NationMaster - Encyclopedia: Polar aurora

Polar aurorae are optical phenomena characterized by colorful displays of light in the night sky. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights; in the Southern Hemisphere it is called the aurora australis. Auroras are the most visible effect of the solar wind upon the Earth's atmosphere. The aurora occur when the Van Allen radiation belts become "overloaded" with energetic particles, which cascade down magnetic field lines and collide with the earth's upper atmosphere. The most powerful aurora tend to occur after coronal mass ejections. Aurora in Latin means dawn and Borealis comes from Boreas, the name of the Greek god of the northern wind. This photograph of the Aurora Borealis or the Northern lights came from the GIMP photo library This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... An optical phenomenon is any observable event which results from the interaction of light and matter. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... This article describes the time of day. ... For other uses, see Sky (disambiguation). ... The Northern Hemisphere is the half of a planets surface (or celestial sphere) that is north of the equator. ... The Southern Hemisphere is the part of Earths surface that is south of the equator. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... Earths atmosphere is the layer of gases surrounding the planet Earth and retained by the Earths gravity. ... Van Allen belts The Van Allen radiation belt is a torus of energetic charged particles around Earth, trapped by Earths magnetic field. ... A coronal mass ejection is a solar event which emits a burst of plasma (electrons and protons). ...

Origin and appearance

The origin of the aurora is 93 million miles (149 million km) from Earth at the Sun. Energetic particles from the Sun are carried out into space along with the ever present hot solar wind. This wind sweeps supersonically toward Earth through interplanetary space at speeds ranging from 300 to over 1000 km per second, carrying with it the solar magnetic field. The solar wind distorts the Earth's magnetic field to create the comet-shaped, plasma-filled magnetosphere. The terrestrial magnetic shield acts as a barrier, protecting the Earth from energetic particles and radiation in the hot solar wind. Particle energy and momentum is transferred from the solar wind to the magnetosphere through a process known as "magnetic reconnection". In this process interplanetary magnetic field lines (originating from the Sun) are coupled to the Earth's magnetic field. Particles in the solar wind can enter this newly created magnetic field line. Auroral physicists call this an open magnetic field line (the field line is open into the solar wind). Due to the dynamic pressure of the solar wind, this newly opened magnetic field line will be convected over the polar cap, and into the tail of the Earth's magnetosphere. Here, a new magnetic reconnection can occur, creating a new closed magnetic field line. The convecting field line will contain solar wind particles. Some of these particles will be able to reach the ionosphere before the field line has reached the magnetospheric tail. These particles will create dayside aurora. Nightside auroras are created from particles accelerated from the magnetopheric tail towards the Earth. These particles will be trapped on the closed field line. Electrons trapped in the Earth's magnetic field (the magnetic mirror effect) are accelerated along the magnetic field toward the polar regions and then strike the atmosphere to form the aurora. Auroras are most intense at times of intense magnetic storms caused by sunspot activity. The distribution of auroral intensity with altitude shows a pronounced maximum near 100 km above the Earth. Download high resolution version (2240x1680, 817 KB)The aurora australis lights the Elevated Station at Amundsen-Scott South Pole Station. ... Download high resolution version (2240x1680, 817 KB)The aurora australis lights the Elevated Station at Amundsen-Scott South Pole Station. ... The Amundsen-Scott South Pole Station is an American research station at Earths South Pole in Antarctica. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... In physics, a magnetic field is an entity produced by moving electric charges (electric currents) that exerts a force on other moving charges. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ... A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. ... Radiation generally means the transmission of objects or information from a source into a surrounding medium or destination. ... A magnetic mirror is a plasma confinement device which uses a cylindrical plasma with strong magnetic coils at the ends. ... 400 year sunspot history A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. ... In physics, intensity is a measure of the time-averaged energy flux. ...

The particles, which stream down the magnetic field of the Earth, reach the neutral atmosphere in a rough circle called the auroral oval. This circle, or annulus, is centered over the magnetic pole and is around 3000 km in diameter during quiet times. The annulus grows larger when the magnetosphere is disturbed. The location of the auroral oval is generally found between 60 and 70 degrees north and south latitude. During intense solar activity, the auroral oval expands, and aurorae have been seen from latitudes as low as 25-30 degrees north and south on extreme occasions. For example, on November 7, 2004, following a Coronal Mass Ejection, they were seen as far south as Arizona. At 45 degrees, aurorae are visible approximately five times per year, while above 55 they are visible almost nightly. An annulus An annulus (from Latin anulus, little ring) is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. ... November 7 is the 311th day of the year (312th in leap years) in the Gregorian Calendar, with 54 days remaining. ... 2004 is a leap year starting on Thursday of the Gregorian calendar. ... A coronal mass ejection is a solar event which emits a burst of plasma (electrons and protons). ...

Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above the Earth's surface and extend upward along its magnetic field for hundreds of kilometers. These arcs or curtains can be as thin as 100 meters while extending from horizon to horizon. Auroral arcs can nearly stand still and then, as though a hand has been run along a tall curtain, the aurora will begin to dance and turn. After magnetic midnight, the aurora can take on a patchy appearance and the patches often blink on and off once every 10 seconds or so until dawn. Most of the auroral features are greenish yellow but sometimes the tall rays will turn red at their tops and along their lower edge. On rare occasions, sunlight will hit the top part of the auroral rays creating a faint blue color. On very rare occasions (once every 10 years or so) the aurora can be a deep blood red color from top to bottom. In addition to producing light, the energetic auroral particles deposit heat. The heat is dissipated by infrared radiation or transported away by strong winds in the upper atmosphere. Download high resolution version (874x576, 51 KB)NASA image of aurora australis taken from Space Shuttle in May 1991. ... Download high resolution version (874x576, 51 KB)NASA image of aurora australis taken from Space Shuttle in May 1991. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ...

The physics of the aurora

The aurora is caused by the interaction of high energy particles (usually electrons) with neutral atoms in the Earth's upper atmosphere. These high energy particles can excite (by collisions) valence electrons that are bound to the neutral atom. The excited electrons can then return to their initial, lower energy state, and in the process release photons (light particles). This process is similar to the plasma discharge in a neon lamp. Properties The electron (sometimes called negatron; commonly represented as e−) is a subatomic particle. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ...

Any particular color of the aurora depends on a specific atmospheric gas and its electrical state, and on the energy of the particle that hits the atmospheric gas. Atomic oxygen is responsible for the two main colors of green (wavelength of 557.7 nm) and red (630.0 nm) from high altitudes. Nitrogen causes the color blue to appear, e.g. at 427.8 nm (molecular ions) as well as the rapidly varying red from the lower borders of active auroral arcs. General Name, Symbol, Number Oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16 (VIA), 2, p Density, Hardness 1. ... The wavelength is the distance between repeating units of a wave pattern. ... General Name, Symbol, Number Nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15 (VA), 2 , p Density 1. ...

Variations on the Sun

The Sun is a star with some features that are highly variable on time scales of hours to hundreds of years. The interplanetary magnetic field direction and solar wind speed and density are driven by the activity on the Sun. They can change drastically and influence the geomagnetic activity. As geomagnetic activity increases, the southern edge of the aurora borealis usually moves to lower latitudes. Similarly, solar mass ejections coincide with larger auroral ovals. If the interplanetary magnetic field is in the opposite direction of the Earth's magnetic field, there can be increased energy flow into the magnetosphere and thus, increased energy flow into the polar regions of the Earth. This will result in an intensification of the auroral displays.

Disturbances in the Earth's magnetosphere are called geomagnetic storms. These, in turn, can produce sudden changes in the brightness and motion of the aurora called auroral substorms. The magnetic fluctuations of these storms and substorms may cause surges in electric power lines and occasional equipment failures in the power grid, resulting in widespread power outages. They can also impact the performance of satellite-to-ground radio communications and navigation systems. Magnetospheric storms can last several hours or even days, and auroral substorms can occur several times a day. Each substorm can deliver several hundred terajoules of energy, as much as the electrical energy consumed in the entire United States over 10 hours. A geomagnetic storm is a storm in the magnetosphere. ... Communication is the process of exchanging information usually via a common system of symbols. ...

Measuring the geomagnetic field

The geomagnetic field can be measured with instruments called magnetometers. Data from many magnetometers allow observers to track the current state of the geomagnetic conditions. The magnetometer data are often given in the form of 3-hourly indices that give a quantitative measure of the level of geomagnetic activity. One such index is called the K-index. The K-index value ranges from 0 to 9 and is directly related to the amount of fluctuation (relative to a quiet day) in the geomagnetic field over a 3-hour interval. The higher the K-index value, the more likely it is that an aurora will occur. The K-index is also, necessarily, tied to a specific observatory location. For locations where there are no observatories, one can only estimate what the local K-index would be by looking at data from the nearest observatory. A global average of auroral activity is converted to the Kp index. A magnetometer is a scientific instrument used to measure the strength of magnetic fields. ...

Auroral sounds?

It is frequently claimed that sightings of aurorae are accompanied by humming and/or crackling sounds. The existence of these sounds is questionable - aurorae occur around 100km above the earth in extremely rarefied conditions which certainly could not transmit audible sounds well enough for them to reach ground level. Most claims of auroral sound report simultaneous auroral movement and noise, impossible given the sound travel time of many minutes between the upper atmosphere and ground level. Hum is a verb which refers to either: A sound with a particular timbre, usually a monotone or with slightly varying tones, often produced by machinery in operation or by insects in flight, or An action whereby a human vocally produces such a sound, but usually with musically varying tonality. ...

One possibility is that electromagnetic waves are transduced into sound waves by objects in the vicinity of the observer. However, the few detailed studies of auroral sounds that have been carried out have come to no definite conclusions. A transducer is a device that converts one type of energy to another, or responds to a physical parameter. ...

These auroral sounds have been likened to the sounds of Dawn Chorus. The electromagnetic dawn chorus is an unexplained phenomenon that occurs most often at sunrise or shortly after, that (with the proper radio equipment) resembles the sound of the birds dawn chorus. ...

Helsinki University of Technology has made examinations and recordings of these sounds and, according to the newspaper Kaleva, found that during bright high level polar aurora, hum, rumble and pops are registered. Helsinki University of Technology is not to be confused with University of Helsinki. ... Kaleva is a newspaper in Oulu, which is in northern Finland. ...

Mythology

In Norse mythology the polar aurora represents the Ride of the Valkyries to War. This article is about the Valkyries, figures of Norse mythology. ...

References in pop culture

On the Simpsons television show Principal Skinner claimed that the aurora borealis was occurring in his kitchen in an attempt to cover up the fact that his stove was on fire. The Simpsons. ...

An episode of the television show Northern Exposure featured the aurora and the Japanese fascination with them. This show also originated the idea that to copulate under the northern lights and conceive a child will bring good luck to that child. Northern Exposure is also the name of a short series of artic-themed trance music albums mixed by Sasha and John Digweed. ... A pair of lions having sexual intercourse in the Maasai Mara, Kenya. ...

In the United Kingdom, the first volume of Philip Pullman's His Dark Materials trilogy is called Northern Lights, named after the equivalent phenomenon in Lyra's world. Philip Pullman Philip Pullman, (born October 19, 1946) is an English writer, educated at Exeter College, Oxford, who is the bestselling author of the His Dark Materials trilogy of fantasy novels and a number of other books, purportedly for children, but attracting increasing attention by adult readers. ... Wills bench and the bridge in the Oxford Botanic Gardens His Dark Materials is a trilogy of novels by the fantasy fiction author Philip Pullman, comprising Northern Lights, The Subtle Knife and The Amber Spyglass. ... Northern Lights (published in the US as The Golden Compass) is the first novel in the His Dark Materials series, written by British novelist Philip Pullman, and published in 1995. ... Lyra Belacqua is a fictional character created by Philip Pullman and the heroine of the His Dark Materials trilogy. ...

Welsh Band Super Furry Animals recorded a song titled "Northern Lites" on their Guerilla album. Rumor says the song is about marijuana smoking, but singer Gruff Rhys says it is all about the spectacle of this natural phenomenon. Super Furry Animals (also known as SFA and the Furries) are a Welsh rock band, with leanings towards psychedelic rock and electronic experimentation. ...

The northern lights are referenced in the song "Farmhouse" by the band Phish. Farmhouse is an album by Phish. ... The official Phish logo. ...

In Salem, Oregon, there is a theater-pub called Northern Lights. It is one of a number of theaters offering limited restaurant and bar menus and services to theatergoers (i.e., delivering drinks and hot meals to customers during their movies). Its name is taken from the arctic phenomenon, and in place of the advertisements it plays slideshows of pictures taken of the Northern Lights. This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... State nickname: Beaver State Other U.S. States Capital Salem Largest city Portland Governor Ted Kulongoski Official languages None Area 255,026 km² (9th) - Land 248,849 km² - Water 6,177 km² (2. ... For other usages see Theatre (disambiguation) Theater (American English) or Theatre (British English and widespread usage among theatre professionals in the US) is that branch of the performing arts concerned with acting out stories in front of an audience using combinations of speech, gesture, music, dance, sound and spectacle — indeed... A public house, usually known as a pub, is a drinking establishment found mainly in the Great Britain, Ireland, Canada, Australia, New Zealand and other countries influenced by British cultural heritage. ...

On Dawson's Creek, A.J. the College Guy tried to hook up with Joey Potter by showing her the Northern Lights.

References

External links

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