Aurora Borealis and the Aurora Australis



 

Learning Objective

  • Recall how and why the aurora forms.

Key Points

    • An aurora is a natural light display in the sky, particularly in the high-latitude (arctic and antarctic) regions, that is caused by the collision of energetic charged particles with atoms in the high-altitude atmosphere.
    • Auroras result from emissions of photons in the Earth’s upper atmosphere (above 80 km), from ionized nitrogen atoms regaining an electron, and from electrons from oxygen and nitrogen atoms returning from an excited state to ground state.
    • The solar wind coming from the sun is the origin of the charged protons and electrons that excite oxygen and nitrogen and cause auroras.
    • The aurora’s color depends on the type of atom that is excited and how its electrons return from those excited states to the ground state.

Terms

  • auroraan atmospheric phenomenon created by charged particles from the sun striking the upper atmosphere, creating colored lights in the sky
  • solar windconsists of free, high-energy electrons and protons that originate at the sun; these particles retain the energy until they collide with the Earth’s atmosphere, causing the northern lights

Auroras

An aurora (plural aurorae or auroras; from the Latin word aurora forsunrise) is a natural light display in the sky, particularly in the high-latitude (arctic and antarctic) regions; it is caused by the collision of atmospheric atoms with energetic, charged particles coming from space. The charged particles originate in the magnetosphere and solar wind and then are directed by the Earth’s magnetic field into the atmosphere. The Earth’s magnetic field directs the charged particles to the Earth’s magnetic poles—as a result, it is the easiest to see the aurora near the poles.

An aurora is classified as either a diffuse or a discrete aurora. A diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone (the area in which auroras are visible). Discrete auroras are sharply-defined features within the diffuse aurora; they vary in brightness from barely visible to bright enough for reading a newspaper at night.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named by Pierre Gassendi in 1621 after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas. The northern lights have had a number of names throughout history: the Cree called the phenomenon the “Dance of the Spirits”; in Europe in the Middle Ages, the auroras were commonly believed to be a sign from God.

Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the sun were rising from an unusual direction. Discrete auroras often display magnetic field lines or curtain-like structures. They can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the winter equinox when it is dark for long periods of time. Clouds in the sky and any light (natural sunlight or man-made light) can prevent the possibility of seeing the aurora from the ground.

The aurora borealis’ southern counterpart, the aurora australis (or the southern lights), has almost identical features. It changes simultaneously with the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia.

Aurora borealisSome images of aurora borealis.

What Causes the Aurora?

Auroras result from emissions of photons in the Earth’s upper atmosphere (above 80 km, or 50 mi), from ionized nitrogen atoms regaining an electron, and from oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles (such as high energy protons and electrons) funneling down and accelerating along the Earth’s magnetic field lines. The excited particles’ energy is lost by the emitting photon or colliding with another atom or molecule.

The solar wind consists of free, high-energy electrons and protons that originate at the sun. In the vacuum of space, they maintain their energy because the normal method of energy loss, collision, is not possible due to the low particle density. Instead, this energetic flow of particles is attracted to the Earth’s magnetic field, where it collides with oxygen and nitrogen in the top regions of the atmosphere. The movement of these charged particles also causes electricity, which serves to further excite the molecules in the Earth’s atmosphere. This energy serves to move the electrons in nitrogen and oxygen from their ground state up to an excited state, where they can then decay back to the ground state by emitting photons of visible light (see the concept on emission spectra for more information). Collisions with other atoms or molecules can absorb the excitation energy and prevent emission.

Oxygen emissions are green or brownish-red, depending on the amount of energy absorbed. Nitrogen emissions are blue if the atom regains an electron after it has been ionized and red if the atom returns to ground state from an excited state. Oxygen is unusual in terms of its return to ground state: it can take three-quarters of a second to emit green light and up to two minutes to emit red. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed, collisions preventing emission are rare enough to allow oxygen the time needed to emit red light.

Collisions become more frequent farther down in the atmosphere, and red emissions do not have time to happen; eventually, even green light emissions are prevented. This is why there is a color differential with altitude: at high altitudes, oxygen’s red emissions remain; then, oxygen’s green emissions and nitrogen’s blue and red emissions; and finally, only nitrogen’s blue and red emissions are left, because collisions prevent oxygen from emitting any light at all. Green is the most common color of all auroras, followed by pink, a mixture of light green and red, pure red, yellow (a mixture of red and green), and pure blue.