Natural Phenomenon: Aurora

Aurora (astronomy)

Images of the aurora australis and aurora borealis from around the world, including those with rarer red and blue lights

An aurora (plural: aurorae or auroras; from the Latin wordaurora, "sunrise" or the Roman goddess of dawn) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone, which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone expands to lower latitudes.

Aurorae are classified as diffuse and discrete. The diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye, even on a dark night. It defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to the naked eye, to bright enough to read a newspaper by at night. Discrete aurorae are usually seen in only the night sky, because they are not as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches or arcs, which are generally subvisual.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn,Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. 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 aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near theequinoxes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.

Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.

Auroral mechanism

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen molecules regaining an electron, and oxygen atoms and nitrogen molecules returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, 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 absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed 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 color 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 green), 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.

Auroral colors

·         Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under some circumstances with more intense solar activity. The low amount of oxygen atoms and their very gradually diminishing concentration is responsible for the faint, gradual appearance of the top parts of the "curtains".

·         Green: At lower altitudes the more frequent collisions suppress this mode and the 557.7 nm emission (green) dominates; fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to high stability of the N₂ molecule) plays its role here as well, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the bottom parts of the curtains.

·         Yellow and pink are a mix of red and green.

·         Blue: At yet lower altitudes atomic oxygen is not common anymore, and ionized molecular nitrogen takes over in visible light emission; it radiates at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the bottoms of the "curtains", show up at the highest levels of solar activity.

A predominantly red aurora australis

Aurora australis (11 September 2005) as captured by NASA's IMAGE satellite, digitally overlaid onto The Blue Marble composite image An animation created using the same satellite data is also available

Sources:http://en.wikipedia.org/wiki/Aurora_(astronomy)