Auroras

Auroras are visual phenomena related to interactions between Earth’s magnetic field and the solar radiation released by the sun. The aurora borealis of the Northern Hemisphere and the aurora australis of the Southern Hemisphere occur year round but are normally visible only at high latitudes.

Introduction

Auroras are complex visual phenomena caused by the interaction of solar radiation with the earth’s atmosphere and magnetic field. Auroras are primarily visible only at high latitudes near the magnetic North and South Poles of the earth.

In the Northern Hemisphere, the phenomenon is called the northern lights or the aurora borealis, which is derived from the name of the Roman goddess of the dawn combined with the Greek word boreas, which refers to the northern wind. In the Southern Hemisphere, these phenomena are called the aurora australis or the southern lights.

Auroras are some of the oldest geophysical phenomena witnessed and recorded in human history, and they have been incorporated into many legends and mythologies. The connection between auroras and solar activity was first proposed by English physicist Richard C. Carrington in 1859, but the hypothesis was not generally accepted until nearly a half-century later as advances in technology allowed scientists to gain a better understanding of Earth’s electromagnetic field and the properties of solar radiation.

Auroras usually appear as curtain- or sheet-like waves of color pulsating along invisible lines in the sky. Typical colors include blue, green, red, and purple, though other combinations are possible. The colors and shapes of auroras are now understood as a product of the shape of Earth’s magnetic field and the composition of gases in the atmosphere.

Optics and Electromagnetism

Optics is a branch of physics concerned with the behavior of light and its interaction with other types of matter. Light is a type of electromagnetic radiation, which is a form of energy that exists as an oscillating wave combining electric and magnetic forces. The relationship between electricity and magnetism causes the attraction between negatively charged and positively charged particles. The adhesion of electrons and protons within an atom is one example of the electromagnetic force in action.

There exist many types of electromagnetic radiation, and only a small fraction can be detected by optical means. Visible light is the portion of the electromagnetic spectrum that can be viewed by the human eye. Because electromagnetic energy exists as a wave, it can be defined partially by its wavelength, which is a measurement of the distance between oscillations of the wave.

Wavelengths are typically measured in nanometer (one billionth of a meter) increments. Visible light falls into the range of approximately 400 to 700 nanometers and makes up the visual spectrum. An optical phenomenon is an interaction involving light that falls within the visual spectrum and is therefore detectable by the human eye.

Earth’s Magnetic Field

In many respects, the earth functions as an electromagnet, generating an electromagnetic field from within its core that reaches thousands of kilometers into space. This field plays a critical role in governing the relationship between the earth and other astrological bodies in the solar system.

The poles of Earth’s magnetic field are shifted several kilometers from the North and South Poles of Earth’s rotation. The earth’s magnetic field also shifts position and may move by more than 15 kilometers (9.3 miles) per year.

The absolute core of the earth is composed primarily of iron that, because of the extreme pressures generated by the earth’s mass, has crystallized into solid form. The solid inner core is surrounded by the outer core, which is filled with iron and other metals that have melted into a liquid state because of the decay of radioactive elements within the core. Heat from radioactive decay generates convection currents within the outer core, which causes the iron and nickel to spin around the inner core. Furthermore, the rotation of the earth causes this spinning metal to align with the North and South Poles of the planet’s rotational axis. The spinning core acts like an electromagnetic generator, creating waves of positive and negative energy that constitute the planet’s magnetic field.

The Magnetosphere

The sun functions as a large nuclear reactor, generating enormous quantities of energy within its core. The sun is composed of plasma, a form of matter that is similar to gas but that contains ionized particles. Astrophysicists estimate that 99 percent of the observable universe is filled with plasma.

The sun comprises primarily hydrogen and helium, with small amounts of other gases. Because of the sun’s mass, the material at its center is subjected to intense pressure and heat; these forces generate nuclear reactions that transform the gases of the sun into superheated plasma.

Because it is constantly generating electric and magnetic energy, the sun releases vast amounts of energy into space. Radiation from the sun passes through the solar system to the ends of the heliosphere, which is the area of space affected by the sun’s properties. Radiation emanates from the outer layer of the sun’s atmosphere, known as the corona. The wave of radiation emanating from the sun is called the solar wind, which consists of millions of charged particles (protons, electrons, and ionized atoms) moving rapidly through space at speeds of approximately 400 km (249 mi) per second.

As the solar wind hits the earth’s magnetic field, it forms the magnetosphere, an “envelope” surrounding the earth that consists of solar radiation particles moving along the lines of positive and negative magnetic current generated by the earth. The magnetosphere blocks much of this solar radiation from reaching the inner atmosphere, where it would be harmful to most life on Earth.

The magnetosphere is shaped like a teardrop with the leading, semispherical end facing the sun and a long tail, called the magnetotail, trailing on the anti-solar side of the earth. Where the solar wind first encounters the magnetic field, the charged particles collide and create a repulsive force that pushes from the earth. This force, called the bow shock, pushes most of the solar wind particles out and to the sides of the earth; the particles continue to flow into the deeper reaches of the solar system.

As these magnetically charged particles move along the magnetic field, they attach to and stretch the magnetic field from the planet, thereby creating the magnetotail. Physicists are uncertain about the length of the magnetotail, but it appears to extend for millions of kilometers into outer space.

Solar Flares and Auroras

In its typical state, the solar wind is too weak to penetrate the magnetic field and is therefore radiated into space along the magnetotail. However, periodically the sun erupts, producing massive energy discharges known as solar flares and coronal mass ejections. When a wave of solar wind produced by one of these phenomena strikes the magnetosphere, the magnetic field distorts and becomes linked with the magnetic field of the solar wind.

At this point, solar particles are drawn along the inside of the magnetosphere and into the magnetotail. However, because this strand of particles is still connected to the leading edge of the earth’s magnetic field, the strand of particles stretches until it snaps. The front part of this strand returns to Earth, while the trailing end is pulled through the magnetotail and into outer space.

The particles attached to the leading edge of the magnetic field are accelerated as the magnetic field snaps into its original shape. As Earth’s magnetic field reconnects, sealing the hole in the magnetosphere, these accelerated particles are ejected into the atmosphere. As they draw closer to the earth, the particles are channeled into Earth’s magnetic field lines, which are concentrated waves of magnetism that surround the earth in a series of lateral circles. Solar particles typically create auroras when they settle into the field lines located at the earth’s poles.

The Aurora Effect

Auroras occur in the portion of the upper atmosphere known as the ionosphere. The lower edge of the auroral curtain may fall to as low as 60 km (100 mi) above sea level while the upper edge may stretch to more than 400 km (249 mi) above the surface. Light given off by an aurora is caused by collisions between electrons from the solar wind and atoms in the ionosphere.

When electrons collide with an oxygen atom, some of the energy from the electron is momentarily transferred to the oxygen atom’s electrons. This places the oxygen in an excited state, which resolves when the oxygen atom releases extra energy in the form of light. Oxygen atoms emit either a greenish light or a dark red light, depending on the time between the collision and the release of light.

When the electrons strike nitrogen atoms, a similar phenomenon will occur, giving off a red light. Additionally, some of the nitrogen atoms struck by charged electrons will undergo a different reaction in which one of the nitrogen atom’s electrons is temporarily dislodged from the atom during the collision. As this electron rejoins with the nitrogen atom, excess energy is produced and released as a blue light.

Because the composition of gases in the atmosphere changes with altitude, the colors of the auroras are divided into levels of different colors. The topmost portion of the auroral curtain appears red because the atmosphere at high altitudes has high concentrations of oxygen and low concentrations of nitrogen. Because of this, oxygen and electron collisions at this level tend to produce red light. In the middle altitudes, oxygen and nitrogen are more evenly mixed, so the light produced from collisions at this level tends to be a mixture of green, blue, and red, ultimately giving rise to a greenish blue pattern.

At the lowest level of the aurora, molecules are so dense that oxygen atoms do not release light. Excited oxygen atoms will release light only if they have sufficient time for the excess energy to be released. If an excited oxygen atom strikes another oxygen atom while in its excited state, it will transfer excess energy to the other oxygen atom, rather than release light. Oxygen atoms are so dense at lower levels of the ionosphere that excited oxygen atoms generally collide with one another rather than release light. Therefore, the light typically observed at the lowest level of the auroral curtain is pinkish and composed of a combination of blue and red light given off by nitrogen atoms.

In some cases, the light present in an aurora may suddenly shift in color and pattern. This rapid shift results from what physicists call a substorm, which is a momentary disruption of the magnetic field that causes an additional pulse of radiation to be released from the magnetotail and injected into the ionosphere. The mass changes in color and shape from substorms are sometimes called auroral eruptions.

Auroras can appear on any day of the year, though they are typically best viewed at night and are generally only visible in the far Northern and Southern Hemispheres. Because the magnetic poles shift from year to year, the optimal location for viewing auroras will shift accordingly.

Solar flares and coronal mass ejections also occur throughout the year, but they tend to increase and decrease according to a regular eleven-year pattern called the solar activity cycle. During the high point of the cycle, called the maxima, solar flares and coronal mass ejections are more common and come with an increase in the potential for auroras to appear.

Principal Terms

corona: the outermost layer of the sun, which extends into space in an irregular pattern surrounding the main body of the star

coronal mass ejection: larger than average burst of solar wind related to deformations and reconfigurations of the sun’s magnetic field

electromagnetic wave: a wave of energy consisting of oscillating electric and magnetic fields

electromagnetism: relationship between electric energy and magnetic energy responsible for attraction between negatively and positively charged particles

heliosphere: portion of space affected by the presence of the sun or another star

ionosphere: portions of the upper atmosphere consisting of part of the mesosphere, thermosphere, and exosphere; characterized by gas ionization through exposure to solar radiation

magnetosphere: outer layer of the earth’s atmosphere constituted by the interaction between the earth’s magnetic field and charged particles released by the sun

optics: branch of physics that deals with the properties and characteristics of light

plasma: state of matter similar to a gas in which a portion of the molecules have become ionized, giving rise to matter built from free ions and electrons

solar wind: charged particles emanating from the sun that extend to the end of the heliosphere

Bibliography

"Aurora." National Geographic Education, education.nationalgeographic.org/resource/aurora/. Accessed 6 Nov. 2024.

Bone, Neil. The Aurora: Sun-Earth Interactions. Wiley, 1996.

Falck-Ytter, Harald. Aurora: The Northern Lights in Mythology, History, and Science. Bell Pond Books, 1999.

Kranking, Carlyn. "Missed the Auroras in May? Here's How to See Them Next Time." Smithsonian Magazine, 6 June 2024, www.smithsonianmag.com/science-nature/missed-the-auroras-in-may-heres-how-to-see-them-next-time-180984480/. Accessed 6 Nov. 2024.

Liu, William, and Masaki Fujimoto, eds. The Dynamic Magnetosphere. Springer, 2011.

National Research Council. The Atmospheric Sciences: Entering the Twenty-First Century. National Academy Press, 2010.

Pielou, Evelyn Chrystalla. A Naturalist’s Guide to the Arctic. University of Chicago Press, 1994.

Wallace, John M., and Peter V. Hobbs. Atmospheric Science: An Introductory Survey. 2d ed. Academic Press, 2006.