Aurora
From The Art and Popular Culture Encyclopedia
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Auroras, also known as northern and southern (polar) lights or aurorae (singular: aurora), are natural light displays in the sky, particularly in the polar regions, and usually observed at night. They typically occur in the ionosphere. They are also referred to as polar auroras. This is a misnomer however, because they are commonly visible between 65 to 72 degrees north and south latitudes, which place them a ring just within the Arctic and Antarctic circles. Aurorae do occur deeper inside the polar regions, but these are infrequent and often invisible to the naked eye.
In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. The aurora borealis is also called the northern polar lights, as it is only visible in the sky from the Northern Hemisphere, with the chance of visibility increasing with proximity to the North Magnetic Pole. (The North Magnetic Pole is currently in the arctic islands of northern Canada.) 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. The aurora borealis most often occurs near the equinoxes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits." In Europe, in the Middle Ages, the auroras were commonly believed a sign from God (see Wilfried Schröder, Das Phänomen des Polarlichts, Darmstadt 1984).
Its southern counterpart, the aurora australis or the southern polar lights, has similar properties, but is only visible from high southern latitudes in Antarctica, South America, or Australasia. Australis is the Latin word for "of the South."
Auroras can be spotted throughout the world and on other planets. They are most visible closer to the poles due to the longer periods of darkness and the magnetic field.
Auroral mechanism
Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 miles), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule:
- oxygen emissions
- Green or brownish-red, depending on the amount of energy absorbed.
- nitrogen emissions
- Blue or red. Blue if the atom regains an electron after it has been ionized. Red if returning 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. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. The very top of the atmosphere is both a higher percentage of oxygen, and so thin that such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.
This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and blue), and lastly pure blue.
Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth's magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind. Seen from space, these fiery curtains form a thin ring in the shape of a monk's tonsure.
See also
