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

AuroraFrom Wikipedia, the free encyclopedia

An aurora, sometimes referred to as a polar light, is a natural light displayin the sky, predominantly seen in the high latitude (Arctic and Antarctic)regions.[nb 1] Auroras are produced when the magnetosphere is sufficientlydisturbed by the solar wind that the trajectories of charged particles in bothsolar wind and magnetospheric plasma, mainly in the form of electrons andprotons, precipitate them into the upper atmosphere(thermosphere/exosphere), where their energy is lost. The resultingionization and excitation of atmospheric constituents emits light of varyingcolour and complexity. The form of the aurora, occurring within bandsaround both polar regions, is also dependent on the amount of accelerationimparted to the precipitating particles. Precipitating protons generallyproduce optical emissions as incident hydrogen atoms after gainingelectrons from the atmosphere. Proton auroras are usually observed at lowerlatitudes. [2] Different aspects of an aurora are elaborated in varioussections below.

Contents1 Occurrence of terrestrial auroras

1.1 Images

1.2 Visual forms and colors

1.3 Other auroral radiation2 Causes of auroras

2.1 Auroral particles

2.2 Auroras and the atmosphere

2.3 Auroras and the ionosphere3 Interaction of the solar wind with Earth

3.1 Magnetosphere

4 Auroral particle acceleration

5 Auroral events of historical significance

6 Historical theories, superstition and mythology

7 Planetary auroras

8 See also

9 References

10 Further reading

11 External links

12 Notes

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Video of the aurora australis taken by the crew ofExpedition 28 on board the International SpaceStation, its sequence of shots was taken 17September 2011 from 17:22:27 to 17:45:12 GMT,on an ascending pass from south of Madagascar tojust north of Australia over the Indian Ocean

Video of the aurora australis taken by the crew ofExpedition 28 on board the International SpaceStation, its sequence of shots was taken 7 September2011 from 17:38:03 to 17:49:15 GMT, from theFrench Southern and Antarctic Lands in the SouthIndian Ocean to southern Australia

Video of the aurora australis taken by the crew ofExpedition 28 on board the International SpaceStation, its sequence of shots was taken 11September 2011 from 13:45:06 to 14:01:51 GMT,from a descending pass near eastern Australia,rounding about to an ascending pass to the east ofNew Zealand

Occurrence of terrestrial auroras

Most auroras occur in a band known as the auroral zone,[3] which is typically 3° to 6° wide in latitude and between 10° and 20° from thegeomagnetic poles at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora iscalled the auroral oval, a band displaced towards the nightside of the Earth. Day-to-day positions of the auroral ovals are posted on theinternet.[4] A geomagnetic storm causes the auroral ovals (north and south) to expand, and bring the aurora to lower latitudes. Early evidencefor a geomagnetic connection comes from the statistics of auroral observations. Elias Loomis (1860), and later Hermann Fritz (1881)[5] and S.Tromholt (1882)[6] in more detail, established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius ofapproximately 2500 km around the Earth's magnetic pole. It was hardly ever seen near the geographic pole, which is about 2000 km away fromthe magnetic pole. The instantaneous distribution of auroras ("auroral oval")[3] is slightly different, being centered about 3–5 degreesnightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between theobserver and the Sun. The aurora can be seen best at this time, which is called magnetic midnight.

In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, andthe Greek name for the north wind, Boreas, by Galileo in 1619.[7] Auroras seen within the auroral oval may be directly overhead, but fromfarther away they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusualdirection.

Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changessimultaneously with changes in the northern auroral zone.[8] It is visible from high southern latitudes in Antarctica, South America, NewZealand, and Australia. Auroras also occur on other planets. Similar to the Earth's aurora, they are also visible close to the planets’ magneticpoles. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs,[9] which can be sub-visual.

Auroras are occasionally seen in latitudes below the auroral zone, when a geomagnetic storm temporarily enlarges the auroral oval. Largegeomagnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after the peak.[10][11] Anaurora may appear overhead as a "corona" of rays, radiating from a distant and apparent central location, which results from perspective. Anelectron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to thelocal geomagnetic field vector B. This angle is known as the “pitch angle” of the particle. The distance, or radius, of the electron from the fieldline at any time is known as its Larmor radius. The pitch angle increases as the electron travels to a region of greater field strength nearer to theatmosphere. Thus it is possible for some particles to return, or mirror, if the angle becomes 90 degrees before entering the atmosphere to collidewith the denser molecules there. Other particles that do not mirror enter the atmosphere and contribute to the auroral display over a range ofaltitudes. Other types of auroras have been observed from space, e.g."poleward arcs" stretching sunward across the polar cap, the related "thetaaurora",[12] and "dayside arcs" near noon. These are relatively infrequent and poorly understood. There are other interesting effects such as

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North America

Eurasia

11. These NOAA maps of North America andEurasia show the local midnight equatorwardboundary of the aurora at different levels ofgeomagnetic activity; a Kp=3 corresponds to lowlevels of geomagnetic activity, while Kp=9represents high levels

25-second exposure of the auroraaustralis from Amundsen-Scott S.P.S.

Aurora timelapse video (40 minutes)

flickering aurora, "black aurora" and sub-visual red arcs. In addition to all these, a weakglow (often deep red) observed around the two polar cusps, the field lines separating theones that close through the Earth from those that are swept into the tail and close remotely.

Images

The altitudes where auroral emissions occur were revealed by Carl Størmer and hiscolleagues who used cameras to triangulate more than 12,000 auroras.[13] They discoveredthat most of the light is produced between 90 and 150 km above the ground, whileextending at times to more than 1000 km. Images of auroras are significantly morecommon today than in the past due to the increase in use of digital cameras that have highenough sensitivities.[14] Film and digital exposure to auroral displays is fraught withdifficulties, particularly if faithfulness of reproduction is an objective. Due to the differentcolor spectrum present, and the temporal changes occurring during the exposure, theresults are somewhat unpredictable. Different layers of the film emulsion responddifferently to lower light levels, and choice of film can be very important. Longerexposures superimpose rapidly changing features, and often blanket the dynamic attributeof a display. Higher sensitivity creates issues with graininess.

The aurora frequently appears either as a diffuse glow or as "curtains" that extendapproximately 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 manyparallel rays, each lined up with the local direction of the magnetic field, consistent withauroras being shaped by Earth's magnetic field. In-situ particle measurements confirm thatauroral electrons are guided by the geomagnetic field, and spiral around them whilemoving toward Earth. The similarity of an auroral display to curtains is often enhanced byfolds within the arcs.

David Malin pioneered multiple exposure using multiple filters for astronomical photography,recombining the images in the laboratory to recreate the visual display more accurately.[15] Forscientific research, proxies are often used, such as ultra-violet, and color-correction to simulate theappearance to humans. Predictive techniques are also used, to indicate the extent of the display, a highlyuseful tool for aurora hunters.[16] Terrestrial features often find their way into aurora images, makingthem more accessible and more likely to be published by major websites.[17] It is possible to takeexcellent images with standard film (using ISO ratings between 100 and 400) and a single-lens reflexcamera with full aperture, a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30seconds, depending on the aurora's brightness.[18]

Early work on the imaging of the auroras was done in 1949 by the University of Saskatchewan usingthe SCR-270 radar.

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Northern lights over Calgary

Red and green auroras, Norway

Aurora borealis fromthe International SpaceStation

Aurora during ageomagnetic storm thatwas most likely causedby a coronal massejection from the Sunon 24 May 2010. Takenfrom the ISS

Diffuse aurora observedby DE-1 satellite fromhigh Earth orbit

Visual forms and colors

Auroras take many different visual forms. The most distinctive and brightest are the curtain-like auroralarcs. They eventually fragment or ‘break-up’ into separate, and rapidly changing, often rayed featuresthat may fill the whole sky. These are the ‘discrete’ auroras, which are at times bright enough to read a newspaper by at night.[19] The ‘diffuse’aurora, on the other hand, is a relatively featureless glow sometimes close to the limit of visibility.[20] It can be distinguished from moonlitclouds by the fact that stars can be seen undiminished through the glow. Diffuse auroras are often composed of patches whose brightnessexhibits regular or near-regular pulsations. The pulsation period can be typically many seconds, so is not always obvious. Occasionally there isa fast, sub-second, flickering. A typical auroral display consists of these forms appearing in the above order throughout the night.[21]

Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes atthis wavelength make this color visible only under more intense solar activity. The low amount of oxygen atoms and their graduallydiminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are themost often-seen hues of red for the auroras.Green: At lower altitudes the more frequent collisions suppress the 630.0 nm (red) mode: rather 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. Theexcited molecular nitrogen (atomic nitrogen being rare due to high stability of the N2 molecule) plays its role here as well, as it cantransfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mixtogether to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible forthe abrupt-looking end of the lower edges of the curtains.Yellow and pink are a mix of red and green or blue. Other shades of red as well as orange may be seen on rare occasions; yellow-greenis moderately common. As red, green, and blue are the primary colours of additive synthesis of colours, in theory practically any colourmight be possible but the ones mentioned in this article comprise a virtually exhaustive list.Blue: At yet lower altitudes, atomic oxygen is uncommon, and ionized molecular nitrogen takes over in producing 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 andpurple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity.[22]

Ultraviolet: Ultraviolet light from auroras (within the optical window but not visible to virtually all humans) has been observed with therequisite equipment, and otherwise invisible auroras of this type were produced on a very small scale by certain HAARPexperiments.[23] Ultraviolet auroras have also been seen on Mars.[24]

Infrared: Infrared light, in wavelengths that are within the optical window, is also part of many auroras.[24][25]

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A predominantly red aurora australis

Other auroral radiation

In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation AKR,discovered in 1972.[26] Ionospheric absorption makes AKR only observable from space. X-ray emissions, originating from the particlesassociated with auroras, have also been detected.[27]

Causes of aurorasA full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves theinteraction of the solar wind with the Earth’s magnetosphere. The varying intensity of the solar wind produces effects of different magnitudes,but includes one or more of the following physical scenarios.

1. A quiescent solar wind flowing past the Earth’s magnetosphere steadily interacts with it and can both inject solar wind particles directlyonto the geomagnetic field lines that are ‘open’, as opposed to being ‘closed’ in the opposite hemisphere, and provide diffusion throughthe bow shock. It can also cause particles already trapped in the radiation belts to precipitate into the atmosphere. Once particles are lostto the atmosphere from the radiation belts, under quiet conditions new ones replace them only slowly, and the loss-cone becomesdepleted. In the magnetotail, however, particle trajectories seem constantly to reshuffle, probably when the particles cross the very weakmagnetic field near the equator. As a result, the flow of electrons in that region is nearly the same in all directions ("isotropic"), andassures a steady supply of leaking electrons. The leakage of electrons does not leave the tail positively charged, because each leakedelectron lost to the atmosphere is replaced by a low energy electron drawn upward from the ionosphere. Such replacement of "hot"electrons by "cold" ones is in complete accord with the 2nd law of thermodynamics. The complete process, which also generates anelectric ring current around the Earth, is uncertain.

2. Geomagnetic disturbance from an enhanced solar wind causes distortions of the magnetotail ("magnetic substorms"). These ‘substorms’tend to occur after prolonged spells (hours) during which the interplanetary magnetic field has had an appreciable southward component.This leads to a higher 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, ‘locked’ together with their resident plasma) from the day side of Earth to the magnetotail, widening theobstacle it presents to the solar wind flow and constricting the tail on the night-side. Ultimately some tail plasma can separate ("magneticreconnection"); some blobs ("plasmoids") are squeezed downstream and are carried away with the solar wind; others are squeezedtoward Earth where their motion feeds strong outbursts of auroras, mainly around midnight ("unloading process"). A geomagnetic stormresulting from greater interaction adds many more particles to the plasma trapped around Earth, also producing enhancement of the "ringcurrent". Occasionally the resulting modification of the Earth's magnetic field can be so strong that it produces auroras visible at middlelatitudes, on field lines much closer to the equator than those of the auroral zone.

3. Acceleration of auroral charged particles invariably accompanies a magnetospheric disturbance that causes an aurora. This mechanism,which is believed to predominantly arise from wave-particle interactions, raises the velocity of a particle in the direction of the guidingmagnetic field. The pitch angle is thereby decreased, and increases the chance of it being precipitated into the atmosphere. Bothelectromagnetic and electrostatic waves, produced at the time of greater geomagnetic disturbances, make a significant contribution to theenergising processes that sustain an aurora. Particle acceleration provides a complex intermediate process for transferring energy from

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Aurora australis (11 September 2005)as captured by NASA's IMAGEsatellite, digitally overlaid onto TheBlue Marble composite image. Ananimation created using the samesatellite data is also available

the solar wind indirectly into the atmosphere.

The details of these phenomena are not fully understood. However it is clear that the prime source ofauroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiationzones, and temporarily magnetically trapped, particles confined by the geomagnetic field, coupled withparticle acceleration processes.[28]

Auroral particles

The immediate cause of the ionization and excitation of atmospheric constituents leading to auroralemissions was discovered in 1960, when a pioneering rocket flight from Fort Churchill in Canadarevealed a flux of electrons entering the atmosphere from above.[29] Since then an extensive collectionof measurements has been acquired painstakingly and with steadily improving resolution since the1960s by many research teams using rockets and satellites to traverse the auroral zone. The mainfindings have been that auroral arcs and other bright forms are due to electrons that have beenaccelerated during the final few 10,000 km or so of their plunge into the atmosphere.[30] Theseelectrons often, but not always, exhibit a peak in their energy distribution, and are preferentially alignedalong the local direction of the magnetic field. Electrons mainly responsible for diffuse and pulsatingauroras have, in contrast, a smoothly falling energy distribution, and an angular (pitch-angle)distribution favouring directions perpendicular to the local magnetic field. Pulsations were discoveredto originate at or close to the equatorial crossing point of auroral zone magnetic field lines.[31] Protonsare also associated with auroras, both discrete and diffuse.

Auroras and the atmosphere

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining anelectron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state.[32] They are ionized or excited by thecollision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost withinthe atmosphere by the emission of a photon, or by collision with another atom or molecule:

oxygen emissionsgreen or orange-red, depending on the amount of energy absorbed.

nitrogen emissionsblue 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 emitred. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the highest atmosphere has a higherpercentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become morefrequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissionsare prevented. This is why there is a color differential with altitude; at high altitudes oxygen red dominates, then oxygen green and nitrogenblue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common color. Then comespink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Auroras and the ionosphere

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;[33] Zmuda and Armstrong, 1973[34]), which flow downinto the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric Elayer (125 km); the rest ("region 2") detours, leaving again through field lines closer to the equator and closing through the "partial ringcurrent" carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so some consider that such currents require a drivingvoltage, which an, as yet unspecified, dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of theorder of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. In another interpretation the currents are the directresult of electron acceleration into the atmosphere by wave/particle interactions.

Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbanceon 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 auroralelectrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral

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Schematic of Earth's magnetosphere

activity. Kristian Birkeland[35] deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowingfrom the dayside toward (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents).

Interaction of the solar wind with EarthThe Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (a gas of free electrons and positive ions) emitted by the Sunin all directions, a result of the two-million-degree temperature of the Sun's outermost layer, the corona. The solar wind reaches Earth with avelocity typically around 400 km/s, a density of around 5 ions/cm3 and a magnetic field intensity of around 2–5 nT (for comparison, Earth'ssurface field is typically 30,000–50,000 nT). During magnetic storms, in particular, flows can be several times faster; the interplanetarymagnetic field (IMF) may also be much stronger. Joan Feynman deduced in the 1970s that the long-term averages of solar wind speedcorrelated with geomagnetic activity.[36] Her work resulted from data collected by the Explorer 33 spacecraft. The solar wind andmagnetosphere consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday's work around 1830) thatwhen an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or iscut by), rather than along, the lines of the magnetic field, an electric current is induced within the conductor. The strength of the currentdepends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distancebetween the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos makeuse of this basic process ("the dynamo effect"), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids.The IMF originates on the Sun, linked to the sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone wouldtend to line them up in the Sun-Earth direction, but the rotation of the Sun angles them at Earth by about 45 degrees forming a spiral in theecliptic plane), known as the Parker spiral. The field lines passing Earth are therefore usually linked to those near the western edge ("limb") ofthe visible Sun at any time.[37] The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should beable in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process ishampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy is more effectivelytransferred by temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly thisprocess is known as magnetic reconnection. As already mentioned, it happens most readily when the interplanetary field is directed southward,in a similar direction to the geomagnetic field in the inner regions of both the north magnetic pole and south magnetic pole.

Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal massejections increase the intensity of the solar wind.[38]

Magnetosphere

Earth's magnetosphere is shaped by the impact of the solar wind on the Earth's magnetic field. Thisforms an obstacle to the flow, diverting it, at an average distance of about 70,000 km (11 Earth radii orRe),[39] producing a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width ofthe magnetosphere abreast of Earth, is typically 190,000 km (30 Re), and on the night side a long"magnetotail" of stretched field lines extends to great distances (> 200 Re). The high latitudemagnetosphere is filled with plasma as the solar wind passes the Earth. The flow of plasma into themagnetosphere increases with additional turbulence, density and speed in the solar wind. This flow isfavoured by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines.[40] The flowpattern of magnetospheric plasma is mainly from the magnetotail toward the Earth, around the Earth and back into the solar wind through themagnetopause on the day-side. In addition to moving perpendicular to the Earth's magnetic field, some magnetospheric plasma travels downalong the Earth's magnetic field lines, gains additional energy and loses it to the atmosphere in the auroral zones. The cusps of themagnetosphere, separating geomagnetic field lines that close through the Earth from those that close remotely allow a small amount of solarwind to directly reach the top of the atmosphere, producing an auroral glow. On 26 February 2008, THEMIS probes were able to determine, forthe first time, the triggering event for the onset of magnetospheric substorms.[41] Two of the five probes, positioned approximately one third thedistance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification.[42]

Geomagnetic storms that ignite auroras may occur more often during the months around the equinoxes. It is not well understood, butgeomagnetic storms may vary with Earth's seasons. Two factors to consider are the tilt of both the solar and Earth’s axis to the ecliptic plane.As the Earth orbits throughout a year, it experiences an interplanetary magnetic field (IMF) from different latitudes of the Sun, which is tilted at8 degrees. Similarly, the 23 degree tilt of the Earth’s axis about which the geomagnetic pole rotates with a diurnal variation, changes the daily

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ISS Expedition 6 team, LakeManicouagan is visible to the bottomleft

Wikinews has relatednews: Aurora Borealiscaused by electrical spacetornadoes

average angle that the geomagnetic field presents to the incident IMF throughout a year. These factors combined can lead to minor cyclicalchanges in the detailed way that the IMF links to the magnetosphere. In turn, this affects the average probability of opening a door throughwhich energy from the solar wind can reach the Earth's inner magnetosphere and thereby enhance auroras.

Auroral particle accelerationThe electrons responsible for the brightest forms of aurora are well accounted for by their acceleration in the dynamic electric fields of plasmaturbulence encountered during precipitation from the magnetosphere into the auroral atmosphere. In contrast, static electric fields are unable totransfer energy to the electrons due to their conservative nature.[43] The electrons and ions that cause the diffuse aurora' dim glow appear not toaccelerate during precipitation. The convergence of magnetic field lines towards the Earth that creates a ‘magnetic mirror’ turns back many ofthe downward flowing electrons as the field strength increases. The bright forms of auroras are produced when downward acceleration not onlyincreases the energy of precipitating electrons but also reduces their pitch angles (angle between electron velocity and the local magnetic fieldvector). This greatly increases the rate of deposition of energy into the atmosphere, and thereby the rates of ionisation, excitation andconsequent auroral light emission. It also enhances the electric current. One early theory proposed for the acceleration of auroral electrons isbased on an assumed static, or quasi-static, electric field and a consequent uni-directional potential drop.[44] The originating charge assemblyand associated equi-potentials are so-far unspecified. However, Poisson’s equation indicates that there can be no configuration of chargeresulting in a net potential drop. This fact prohibits the concept of a uni-directional potential drop. The electric field theory proposed for auroralparticle acceleration is therefore highly questionable as it appears to violate a basic principle of physics. A more credible theory is based onacceleration by Landau [45] resonance in the turbulent electric fields of the acceleration region. This process is essentially the same as thatemployed in plasma fusion laboratories throughout the world,[46] and appears well able to account in principle for most – if not all – detailedproperties of the electrons responsible for the brightest forms of auroras, above, below and within the acceleration region.[47]

Other mechanisms have also been proposed, in particular, Alfvén waves, wave modes involving themagnetic field first noted by Hannes Alfvén (1942), which have been observed in the laboratory and inspace. The question is whether these waves might just be a different way of looking at the aboveprocess, however, because this approach does not point out a different energy source, and many plasmabulk phenomena can also be described in terms of Alfvén waves.

Other processes are also involved in the aurora, and muchremains to be learned. Auroral electrons created by largegeomagnetic storms often seem to have energies below 1 keV,and are stopped higher up, near 200 km. Such low energiesexcite mainly the red line of oxygen, so that often such aurorasare red. On the other hand, positive ions also reach the ionosphere at such time, with energies of 20–30keV, 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. Some O+ ions ("conics") also seem accelerated indifferent ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves in directions mainly perpendicularto the field lines. They therefore start at their "mirror points" and can travel only upward. As they do so, the "mirror effect" transforms theirdirections of motion, from perpendicular to the field line to a cone around it, which gradually narrows down, becoming increasingly parallel atlarge distances where the field is much weaker.

Auroral events of historical significanceThe auroras that resulted from the "great geomagnetic storm" on both 28 August and 2 September 1859 are thought to be the most spectacularin recent recorded history. In a paper to the Royal Society on 21 November 1861, Balfour Stewart described both auroral events as documentedby a self-recording magnetograph at the Kew Observatory and established the connection between the 2 September 1859 auroral storm and theCarrington-Hodgson flare event when he observed that, "It is not impossible to suppose that in this case our luminary was taken in the act."[48]

The second auroral event, which occurred on 2 September 1859 as a result of the exceptionally intense Carrington-Hodgson white light solarflare on 1 September 1859, produced auroras, so widespread and extraordinarily bright, that they were seen and reported in published scientificmeasurements, ship logs, and newspapers throughout the United States, Europe, Japan, and Australia. It was reported by the New York Timesthat in Boston on Friday 2 September 1859 the aurora was "so brilliant that at about one o'clock ordinary print could be read by the light".[49]

One o'clock EST time on Friday 2 September, would have been 6:00 GMT and the self-recording magnetograph at the Kew Observatory was

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Star falling into the aurora over theKewaunee River.

recording the geomagnetic storm, which was then one hour old, at its full intensity. Between 1859 and 1862, Elias Loomis published a series ofnine papers on the Great Auroral Exhibition of 1859 in the American Journal of Science where he collected world-wide reports of the auroralevent.

That aurora is thought to have been produced by one of the most intense coronal mass ejections in history. It is also notable for the fact that it isthe first time where the phenomena of auroral activity and electricity were unambiguously linked. This insight was made possible not only dueto scientific magnetometer measurements of the era, but also as a result of a significant portion of the 125,000 miles (201,000 km) of telegraphlines then in service being significantly disrupted for many hours throughout the storm. Some telegraph lines, however, seem to have been ofthe appropriate length and orientation to produce a sufficient geomagnetically induced current from the electromagnetic field to allow forcontinued communication with the telegraph operator power supplies switched off. The following conversation occurred between twooperators of the American Telegraph Line between Boston and Portland, Maine, on the night of 2 September 1859 and reported in the BostonTraveler:

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 neutralizeand augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work withoutbatteries 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.[49] Such events led to thegeneral conclusion that

The effect of the aurorae on the electric telegraph is generally to increase or diminish the electric current generated in workingthe wires. Sometimes it entirely neutralizes them, so that, in effect, no fluid is discoverable in them. The aurora borealis seemsto be composed of a mass of electric matter, resembling in every respect, that generated by the electric galvanic battery. Thecurrents from it change coming on the wires, and then disappear: the mass of the aurora rolls from the horizon to thezenith.[50]

Historical theories, superstition and mythologyMagnetic control of the aurora was mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter,and Celsius described in 1741 evidence that large magnetic fluctuations occurred whenever the aurorawas observed overhead. It was also later realized that large electric currents were associated with theaurora, flowing in the region where auroral light originated.

Multiple superstitions and obsolete theories explaining the aurora have emerged over the centuries.

Seneca speaks diffusely on auroras in the first book of his Naturales Quaestiones, drawing mainly fromAristotle; he classifies them "putei" or wells when they are circular and "rim a large hole in the sky","pithaei" when they look like casks, "chasmata" from the same root of the English chasm, "pogoniae"when they are bearded, "cyparissae" when they look like cypresses), describes their manifold colors andasks himself whether they are above or below the clouds. He recalls that under Tiberius, an auroraformed above Ostia, so intense and so red that a cohort of the army, stationed nearby for fireman duty,

galloped to the city.Walter William Bryant wrote in his book Kepler (1920) that Tycho Brahe "seems to have been something of a homœopathist, for herecommends sulfur to cure infectious diseases “brought on by the sulphurous vapours of the Aurora Borealis."[51]

Benjamin Franklin theorized that the "mystery of the Northern Lights" was caused by a concentration of electrical charges in the polarregions intensified by the snow and other moisture.[52]

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Frederic Edwin Church's 1865 painting "AuroraBorealis"

The northern lights have had a number of names throughout history. The Cree called the phenomenon the "Dance of the Spirits". In MedievalEurope, the auroras were commonly believed to be a sign from God.[53]

There is the claim from 1855 that in Norse mythology:

The Valkyrior are warlike virgins, mounted upon horses and armed with helmets and spears. /.../ When they ride forth on theirerrand, their armour sheds a strange flickering light, which flashes up over the northern skies, making what Men call the "auroraborealis", or "Northern Lights".[54]

While a striking notion, there is not a vast body of evidence in the Old Norse literature giving this interpretation, or even much reference toauroras. Although auroral activity is common over Scandinavia and Iceland today, it is possible that the Magnetic North Pole was considerablyfarther away from this region during the relevant period of Norse mythology.[55] Today, the Northern Lights are visible in Iceland fromSeptember to April.[56]

The first Old Norse account of norðrljós is found in the Norwegian chronicle Konungs Skuggsjá from AD 1230, (long after the Viking age).The chronicler has heard about this phenomenon from compatriots returning from Greenland, and he gives three possible explanations: that theocean 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 thatthey eventually became fluorescent.[57]

In ancient Roman mythology, Aurora is the goddess of the dawn, renewing herself every morning to fly across the sky, announcing the arrivalof the sun. The persona of Aurora the goddess has been incorporated in the writings of Shakespeare, Lord Tennyson, and Thoreau. The nameAurora, however, simply comes from the Latin word for the dawn. The goddess was not associated with polar light phenomena, in Romanmyth.

In the traditions of Aboriginal Australians, the Aurora Australis is commonly associated with fire. For example, the Gunditjmara people ofwestern Victoria called auroras "Puae buae", meaning "ashes", while the Gunai people of eastern Victoria perceived auroras as bushfires in thespirit world. When the Dieri people of South Australia said that an auroral display was "Kootchee", an evil spirit creating a large fire. Similarly,the Ngarrindjeri people of South Australia referred to auroras seen over Kangaroo Island as the campfires of spirits in the ‘Land of the Dead’.Aboriginal people in southwest Queensland believed the auroras to be the fires of the "Oola Pikka", ghostly spirits who spoke to the peoplethrough auroras. Sacred law forbade anyone except male elders from watching or interpreting the messages of ancestors they believed weretransmitted through auroras.[58]

After the Battle of Fredericksburg, the lights could be seen from the battlefield that night. TheConfederate Army took it as a sign that God was on their side during the battle as it was veryrare that one could see the lights in Virginia. The painting Aurora Borealis (see AuroraBorealis (http://commons.wikimedia.org/wiki/File:Church_1911.4.1.jpg)) (1865) by Americanlandscape painter Frederic Edwin Church is widely interpreted to represent the conflict of theAmerican Civil War.[59]

A variety of Native American myths surround the spectacle. Early European explorer SamuelHearne traveled with Chipewyan Dene in 1771 and recorded their views on the aurora borealis,or the "ed-thin", as they called it, meaning caribou. Dene experience was that stroking cariboufur created sparks much like the aurora. They also believed that the lights were the spirits oftheir departed friends dancing in the sky, and when the lights shined the brightest it meant thattheir deceased friends were very happy.[60]

Planetary aurorasBoth Jupiter and Saturn have magnetic fields much stronger than Earth's (Jupiter's equatorial field strength is 4.3 gauss, compared to 0.3 gaussfor Earth), and both have extensive radiation belts. Auroras have been observed on both, most clearly with the Hubble Space Telescope. Uranusand Neptune have also been observed to have auroras.[61]

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Jupiter aurora; the bright spot at farleft connects magnetically to Io; spotsat bottom lead to Ganymede andEuropa.

An aurora high above the northernpart of Saturn; image taken by theCassini spacecraft. A movie showsimages from 81 hours of observationsof Saturn's aurora

The auroras on the gas giants seem, like Earth's, to be powered by the solar wind. In addition, however, Jupiter's moons, especially Io, arepowerful sources of auroras on Jupiter. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamomechanism due to the relative motion between the rotating planet and the moving moon. Io, which has active volcanism and an ionosphere, is aparticularly strong source, and its currents also generate radio emissions, studied since 1955. Auroras also have been observed on the surfacesof Io, Europa, and Ganymede, using the Hubble Space Telescope. Auroras have also been observed on Venus and Mars. Because Venus has nointrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varyingshape and intensity, sometimes distributed across the full planetary disc. Venusian auroras are producedby the impact of electrons originating from the solar wind and precipitating in the night-sideatmosphere. An aurora was also detected on Mars, on 14 August 2004, by the SPICAM instrumentaboard Mars Express. The aurora was located at Terra Cimmeria, in the region of 177° East, 52° South.The total size of the emission region was about 30 km across, and possibly about 8 km high. Byanalyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor,scientists observed that the region of the emissions corresponded to an area where the strongestmagnetic field is localized. This correlation indicates that the origin of the light emission was a flux ofelectrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.[61][62]

The brown dwarf star LSR J1835+3259 was discovered to have auroras in July 2015, the first extra-solar auroras discovered.[63] The aurora is a million times brighter than the Northern Lights, mainly redin colour, because the charged particles are interacting with hydrogen in its atmosphere. It is not knownwhat the cause is. Some have speculated that material maybe being stripped off the surface of the browndwarf via stellar winds to produce its own electrons. Another possible explanation is an as-yet-undetected planet or moon around the dwarf, which is throwing off material to light it up, as is the casewith Jupiter and its moon Io.[64]

See alsoAurora (heraldry)HeliophysicsList of solar stormsList of plasma (physics) articlesPaschen's lawSpace weather

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doi:10.1029/jz065i002p00551.28. Burch, J L (1987). Akasofu S-I and Y Kamide, ed. The solar wind and the Earth. D. Reidel. p. 103. ISBN 90-277-2471-7.29. McIlwain, C E (1960). "Direct Measurement of Particles Producing Visible Auroras". Journal of Geophysical Research 65: 2727.

Bibcode:1960JGR....65.2727M. doi:10.1029/JZ065i009p02727.30. Reiff, P. H.; Collin, H. L.; Craven, J. D.; Burch, J. L.; Winningham, J. D.; Shelley, E. G.; Frank, L. A.; Friedman, M. A. (1988). "Determination of auroral

electrostatic potentials using high- and low-altitude particle distributions". Journal of Geophysical Research 93: 7441. Bibcode:1988JGR....93.7441R.doi:10.1029/JA093iA07p07441.

31. Bryant, D. A.; Collin, H. L.; Courtier, G. M.; Johnstone, A. D. (1967). "Evidence for Velocity Dispersion in Auroral Electrons". Nature 215 (5096): 45.Bibcode:1967Natur.215...45B. doi:10.1038/215045a0.

32. "Ultraviolet Waves".33. Schield, M. A.; Freeman, J. W.; Dessler, A. J. (1969). "A Source for Field-Aligned Currents at Auroral Latitudes". Journal of Geophysical Research 74:

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Journal of Geophysical Research 78 (28): 6802–6807. Bibcode:1973JGR....78.6802A. doi:10.1029/JA078i028p06802.35. Birkeland, Kristian (1908). The Norwegian Aurora Polaris Expedition 1902–1903. New York: Christiania (Oslo): H. Aschehoug & Co. p. 720. out-of-print,

full text online36. Crooker, N. U.; Feynman, J.; Gosling, J. T. (1 May 1977). "On the high correlation between long-term averages of solar wind speed and geomagnetic

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41. "NASA – THEMIS Satellites Discover What Triggers Eruptions of the Northern Lights". Nasa.gov. Archived from the original on 29 June 2011. Retrieved26 July 2011.

42. Angelopoulos, V.; McFadden, J. P.; Larson, D.; Carlson, C. W.; Mende, S. B.; Frey, H.; Phan, T.; Sibeck, D. G.; Glassmeier, K.-H.; Auster, U.; Donovan, E.;Mann, I. R.; Rae, I. J.; Russell, C. T.; Runov, A.; Zhou, X.-Z.; Kepko, L. (2008). "Tail Reconnection Triggering Substorm Onset". Science 321 (5891): 931–5. Bibcode:2008Sci...321..931A. doi:10.1126/science.1160495. PMID 18653845.

43. Bryant, Duncan (1998). Electron Acceleration in the Aurora and Beyond. Bristol & Philadelphia: Institute of Physics Publishing Ltd. p. 163.ISBN 0750305339.

44. Evans, D S (1975). Hot Plasma in the Magnetosphere. New York and London: Plenum Press. pp. 319–340. ISBN 0306337002.45. "Lev Davidovich Landau". history.mcs.st-andrews.ac.uk.46. Cairns, R A (1993). Dendy, R O, ed. Plasma Physics:An Introductory Course. Cambridge University Press. pp. 391–410. ISBN 0521433096.47. Bryant D A; Perry C H (1995). "Velocity-space distributions of wave-accelerated auroral electrons". Journal of Geophysical Research 100 (A12): 23,711–

23,725. Bibcode:1995JGR...10023711B. doi:10.1029/95ja00991.48. Balfour Stewart (1860–1862). "On the Great Magnetic Disturbance of 28 August to 7 September 1859, as Recorded by Photography at the Kew

Observatory". Proceedings of the Royal Society of London 11: 407–410. doi:10.1098/rspl.1860.0086. JSTOR 111936.49. Green, J; Boardsen, S; Odenwald, S; Humble, J; Pazamickas, K (2006). "Eyewitness reports of the great auroral storm of 1859". Advances in Space

Research 38 (2): 145–154. Bibcode:2006AdSpR..38..145G. doi:10.1016/j.asr.2005.12.021.50. The British Colonist, Vol. 2 No. 56, 19 October 1859, page 1, accessed online at BritishColonist.ca (http://britishcolonist.ca/display.php?

issue=18591019&pages=001,&terms=aurora), on 19 February 2009.51. Walter William Bryant, Kepler. Macmillan Co. (1920) p.23.52. "Scientist and Inventor: Benjamin Franklin: In His Own Words... (AmericanTreasures of the Library of Congress)". Loc.gov. 16 August 2010. Archived

from the original on 28 June 2011. Retrieved 26 July 2011.53. Wilfried Schröder, Das Phänomen des Polarlichts, Darmstadt 198454. "Bullfinch's Mythology". Mythome.org. 10 February 1996. Retrieved 5 August 2010.55. "The Aurora Borealis and the Vikings". Vikinganswerlady.com. Retrieved 5 August 2010.

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Wikimedia Commons hasmedia related to Aurora.

Wikiquote has quotationsrelated to: Aurora

Wikivoyage has a travelguide for Northern Lights.

Wikisource has the text ofthe 1879 AmericanCyclopædia article AuroraBorealis.

Further reading

External linksAurora – FAQ (http://odin.gi.alaska.edu/FAQ/).Aurora – Forecasting (http://www.gi.alaska.edu/AuroraForecast/).Official MET aurora forecasting in Iceland (http://www.northernlightsiceland.com/northern-lights-forecast/).Aurora Borealis –Predicting (http://www.aurorahunter.com/aurora-prediction.php).Solar Terrestrial Data (http://www.hamqsl.com/solar1.html#converters) – Online Converter –Northern Lights Latitude.NASA –Carrington Super Flare(http://science.nasa.gov/headlines/y2008/06may_carringtonflare.htm?).Aurora Live Europe (http://www.auroralive.eu) – Aurora forecasts for middle latitudes in Europe.Multimedia:

Amazing time-lapse video of Aurora Borealis(http://vimeo.com/user10702000/fireinthesky) – Shot in Iceland over the winter of2013/2014.Popular video of Aurora Borealis(http://nrk.no/nyheter/distrikt/troms_og_finnmark/1.7467857) – Taken in Norway in 2011.Aurora Photo Gallery (http://www.aurora-northern-lights.com) – Views taken 2009–2011.Aurora Photo Gallery (http://apod.nasa.gov/apod/ap120103.html) – "Full-Sky Aurora" over Eastern Norway. December 2011.Videos and Photos – Auroras at Night (http://www.twanight.org/newTWAN/gallery.asp?Gallery=Aurora&page=1).Video (04:49) (http://www.youtube.com/watch?v=lT3J6a9p_o8) – Aurora Borealis –How The Northern Lights Are Created.Video (47:40) (http://www.nfb.ca/film/northern_lights) – Northern Lights –Documentary.Video (5:00) (https://vimeo.com/62602652) – Northern lights video in real timeVideo (01:42) (http://www.vimeo.com/27315234) – Northern Lights – Story of Geomagnetc Storm (Terschelling Island – 6/7 April2000).Video (01:56) (http://www.youtube.com/watch?v=Lc3FxNXjBs0) (Time-Lapse) -Auroras – Ground-Level View from FinnishLapland 2011.

55. "The Aurora Borealis and the Vikings". Vikinganswerlady.com. Retrieved 5 August 2010.56. "Northern Lights in Iceland". GuideToIceland.is. Retrieved 22 April 2015.57. "Norrsken history". Irf.se. 12 November 2003. Archived from the original on 21 July 2011. Retrieved 26 July 2011.58. Hamacher, D.W. (2013). "Aurorae in Australian Aboriginal Traditions" (PDF). Journal of Astronomical History and Heritage 16 (2): 207–219.

arXiv:1309.3367. Bibcode:2013JAHH...16..207H.59. "Aurora Borealis at the American Art Museum".60. Hearne, Samuel (1958). A Journey to the Northern Ocean: A journey from Prince of Wales' Fort in Hudson's Bay to the Northern Ocean in the years 1769,

1770, 1771, 1772. Richard Glover (ed.). Toronto: The MacMillan Company of Canada. pp. 221–222.61. "ESA Portal – Mars Express discovers auroras on Mars". Esa.int. 11 August 2004. Retrieved 5 August 2010.62. "Mars Express Finds Auroras on Mars". Universe Today. 18 February 2006. Retrieved 5 August 2010.63. O'Neill, Ian (29 July 2015). "Monstrous Aurora Detected Beyond our Solar System". news.discovery.com. Discovery. Retrieved 29 July 2015.64. Q. Choi, Charles (29 July 2015). "First Alien Auroras Found, Are 1 Million Times Brighter Than Any On Earth". space.com. Retrieved 29 July 2015.

Stern, David P. (1996). "A Brief History of Magnetospheric Physics During the Space Age". Reviews of Geophysics 34: 1–31. Bibcode:1996RvGeo..34....1S.doi:10.1029/95rg03508.Stern, David P. and Peredo, Mauricio. "The Exploration of the Earth's Magnetosphere". phy6.org.Eather, Robert H. (1980). Majestic Lights: The Aurora in Science, History, and The Arts. Washington, DC: American Geophysical Union. p. 323. ISBN 0-87590-215-4.Syun-Ichi Akasofu (April 2002). "Secrets of the Aurora Borealis". Alaska Geographic Series (Graphic Arts Center Publishing Company) 29 (1).Daglis, Ioannis and Syun-Ichi Akasofu (November 2004). "Aurora - The magnificent northern lights" (PDF). Recorder. Canadian Society of EplorationGeophysicists. pp. 45–48.Savage, Candace Sherk (1994). Aurora: The Mysterious Northern Lights. San Francisco: Sierra Club Books / Firefly Books. ISBN 0-87156-419-X. (144pages)Hultqvist, Bengt (2007). "The Aurora". In Kamide, Y.; Chian, A. Handbook of the Solar-Terrestrial Environment. Berlin Heidelberg: Springer-Verlag.pp. 331–354. doi:10.1007/978-3-540-46315-3_13. ISBN 978-3-540-46314-6.Sandholt, Even; Carlson, Herbert C.; Egeland, Alv (2002). "Optical Aurora". Dayside and Polar Cap Aurora. Netherlands: Springer Netherlands. pp. 33–51.doi:10.1007/0-306-47969-9_3. ISBN 978-0-306-47969-4.Phillips, Tony (21 October 2001). " 'tis the Season for Auroras". NASA. Archived from the original on 11 April 2006. Retrieved 15 May 2006.

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Video (02:43) (http://www.youtube.com/watch?v=Mb87D5PAMuY) (Time-Lapse) -Auroras – Ground-Level View from Tromsø,Norway. 24 November 2010.Video (00:27) (http://www.youtube.com/watch?v=l6ahFFFQBZY) (Time-Lapse) –Earth and Auroras – Viewed from TheInternational Space Station.

Notes1. The name ”auroras” is now the more common plural of ”aurora”, however ”aurorae” is the original Latin plural and is often used by scientists; in some

contexts Aurora is an uncountable noun, multiple sightings being referred to as ”the Aurora”. Modern style guides recommend that the names ofmeteorological phenomena, such as aurora borealis, be uncapitalized.[1]

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