The Northern Lights, also known by its scientific name aurora borealis, has been one of the most notable natural occurrences in the northernmost part of the world for millennia. At night, the sky lights up with a mosaic of natural colors, illuminating the ground for miles around and providing an experience unlike you have ever seen. But you might be wondering why the Northern Lights exists, what causes it, and why it is generally only visible in and around the Arctic Circle.
The Northern Lights is a natural phenomenon that has been occurring since long before the dawn of mankind itself. Read on to learn what causes the Northern Lights and how you can see it for yourself.
The Northern Lights, the Sun, and the Magnetosphere
The Northern Lights has been observed by humans for as long as humans have inhabited the globe, and was often explained as the result of supernatural forces. In recent decades, as humans have learned more about the mechanics of the natural world, the phenomenon behind the Northern Lights has become better understood.
Auroras occur when the Earth’s magnetosphere is disturbed by solar wind. But what is solar wind, and what is the magnetosphere?
The Sun is a major part of what makes Earth habitable, as its life-giving rays allow for plants to grow and animals to thrive. The sun provides natural light during the day and sun exposure is necessary for humans and other animals to remain healthy through the production of Vitamin D, though excessive sun exposure without protection can lead to sunburns, sun poisoning, and skin cancer.
However, not all of the sun’s effects are beneficial. In addition to light, the sun gives off radiation and other particles that are deadly to life. Solar wind is the name used to describe the sun’s process of emitting waves of charged particles: electrons, protons, and alpha particles that are infused with kinetic energy. Solar wind is regularly emitted by the sun and affects all objects in the solar system, including Earth. It is in part because of solar wind and solar radiation, for example, that spacecraft cannot be built with windows, because windows would allow radiation to enter the ship and harm the astronauts inside.
So why aren’t human beings affected by solar wind? The answer is because of the magnetosphere. The magnetosphere is an invisible region surrounding the Earth that is responsible for deflecting solar wind and other types of interstellar radiation. As you might guess from the name, the magnetosphere is generated by the Earth’s magnetic field, concentrated at the North and South Poles.
Why Do Auroras Occur?
Auroras occur due to the interaction between solar wind and the magnetosphere. When charged particles collide with other atoms, they cause those atoms to become excited. Atoms consist of a nucleus, a core consisting of neutrons and protons, and one or more electrons orbiting the nucleus in a manner similar to how the Earth orbits the sun.
Ordinarily, atoms are balanced between electrons and protons due to the fact that each carries an electrical charge. Protons have a positive charge, while electrons have a negative charge. When charged particles collide with balanced atoms, the excess energy causes electrons in those atoms to migrate to a higher-energy orbit. When the charged energy is depleted, the electron returns to its original orbit, releasing a photon—a particle of light—in the process.
When many charged particles collide with many atoms, a large amount of light is created in the process. When this occurs as a result of solar wind interacting with the magnetosphere, it creates the Northern Lights. The process is similar to how neon signs function. In a neon tube, electricity is used to excite particles within neon gas, resulting in them giving off light.
The Color, Shape, and Sound of the Northern Lights
Auroras appear to the naked eye as a brilliant mosaic of colors, but the specific colors that appear are due to solar wind interacting with different types of atoms. The Earth’s atmosphere is largely comprised of nitrogen and oxygen, two gases with different atomic makeups, and their appearance when excited by charged particles differs as a result.
The most common type of auroras are green auroras, created due to solar wind interacting with oxygen and nitrogen particles in the lower atmosphere. Blue auroras are created when solar wind interaction extends into lower altitudes, generated largely by nitrogen particles due to an absence of oxygen at this level of the atmosphere. On the opposite end of the spectrum, red auroras are generated through solar interaction in the outermost parts of the atmosphere, but red auroras are generally only visible during exceptionally high periods of solar activity.
The Northern Lights can also manifest in the form of ultraviolet and infrared auroras. Ultraviolet auroras are sometimes visible but are difficult to spot with the naked eye, while infrared auroras generally cannot be observed without proper monitoring equipment. In addition, interactions between atoms in different parts of the atmosphere can result in colors mixing to form yellow, pink, and purple auroras.
Auroras most commonly appear in the shape of giant curtains that stretch across the night sky. However, auroras will sometimes manifest as arcs or spirals. This occurs during periods of strong magnetic activity, drawing auroras to follow lines of force that are present in the magnetic field. During periods of intense solar activity, auroras are larger and brighter.
Finally, auroras often give off sound in addition to light. Aurora noise manifests as a hissing or crackling sensation and appears at around 230 feet (70 meters) from the Earth’s surface. Auroras create noise when an inversion layer is created in the atmosphere due to cold weather. When charged particles collide with the inversion layer, they discharge, creating the noise. If you are high enough when observing the Northern Lights, you will be able to hear aurora noise.
Why Are the Northern Lights Only Visible in the Arctic Circle?
While auroras can sometimes occur in other parts of the world, they are most commonly observed in and around the North Pole: the northernmost fringes of Russia, Canada, Scandinavia, Greenland, and Alaska. Auroras are also visible from the South Pole, where they are known as aurora australis. The bands of territory where auroras occur most frequently are known as the “auroral zones.”
The reason why auroras are generally only visible around the North and South Poles is due to the fact that these are the two places on Earth where magnetic field lines connect to the atmosphere. The Earth’s magnetic field converges on both the North and South Poles, and magnetic field lines connect both poles to each other, bypassing the equator and regions close to it. Auroras are constricted to traveling along magnetic field lines and cannot escape them.
When solar wind reaches the Earth, the magnetosphere draws particles to the North and South Poles, the source of the planet’s magnetic field. Because of this, relatively few particles can reach the equator, preventing auroras from forming.
On a more practical level, auroras are best observed in and around the Arctic Circle due to a lack of human habitation and longer nights during winter. Light pollution from cities and other human developments makes it more difficult to observe stars and planets at night, and it also obscures auroras. The population of the Arctic Circle is relatively small due to its extreme climate, resulting in less light pollution and more unspoiled wilderness.
Finally, due to the Earth’s axial tilt, locations that are further away from the equator have more extreme shifts in daylight in summer and winter. During the fall and winter, nights in Alaska and other northerly locales are extremely long, and the North Pole itself receives no daylight at all during the winter. Auroras can only be seen at night due to the absence of the sun, and thus long nights are necessary for effective aurora viewing.
On the rare occasions that auroras occur outside the auroral zones, it is due to increased geomagnetic storm activity. Additionally, during periods of magnetic reversal, auroras occur with more regularity around the equator and other parts of the globe. A magnetic reversal is when the magnetic North and South Poles reverse polarity, with reversals occurring roughly every 450,000 years. Some scientists believe that the Earth will undergo another reversal in the next few thousand years.
While science has made great strides in studying the Northern Lights, there are still many things about them that we don’t know. What we do know is that the Northern Lights are a natural, harmless occurrence that results from the Earth’s interaction with the sun. The result of solar wind being deflected by our planet’s protective magnetosphere, the Northern Lights have been a source of amazement and inspirations for generations of humans around the globe. If you’re curious about the Northern Lights, why not book a tour and see for yourself?