theory of the northern and southern lights.
Credit: Austin Montelius, University of Iowa
The last step in creating the auroras after the Alfven waves is exciting atoms, an occurrence largely dependent on the position of its electrons. This happens when these charged particles in the plasma hit the atoms in our atmosphere. Each electron has a ground state that is the energy level it occupies normally and is the lowest energy state it can have. It also has a maximum energy level that it can occupy and still remain a part of the atom; if it moves further than this maximum, the electron is no longer a part of the atom and the atom is said to be ionized. When an atom is excited, the electrons move to higher energy orbits, further away from the nucleus, between the ground state and the maximum level. However, the electrons do not remain excited for long; they return to their original, ground, state and emit a photon (particle of light) in the process. As photons are emitted we see the auroras.
Auroras come in different shapes and colours. The colours depend on the gas that is being excited as well as the altitude of the collision. Oxygen and nitrogen are the most common gases in the atmosphere so most collisions happen with atoms of these two gases. At high altitudes in the atmosphere (approximately 300 km), collisions with oxygen produce a rare red aurora. Otherwise, oxygen usually emits the green and yellow auroras at lower altitudes (100k – 300km). Nitrogen collisions give off a characteristic red, violet or blue; reds are produced above 240km, violet and purple above 100km and blue between 80 and 100km. Researchers today recognize two main aural shapes: the arc, and the pulsating aurora, which are created by slight differences in the physical process as particles leave the magnetosphere and approach Earth.
Discussing why auroras are most common at the poles, we revert to the Earth’s magnetism for an explanation. Since these particles are guided by or travel along the Earth’s magnetic field lines, they move from toward magnetic south and magnetic north which happen to be near the geographical north and south as well. Since the field lines do not enter or leave the equator, not enough particles reach equatorial countries for there to be a visible aurora. Therefore, places like Alaska, Canada, Greenland, Iceland, and Scandinavia in the North and Antarctica, Australia, New Zealand, and Tasmania in the South host some of the most beautiful lights. Lastly, auroras can be chaotic and unpredictable because they are formed from plasma, whose unstable nature describes the erratic movements of the northern light, and even today scientists continue investigating the movements of the northern and southern lights to better understand how the Earth’s magnetosphere and atmosphere interact.
From a plasma stream that turns into the Earth’s magnetosphere to particles surfing on Alfven waves to finally excited atoms, the aurora borealis and australis not only offer a memorable sensory experience, but also display fascinating physical concepts that lead to their formation. To think that when we see the lights, we see the work of the Sun thousands of kilometres away and its interactions with the Earth’s magnetism makes me believe in Rumi’s quote that fits, with frightening precision, the phenomenon of the auroras.
Newtonics 