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I’ve tried to present here a somewhat basic, yet still comprehensive explanation of the phenomenon commonly known as the northern lights or aurora borealis (aurora australis in the southern hemisphere). We observe the aurora, commonly referred to as the northern lights, as a colorful dancing array of colors in the night sky, frequently seen in the far northern or southern hemispheres. They can be remarkably beautiful and mysterious. I’ve distilled the concept down to its basics but included additional content and links so you can learn as much as you like.
While scientists have gained a lot of understanding of the aurora in recent years, many details about the processes still need investigation. There’s also not a singular process that generates the aurora. Rather, there are a few different Earth-solar wind interactions that give rise to the lights.
- What is the Aurora?
- The altitude of Aurora and the Thermosphere
- Earth’s Geomagnetic Field
- The Solar Wind
- Interaction of Earth’s Geomagnetic Field and the Solar Wind
- Light of Aurora
- References and Further Reading
What is the Aurora?
The aurora is a display of natural lights that occur at high altitudes in the night sky. They are most often seen at high (aurora borealis) or low (aurora australis) latitudes, near the Arctic and Antarctic circles.
The altitude of Aurora and the Thermosphere
The aurora occurs at very high altitude, primarily in a part of the atmosphere known as the thermosphere. The thermosphere is the second-highest layer of Earth’s atmosphere, beginning at approximately 80 km (50 miles) above the surface. This layer extends up to 550 km (342 miles) above the surface.
The density of molecules at this altitude is very low, so there aren’t many collisions between molecules and atoms. The composition of the lower thermosphere is primarily diatomic nitrogen and diatomic oxygen (N2 and O2). In the higher regions, atomic oxygen (O) becomes abundant and is dominant in the uppermost thermosphere. You’ll soon see why this chemical composition is important in how the aurora gets its distinctive colors.
While most of the visible light emitted by the aurora is 90-150 km (55-93 miles) above the Earth’s surface, auroral emission can extend beyond the thermosphere into the exosphere and above an altitude of 1000 km (621 miles). However, this is unlikely to be visible due to the very low density of the atoms and molecules from which the light is emitted.
While technically the thermosphere is a part of the Earth’s atmosphere, we typically think of most of the thermosphere as space. The International Space Station (ISS) orbits at an altitude of approximately 400 km (250 miles). That’s about 150 km (90 miles) from the thermopause, the boundary layer between the thermosphere and the outermost atmospheric layer, the exosphere. In fact, the ISS gets quite a unique view of the aurora!
Earth’s Geomagnetic Field
The Earth’s geomagnetic field is a magnetic field generated in the Earth’s outer, liquid core. A magnetic field exhibits a force that acts on other magnetic fields and on moving electric charges or charged particles. The geomagnetic field is a dipole field, similar to a bar magnet with north and south poles. The magnetic field lines are closed loops in space connecting the north and south poles.
Earth isn’t the only planet with a magnetic field. Earth’s magnetic field is generated from the motion of the electrically conductive molten core. Likewise, other planets that rotate relatively quickly and likely have conductive cores also produce magnetic fields. We have even detected aurora on some other planets that are known to have magnetic fields, like Jupiter and Saturn.
The Earth’s magnetic field creates a “bubble” around the Earth known as the magnetosphere. Moving particles with electric charge can’t cross magnetic field lines, but can only spiral around them. Because of this, ions and electrons from space can’t penetrate the magnetosphere and are instead deflected around it. This effectively protects Earth’s atmosphere and surface from dangerous charged particles from the sun and from outer space.
The Earth’s magnetic field is always changing. It changes in response to internal dynamics within the Earth’s core, typically on very long time scales. The north and south poles drift slowly, about 50 km (30 miles) per year, and even reverse every one hundred thousand to one million years. More related to the aurora, the magnetosphere also changes on much faster timescales in response to the solar wind, our next topic.
The Solar Wind
The driver of the aurora is the solar wind. The solar wind is a stream of charged particles, primarily protons (ionized hydrogen) and free electrons emanating from the Sun’s atmosphere. Both particles carry a charge (protons are positively charged and electrons are negatively charged) and moving charged particles produce a magnetic field. The solar wind is essentially a fluid composed of ions (charged particles), which is a state of matter known as a plasma.
This solar wind is fast. The mean speed at Earth’s orbit is about 400-500 km/sec, that’s about 1 million miles per hour! However, it varies greatly depending on solar activity and typically ranges between 300 km/sec and 800 km/sec. The fastest wind speeds occur from regions of coronal holes (colder areas of the Sun where the solar magnetic field is “open” allowing the plasma to escape) and during coronal mass ejections (fast release of plasma and magnetic field, often accompanied by a solar flare).
Heliospheric Magnetic Field
The solar wind plasma drags out part of the solar magnetic field from the corona of the Sun, filling the solar system with what is referred to as the heliospheric magnetic field (HMF), or interplanetary magnetic field (IMF). The name heliospheric magnetic field is most used today to avoid confusion with the interstellar magnetic field or local interstellar magnetic field, the magnetic field outside of our solar system’s heliosphere. The HMF is “frozen in” to the solar wind as it is unable to diffuse through the plasma. You can think of the two as being magnetically bound to each other.
The Sun, like Earth, has a magnetic dipole with a north and south magnetic field. This magnetic field twists into a spiral shape as the Sun rotates, and the solar wind flows outward into a form known as the Parker Spiral. This spiral “sheet” is called the heliospheric current sheet because a small electric current flows through it (the result of a moving magnetic field). The heliospheric current sheet is the largest structure in the solar system.
The Earth (and other planets) are perpetually dipping in and out of this current sheet where the direction of the polarity, or direction of the Sun’s magnetic field changes.
Why the Solar Wind and HMF Matter
It is both the solar wind and the heliospheric magnetic field that interact with the Earth’s Geomagnetic Field. The charged particles of protons and electrons from the solar wind interact with the atoms and molecules in the thermosphere that create the light of the aurora.
Interaction of Earth’s Geomagnetic Field and the Solar Wind
As mentioned before, charged particles, like those composing the solar wind cannot penetrate Earth’s magnetic field. But, the HMF and the solar wind does impart a force on the Earth’s magnetic field. Because of the pressure exerted on Earth’s geomagnetic field by the HMF and solar wind, the magnetosphere takes on the shape of the popular image of a comet. That is, it is compressed in the “front” and has a long, extended tail (called the magnetotail) on the side away from the sun.
Van Allen Radiation Belts
The field lines of Earth’s magnetic field converge near the poles and the magnetic strength increases greatly where they converge. As a result, charged particles from the solar wind spiraling along the Earth’s magnetic field lines get magnetically “squeezed” back as they approach the pole and are reflected in the opposite direction. This causes them to “bounce” back and forth between the poles and the charged particles are trapped in the magnetic field. These regions of trapped, very high energy particles are called the Van Allen Radiation Belts.
There are two toroidal (donut-shaped) Van Allen Belts, and outer belt mostly filled with electrons, and an inner belt mostly full of protons. The inner belt is relatively stable compared to the outer belt, which shrinks and expands. It is the outer belt that is most influenced by solar wind speed and geomagnetic storms. It changes both in spatial extent as well as composition from solar storms and changing solar wind speed or density. For instance, long periods of slow solar wind speeds (less than 300 km/s) can leave it nearly drained of high energy electrons. In contrast, high wind speeds or disruptions by solar storms can significantly increase the number of electrons within the belt.
It is largely the radiation in the outer belt that imparts the energy on the atmosphere that causes the aurora. This can happen in a few different ways, some of which are beyond the scope of this article. One way this happens is the high energy electrons and protons precipitate (leak out of the Van Allen Belts) of the high energy into the atmosphere. This would be the case when the solar wind is “quiet”. Also, some of the solar wind does manage to diffuse through the magnetosphere. Although, with such low counts of high energy particles, not much light would be generated as we see during some of the great auroral displays.
Another way these particles enter the atmosphere happens during solar storms and generates much more dramatic aurora displays. This would be the case when there is an Earth-directed Coronal Mass Ejection (CME – often associated with solar flares). In this case, when the magnetic field from the CME comes into contact with and is opposite Earth’s geomagnetic field, the magnetic fields combine and “break off” in a process known as magnetic reconnection.
When magnetic reconnection occurs on the Sun side of the geomagnetic field, the field is swept off into the magnetotail (see diagram below). Inward pressure on the magnetotail causes reconnection in the interior of that region, pinching off a piece of the tail and forcing the remaining section to “snap” back toward Earth. The process accelerates the high energy particles (mainly electrons) in the Van Allen belts even more toward the poles, entering the atmosphere.
Now we know where the energy comes from to drive the aurora, but there is still the question of how these energetic particles cause light when they enter the thermosphere.
Light of Aurora
The light of the aurora that we see is directly tied to the atoms or molecules in the thermosphere and their possible energy levels. When the energetic particles (mainly electrons) enter the atmosphere after magnetic reconnection, they collide with atoms and molecules in the atmosphere, imparting some of their energy on them. But, those atoms and molecules don’t hold on to that energy for long, instead they radiate that energy off in the form of light.
Atoms and Molecules Involved
The two atoms involved in causing the light from aurora are oxygen and nitrogen. Specifically, the colors we see are due to photon emission from monatomic oxygen (O), monatomic nitrogen (N), diatomic nitrogen (N2), diatomic oxygen (O2), and ionized diatomic nitrogen (N2+).
Recall from the section on the Altitude of Aurora and the Thermosphere that in the lower reaches of the Thermosphere the composition is primarily N2 and O2, although the atomic oxygen level is still high in the lower Thermosphere. Monatomic oxygen (O) dominates in the upper region. Most of the visible aurora is from atomic oxygen (O) and molecular nitrogen (N2)
Discrete Energy States
Atoms and molecules behave strangely compared to what we experience daily. The nucleus or core of an atom is composed of protons and neutrons. “Orbiting” the nucleus is another particle called an electron. I put “orbiting” in quotations because that’s not an exact representation of what is happening, but it is a representation that can be visualized easily. To go further, we need to delve into the complicated subject of quantum mechanics, which is also beyond the scope of this writing.
These electrons can’t just “orbit” the nucleus at any old place they choose. They can only orbit at a number of precise locations that depend on the type of the atom (helium, oxygen, nitrogen, etc.). These precise locations are called orbitals. Each orbital can only contain two electrons (due to the Pauli exclusion principle), so once an orbital is full, any additional electrons must be in “higher” orbitals.
When all of the lowest orbitals of an atom are full of electrons, and there are no electrons in “higher” orbitals, an atom is said to be at its ground energy state. This ground state is the most stable for an atom or molecule, which is why they quickly return to that state after being “hit” into a higher energy (like from a high energy electron).
When atoms and molecules in the thermosphere are hit by these highly energetic electrons, the atom or molecule jumps into a higher energy state. These can’t be any energy, but must be the exact energy corresponding to the next electron orbital energy. In other words, each higher orbital has a precise energy difference from the ground state energy. In a very short amount of time (often much less than a second) the atom returns to its ground state, emitting a photon at exactly the same energy difference as the higher energy state to the ground state. That energy is directly related to the frequency (and wavelength – for visible light this is on the scale of hundreds nanometers – nm for short) of the emitted photon, which is why the aurora emits the common colors of green (557.7 nm wavelength) or higher altitude reds (630 nm wavelength); those colors are directly tied to the possible energy levels of atomic oxygen.
In fact, the visible spectrum is just a small portion of the electromagnetic spectrum that includes gamma rays and even radio waves. Of the visible spectrum, violet light has the highest energy (lowest wavelength) and red light has the lowest energy (highest wavelength).
Color of Aurora
Most of the visible light we see from the aurora comes from either atomic oxygen or molecular nitrogen.
The most common color we see in the aurora is green light (557.7 nm wavelength). This color originates from oxygen atoms in the thermosphere between approximately 95-200 km (60-120 miles) above Earth’s surface. Occasionally we can see the higher altitude red (630 nm), but is often much fainter, mainly because of the lower density of oxygen atoms at higher altitudes (among other things like transition lifetime). Digital cameras are much more sensitive to these wavelengths, and can often see the red colors in the image even if they weren’t visible to the eye.
Also at high altitudes, nitrogen molecules can emit purple light at high altitudes (not depicted in the diagram above). This color is also faint and often difficult to detect.
During episodes of intense geomagnetic activity, electrons can excite nitrogen molecules much lower in the Thermosphere (below 60 miles). The nitrogen molecules can emit blue light as well as red light that combine to look crimson or pink from the ground. It’s really incredible to see those low-altitude pink colors because the aurora typically is moving incredibly fast, dancing overhead as it has so much energy!
Thank you for reading! There’s still quite a bit more to talk about, but this article was meant to be a foundation for many more related to the science behind the aurora. Be sure to subscribe to my newsletter for updates!
References and Further Reading
Thermosphere; S.C. Solomon, R.G. Roble, in Encyclopedia of Atmospheric Sciences (Second Edition), 2015
The Physical Universe: An Introduction to Astronomy; Shu, Frank; University Science Books, Mill Valley, California; 1982
The Heliospheric Current Sheet, NASA, 2013 – updated 2017
The Heliospheric Magnetic Field, Mathew J. Owens & Robert J. Forsyth; Living Reviews in Solar Physics
Further Understanding of Geomagnetism, National Centers For Environmental Information
Modeling Earth’s Geomagnetic Fields, National Centers for Environmental Information: NOAA
Lummerzheim, Dirk. (2009). The Colors of the Aurora. Park Science. 8. 37.
Baker, D.N., Erickson, P.J., Fennell, J.F. et al. Space Weather Effects in the Earth’s Radiation Belts. Space Sci Rev 214, 17 (2018). https://doi.org/10.1007/s11214-017-0452-7
Li, W., & Hudson, M. K. (2019). Earth’s Van Allen radiation belts: From discovery to the Van Allen Probes era. Journal of Geophysical Research: Space Physics, 124, 8319– 8351. https://doi.org/10.1029/2018JA025940
Akasofu S-I (2019) The Explosive Characteristics of the Aurora: The Electric Current Line Approach. Front. Astron. Space Sci. 6:1. doi: 10.3389/fspas.2019.00001
All online sources accessed August and September 2020 unless otherwise noted