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Encyclopedia > Aurora (astronomy)
The Aurora Borealis, or Northern Lights, shines above Bear Lake
The Aurora Borealis, or Northern Lights, shines above Bear Lake
Aurora Borealis as seen over Canada at 11,000m (36,000 feet)
Aurora Borealis as seen over Canada at 11,000m (36,000 feet)
Red and green Aurora in Fairbanks, Alaska
Red and green Aurora in Fairbanks, Alaska

Auroras (or aurorae) [snglr.: aurora] are natural different colored light displays, which are usually observed in the night sky, particularly in the polar zone. Some scientists therefore call them "polar auroras" (or "aurorae polaris"). In northern latitudes, it is known as the aurora borealis, named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a greenish glow (or sometimes a faint red), as if the sun were rising from an unusual direction. The aurora borealis is also called the northern lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. Image File history File linksMetadata Download high-resolution version (3008x1960, 3983 KB) The original version before retouching can be found here. ... Image File history File linksMetadata Download high-resolution version (3008x1960, 3983 KB) The original version before retouching can be found here. ... Dear people who run this website, Dont you think it would be smart to add the whole detail in the western hemisphere and the southern hemisphere? like where the gulfs are and the mountain ranges!!! Just a suggestion. ... Image File history File linksMetadata Size of this preview: 800 × 500 pixelsFull resolution (1600 × 1000 pixel, file size: 620 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Aurora (astronomy) Metadata This file contains... Image File history File linksMetadata Size of this preview: 800 × 500 pixelsFull resolution (1600 × 1000 pixel, file size: 620 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Aurora (astronomy) Metadata This file contains... Image File history File links Size of this preview: 800 × 596 pixelsFull resolution (1536 × 1145 pixels, file size: 1. ... Image File history File links Size of this preview: 800 × 596 pixelsFull resolution (1536 × 1145 pixels, file size: 1. ... Fairbanks redirects here. ... Aurora Borealis can mean: For the astronomical phenomenon known as Aurora Borealis, a bright glow observed in the northern night sky see Aurora (astronomy). ... // Arts Literature Aurora (comics), superhero in the Marvel Universe Aurora (planet), Spacer world in Isaac Asimovs fiction Aurora Award (Prix Aurora), Canadian science-fiction and fantasy Aurora consurgens, manuscripts Aurora Leigh, a lengthy poem by Elizabeth Barrett Browning Aurora, SF society, a science fiction & fantasy society from Rijeka, Croatia... Amateur astronomy, often called back yard astronomy, is a hobby whose participants enjoy observing celestial objects. ... A geographical pole is either of two fixed points on the surface of a spinning body or planet, at 90 degrees from the equator, based on the axis around which a body spins. ... Aurora, by Guercino, 1621-23 (ceiling fresco in the Casino Ludovisi, Rome), a classic example of Baroque illusionistic painting Aurora was the ancient Roman equivalent of Eos, the ancient Greek goddess of the dawn. ... Zephyrus, the Greek god of the west wind and the goddess Chloris, from a 1875 engraving by William-Adolphe Bouguereau In Greek mythology, the Anemoi (in Greek, Άνεμοι — winds) were wind gods who were each ascribed a cardinal direction, from which their respective winds came, and were each associated with various... Northern hemisphere highlighted in yellow. ...


Its southern counterpart, aurora australis, has similar properties. Australis is the Latin word for "of the South".

Contents

Auroral mechanism

Auroras are produced by the collision of charged particles, mostly electrons but also protons and heavier particles, from the magnetosphere, with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km). The particles have energies from 1 - 100 keV. Most originate from the sun and arrive at the vicinity of earth in the relatively low energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with that of the earth and solar particles then enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards earth. A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... “km” redirects here. ... The electronvolt (symbol eV) is a unit of energy. ...


The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emission from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae.


Auroral forms and magnetism

An aurora
An aurora

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by the earth's magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving earthwards. Image File history File links Aurora2. ... Image File history File links Aurora2. ...


The curtains often show folds called "striations", which are curtain-like. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.


Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908)[1] deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents). Note: This article contains special characters. ... See also explorations, sea explorers, astronaut, conquistador, travelogue, the History of Science and Technology and Biography. ... A geographer is a crazy psycho whose area of study is geocrap, the pseudoscientific study of Earths physical environment and human habitat and the study of boring students to death. ... Pytheas (Πυθέας(Pitheas), ca. ... Olof Petrus (Peter) Hiorter (1696-1750) was a Swedish astronomer. ... Anders Celsius The observatory of Anders Celsius, from a contemporary engraving. ... Electric current is the flow (movement) of electric charge. ... Kristian Birkeland Kristian Birkeland (December 13, 1867 - June 15, 1917) was born in Christiania (Oslo today) and wrote his first scientific paper at the age of 18. ... The aurora on Jupiter, powered by Jovian Birkeland currents [Ref. ...


Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881)[2] established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km around the magnetic pole of the earth, not its geographic one. It was hardly ever seen near that pole itself. The instantaneous distribution of auroras ("auroral oval", Yasha [or Yakov] Felds[h]tein 1963[3]) is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest towards the equator around midnight. The aurora can be seen best at this time. An editor has expressed a concern that the subject of the article does not satisfy the notability guideline for Biographies. ...

An Aurora australis
An Aurora australis

Image File history File links Download high resolution version (5109x1200, 412 KB) Aurora Australis appearing in the night sky (10:50 pm Australian time (GMT = 8:50 am) at Swifts Creek, 100km north of Lakes Entrance, Victoria, Australia. ... Image File history File links Download high resolution version (5109x1200, 412 KB) Aurora Australis appearing in the night sky (10:50 pm Australian time (GMT = 8:50 am) at Swifts Creek, 100km north of Lakes Entrance, Victoria, Australia. ...

The solar wind and magnetosphere

Schematic of Earth's magnetosphere
Schematic of Earth's magnetosphere

The earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the sun in all directions, a result of the million-degree heat of the sun's outermost layer, the solar corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT (nanoteslas; the earth's surface field is typically 30,000–50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger. Image File history File links Schematic of magnetosphere. ... Image File history File links Schematic of magnetosphere. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... This article is about the astronomical term. ... SI unit. ... A geomagnetic storm is a temporary disturbance of the Earths magnetosphere. ... The Interplanetary Magnetic Field (IMF) is the term for the Sun’s magnetic field among the planets of the Solar System. ...


The IMF originates on the sun, related to the field of sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the sun-earth direction, but the rotation of the sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.[4] For other uses, see Sunspot (disambiguation). ... For other senses of this word, see magnetism (disambiguation). ...


The earth's magnetosphere is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around it, at a distance of about 70,000 km (before it reaches that boundary, typically 12,000–15,000 km upstream, a bow shock forms). The width of the magnetospheric obstacle, abreast of Earth, is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances. A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... In a planetary magnetosphere, the bow shock is the boundary at which the solar wind abruptly drops as a result of its approach to the magnetopause. ...


When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.


Frequency of occurrence

Aurora australis 1994 from latitude 47 degrees south
Aurora australis 1994 from latitude 47 degrees south

The aurora is a common occurrence in the Poles. It is occasionally seen in temperate latitudes, when a strong magnetic storm temporarily expands the auroral oval. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak.[citation needed] However, within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of IMF lines (known as Bz, pronounced "bee-sub-zed" or "bee-sub-zee"), being greater with southward slants. Image File history File links AuroraAustralisPaulMoss. ... Image File history File links AuroraAustralisPaulMoss. ... For other uses, see Sunspot (disambiguation). ...


Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes. It is not well understood why geomagnetic storms are tied to the earth's seasons when polar activity is not. It is known, however, that during spring and autumn, the earth's and the interplanetary magnetic field link up. At the magnetopause, Earth's magnetic field points north. When Bz becomes large and negative (i.e., the IMF tilts south), it can partially cancel Earth's magnetic field at the point of contact. South-pointing Bz's open a door through which energy from the solar wind can reach Earth's inner magnetosphere. A geomagnetic storm is a temporary disturbance of the Earths magnetosphere. ... For other uses, see Equinox (disambiguation). ... A magnetopause flows along the boundary between a magnetic field, (see: magnetosphere) and surrounding plasma. ...


The peaking of Bz during this time is a result of geometry. The interplanetary magnetic field comes from the sun and is carried outward the solar wind. Because the sun rotates the IMF has a spiral shape. Earth's magnetic dipole axis is most closely aligned with the Parker spiral in April and October. As a result, southward (and northward) excursions of Bz are greatest then. Heliospheric current sheet, the largest structure in the Solar System, is the three-dimensional form of the Parker spiral, that results from the influence of the Suns rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [1]. (click to enlarge) The Parker spiral is the shape...


However, Bz is not the only influence on geomagnetic activity. The Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. Because the solar wind blows more rapidly from the sun's poles than from its equator, the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest — by about 50 km/s, on average — around September 5 and March 5 when Earth lies at its highest heliographic latitude. is the 248th day of the year (249th in leap years) in the Gregorian calendar. ... This article is about the day. ...


Still, neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variation.


Auroral events of historical significance

The auroras which occurred as a result of the "great geomagnetic storm" on both August 28, 1859 and September 2, 1859 are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history. The latter, which occurred on September 2, 1859 as a result of the exceptionally intense Carrington-Hodgson white light solar flare on September 1, 1859 produced aurora so widespread and extraordinarily brilliant that they were seen and reported in published scientific measurements, ship's logs and newspapers throughout the United States, Europe, Japan and Australia. It was said in the New York Times[specify] that "ordinary print could be read by the light [of the aurora]". The aurora is thought to have been produced by one of the most intense coronal mass ejections in history, very near the maximum intensity that the sun is thought to be capable of producing. It is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked. This insight was made possible not only due to scientific magnetometer measurements of the era but also as a result of a significant portion of the 125,000 miles of telegraph lines then in service being significantly disrupted for many hours throughout the storm. Some telegraph lines however, seem to have been of the appropriate length and orientation which allowed a current (geomagnetically induced current) to be induced in them (due to Earth's severely fluctuating magnetosphere) and actually used for communication. The following conversation was had between two operators of the American Telegraph Line between Boston and Portland, ME, on the night of September 2, 1859 and reported in the Boston Traveler: is the 245th day of the year (246th in leap years) in the Gregorian calendar. ... is the 245th day of the year (246th in leap years) in the Gregorian calendar. ... A solar flare observed by Hinode in the G-band. ... is the 244th day of the year (245th in leap years) in the Gregorian calendar. ... The New York Times is an internationally known daily newspaper published in New York City and distributed in the United States and many other nations worldwide. ... A composite image showing two CMEs (at 2 oclock and 8 oclock), with the sun at center. ... A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. ... Telegraphy (from the Greek words tele = far away and grapho = write) is the long distance transmission of written messages without physical transport of letters, originally over wire. ... Geomagnetically induced currents (GIC), affecting the normal operation of long technological conductor systems, are a manifestation at ground level of space weather. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Nickname: City on the Hill, Beantown, The Hub (of the Universe)1, Athens of America, The Cradle of Revolution, Puritan City, Americas Walking City Location in Massachusetts, USA Counties Suffolk County Mayor Thomas M. Menino(D) Area    - City 232. ... Waterfront of Portland along the Fore River Portland is the largest city in Maine with a population of 64,249 citizens as of 2000. ... is the 245th day of the year (246th in leap years) in the Gregorian calendar. ...

Boston operator (to Portland operator): "Please cut off your battery [power source] entirely for fifteen minutes."
Portland operator: "Will do so. It is now disconnected."
Boston: "Mine is disconnected, and we are working with the auroral current. How do you receive my writing?"
Portland: "Better than with our batteries on. - Current comes and goes gradually."
Boston: "My current is very strong at times, and we can work better without the batteries, as the aurora seems to neutralize and augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work without batteries while we are affected by this trouble."
Portland: "Very well. Shall I go ahead with business?"
Boston: "Yes. Go ahead."

The conversation was carried on for around two hours using no battery power at all and working solely with the current induced by the aurora, and it was said that this was the first time on record that more than a word or two was transmitted in such manner.[5]


The origin of the aurora

Aurora australis (September 11, 2005) as captured by NASA's IMAGE satellite, digitally overlaid onto the Blue Marble composite image.
Aurora australis (September 11, 2005) as captured by NASA's IMAGE satellite, digitally overlaid onto the Blue Marble composite image.

The ultimate energy source of the aurora is the solar wind flowing past the Earth. Image File history File links Aurora_australis_20050911. ... Image File history File links Aurora_australis_20050911. ... is the 254th day of the year (255th in leap years) in the Gregorian calendar. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... Look up image in Wiktionary, the free dictionary. ... The Blue Marble is a famous photograph of Earth taken on 7 December 1972 by the crew of the Apollo 17 spacecraft at a distance of about 45,000 km. ...


Both the magnetosphere and the solar wind consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday's [1791 - 1867] work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cut by), rather than along, the lines of the magnetic field, an electrical current is said to be induced into that conductor and electrons will flow within it. The amount of current flow is dependent upon a) the rate of relative motion and b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process ("the dynamo effect"), any and all conductors, solid or otherwise are so affected including plasmas or other fluids. For other uses, see Plasma. ... Michael Faraday, FRS (September 22, 1791 – August 25, 1867) was an English chemist and physicist (or natural philosopher, in the terminology of that time) who contributed to the fields of electromagnetism and electrochemistry. ... Dynamo, or Dinamo, may refer to: Dynamo, an electrical generator Dynamo (sports society) of the Soviet Union Operation Dynamo, the 1940 mass evacuation at Dunkirk Dynamo, the rock band based in Belfast Dynamo theory, a theory relating to magnetic fields of celestial bodies Dynamo Open Air, annual heavy metal music... The Dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field. ...


In particular the solar wind and the magnetosphere are two electrically conducting fluids with such relative motion and should be able (in principle) to generate electric currents by "dynamo action", in the process also extracting energy from the flow of the solar wind. The process is hampered by the fact that plasmas conduct easily along magnetic field lines, but not so easily perpendicular to them. It is therefore important that a temporary magnetic interconnection be established between the field lines of the solar wind and those of the magnetosphere, by a process known as magnetic reconnection. It happens most easily with a southward slant of interplanetary field lines, because then field lines north of Earth approximately match the direction of field lines near the north magnetic pole (namely, into the earth), and similarly near the southern pole. Indeed, active auroras (and related "substorms") are much more likely at such times. Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing the overall topology of a magnetic field. ...


Electric currents originating in such fashion apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons: some are magnetically trapped, some reside in the magnetotail, and some exist in the upward extension of the ionosphere, which may extend (with diminishing density) some 25,000 km around the earth. Properties The electron (also called negatron, commonly represented as e−) is a subatomic particle. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Relationship of the atmosphere and ionosphere The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. ...

Aurora borealis as seen from Space Shuttle Atlantis during STS-117 mission.

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;[6] Zmuda and Armstrong, 1973[7]) which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest ("region 2") detours, leaving again through field lines closer to the equator and closing through the "partial ring current" carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so such currents require a driving voltage, which some dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The aurora on Jupiter, powered by Jovian Birkeland currents [Ref. ... A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohms law: V = IR Ohms law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i. ...


Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity. The Hall effect refers to the potential difference (voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a Hall bar or a van der Pauw element through which an electric current is flowing, created by a magnetic field applied perpendicular to the...


However, ohmic resistance is not the only obstacle to current flow in this circuit. The convergence of magnetic field lines near Earth creates a "mirror effect" which turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity. To overcome this, part of the available voltage appears along the field line ("parallel to the field"), helping electrons overcome that obstacle by widening the bundle of trajectories reaching Earth; a similar "parallel voltage" is used in "tandem mirror" plasma containment devices. A feature of such voltage is that it is concentrated near Earth (potential proportional to field intensity; Persson, 1963[8]), and indeed, as deduced by Evans (1974) and confirmed by satellites, most auroral acceleration occurs below 10,000 km. Another indicator of parallel electric fields along field lines are beams of upwards flowing O+ ions observed on auroral field lines.


While this mechanism is probably the main source of the familiar auroral arcs, formations conspicuous from the ground, more energy might go to other, less prominent types of aurora, e.g. the diffuse aurora (below) and the low-energy electrons precipitated in magnetic storms (also below).

The Aurora Borealis as viewed from the ISS Expedition 6 team. Lake Manicouagan is visible to the bottom left.

Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves, in directions mainly perpendicular to the field lines. They therefore start at their own "mirror points" and can travel only upwards. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the line to lying on a cone around it, which gradually narrows down. Aurora Borealis over Canada photographed by Expedition Six. ... Aurora Borealis over Canada photographed by Expedition Six. ... ISS redirects here. ... // Crew Kenneth Bowersox (5), Commander - U.S.A. Nikolai Budarin (3), Flight Engineer - Russia Donald Pettit (1), Flight Engineer - U.S.A. (1) number of spaceflights each crew member has completed, including this mission. ... Lake Manicouagan as seen from earth orbit. ...


In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972). Ionospheric absorption makes AKR observable from space only. Auroral kilometric radiation (AKR) is the intense radio radiation emitted in the acceleration zone (at a height of three times the radius of the Earth) of the polar lights. ...


These "parallel voltages" accelerate electrons to auroral energies and seem to be a major source of aurora. Other mechanisms have also been proposed, in particular, Alfvén waves, wave modes involving the magnetic field first noted by Hannes Alfvén (1942), which have been observed in the lab and in space. The question is however whether this might just be a different way of looking at the above process, because this approach does not point out a different energy source, and many plasma bulk phenomena can also be described in terms of Alfvén waves. A cluster of double layers forming in an Alfvén wave, about a sixth of the distance from the left. ... Hannes Alfvén (1908-1995) at the 1970 Nobel Prize ceremonies Hannes Olof Gösta Alfvén (May 30, 1908; Norrköping, Sweden – April 2, 1995; Djursholm, Sweden) was a Swedish plasma physicist and Nobel Prize in Physics laureate for his work on the theory of magnetohydrodynamics. ...


Other processes are also involved in the aurora, and much remains to be learned. Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red. On the other hand, positive ions also reach the ionosphere at such time, with energies of 20-30 keV, suggesting they might be an "overflow" along magnetic field lines of the copious "ring current" ions accelerated at such times, by processes different from the ones described above.

Aurora Borealis
Aurora Borealis from ISS
Wikipedia:Media help

Image File history File links Aurora_Borealis_from_Expedition_6. ...

Sources and types of aurora

Again, our understanding is very incomplete. A rough guess may point out three main sources:

  1. Dynamo action with the solar wind flowing past Earth, possibly producing quiet auroral arcs ("directly driven" process). The circuit of the accelerating currents and their connection to the solar wind are uncertain.
  2. Dynamo action involving plasma squeezed earthward by sudden convulsions of the magnetotail ("magnetic substorms"). Substorms tend to occur after prolonged spells (hours) during which the interplanetary magnetic field has an appreciable southward component, leading to a high rate of interconnection between its field lines and those of Earth. As a result the solar wind moves magnetic flux (tubes of magnetic field lines, moving together with their resident plasma) from the day side of Earth to the magnetotail, widening the obstacle it presents to the solar wind flow and causing it to be squeezed harder. Ultimately the tail plasma is torn ("magnetic reconnection"); some blobs ("plasmoids") are squeezed tailwards and are carried away with the solar wind; others are squeezed earthwards where their motion feeds large outbursts of aurora, mainly around midnight ("unloading process"). Geomagnetic storms have similar effects, but with greater vigor. The big difference is the addition of many particles to the plasma trapped around Earth, enhancing the "ring current" which it carries. The resulting modification of the earth's field allows aurora to be visible at middle latitudes, on field lines much closer to the equator.
  3. Satellite images of the aurora from above show a "ring of fire" along the auroral oval (see above), often widest at midnight. That is the "diffuse aurora", not distinct enough to be seen by the eye. It does not seem to be associated with acceleration by electric currents (although currents and their arcs may be embedded in it) but to be due to electrons leaking out of the magnetotail.

Any magnetic trapping is leaky--there always exists a bundle of directions ("loss cone") around the guiding magnetic field lines where particles are not trapped but escape. In the radiation belts of Earth, once particles on such trajectories are gone, new ones only replace them very slowly, leaving such directions nearly "empty". In the magnetotail, however, particle trajectories seem to be constantly reshuffled, probably when the particles cross the very weak field near the equator. As a result, the flow of electrons in all directions is nearly the same ("isotropic"), and that assures a steady supply of leaking electrons. The Dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing the overall topology of a magnetic field. ... A plasmoid is a coherent structure of plasma and magnetic fields. ... Image File history File links DEaurora. ... Van Allen radiation belts The Van Allen Radiation Belt is a torus of energetic charged particles (plasma) around Earth, held in place by Earths magnetic field. ...


The energization of such electrons comes from magnetotail processes. The leakage of negative electrons does not leave the tail positively charged, because each leaked electron lost to the atmosphere is quickly replaced by a low energy electron drawn upwards from the ionosphere. Such replacement of "hot" electrons by "cold" ones is in complete accord with the 2nd law of thermodynamics. The second law of thermodynamics, in its most concise form, states that the total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value. ...


Other types of aurora have been observed from space, e.g.[citation needed] "poleward arcs" stretching sunward across the polar cap, the related "theta aurora", and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Space does not allow discussion of other effects such as flickering aurora, "black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) has been observed around the two polar cusps, the "funnels" of field lines separating the ones that close on the day side of Earth from lines swept into the tail. The cusps allow a small amount of solar wind to reach the top of the atmosphere, producing an auroral glow.


Auroras on other planets

Jupiter aurora. The bright spot at far left is the end of field line to Io; spots at bottom lead to Ganymede and Europa.
Jupiter aurora. The bright spot at far left is the end of field line to Io; spots at bottom lead to Ganymede and Europa.

Both Jupiter and Saturn have magnetic fields much stronger than Earth's (in Jupiter is about 60000 Rayleigh and in Saturn is almost between 2000-5000 Rayleigh) (Uranus, Neptune and Mercury are also magnetic), and both have large radiation belts. Aurora has been observed on both, most clearly with the Hubble Space Telescope. Image File history File links Jupiter. ... Image File history File links Jupiter. ... For other uses, see Jupiter (disambiguation). ... This article is about the natural satellite of Jupiter. ... Apparent magnitude: 5. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble. ...


These auroras seem, like Earth's, to be powered by the solar wind. In addition, however, Jupiter's moons, especially Io, are powerful sources of auroras. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to relative motion between the rotating planet and the moving moon. Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions, studied since 1955. Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ...


An aurora has recently been detected on Mars, even though it was thought that the lack of a strong magnetic field would not make one possible.[9]


History of Aurora theories

In the past theories have been proposed to explain the phenomenon. These theories are now obsolete.

  • Auroral electrons come from beams emitted by the sun. This was claimed around 1900 by Kristian Birkeland, whose experiments in a vacuum chamber with electron beams and magnetized spheres (miniature models of the earth or "terrellas") showed that such electrons would be guided towards the polar regions. Problems with this model included absence of aurora at the poles themselves, self-dispersal of such beams by their negative charge, and more recently, lack of any observational evidence in space.
  • The aurora is the overflow of the radiation belt ("leaky bucket theory"). This was first disproved around 1962 by James Van Allen and co-workers, who showed that the high rate at which energy was dissipated by the aurora would quickly drain all that was available in the radiation belt. Soon afterwards it became clear that most of the energy in trapped particles resided in positive ions, while auroral particles were almost always electrons, of relatively low energy.
  • The aurora is produced by solar wind particles guided by the earth's field lines to the top of the atmosphere. This holds true for the cusp aurora, but outside the cusp, the solar wind has no direct access. In addition, the main energy in the solar wind resides in positive ions; electrons only have about 0.5 eV (electron volt), and while in the cusp this may be raised to 50–100 eV, that still falls short of auroral energies.

Kristian Birkeland Kristian Birkeland (December 13, 1867 - June 15, 1917) was born in Christiania (Oslo today) and wrote his first scientific paper at the age of 18. ... Van Allen radiation belts The Van Allen Radiation Belt is a torus of energetic charged particles (plasma) around Earth, held in place by Earths magnetic field. ... James Van Allen at National Air & Space Museum (NASM), 1981, Photo courtesy of NASM. Explorer I model and Pioneer H probe in background James Alfred Van Allen (September 7, 1914 – August 9, 2006) was an American space scientist at the University of Iowa. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ...

Auroral images

Images of aurora are significantly more common today due to the rise in digital camera use with high enough sensitivities.[10] Film and digital exposure to auroral displays is fraught with many difficulties, particularly if faithfulness of reproduction is an important objective. Due to the different spectral energy present, and changing dynamically throughout the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, and choice of film can be very important. Longer exposures aggregate the rapidly changing energy and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess. David Malin pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately. [1] For scientific research, proxies are often used, such as ultra-violet, and re-coloured to simulate the appearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highly useful tool for aurora hunters. [2] Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by the major websites. [11] It is possible to take excellent images with standard film (employing ISO ratings between 100 and 400) and an SLR with full aperture, a fast lens (f1.4 50mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's display strength.[12] David Malin (born 28 March 1941) is a British-Australian astronomer and photographer. ... Film speed is the measure of a photographic films sensitivity to light. ... This article is about SLR cameras in general. ...


Aurora in folklore

In Bulfinch's Mythology from 1855 by Thomas Bulfinch there is the claim that in Norse mythology: Thomas Bulfinch (July 15, 1796 - May 27, 1867) was an American writer, born in Newton, Massachusetts to a highly-educated but not rich Bostonian merchant family. ... Thomas Bulfinch (July 15, 1796 - May 27, 1867) was an American writer, born in Newton, Massachusetts to a highly-educated but not rich Bostonian merchant family. ... Norse, Viking or Scandinavian mythology comprises the indigenous pre-Christian religion, beliefs and legends of the Scandinavian peoples, including those who settled on Iceland, where most of the written sources for Norse mythology were assembled. ...

The Valkyrior are warlike virgins, mounted upon horses and armed with helmets and spears. /.../ When they ride forth on their errand, their armour sheds a strange flickering light, which flashes up over the northern skies, making what men call the "aurora borealis", or "Northern Lights". [13]

While a striking notion, there is nothing in the Old Norse literature supporting this assertion. Although auroral activity is common over Scandinavia and Iceland today, it is possible that the Magnetic North Pole was considerably further away from this region during the centuries before the documentation of Norse mythology, thus explaining the absent references.[14] The Valkyries Vigil, by the Pre-Raphaelite painter Edward Robert Hughes. ...


The first Old Norse account of norðurljós is instead found in the Norwegian chronicle Konungs Skuggsjá from AD 1230. The chronicler has heard about this phenomenon from compatriots returning from Greenland, and he gives three possible explanations: that the ocean was surrounded by vast fires, that the sun flares could reach around the world to its night side, or that glaciers could store energy so that they eventually became fluorescent.[15] A page from Konungs skuggsjá. Konungs skuggsjá (Old Norse for Kings mirror; Latin: Speculum regale, modern Norwegian: Kongespeilet) is a Norwegian educational scripture from around 1250, dealing with politics and moral. ... This article is about the geological formation. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ...


An old Scandinavian name for northern lights translates as "herring flash". It was believed that northern lights were the reflections cast by large swarms of herring onto the sky. For other uses, see Scandinavia (disambiguation). ... Species Clupea alba Clupea bentincki Clupea caspiopontica Clupea chrysotaenia Clupea elongata Clupea halec Clupea harengus Clupea inermis Clupea leachii Clupea lineolata Clupea minima Clupea mirabilis Clupea pallasii Clupea sardinacaroli Clupea sulcata Herrings are small oily fish of the genus Clupea found in the shallow, temperate waters of the North Atlantic...


Another Scandinavian source refers to "the fires that surround the North and South edges of the world". This has been put forward as evidence that the Norse ventured as far as Antarctica, although this is insufficient to form a solid conclusion.


The Finnish name for northern lights is revontulet, fox fires. According to legend, foxes made of fire lived in Lapland, and revontulet were the sparks they whisked up into the atmosphere with their tails. This article is about the animal. ... Lappi, or the Province of Lapland is one of the Provinces of Finland, and a part of the larger geographical area of Lapland, which spans over four countries. ...


In Estonian they are called virmalised, spirit beings of higher realms. In some legends they are given negative characters, in some positive ones.


The Sami people believed that one should be particularly careful and quiet when observed by the northern lights (called guovssahasat in Northern Sami). Mocking the northern lights or singing about them was believed to be particularly dangerous and could cause the lights to descend on the mocker and kill him/her. The Sami people (also Sámi, Saami, Lapps, sometimes also Laplanders) are the indigenous people of Sápmi, which today encompasses parts of northern Sweden, Norway, Finland and the Kola Peninsula of Russia. ... Northern or North Sami (also written Sámi or Saami; formerly Lappish or Lapp) is the most widely spoken of all Sami languages. ...


The Algonquin believed the lights to be their ancestors dancing around a ceremonial fire. This article is about the Native American tribe. ...


In Latvian folklore northern lights, especially if red and observed in winter, are believed to be fighting souls of dead warriors, an omen foretelling disaster (especially war or famine). This page is about the core essence of a being. ... Warriors may refer to Warriors (book series) is a series of fantasy novels written by Kate Cary and Cherith Baldry, under the pen name Erin Hunter. ... Examples of omens from the Nuremberg Chronicle (1493): natural phenomena and strange births. ... <nowiki>Insert non-formatted text hereBold text</nowiki>A famine is a social and economic crisis that is commonly accompanied by widespread malnutrition, starvation, epidemic and increased mortality. ...


In Russian folklore aurora borealis was associated with dragon ("Ognenniy Zmey"), who came to women and seduced them in the absence of their husbands.


In Scotland, the northern lights were known as "the merry dancers" or na fir-chlis. There are many old sayings about them, including the Scottish Gaelic proverb "When the merry dancers play, they are like to slay." The playfulness of the merry dancers was supposed to end occasionally in quite a serious fight, and next morning when children saw patches of red lichen on the stones, they say amongst themselves that "the merry dancers bled each other last night". The appearance of these lights in the sky was considered a sign of the approach of unsettled weather. This article is about the country. ... Look up saying in Wiktionary, the free dictionary. ... Scottish Gaelic (Gàidhlig) is a member of the Goidelic branch of Celtic languages. ... Look up proverb in Wiktionary, the free dictionary. ... For other uses, see Lichen (disambiguation). ...


Many prospectors during the Klondike Gold Rush believed that the Northern Lights were the reflection of the mother lode of all gold. A typical gold mining operation, on Bonanza Creek. ... Mother Lode is a term associated with the mining of gold. ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ...


References

Specific references:

  1. ^ Birkeland, Kristian (1908). "The Norwegian Aurora Polaris Expedition 1902-3."
  2. ^ Fritz, Hermann (1881). "Das Polarlicht."
  3. ^ Feldshtein, Y. (1963). "Some problems concerning the morphology of auroras and magnetic disturbances at high latitudes", Geomagnetism and Aeronomy, 3, 183-192.
  4. ^ Solar wind forecast from a University of Alaska website
  5. ^ Green J. L., Boardsen S., Odenwald S., Humble J., Pazamickas K. A.. (2005) Eyewitness reports of the great auroral storm of 1859. Adv. in Space Res. 38 (2006) 145-154.
  6. ^ Schield, M. A.; Freeman, J. W.; & Dessler, A. J. (1969) "A Source for Field-Aligned Currents at Auroral Latitudes", Journal of Geophysical Research, 74, 247-256.
  7. ^ Armstrong J. C., & Zmuda, A. J. (1973). "Triaxial magnetic measurements of field-aligned currents at 800 kilometers in the auroral region: Initial results", Journal of Geophysical Research, 78, 6802-6807.
  8. ^ Persson, Hans (1963). "Electric field along a magnetic line of force in a low-density plasma", Physics of Fluids, 6, 1756-1759.
  9. ^ http://www.universetoday.com/am/publish/mars_express_aurorae.html?1722006
  10. ^ http://www.spaceweather.com/
  11. ^ http://www.spaceweather.com/index.cgi
  12. ^ http://www.spaceweather.com/aurora/images/24nov01/Moss1.jpg 2001 image
  13. ^ http://www.mythome.org/bxxxviii.html
  14. ^ http://www.vikinganswerlady.com/njordrljos.htm
  15. ^ http://quasar.irf.se/norrsken/Norrsken_history.html

General references: The University of Alaska is a Land-Grant, Sea-Grant, and Space Grant university founded in 1922 in Fairbanks, Alaska. ...

  • "Secrets of the Polar Aurora"
  • "Exploration of the Earth's magnetosphere" - overview of the magnetosphere, including auroras; and including extensive bibliographies of scientific articles
  • Dr. Thomas K Hatton of the Astronomy Department- University of Chicago
  • Eather, Robert H. (1980). Majestic Lights: The Aurora in Science, History, and The Arts. Washington, DC: American Geophysical Union. ISBN 0-87590-215-4.  (323 pages)
  • Syun-Ichi Akasofu (April 2002). "Secrets of the Aurora Borealis". Alaska Geographic Series 29 (1). Graphic Arts Center Publishing Company. 
  • Savage, Candace Sherk (1994 / 2001). Aurora: The Mysterious Northern Lights. San Francisco: Sierra Club Books / Firefly Books. ISBN 0-87156-419-X.  (144 pages)
  • Phillips, Tony (October 21, 2001). 'tis the Season for Auroras. NASA. Retrieved on 2006-05-15.
  • [3] image gallery

is the 294th day of the year (295th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 135th day of the year (136th in leap years) in the Gregorian calendar. ... The Golden Compass redirects here. ... 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. ...

External links

Wikimedia Commons has media related to:
Aurora

Image File history File links Broom_icon. ... Image File history File links Commons-logo. ... Allison Louise Crowe (born November 16, 1981) is a Canadian singer, songwriter, and pianist from Nanaimo, British Columbia who now lives in Corner Brook, Newfoundland and Labrador, Canada. ...


  Results from FactBites:
 
Aurora (astronomy) - Wikipedia, the free encyclopedia (4764 words)
The aurora is a common occurrence in the ring-shaped zone.
As a result the solar wind moves magnetic flux (tubes of magnetic field lines, moving together with their resident plasma) from the day side of Earth to the magnetotail, widening the obstacle it presents to the solar wind flow, and causing it to be squeezed harder.
The aurora is the overflow of the radiation belt ("leaky bucket theory").
  More results at FactBites »

 
 

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