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| − | {{short description|Natural light display that occurs in the sky, primarily at high latitudes (near the Arctic and Antarctic)}} |
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| − | {{redirect-several|Aurora|Aurora Australis|Aurora Borealis|Northern Lights|Southern Lights}} |
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| − | {{Use dmy dates|date=September 2019}} |
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| − | | image1 = Aurora Borealis and Australis Poster.jpg |
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| − | | image2 = Aurora australis panorama.jpg |
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| − | | caption2 = Images of auroras from around the world, including those with rarer red and blue lights |
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| − | | image3 = Aurora australis ISS.jpg |
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| − | | caption3 = Aurora australis from the [[International Space Station|ISS]], 2017. Video of this encounter: [https://eol.jsc.nasa.gov/BeyondThePhotography/CrewEarthObservationsVideos/videos/slights_iss_20170817/slights_iss_20170817.mp4] |
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| − | An '''aurora''' (plural: '''auroras''' or '''aurorae'''),{{efn|The name "auroras" is now the more common plural of "aurora"{{citation needed|reason=the only provided ref does not state this|date=October 2019}}, 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". |
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| − | Modern style guides recommend that the names of [[meteorological phenomena]], such as aurora borealis, be uncapitalized.<ref>{{cite web |url=http://www1.umn.edu/urelate/style/sciterminology.html#Anchor-37516 |title=University of Minnesota Style Manual |publisher=.umn.edu |date=18 July 2007 |accessdate=5 August 2010 |archiveurl=https://web.archive.org/web/20100722101345/http://www1.umn.edu/urelate/style/sciterminology.html#Anchor-37516 |archivedate=22 July 2010 |url-status=dead}}</ref>}} sometimes referred to as '''polar lights''' (aurora polaris), '''northern lights''' (aurora borealis), or '''southern lights''' (aurora australis), is a natural light display in the Earth's sky, predominantly seen in [[polar regions of Earth|high-latitude regions]] (around the [[Arctic]] and [[Antarctic]]). |
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| − | Auroras are the result of disturbances in the [[magnetosphere]] caused by [[solar wind]]. These disturbances are sometimes strong enough to alter the trajectories of [[charged particle]]s in both solar wind and magnetospheric plasma. These particles, mainly [[electron]]s and [[proton]]s, [[Electron precipitation|precipitate]] into the upper atmosphere ([[thermosphere]]/[[exosphere]]). |
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| − | The resulting [[ionization]] and excitation of atmospheric constituents emit light of varying color and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons generally produce optical emissions as incident [[hydrogen]] atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.<ref>{{cite web|url=http://auspace.athabascau.ca/handle/2149/518 |title=Simultaneous ground and satellite observations of an isolated proton arc at sub-auroral latitudes |publisher=Journal of Geophysical Research |date=2007 |accessdate=5 August 2015}}</ref> |
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| − | ==Etymology== |
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| − | The word "aurora" is derived from the name of the Roman goddess of the dawn, [[Aurora (mythology)|Aurora]], who travelled from east to west announcing the coming of the sun.<ref>{{cite dictionary|url=https://www.etymonline.com/word/aurora|title=Aurora|editor-last=Harper|editor-first=Douglas|dictionary=[[Online Etymology Dictionary]]|access-date=14 February 2019}}</ref> Ancient Greek poets used the name metaphorically to refer to dawn, often mentioning its play of colours across the otherwise dark sky (''e.g.'', "rosy-fingered dawn"). {{Citation needed|reason=No citation and the pages 'Linguistic relativity and the color naming debate' and'Studies on Homer and the Homeric Age § Colour controversy' on Wikipedia show how this is rather unlikely, or at least a mistranslation|date=June 2020}} |
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| − | Most auroras occur in a band known as the "auroral zone",<ref name="feldstein86">{{cite journal |last=Feldstein |first=Y. I. |year=2011 |title=A Quarter Century with the Auroral Oval |journal=EOS |volume=67 |issue=40 |page=761 |doi=10.1029/EO067i040p00761-02 |bibcode=1986EOSTr..67..761F }}</ref> which is typically 3° to 6° wide in latitude and between 10° and 20° from the [[geomagnetic pole]]s at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora is called the "auroral oval", a band displaced by the solar wind towards the night side of the Earth.<ref>{{Cite book|url=https://books.google.com/books?id=9gLwCAAAQBAJ&pg=PA190|title=Illustrated Glossary for Solar and Solar-Terrestrial Physics|last1=Bruzek|first1=A.|last2=Durrant|first2=C. J.|date=2012|publisher=Springer Science & Business Media|isbn=978-94-010-1245-4|page=190}}</ref> Early evidence for a geomagnetic connection comes from the statistics of auroral observations. [[Elias Loomis]] (1860),<ref name="Loomis" /> and later Hermann Fritz (1881)<ref>{{cite book |last1=Fritz |first1=Hermann |title=Das Polarlicht |trans-title=The Aurora |date=1881 |publisher=F. A. Brockhaus |location=Leipzig, Germany |url=https://babel.hathitrust.org/cgi/pt?id=nnc1.cu50485466&view=1up&seq=11 |language=German}}</ref> and Sophus Tromholt (1881)<ref>{{cite book |last1=Tromholt |first1=Sophus |title=Meteorologisk Aarbog for 1880. Part 1. |date=1881 |publisher=Danske Meteorologiske Institut |location=Copenhagen, Denmark |pages=I–LX |url=https://archive.org/details/meteorologiska1880dansuoft/page/n191 |language=Danish, French |chapter=Om Nordlysets Perioder / Sur les périodes de l'aurore boréale [On the periods of the aurora borealis]}}</ref> in more detail, established that the aurora appeared mainly in the auroral zone. Day-to-day positions of the auroral ovals are posted on the Internet.<ref>{{cite web |website=SpaceWeather |url=http://www.spaceweather.com/ |title=Current Auroral Oval |accessdate=19 December 2014 }}</ref> |
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| − | In northern [[latitude]]s, the effect is known as the aurora borealis or the northern lights. The former term was coined by [[Galileo]] in 1619, from the [[Ancient Rome|Roman]] [[Aurora (mythology)|goddess of the dawn]] and the [[Greek language|Greek]] name for the north wind.<ref>{{Cite book |doi=10.1029/HG002p0011 |chapter=An historical footnote on the origin of 'aurora borealis' |title=History of Geophysics: Volume 2 |journal=History of Geophysics: Volume 2. Series: History of Geophysics |volume=2 |pages=11–14 |series=History of Geophysics |year=1986 |authorlink1=George Siscoe |last1=Siscoe |first1=G. L. |isbn=978-0-87590-276-0|bibcode = 1986HGeo....2...11S }}</ref><ref>{{cite book |last1=Guiducci |first1=Mario |last2=Galilei |first2=Galileo |title=Discorso delle Comete |trans-title=Discourse on Comets |date=1619 |publisher=Pietro Cecconcelli |location=Firenze (Florence), Italy |page=39 |url=https://books.google.com/books?id=_EtbAAAAcAAJ&pg=PA39 |language=Italian}} On p. 39, Galileo explains that auroras are due to sunlight reflecting from thin, high clouds. From p. 39: ''" … molti di voi avranno più d'una volta veduto 'l Cielo nell' ore notturne, nelle parti verso Settentrione, illuminato in modo, che di lucidità non-cede alla piu candida Aurora, ne lontana allo spuntar del Sole; effetto, che per mio credere, non-ha origine altrode, che dall' essersi parte dell' aria vaporosa, che circonda la terra, per qualche cagione in modo più del consueto assottigliata, che sublimandosi assai più del suo consueto, abbia sormontato il cono dell' ombra terrestre, si che essendo la sua parte superiore ferita dal Sole abbia potuto rifletterci il suo splendore, e formarci questa boreale aurora."'' ( … many of you will have seen, more than once, the sky in the night hours, in parts towards the north, illuminated in a way that the clear [sky] does not yield to the brighter aurora, far from the rising of the sun; an effect that, by my thinking, has no other origin than being part of the vaporous air that surrounds the Earth, for some reason thinner than usual, which, being sublimated far more than usual, has risen above the cone of the Earth's shadow, so that its upper part, being struck by the sun['s light], has been able to reflect its splendor and to form this aurora borealis.)</ref> The southern counterpart, the aurora australis or the southern lights, has features almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone.<ref>{{Cite journal |doi=10.1016/j.jastp.2006.05.026 |title=Auroral conjugacy studies based on global imaging |journal=Journal of Atmospheric and Solar-Terrestrial Physics |volume=69 |issue=3 |pages=249 |year=2007 |last1=Østgaard |first1=N. |last2=Mende |first2=S. B. |last3=Frey |first3=H. U. |last4=Sigwarth |first4=J. B. |last5=Åsnes |first5=A. |last6=Weygand |first6=J. M. |bibcode=2007JASTP..69..249O}}</ref> The aurora australis is visible from high southern latitudes in [[Antarctica]], [[Chile]], [[Argentina]], [[New Zealand]], and [[Australia]]. |
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| − | A [[geomagnetic storm]] causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes. The instantaneous distribution of auroras ("auroral oval")<ref name="feldstein86" /> is slightly different, being centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the [[Poles of astronomical bodies#Magnetic poles|magnetic pole]] in question is in between the observer and the [[Sun]]. The aurora can be seen best at this time, which is called [[magnetic midnight]]. |
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| − | Auroras seen within the auroral oval may be directly overhead, but from farther away, they illuminate the poleward horizon as a greenish glow, or sometimes a faint red, as if the Sun were rising from an unusual direction. Auroras also occur poleward of the auroral zone as either diffuse patches or arcs,<ref>{{cite journal |last=Frey |first=H. U. |year=2007 |title=Localized aurora beyond the auroral oval |doi=10.1029/2005RG000174 |journal=Rev. Geophys. |volume=45 |issue=1 |pages=RG1003 |bibcode=2007RvGeo..45.1003F }}</ref> which can be subvisual. |
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| − | {{multiple image |
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| − | | title = Videos of the aurora australis taken by the crew of [[Expedition 28]] on board the International Space Station |
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| − | | image1 = Aurora Australis.ogv |
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| − | | caption1 = This sequence of shots was taken 17 September 2011 from 17:22:27 to 17:45:12 GMT,<br>on an ascending pass from south of [[Madagascar]] to just north of [[Australia]] over the [[Indian Ocean]] |
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| − | | image2 = Aurora Australis over Indian Ocean.ogv |
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| − | | caption2 = This sequence of shots was taken 7 September 2011 from 17:38:03 to 17:49:15 GMT,<br>from the [[French Southern and Antarctic Lands]] in the South Indian Ocean to southern Australia |
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| − | | image3 = Aurora Australis south of Australia.ogv |
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| − | | caption3 = This sequence of shots was taken 11 September 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 of [[New Zealand]] |
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| − | | title = [[National Oceanic and Atmospheric Administration|NOAA]] maps of North America and Eurasia |
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| − | | footer = These maps show the local midnight equatorward boundary of the aurora at different levels of geomagnetic activity.<br>A Kp=3 corresponds to low levels of geomagnetic activity, while Kp=9 represents high levels. |
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| − | | image1 = Aurora Kp Map North America.gif |
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| − | | alt1 = Kp map of North America |
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| − | | caption1 =North America |
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| − | | image2 = Aurora Kp Map Eurasia.gif |
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| − | | alt2 = Kp map of Eurasia |
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| − | | caption2 =Eurasia |
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| − | }} |
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| − | </div> |
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| − | Auroras are occasionally seen in latitudes below the auroral zone, when a geomagnetic storm temporarily enlarges the auroral oval. Large geomagnetic storms are most common during the peak of the 11-year [[sunspot|sunspot cycle]] or during the three years after the peak.<ref>{{cite journal |last1=Stamper |first1=J. |first2=M. |last2=Lockwood |first3=M. N. |last3=Wild |title=Solar causes of the long-term increase in geomagnetic activity |journal=J. Geophys. Res. |date=December 1999 |volume=104 |issue=A12 |pages=28,325–28,342 |doi=10.1029/1999JA900311 |bibcode=1999JGR...10428325S |url=http://centaur.reading.ac.uk/38740/1/180_Stamperetal_1999JA900311.pdf }}</ref><ref>{{cite journal |last1=Papitashvili |first1=V. O. |last2=Papitashva |first2=N. E. |last3=King |first3=J. H. |title=Solar cycle effects in planetary geomagnetic activity: Analysis of 36-year long OMNI dataset |journal=Geophys. Res. Lett. |date=September 2000 |volume=27 |issue=17 |pages=2797–2800 |doi=10.1029/2000GL000064 |bibcode=2000GeoRL..27.2797P |url=https://deepblue.lib.umich.edu/bitstream/2027.42/94796/1/grl13462.pdf |hdl=2027.42/94796 }}</ref> |
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| − | An electron spirals (gyrates) about a field line at an angle that is determined by its velocity vectors, parallel and perpendicular, respectively, to the local geomagnetic field vector B. This angle is known as the "pitch angle" of the particle. The distance, or radius, of the electron from the field line 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 the atmosphere. Thus, it is possible for some particles to return, or mirror, if the angle becomes 90° before entering the atmosphere to collide with the denser molecules there. Other particles that do not mirror enter the atmosphere and contribute to the auroral display over a range of altitudes. |
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| − | Other types of auroras have been observed from space, e.g."poleward arcs" stretching sunward across the polar cap, the related "theta aurora",<ref>{{Cite journal |doi=10.1029/2003GL017914 |title=Observations of non-conjugate theta aurora |journal=Geophysical Research Letters |volume=30 |issue=21 |pages=2125 |year=2003 |last1=Østgaard |first1=N. |bibcode=2003GeoRL..30.2125O }}</ref> and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as flickering aurora, "black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) observed around the two polar cusps, the field lines separating the ones that close through the Earth from those that are swept into the tail and close remotely. |
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| − | ===Images=== |
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| − | [[File:Aurora Borealis from Expedition 6.ogv|thumb|Video of the aurora borealis from the International Space Station]] |
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| − | The altitudes where auroral emissions occur were revealed by [[Carl Størmer]] and his colleagues, who used cameras to triangulate more than 12,000 auroras.<ref>{{cite journal |last1=Størmer |first1=Carl |title=Frequency of 12,330 measured heights of aurora from southern Norway in the years 1911–1944 |journal=Terrestrial Magnetism and Atmospheric Electricity |year=1946 |volume=51 |issue=4 |pages=501–504 |doi=10.1029/te051i004p00501 |bibcode=1946TeMAE..51..501S }}</ref> They discovered that most of the light is produced between 90 and 150 km above the ground, while extending at times to more than 1000 km. |
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| − | Images of auroras are significantly more common today than in the past due to the increase in the use of [[digital camera]]s that have high enough sensitivities.<ref>{{cite web|url=http://www.spaceweather.com/ |title=News and information about meteor showers, solar flares, auroras, and near-Earth asteroids |publisher=SpaceWeather.com |accessdate=5 August 2010| archiveurl= https://web.archive.org/web/20100804164127/http://spaceweather.com//| archivedate= 4 August 2010 | url-status=live}}</ref> Film and digital exposure to auroral displays is fraught with difficulties. Due to the different color spectra present, and the temporal changes occurring during the exposure, the results are somewhat unpredictable. Different layers of the film emulsion respond differently to lower light levels, and choice of a film can be very important. Longer exposures superimpose rapidly changing features, and often blanket the dynamic attribute of a display. Higher sensitivity creates issues with graininess. |
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| − | [[David Malin]] pioneered multiple exposure using multiple filters for astronomical photography, recombining the images in the laboratory to recreate the visual display more accurately.<ref>{{cite web |url=http://www.davidmalin.com/index.html |title=Astronomical photographs from David Malin Images |publisher=davidmalin.com |accessdate=3 August 2010 }}</ref> For scientific research, proxies are often used, such as ultraviolet, and color-correction 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.<ref>{{cite web |url=http://www.swpc.noaa.gov/pmap/index.html |title=NOAA POES Auroral Activity |publisher=swpc.noaa.gov |accessdate=3 August 2010 |archiveurl=https://web.archive.org/web/20100728192250/http://www.swpc.noaa.gov/pmap/index.html |archivedate=28 July 2010 |url-status=dead}}</ref> Terrestrial features often find their way into aurora images, making them more accessible and more likely to be published by major websites.<ref>{{cite web|url=http://www.spaceweather.com/ |title=What's up in space: Auroras Underfoot|publisher=SpaceWeather.com |accessdate=26 July 2011| archiveurl= https://web.archive.org/web/20110717093810/http://www.spaceweather.com/| archivedate= 17 July 2011 | url-status=live}}</ref> Excellent images are possible with standard film (using [[Film speed|ISO ratings]] between 100 and 400) and a [[single-lens reflex camera]] with full [[aperture]], a fast lens (f1.4 50 mm, for example), and exposures between 10 and 30 seconds, depending on the aurora's brightness.<ref>[http://www.spaceweather.com/aurora/images/24nov01/Moss1.jpg Aurora image] (JPG)</ref> |
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| − | Early work on the imaging of the auroras was done in 1949 by the [[University of Saskatchewan]] using the [[SCR-270]] radar. |
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| − | <gallery mode="packed" widths="180" heights="180"> |
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| − | File:Aurora Australis From ISS.JPG|Aurora during a [[geomagnetic]] storm that was most likely caused by a [[coronal mass ejection]] from the Sun on 24 May 2010, taken from the ISS |
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| − | File:DEaurora.gif|Diffuse aurora observed by DE-1 satellite from high Earth orbit |
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| − | File:Virmalised 18.03.15 (4).jpg|[[Estonia]], {{nowrap|18 March 2015}} |
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| − | </gallery> |
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| − | ===Forms of auroras=== |
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| − | According to Clark (2007), there are four main forms that can be seen from the ground, from least to most visible:<ref name=Clake2007>{{Cite journal|doi=10.1016/j.endeavour.2007.07.004|title=Astronomical fire: Richard Carrington and the solar flare of 1859|journal= Endeavour|volume=31|issue=3|pages=104–109|year=2007|last1=Clark|first1=Stuart|pmid=17764743}}</ref> |
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| − | [[File:Aurora shapes.jpg|thumb|Different forms]] |
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| − | * A mild ''glow'', near the horizon. These can be close to the limit of visibility,<ref>{{Cite journal |doi=10.1016/S1364-6826(96)00113-7 |title=Polar cap arcs: A review |journal=Journal of Atmospheric and Solar-Terrestrial Physics |volume=59 |issue=10 |pages=1087 |year=1997 |last1=Zhu |first1=L. |last2=Schunk |first2=R. W. |last3=Sojka |first3=J. J. |bibcode=1997JASTP..59.1087Z }}</ref> but can be distinguished from moonlit clouds because stars can be seen undiminished through the glow. |
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| − | * ''Patches'' or ''surfaces'' that look like clouds. |
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| − | * ''Arcs'' curve across the [[sky]]. |
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| − | * ''Rays'' are light and dark stripes across arcs, reaching upwards by various amounts. |
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| − | * ''Coronas'' cover much of the sky and diverge from one point on it. |
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| − | Brekke (1994) also described some auroras as ''curtains''.<ref name=Brekke1994>{{cite book |last1=A |first1=Brekke |last2=A |first2=Egeland |title=The Northern Lights |date=1994 |publisher=Grøndahl and Dreyer, Oslo |isbn=978-82-504-2105-9 |page=137}}</ref> The similarity to curtains is often enhanced by folds within the arcs. Arcs can fragment or break up into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are also known as ''discrete auroras'', which are at times bright enough to read a newspaper by at night.<ref name= Yahninet1997>{{Cite journal |doi=10.1007/s00585-997-0943-z |title=Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles |journal=Annales Geophysicae |volume=15 |issue=8 |pages=943 |year=1997 |last1=Yahnin |first1=A. G. |last2=Sergeev |first2=V. A. |last3=Gvozdevsky |first3=B. B. |last4=Vennerstrøm |first4=S. |bibcode=1997AnGeo..15..943Y |doi-access=free }}</ref> |
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| − | These forms are consistent with auroras' being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the [[Perspective (graphical)|shapes of the luminous parts of the atmosphere and a viewer's position]].<ref name=Thomson1917>{{Cite journal |doi=10.1073/pnas.3.1.1|pmid=16586674|pmc=1091158|title=Inferences concerning auroras|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=3|issue=1|pages=1–7|year=1917|last1= Thomson |first1=E.|bibcode=1917PNAS....3....1T}}</ref> |
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| − | ===Colors and wavelengths of auroral light=== |
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| − | * '''Red''': At its highest altitudes, excited atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. The low number of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are the most often-seen hues of red for the auroras. |
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| − | * '''Green''': At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to the high stability of the N<sub>2</sub> molecule) plays a role here, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to [[forbidden transition]]s of atomic oxygen, a slow mechanism responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading. |
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| − | * '''Blue''': At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity.<ref>{{cite web|title=Windows to the Universe – Auroral colors and spectra|url=http://www.windows2universe.org/earth/Magnetosphere/tour/tour_earth_magnetosphere_09.html}}</ref> The molecular nitrogen transitions are much faster than the atomic oxygen ones. |
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| − | * '''Ultraviolet''': Ultraviolet radiation from auroras (within the optical window but not visible to virtually all{{Clarify|reason=vague|date=July 2020}} humans) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars,<ref name="sci-news.com">{{cite web|url=http://www.sci-news.com/space/science-nasas-maven-ultraviolet-aurora-mars-02614.html|title=NASA's MAVEN Orbiter Detects Ultraviolet Aurora on Mars | Space Exploration | Sci-News.com|publisher=sci-news.com|accessdate=16 August 2015}}</ref> Jupiter and Saturn. |
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| − | * '''Infrared''': Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras.<ref name="sci-news.com"/><ref name="dapep">{{cite web|url=http://www.dapep.org/DAPT/EM-Wiki/aurora-borealis.html|title=Aurora Borealis|publisher=dapep.org|accessdate=16 August 2015}}</ref> |
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| − | * '''Yellow''' and '''pink''' are [[Additive colour|a mix]] of red and green or blue. Other shades of red, as well as orange, may be seen on rare occasions; yellow-green is moderately common.{{Clarify|reason=vague|date=July 2020}} As red, green, and blue are the primary colors of additive synthesis of colors, in theory, practically any color might be possible, but the ones mentioned in this article comprise a virtually exhaustive list. |
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| − | |||
| − | ===Changes with time=== |
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| − | Auroras change with time. Over the night, they begin with glows and progress towards coronas, although they may not reach them. They tend to fade in the opposite order.<ref name=Brekke1994 /> |
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| − | At shorter time scales, auroras can change their appearances and intensity, sometimes so slowly as to be difficult to notice, and at other times rapidly down to the sub-second scale.<ref name = Yahninet1997 /> The phenomenon of pulsating auroras is an example of intensity variations over short timescales, typically with periods of 2–20 seconds. This type of aurora is generally accompanied by decreasing peak emission heights of about 8 km for blue and green emissions and above average solar wind speeds (~ 500 km/s).<ref>{{Cite journal|last1=Partamies|first1=N.|last2=Whiter|first2=D.|last3=Kadokura|first3=A.|last4=Kauristie|first4=K.|last5=Tyssøy|first5=H. Nesse|last6=Massetti|first6=S.|last7=Stauning|first7=P.|last8=Raita|first8=T.|date=2017|title=Occurrence and average behavior of pulsating aurora|journal=Journal of Geophysical Research: Space Physics|language=en|volume=122|issue=5|pages=5606–5618|doi=10.1002/2017JA024039|bibcode=2017JGRA..122.5606P|issn=2169-9402|url=http://urn.fi/urn:nbn:fi-fe2019092429533}}</ref> |
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| − | |||
| − | ===Other auroral radiation=== |
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| − | In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as [[auroral kilometric radiation]] (AKR), discovered in 1972.<ref>{{cite journal |last1=Gurnett |first1=D.A. |title=The Earth as a radio source |journal=Journal of Geophysical Research |year=1974 |volume=79 |issue=28 |page=4227 |bibcode=1974JGR....79.4227G |doi=10.1029/JA079i028p04227 }}</ref> Ionospheric absorption makes AKR only observable from space. X-ray emissions, originating from the particles associated with auroras, have also been detected.<ref>{{cite journal |last1=Anderson |first1=K.A. |title=Balloon observations of X-rays in the auroral zone |journal=Journal of Geophysical Research |year=1960 |volume=65 |issue=2 |pages=551–564 |doi=10.1029/jz065i002p00551 |bibcode=1960JGR....65..551A }}</ref> |
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| − | |||
| − | === Aurora noise === |
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| − | Aurora [[noise]], similar to a hissing, or crackling noise, begins about {{convert|70|m|ft|abbr=on}} above the Earth's surface and is caused by charged particles in an [[Inversion (meteorology)|inversion]] layer of the atmosphere formed during a cold night. The charged particles discharge when particles from the Sun hit the inversion layer, creating the noise.<ref>{{Cite web|url=http://news.nationalgeographic.com/2016/06/auroras-sounds-noises-explained-earth-space-astronomy|title=Auroras Make Weird Noises, and Now We Know Why|date=27 June 2016|access-date=28 June 2016}}</ref><ref>{{Cite web|url=http://elec.aalto.fi/en/current/news/2016-06-22/|title=News: Acoustics researcher finds explanation for auroral sounds|date=21 June 2016|access-date=28 June 2016}}</ref> |
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| − | |||
| − | ===Atypical auroras=== |
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| − | |||
| − | ====STEVE==== |
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| − | In 2016 more than fifty [[citizen science]] observations described what was to them an unknown type of aurora which they named "[[Steve (atmospheric phenomenon)|STEVE]]," for "Strong Thermal Emission Velocity Enhancement." But STEVE is not an aurora but is caused by a {{convert|25|km|mi|abbr=on}} wide ribbon of hot [[plasma (physics)|plasma]] at an altitude of {{convert|450|km|mi|abbr=on}}, with a temperature of {{convert|6000|K|C F|abbr=on}} and flowing at a speed of {{convert|6|km/s|mi/s|abbr=on}} (compared to {{convert|10|m/s|ft/s|abbr=on}} outside the ribbon).<ref name=PhysOrg>{{Cite news|url=https://phys.org/news/2018-08-kind-aurora.html|title=New kind of aurora is not an aurora at all|last=American Geophysical Union|date=20 August 2018|work=Physorg.com|access-date=21 August 2018}}</ref> |
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| − | |||
| − | ====Picket-fence aurora==== |
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| − | The processes that cause STEVE also are associated with a picket-fence aurora, although the latter can be seen without STEVE.<ref>{{cite web |last1=Andrews |first1=Robin George |title=Steve the odd 'aurora' revealed to be two sky shows in one |url=https://www.nationalgeographic.com/science/2019/05/odd-aurora-named-steve-revealed-to-be-two-different-sky-shows-in-one/ |website=National Geographic |publisher=National Geographic |accessdate=4 May 2019 |date=3 May 2019}}</ref><ref name="ReferenceA">{{cite journal |last1=Nishimura |first1=Y. |last2=Gallardo‐Lacourt |first2=B. |last3=Zou |first3=Y. |last4=Mishin |first4=E. |last5=Knudsen |first5=D.J. |last6=Donovan |first6=E.F. |last7=Angelopoulos |first7=V. |last8=Raybell |first8=R. |title=Magnetospheric signatures of STEVE: Implication for the magnetospheric energy source and inter‐hemispheric conjugacy |journal=Geophysical Research Letters |volume=46 |issue=11 |pages=5637–5644 |date=16 April 2019 |doi=10.1029/2019GL082460|bibcode=2019GeoRL..46.5637N |doi-access=free }}</ref> It is an aurora because it is caused by precipitation of electrons in the atmosphere but it appears outside the auroral oval,<ref>{{cite web |last1=Lipuma |first1=Lauren |title=Scientists discover what powers celestial phenomenon STEVE |url=https://news.agu.org/press-release/scientists-discover-what-powers-celestial-phenomenon-steve/ |website=AGU News |publisher=American Geophysical Union |accessdate=4 May 2019}}</ref> closer to the [[equator]] than typical auroras.<ref>{{Cite web|url=https://www.theguardian.com/science/shortcuts/2018/mar/19/steve-mystery-purple-aura-rivals-northern-lights-alberta-canada-nasa|title='Steve': the mystery purple aurora that rivals the northern lights|last=Saner|first=Emine|date=19 March 2018|website=the Guardian|language=en|access-date=22 March 2018}}</ref> When the picket-fence aurora appears with STEVE, it is below.<ref name="ReferenceA"/> |
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| − | |||
| − | ==Causes== |
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| − | A full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves the interaction 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. |
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| − | # A quiescent solar wind flowing past the Earth's magnetosphere steadily interacts with it and can both inject solar wind particles directly onto the geomagnetic field lines that are 'open', as opposed to being 'closed' in the opposite hemisphere, and provide diffusion through the [[bow shock]]. It can also cause particles already trapped in the [[Van Allen radiation belt|radiation belts]] to precipitate into the atmosphere. Once particles are lost to the atmosphere from the radiation belts, under quiet conditions, new ones replace them only slowly, and the loss-cone becomes depleted. In the magnetotail, however, particle trajectories seem constantly to reshuffle, probably when the particles cross the very weak magnetic field near the equator. As a result, the flow of electrons in that region is nearly the same in all directions ("isotropic") and assures a steady supply of leaking electrons. The leakage of electrons does not leave the tail positively charged, because each leaked electron 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 an electric ring current around the Earth, is uncertain. |
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| − | # Geomagnetic disturbance from an enhanced [[solar wind]] causes distortions of the [[magnetosphere|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 the obstacle it presents to the solar wind flow and constricting the tail on the night-side. Ultimately some tail plasma can separate ("[[magnetic reconnection]]"); some blobs ("[[plasmoid]]s") are squeezed downstream and are carried away with the solar wind; others are squeezed toward Earth where their motion feeds strong outbursts of auroras, mainly around midnight ("unloading process"). A geomagnetic storm resulting from greater interaction adds many more particles to the plasma trapped around Earth, also producing enhancement of the "ring current". Occasionally the resulting modification of the Earth's magnetic field can be so strong that it produces auroras visible at middle latitudes, on field lines much closer to the equator than those of the auroral zone. |
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| − | #: [[File:Moon and Aurora.jpg|thumb|Moon and Aurora]] |
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| − | # Acceleration of auroral charged particles invariably accompanies a magnetospheric disturbance that causes an aurora. This mechanism, which is believed to predominantly arise from strong electric fields along the magnetic field or wave-particle interactions, raises the velocity of a particle in the direction of the guiding magnetic field. The pitch angle is thereby decreased and increases the chance of it being precipitated into the atmosphere. Both electromagnetic and electrostatic waves, produced at the time of greater geomagnetic disturbances, make a significant contribution to the energizing processes that sustain an aurora. Particle acceleration provides a complex intermediate process for transferring energy from the solar wind indirectly into the atmosphere. |
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| − | [[File:Aurora australis 20050911.jpg|right|thumb|Aurora australis (11 September 2005) as captured by NASA's [[IMAGE (spacecraft)|IMAGE]] satellite, digitally overlaid onto ''[[The Blue Marble]]'' composite image. |
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| − | [[:Image:Aurora Australis.gif|An animation]] created using the same satellite data is also available]] |
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| − | The details of these phenomena are not fully understood. However, it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiation zones and temporarily magnetically-trapped particles confined by the geomagnetic field, coupled with particle acceleration processes.<ref>{{cite book |last1=Burch |first1=J L |editor1-last=Akasofu S-I and Y Kamide |title=The solar wind and the Earth |date=1987 |publisher=D. Reidel |isbn=978-90-277-2471-7 |page=103}}</ref> |
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| − | |||
| − | ===Auroral particles=== |
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| − | The immediate cause of the ionization and excitation of atmospheric constituents leading to auroral emissions was discovered in 1960, when a pioneering rocket flight from Fort Churchill in Canada revealed a flux of electrons entering the atmosphere from above.<ref>{{cite journal |last1=McIlwain |first1=C E |title=Direct Measurement of Particles Producing Visible Auroras |journal=Journal of Geophysical Research |year=1960 |volume=65 |issue=9 |page=2727 |doi=10.1029/JZ065i009p02727 |bibcode=1960JGR....65.2727M}}</ref> Since then an extensive collection of measurements has been acquired painstakingly and with steadily improving resolution since the 1960s by many research teams using rockets and satellites to traverse the auroral zone. The main findings have been that auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10,000 km or so of their plunge into the atmosphere.<ref>{{Cite journal |doi=10.1029/JA093iA07p07441 |title=Determination of auroral electrostatic potentials using high- and low-altitude particle distributions |journal=Journal of Geophysical Research |volume=93 |issue=A7 |pages=7441 |year=1988 |last1=Reiff |first1=P. H. |last2=Collin |first2=H. L. |last3=Craven |first3=J. D. |last4=Burch |first4=J. L. |last5=Winningham |first5=J. D. |last6=Shelley |first6=E. G. |last7=Frank |first7=L. A. |last8=Friedman |first8=M. A. |bibcode=1988JGR....93.7441R }}</ref> These electrons often, but not always, exhibit a peak in their energy distribution, and are preferentially aligned along the local direction of the magnetic field. |
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| − | Electrons mainly responsible for diffuse and pulsating auroras have, in contrast, a smoothly falling energy distribution, and an angular (pitch-angle) distribution favouring directions perpendicular to the local magnetic field. Pulsations were discovered to originate at or close to the equatorial crossing point of auroral zone magnetic field lines.<ref>{{Cite journal |doi=10.1038/215045a0 |title=Evidence for Velocity Dispersion in Auroral Electrons |journal=Nature |volume=215 |issue=5096 |pages=45 |year=1967 |last1=Bryant |first1=D. A. |last2=Collin |first2=H. L. |last3=Courtier |first3=G. M. |last4=Johnstone |first4=A. D. |bibcode=1967Natur.215...45B |s2cid=4173665 }}</ref> Protons are also associated with auroras, both discrete and diffuse. |
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| − | |||
| − | === Auroras and the atmosphere === |
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| − | Auroras result from emissions of [[photon]]s in the Earth's upper [[Earth's atmosphere|atmosphere]], above {{convert|80|km|mi|sp=us|abbr=on}}, from [[ionized]] [[nitrogen]] atoms regaining an electron, and [[oxygen]] atoms and [[nitrogen]] based molecules returning from an [[excited state]] to [[ground state]].<ref>{{cite web|title=Ultraviolet Waves|url=http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|url-status=dead|archiveurl=https://web.archive.org/web/20110127004149/http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|archivedate=27 January 2011}}</ref> They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule: |
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| − | ;[[oxygen]] emissions: green or orange-red, depending on the amount of energy absorbed. |
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| − | ;[[nitrogen]] emissions:blue, purple or red; blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state. |
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| − | |||
| − | Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light and up to two minutes for the red 630.0 nm emission. Collisions with other atoms or molecules absorb the excitation energy and prevent emission, this process is called [[Quenching (fluorescence)|collisional quenching]]. Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so that red emissions do not have time to happen, and eventually, even green light emissions are prevented. |
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| − | This is why there is a color differential with altitude; at high altitudes oxygen red dominates, then oxygen green and nitrogen blue/purple/red, then finally nitrogen blue/purple/red when collisions prevent oxygen from emitting anything. Green is the most common color. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue. |
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| − | |||
| − | ===Auroras and the ionosphere=== |
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| − | Bright auroras are generally associated with [[Birkeland current]]s (Schield et al., 1969;<ref>{{cite journal |doi=10.1029/JA074i001p00247 |last1=Schield |first1=M. A. |last2=Freeman |first2=J. W. |last3=Dessler |first3=A. J. |year=1969 |title=A Source for Field-Aligned Currents at Auroral Latitudes |journal=Journal of Geophysical Research |volume=74 |issue=1 |pages=247–256 |bibcode=1969JGR....74..247S}}</ref> Zmuda and Armstrong, 1973<ref>{{cite journal |doi=10.1029/JA078i028p06802 |last1=Armstrong |first1=J. C. |last2=Zmuda |first2=A. J. |year=1973 |title=Triaxial magnetic measurements of field-aligned currents at 800 kilometers in the auroral region: Initial results |journal=Journal of Geophysical Research |volume=78 |issue=28 |pages=6802–6807 |bibcode=1973JGR....78.6802A}}</ref>), 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 [[Ohm's law|ohmic conductor]], so some consider that such currents require a driving voltage, which an, as yet unspecified, 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. In another interpretation, the currents are the direct result of electron acceleration into the atmosphere by wave/particle interactions. |
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| − | |||
| − | 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. [[Kristian Birkeland]]<ref name="NAPE">{{cite book |last=Birkeland |first=Kristian |title=The Norwegian Aurora Polaris Expedition 1902–1903 |date=1908 |publisher=H. Aschehoug & Co. |location=New York: Christiania (Oslo) |page=720 |url=https://archive.org/details/norwegianaurorap01chririch}} out-of-print, full text online</ref> deduced that the currents flowed in the east–west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named "auroral electrojets" (see also [[Birkeland current]]s). |
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| − | |||
| − | ==Interaction of the solar wind with Earth== |
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| − | The Earth is constantly immersed in the [[solar wind]], a rarefied flow of magnetized hot plasma (a gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree temperature of the Sun's outermost layer, the [[solar corona|corona]]. The quiescent solar wind reaches Earth with a velocity typically around 400 km/s, a density of around 5 ions/cm<sup>3</sup> and a magnetic field intensity of around 2–5 nT (for comparison, Earth's surface field is typically 30,000–50,000 nT). During [[Geomagnetic storm|magnetic storms]], in particular, flows can be several times faster; the [[interplanetary magnetic field]] (IMF) may also be much stronger. [[Joan Feynman]] deduced in the 1970s that the long-term averages of solar wind speed correlated with geomagnetic activity.<ref>{{cite journal|url=https://ntrs.nasa.gov/search.jsp?R=19770051690|title=On the high correlation between long-term averages of solar wind speed and geomagnetic activity|journal=Journal of Geophysical Research|author1=Crooker, N. U. |author2=Feynman, J. |author3=Gosling, J. T. |date=1 May 1977|volume=82|issue=13|page=1933|doi=10.1029/JA082i013p01933|bibcode=1977JGR....82.1933C}}</ref> Her work resulted from data collected by the [[Explorer 33]] spacecraft. |
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| − | The solar wind and magnetosphere consist of [[Plasma (physics)|plasma]] (ionized gas), which conducts electricity. It is well known (since [[Michael Faraday]]'s 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 electric current is induced within the conductor. The strength of the current depends on a) the rate of relative motion, 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. [[Dynamo]]s make use of this basic process ("the [[dynamo theory|dynamo effect]]"), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids. |
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| − | The IMF originates on the Sun, linked to the [[sunspot]]s, and its [[magnetism|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 angles them at Earth by about 45 degrees forming a spiral in the ecliptic plane), known as the [[Eugene Parker|Parker spiral]]. The field lines passing Earth are therefore usually linked to those near the western edge ("limb") of the visible Sun at any time.<ref>[http://gse.gi.alaska.edu/recent/javascript_movie.html Alaska.edu] {{webarchive|url=https://web.archive.org/web/20061220050940/http://gse.gi.alaska.edu/recent/javascript_movie.html |date=20 December 2006 }}, Solar wind forecast from a [[University of Alaska]] website</ref> |
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| − | The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process is hampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy is more effectively transferred by the temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly this process 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]]. |
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| − | [[File:Structure_of_the_magnetosphere_LanguageSwitch.svg|lang=en|thumb|Schematic of Earth's [[magnetosphere]]]] |
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| − | Auroras are more frequent and brighter during the intense phase of the solar cycle when [[coronal mass ejections]] increase the intensity of the solar wind.<ref>{{cite web|url=http://www.nasa.gov/worldbook/aurora_worldbook.html |archive-url=https://web.archive.org/web/20050905165404/http://www.nasa.gov/worldbook/aurora_worldbook.html |url-status=dead |archive-date=5 September 2005 |title=NASA – NASA and World Book |publisher=Nasa.gov |date=7 February 2011 |accessdate=26 July 2011}}</ref> |
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| − | |||
| − | ===Magnetosphere=== |
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| − | Earth's [[magnetosphere]] is shaped by the impact of the solar wind on the Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re),<ref>{{cite journal |last1=Shue |first1=J.-H |first2=J. K. |last2=Chao |first3=H. C. |last3=Fu |first4=C. T. |last4=Russell |first5=P. |last5=Song |first6=K. K. |last6=Khurana |first7=H. J. |last7=Singer |title=A new functional form to study the solar wind control of the magnetopause size and shape |journal=J. Geophys. Res. |date=May 1997 |volume=102 |issue=A5 |pages=9497–9511 |doi=10.1029/97JA00196 |bibcode=1997JGR...102.9497S }}</ref> producing a [[bow shock]] 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of the 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). |
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| − | The high latitude magnetosphere is filled with plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favored by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines.<ref>{{cite journal |last1=Lyons |first1=L. R. |first2=H.-J. |last2=Kim |first3=X. |last3=Xing |first4=S. |last4=Zou |first5=D.-Y. |last5=Lee |first6=C. |last6=Heinselman |first7=M. J. |last7=Nicolls |first8=V. |last8=Angelopoulos |first9=D. |last9=Larson |first10=J. |last10=McFadden |first11=A. |last11=Runov |first12=K.-H. |last12=Fornacon |title=Evidence that solar wind fluctuations substantially affect global convection and substorm occurrence |journal=J. Geophys. Res. |year=2009 |volume=114 |issue=A11306 |pages=1–14 |doi=10.1029/2009JA014281 |bibcode=2009JGRA..11411306L }}</ref> The flow pattern of magnetospheric plasma is mainly from the magnetotail toward the Earth, around the Earth and back into the solar wind through the [[magnetopause]] on the day-side. In addition to moving perpendicular to the Earth's magnetic field, some magnetospheric plasma travels down along the Earth's magnetic field lines, gains additional energy and loses it to the atmosphere in the auroral zones. The cusps of the magnetosphere, separating geomagnetic field lines that close through the Earth from those that close remotely allow a small amount of solar wind to directly reach the top of the atmosphere, producing an auroral glow. |
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| − | On 26 February 2008, [[THEMIS]] probes were able to determine, for the first time, the triggering event for the onset of [[magnetospheric substorm]]s.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/themis/auroras/themis_power.html |title=NASA – THEMIS Satellites Discover What Triggers Eruptions of the Northern Lights |publisher=Nasa.gov |accessdate=26 July 2011| archiveurl= https://web.archive.org/web/20110629043044/http://www.nasa.gov/mission_pages/themis/auroras/themis_power.html| archivedate= 29 June 2011 | url-status=live}}</ref> Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a [[magnetic reconnection]] event 96 seconds prior to auroral intensification.<ref>{{cite journal |doi=10.1126/science.1160495 |title=Tail Reconnection Triggering Substorm Onset |year=2008 |last1=Angelopoulos |first1=V. |last2=McFadden |first2=J. P. |last3=Larson |first3=D. |last4=Carlson |first4=C. W. |last5=Mende |first5=S. B. |last6=Frey |first6=H. |last7=Phan |first7=T. |last8=Sibeck |first8=D. G. |last9=Glassmeier |first9=K.-H. |journal=Science |volume=321 |issue=5891 |pages=931–5 |pmid=18653845 |bibcode=2008Sci...321..931A |last10=Auster |first10=U. |last11=Donovan |first11=E. |last12=Mann |first12=I. R. |last13=Rae |first13=I. J. |last14=Russell |first14=C. T. |last15=Runov |first15=A. |last16=Zhou |first16=X.-Z. |last17=Kepko |first17=L. |s2cid=206514133 }}</ref> |
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| − | [[Geomagnetic storm]]s that ignite auroras may occur more often during the months around the [[equinox]]es. It is not well understood, but geomagnetic 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 at 8 degrees. Similarly, the 23-degree tilt of the Earth's axis about which the geomagnetic pole rotates with a diurnal variation changes the daily average angle that the geomagnetic field presents to the incident IMF throughout a year. These factors combined can lead to minor cyclical changes in the detailed way that the IMF links to the magnetosphere. In turn, this affects the average probability of opening a door through which energy from the solar wind can reach the Earth's inner magnetosphere and thereby enhance auroras. |
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| − | ==Auroral particle acceleration== |
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| − | The electrons responsible for the brightest forms of the aurora are well accounted for by their acceleration in the dynamic electric fields of plasma turbulence encountered during precipitation from the magnetosphere into the auroral atmosphere. In contrast, static electric fields are unable to transfer energy to the electrons due to their conservative nature.<ref>{{cite book |last1=Bryant |first1=Duncan |title=Electron-Acceleration-in-the-Aurora-and-Beyond |date=1998 |publisher=Institute of Physics Publishing Ltd |location=Bristol & Philadelphia |isbn=978-0750305334 |url=https://www.crcpress.com/Electron-Acceleration-in-the-Aurora-and-Beyond/Bryant/9780750305334 |page=163}}</ref> The electrons and ions that cause the diffuse aurora appear not to be accelerated during precipitation. |
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| − | The increase in strength of magnetic field lines towards the Earth creates a 'magnetic mirror' that turns back many of the downward flowing electrons. The bright forms of auroras are produced when downward acceleration not only increases the energy of precipitating electrons but also reduces their pitch angles (angle between electron velocity and the local magnetic field vector). This greatly increases the rate of deposition of energy into the atmosphere, and thereby the rates of ionization, excitation and consequent emission of auroral light. Acceleration also increases the electron current flowing between the atmosphere and magnetosphere. |
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| − | One early theory proposed for the acceleration of auroral electrons is based on an assumed static, or quasi-static, electric field creating a uni-directional potential drop.<ref name="Evidence for the low altitude acceleration of auroral particles">{{cite book |last1=Evans |first1=D S |title=Hot Plasma in the Magnetosphere |date=1975 |publisher=Plenum Press |location=New York and London |isbn=978-0306337000 |pages=319–340}}</ref> No mention is provided of either the necessary space-charge or equipotential distribution, and these remain to be specified for the notion of acceleration by double layers to be credible. Fundamentally, [[Poisson's equation]] indicates that there can be no configuration of charge resulting in a net potential drop. Inexplicably though, some authors<ref>Boström, Rolf "Observations of weak double layers on auroral field lines" (1992) IEEE Transactions on Plasma Science ({{ISSN|0093-3813}}), vol. 20, no. 6, pp. 756–763</ref><ref>Ergun, R. E., et al. "Parallel electric fields in the upward current region of the aurora: Indirect and direct observations" (2002) Physics of Plasmas, Volume 9, Issue 9, pp. 3685–3694</ref> still invoke quasi-static parallel electric fields as net accelerators of auroral electrons, citing interpretations of transient observations of fields and particles as supporting this theory as firm fact. In another example,<ref>{{cite journal |last1=Carlson |first1=C.W., R.F.Pfaff and J.G.Watzin |title=The Fast Auroral SnapshoT (FAST) mission |journal=Geophysical Research Letters |date=June 1998 |volume=25 |issue=12 |pages=2013–2016|bibcode = 1998GeoRL..25.2013C |doi = 10.1029/98GL01592 }}</ref> there is little justification given for saying 'FAST observations demonstrate detailed quantitative agreement between the measured electric potentials and the '''ion beam''' energies...., leaving no doubt that parallel potential drops are a dominant source of '''auroral particle''' acceleration'. |
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| − | Another theory is based on acceleration by Landau<ref>{{cite web|url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Landau_Lev.html|website=history.mcs.st-andrews.ac.uk |title=Lev Davidovich Landau}}</ref> resonance in the turbulent electric fields of the acceleration region. This process is essentially the same as that employed in plasma fusion laboratories throughout the world,<ref name="Radio-frequency Plasma Heating">{{cite book |last1=Cairns |first1=R A |editor1-last=Dendy |editor1-first=R O |title=Plasma Physics:An Introductory Course |date=1993 |publisher=Cambridge University Press |isbn=978-0521433099 |pages=391–410}}</ref> and appears well able to account in principle for most – if not all – detailed properties of the electrons responsible for the brightest forms of auroras, above, below and within the acceleration region.<ref>{{cite journal |last1=Bryant |first1=D A |last2=Perry |first2=C H |title=Velocity-space distributions of wave-accelerated auroral electrons |journal=Journal of Geophysical Research |year=1995 |volume=100 |issue=A12 |pages=23,711–23,725 |doi=10.1029/95ja00991 |bibcode=1995JGR...10023711B }}</ref> |
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| − | [[File:Aurora Borealis.jpg|thumb|left|The aurora borealis as viewed from the International Space Station|ISS [[Expedition 6]] team, [[Lake Manicouagan]] is visible to the bottom left]] |
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| − | Other mechanisms have also been proposed, in particular, [[Alfvén waves]], wave modes involving the magnetic field first noted by [[Hannes Alfvén]] (1942),<ref>{{cite journal |last1=Alfvén |first1=Hannes |title=Existence of electromagnetic-hydrodynamic waves |journal=Nature |date=3 October 1942 |volume=150 |issue=3805 |pages=405–406|doi=10.1038/150405d0 |bibcode=1942Natur.150..405A |s2cid=4072220 }}</ref> which have been observed in the laboratory and in space. The question is whether these waves might just be a different way of looking at the above process, however, 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. |
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| − | 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. |
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| − | 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 "mirror points" and can travel only upward. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the field line to a cone around it, which gradually narrows down, becoming increasingly parallel at large distances where the field is much weaker. |
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| − | ==Auroral events of historical significance== |
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| − | The discovery of a 1770 Japanese [[diary]] in 2017 depicting auroras above the ancient Japanese capital of [[Kyoto]] suggested that the storm may have been 7% larger than the [[Carrington event]], which affected telegraph networks.<ref name="1770 Japanese diary">{{Cite web |url=http://www.atlasobscura.com/articles/aurora-kyoto-1770-painting-science-magnetic-storm |title=1770 Kyoto Diary |last=Frost |first=Natasha |date=4 October 2017 |website=Atlas Obscura |access-date=13 October 2017}}</ref><ref name="Inclined zenith aurora over Kyoto on 17 September 1770: Graphical evidence of extreme magnetic storm">{{Cite journal |title=Inclined zenith aurora over Kyoto on 17 September 1770: Graphical evidence of extreme magnetic storm |journal=Space Weather |volume=15 |issue=10 |pages=1314–1320 |date=17 September 2017 |doi = 10.1002/2017SW001690|last1 = Kataoka|first1 = Ryuho|last2=Iwahashi |first2=Kiyomi |bibcode=2017SpWea..15.1314K }}</ref> |
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| − | The auroras that resulted from the "[[Solar storm of 1859|great geomagnetic storm]]" on both 28 August and 2 September 1859, however, are thought to be the most spectacular in recent recorded history. In a paper to the [[Royal Society]] on 21 November 1861, Balfour Stewart described both auroral events as documented by a self-recording [[magnetograph]] at the [[Kew Observatory]] and established the connection between the 2 September 1859 auroral storm and the [[Richard Christopher Carrington|Carrington]]-Hodgson flare event when he observed that "It is not impossible to suppose that in this case our luminary was taken ''in the act''."<ref>{{cite journal |last1=Stewart |first1=Balfour |title=On the Great Magnetic Disturbance of 28 August to 7 September 1859, as Recorded by Photography at the Kew Observatory |journal=Philosophical Transactions of the Royal Society of London |date=1861 |volume=151 |pages=423–430 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000054593107&view=1up&seq=461|doi=10.1098/rstl.1861.0023 |doi-access=free }} See p. 428.</ref> The second auroral event, which occurred on 2 September 1859 as a result of the exceptionally intense Carrington-Hodgson white light [[solar flare]] on 1 September 1859, produced auroras, so widespread and extraordinarily bright, that they were seen and reported in published scientific measurements, ship logs, and newspapers throughout the United States, Europe, Japan, and Australia. It was reported by ''[[The New York Times]]'' that 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".<ref name="ADVSR1">{{cite journal |doi=10.1016/j.asr.2005.12.021 |title=Eyewitness reports of the great auroral storm of 1859 |journal=Advances in Space Research |volume=38 |issue=2 |year=2006 |pages=145–54 |last1=Green |first1=J |last2=Boardsen |first2=S |last3=Odenwald |first3=S |last4=Humble |first4=J |last5=Pazamickas |first5=K |bibcode=2006AdSpR..38..145G|hdl=2060/20050210157 |hdl-access=free }}</ref> 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 recording the [[geomagnetic storm]], which was then one hour old, at its full intensity. Between 1859 and 1862, [[Elias Loomis]] published a series of nine papers on the [[Elias Loomis#Great Auroral Exhibition of 1859|Great Auroral Exhibition of 1859]] in the ''[[American Journal of Science]]'' where he collected worldwide reports of the auroral event.<ref name="Loomis">See: |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September, 1859 |journal=The American Journal of Science |date=November 1859 |volume=28 |pages=385–408 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679510&view=1up&seq=403 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—2nd article |journal=The American Journal of Science |date=January 1860 |volume=29 |pages=92–97 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=112 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—3rd article |journal=The American Journal of Science |date=February 1860 |volume=29 |pages=249–266 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=269 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—4th article |journal=The American Journal of Science |date=May 1860 |volume=29 |pages=386–399 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=406 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859, and the geographical distribution of auroras and thunder storms—5th article |journal=The American Journal of Science |date=July 1860 |volume=30 |pages=79–100 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679512&view=1up&seq=93 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—6th article |journal=The American Journal of Science |date=November 1860 |volume=30 |pages=339–361 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679512&view=1up&seq=363 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—7th article |journal=The American Journal of Science |date=July 1861 |volume=32 |pages=71–84 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679513&view=1up&seq=85 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=On the great auroral exhibition of August 28 to September 4, 1859, and auroras generally—8th article |journal=The American Journal of Science |date=September 1861 |volume=32 |pages=318–335 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679513&view=1up&seq=334 |series=2nd series}} |
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| − | * {{cite journal |last1=Loomis |first1=Elias |title=On electrical currents circulating near the earth's surface and their connection with the phenomena of the aurora polaris—9th article |journal=The American Journal of Science |date=July 1862 |volume=34 |pages=34–45 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679515&view=1up&seq=62 |series=2nd series}}</ref> |
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| − | That aurora is thought to have been produced by one of the most intense [[coronal mass ejection]]s in history. 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 {{convert|125000|mi|km}} of [[electrical telegraph|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 to produce a sufficient [[geomagnetically induced current]] from the [[electromagnetic field]] to allow for continued communication with the telegraph operator power supplies switched off.<ref>{{cite journal |last1=Loomis |first1=Elias |title=The great auroral exhibition of August 28 to September 4, 1859—2nd article |journal=The American Journal of Science |date=January 1860 |volume=29 |pages=92–97 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=112 |series=2nd series}}</ref> The following conversation occurred between two operators of the American Telegraph Line between [[Boston]] and [[Portland, Maine]], on the night of 2 September 1859 and reported in the ''Boston Traveler'': |
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| − | {{Quotation| |
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| − | '''Boston operator (to Portland operator):''' "Please cut off your battery [power source] entirely for fifteen minutes."<br /> |
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| − | '''Portland operator:''' "Will do so. It is now disconnected."<br /> |
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| − | '''Boston:''' "Mine is disconnected, and we are working with the auroral current. How do you receive my writing?"<br /> |
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| − | '''Portland:''' "Better than with our batteries on. – Current comes and goes gradually."<br /> |
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| − | '''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."<br /> |
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| − | '''Portland:''' "Very well. Shall I go ahead with business?"<br /> |
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| − | '''Boston:''' "Yes. Go ahead." |
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| − | }} |
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| − | The conversation was carried on for around two hours using no [[Battery (electricity)|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.<ref name="ADVSR1" /> Such events led to the general conclusion that |
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| − | {{Quotation|The effect of the aurorae on the electric telegraph is generally to increase or diminish the electric current generated in working the wires. Sometimes it entirely neutralizes them, so that, in effect, no fluid [current] is discoverable in them. The aurora borealis seems to be composed of a mass of electric matter, resembling in every respect, that generated by the electric galvanic battery. The currents from it change coming on the wires, and then disappear the mass of the aurora rolls from the horizon to the zenith.<ref>''The British Colonist'', Vol. 2 No. 56, 19 October 1859, p. 1, accessed online at [http://britishcolonist.ca/display.php?issue=18591019&pages=001,&terms=aurora BritishColonist.ca] {{webarchive|url=https://web.archive.org/web/20090831112729/http://britishcolonist.ca/display.php?issue=18591019&pages=001%2C&terms=aurora |date=31 August 2009 }}, on 19 February 2009.</ref>}} |
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| − | ==Historical theories, superstition and mythology== |
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| − | An aurora was described by the [[Ancient Greece|Greek]] [[explorer]] [[Pytheas]] in the 4th century BC.<ref>Macleod, ''Explorers: Great Tales of Adventure and Endurance'', p.21.</ref> [[Seneca the Younger|Seneca]] wrote about auroras in the first book of his ''[[Naturales Quaestiones]]'', classifying them, for instance as ''pithaei'' ('barrel-like'); ''chasmata'' ('chasm'); ''pogoniae'' ('bearded'); ''cyparissae'' ('like [[cypress]] trees'), and describing their manifold colors. He wrote about whether they were above or below the [[clouds]], and recalled that under [[Tiberius]], an aurora formed above the port city of [[Ostia Antica|Ostia]] that was so intense and red that a cohort of the army, stationed nearby for fire duty, galloped to the rescue.<ref>Clarke, J.,[https://archive.org/stream/physicalsciencei00seneiala#page/38/mode/2up ''Physical Science in the time of Nero''] p.39-41, London, Macmillan, (1910), accessed online on 1 January 2017.</ref> It has been suggested that [[Pliny the Elder]] depicted the aurora borealis in his ''[[Natural History (Pliny)|Natural History]]'', when he refers to ''trabes'', ''chasma'', 'falling red flames' and 'daylight in the night'.<ref>Bostock, J. and Riley, H.T., ''The Natural History of Pliny: Volume II'', London, Bohn (1855), accessed online at [https://archive.org/stream/naturalhistoryof11855plin#page/62/mode/2up/search/aurora], on 1 January 2017.</ref> |
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| − | The history of [[Chinese History|China]] has rich and, possibly the oldest, records of the aurora borealis. On an autumn around 2000 BC, according to a legend, a young woman named Fubao was sitting alone in the wilderness by a bay, when suddenly an "magical band of light" appeared like "moving clouds and flowing water", turning into a bright [[Halo (optical phenomenon)|halo]] around the [[Big Dipper]], which cascaded a pale silver brilliance, illuminating the earth and making shapes and shadows seem alive. Moved by this sight, Fubao became pregnant and gave birth to a song, the Emperor [[Xuanyuan]], known legendarily as the initiator of [[Chinese culture]] and the ancestor of all Chinese people.In the [[Shanhaijing]], a creature named 'Shilong' is described to be like a red dragon shining in the night sky with a body a thousand miles long.In ancient times, the Chinese did not have a fixed word for the aurora, so it was named according to the different shapes of the aurora, such as "Sky Dog(“天狗”)", "Sword/Knife Star(“刀星”)", "Chiyou banner(“蚩尤旗”)", "Sky's Open Eyes(“天开眼”)", and "Stars like Rain(“星陨如雨”)". |
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| − | In [[Japanese folklore]], [[pheasants]] were considered messengers from heaven. However, researchers from Japan's Graduate University for Advanced Studies and National Institute of Polar Research claimed in March 2020 that red pheasant tails witnessed across the night sky over Japan in 620 A.D., might be a red aurora produced during a magnetic storm.<ref>{{cite web|url=https://phys.org/news/2020-03-modern-science-reveals-ancient-secret.html|title=Modern science reveals ancient secret in Japanese literature|website=phys.org|date=30 March 2020}}</ref> |
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| − | [[File:Aurora australis.jpg|thumb|The Aboriginal Australians associated auroras (which are mainly low on the horizon and predominantly red) with fire.]] |
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| − | In the traditions of [[Aboriginal Australians]], the Aurora Australis is commonly associated with fire. For example, the [[Gunditjmara people]] of western [[Victoria (Australia)|Victoria]] called auroras ''puae buae'' ('ashes'), while the [[Gunai people]] of eastern Victoria perceived auroras as [[Wildfire|bushfires]] in the spirit world. The [[Diyari|Dieri]] people of [[South Australia]] say that an auroral display is ''kootchee'', an evil spirit creating a large fire. Similarly, the [[Ngarrindjeri|Ngarrindjeri people]] of South Australia refer to auroras seen over [[Kangaroo Island]] as the campfires of spirits in the 'Land of the Dead'. Aboriginal people in southwest [[Queensland]] believe the auroras to be the fires of the ''Oola Pikka'', ghostly spirits who spoke to the people through auroras. Sacred law forbade anyone except male elders from watching or interpreting the messages of ancestors they believed were transmitted through an aurora.<ref>{{cite journal |last=Hamacher |first=D.W. |title=Aurorae in Australian Aboriginal Traditions |journal=Journal of Astronomical History and Heritage |year=2013 |volume=16 |issue=2 |pages=207–219 |url=http://www.narit.or.th/en/files/2013JAHHvol16/2013JAHH...16..207H.pdf |arxiv=1309.3367 |bibcode=2013JAHH...16..207H |access-date=19 October 2013 |archive-url=https://web.archive.org/web/20131020181951/http://www.narit.or.th/en/files/2013JAHHvol16/2013JAHH...16..207H.pdf |archive-date=20 October 2013 |url-status=dead }}</ref> |
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| − | ''[[Bulfinch's Mythology]]'' relates that in [[Norse mythology]], the armour of the [[Valkyrie|Valkyrior]] "sheds a strange flickering light, which flashes up over the northern skies, making what Men call the 'aurora borealis', or 'Northern Lights' ".<ref>{{cite web |url=http://www.mythome.org/bxxxviii.html |title=Bullfinch's Mythology |publisher=Mythome.org |date=10 February 1996 |accessdate=5 August 2010 |url-status=dead |archiveurl=https://web.archive.org/web/20110214105537/http://www.mythome.org/bxxxviii.html |archivedate=14 February 2011}}</ref> There appears to be no evidence in [[Old Norse literature]] to substantiate this assertion.<ref>{{cite web|url=http://www.vikinganswerlady.com/njordrljos.htm |title=The Aurora Borealis and the Vikings |publisher=Vikinganswerlady.com |accessdate=5 August 2010}}</ref> The first Old Norse account of ''norðrljós'' is 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 [[glacier]]s could store energy so that they eventually became [[Fluorescence|fluorescent]].<ref>{{cite web |url=http://www.irf.se/norrsken/Norrsken_history.html |title=Norrsken history |publisher=Irf.se |date=12 November 2003 |accessdate=26 July 2011 |archiveurl=https://web.archive.org/web/20110721215920/http://www.irf.se/norrsken/Norrsken_history.html |archivedate=21 July 2011 |url-status=dead}}</ref> |
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| − | * Walter William Bryant wrote in his book [[q:Kepler|''Kepler'']] (1920) that [[Tycho Brahe]] "seems to have been something of a [[Homeopathy|homœopathist]], for he recommends [[Sulfur#History|sulfur]] to cure infectious diseases "brought on by the sulphurous vapours of the Aurora Borealis."<ref>Walter William Bryant, {{Ws | [[s: Kepler|''Kepler'']]}} Macmillan Co. (1920) {{Ws | [[s: Kepler/Chapter 3#23|p.23]]}}</ref> |
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| − | In 1778, [[Benjamin Franklin]] theorized in his paper ''Aurora Borealis, Suppositions and Conjectures towards forming an Hypothesis for its Explanation'' that an aurora was caused by a concentration of electrical charge in the polar regions intensified by the snow and moisture in the air:<ref>The original English text of Benjamin Franklin's article on the cause of auroras is available at: [https://founders.archives.gov/documents/Franklin/01-28-02-0150 U.S. National Archives: Founders Online] |
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| − | * A translation into French of Franklin's article was read to the French Royal Academy of Sciences and an excerpt of it was published in: {{cite journal |last1=Francklin |title=Extrait des suppositions et des conjectures sur la cause des Aurores Boréales |journal=Journal de Physique |date=June 1779 |volume=13 |pages=409–412 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015077781162&view=1up&seq=439 |trans-title=Extract of Suppositions and conjectures on the cause of auroras borealis |language=French}}</ref><ref>{{cite book|editor=Goodman, N.|title=The Ingenious Dr. Franklin: Selected Scientific Letters of Benjamin Franklin|url=https://books.google.com/books?id=Wojw-wmYrNwC&pg=PA3|year=2011|publisher=University of Pennsylvania Press|location=Philadelphia|isbn=978-0-8122-0561-9|page=3}}</ref> {{Quote|text=May not then the great quantity of electricity brought into the polar regions by the clouds, which are condens'd there, and fall in snow, which electricity would enter the earth, but cannot penetrate the ice; may it not, I say (as a bottle overcharged) break thro' that low atmosphere and run along in the vacuum over the air towards the equator, diverging as the degrees of longitude enlarge, strongly visible where densest, and becoming less visible as it more diverges; till it finds a passage to the earth in more temperate climates, or is mingled with the upper air? |author=Benjamin Franklin|source=}} |
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| − | Observations of the rhythmic movement of compass needles due to the influence of an aurora were confirmed in the Swedish city of [[Uppsala]] by [[Anders Celsius]] and [[Olof Hiorter]]. In 1741, Hiorter was able to link large magnetic fluctuations with an aurora being observed overhead. This evidence helped to support their theory that 'magnetic storms' are responsible for such compass fluctuations.<ref>J. Oschman (2016), ''Energy Medicine: The Scientific Basis'' (Elsevier, Edinburgh) p. 275.</ref> |
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| − | [[File:Church 1911.4.1.jpg|thumb|[[Frederic Edwin Church|Church's]] 1865 painting ''[[Aurora Borealis (painting)|Aurora Borealis]]'']] |
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| − | A variety of [[Indigenous peoples of the Americas|Native American]] myths surround the spectacle. The European explorer [[Samuel Hearne]] traveled with [[Chipewyan|Chipewyan Dene]] in 1771 and recorded their views on the ''ed-thin'' ('caribou'). According to Hearne, the Dene people saw the resemblance between an aurora and the sparks produced when [[caribou]] fur is stroked. They believed that the lights were the spirits of their departed friends dancing in the sky, and when they shone brightly it meant that their deceased friends were very happy.<ref>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.</ref> |
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| − | During the night after the [[Battle of Fredericksburg]], an aurora was seen from the battlefield. The [[Confederate Army]] took this as a sign that God was on their side, as the lights were rarely seen so far south. The painting ''[[Aurora Borealis (painting)|Aurora Borealis]]'' by [[Frederic Edwin Church]] is widely interpreted to represent the conflict of the [[American Civil War]].<ref>{{cite web|url=http://www.americanart.si.edu/collections/search/artwork/?id=4806|title=''Aurora Borealis'' at the American Art Museum}}</ref> |
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| − | A mid 19th-century British source says auroras were a rare occurrence before the 18th-century.<ref>''The National Cyclopaedia of Useful Knowledge, Vol.II'', (1847), London, Charles Knight, p.496</ref> It quotes [[Edmond Halley|Halley]] as saying that before the aurora of 1716, no such phenomenon had been recorded for more than 80 years, and none of any consequence since 1574. It says no appearance is recorded in the [[French Academy of Sciences|''Transactions of the French Academy of Sciences'']] between 1666 and 1716. And that one aurora recorded in ''Berlin Miscellany'' for 1797 was called a very rare event. One observed in 1723 at [[Bologna]] was stated to be the first ever seen there. [[Anders Celsius|Celsius]] (1733) states the oldest residents of [[Uppsala]] thought the phenomenon a great rarity before 1716. The period between approximately 1645 to 1715 corresponds to the [[Maunder minimum]] in sunspot activity. |
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| − | It was the Norwegian scientist [[Kristian Birkeland]] who, in the early 1900s, laid the foundation for our current understanding of geomagnetism and polar auroras. |
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| − | ==Non-terrestrial auroras== |
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| − | {{see also|Magnetosphere of Jupiter#Aurorae}} |
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| − | [[Image:Jupiter.Aurora.HST.UV.jpg|thumb|[[Jupiter]] aurora; the far left bright spot connects magnetically to [[Io (moon)|Io]]; the spots at the bottom of the image lead to [[Ganymede (moon)|Ganymede]] and [[Europa (moon)|Europa]].]] |
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| − | [[File:Saturns Northern Aurora still.jpg|thumb|An aurora high above the northern part of Saturn; image taken by the [[Cassini spacecraft]]. |
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| − | [[:Image:Saturns Northern Aurora in Motion.gif|A movie]] shows images from 81 hours of observations of Saturn's aurora]] |
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| − | |||
| − | Both [[Jupiter]] and [[Saturn]] have magnetic fields that are stronger than Earth's (Jupiter's equatorial field strength is 4.3 gauss, compared to 0.3 gauss for Earth), and both have extensive radiation belts. Auroras have been observed on both gas planets, most clearly using the [[Hubble Space Telescope]], and the [[Cassini–Huygens|''Cassini'']] and [[Galileo (spacecraft)|''Galileo'']] spacecraft, as well as on [[Uranus]] and [[Neptune]].<ref name="esa">{{cite web|url=http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html |title=ESA Portal – Mars Express discovers auroras on Mars |publisher=Esa.int |date=11 August 2004 |accessdate=5 August 2010}}</ref> |
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| − | The aurorae on Saturn seem, like Earth's, to be powered by the solar wind. However, Jupiter's aurorae are more complex. The Jupiter's main auroral oval is associated with the plasma produced by the volcanic moon, [[Io (moon)|Io]] and the transport of this plasma within the planet's [[Magnetosphere of Jupiter|magnetosphere]]. An uncertain fraction of Jupiter's aurorae are powered by the solar wind. In addition, the moons, especially Io, are also powerful sources of aurora. These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the 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, which have been studied since 1955. Using the Hubble Space Telescope, auroras over Io, Europa and Ganymede have all been observed. |
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| − | Auroras have also been observed on [[Venus]] and [[Mars]]. Venus has no magnetic field and so Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed over the full disc of the planet. A Venusian aurora originates when electrons from the solar wind collide with the night-side atmosphere. |
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| − | An aurora was detected on Mars, on 14 August 2004, by the SPICAM instrument aboard [[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. By analyzing 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 strongest magnetic field is localized. This correlation indicated that the origin of the light emission was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.<ref name="esa"/><ref>{{cite news|date=18 February 2006 |url=http://www.universetoday.com/am/publish/mars_express_aurorae.html?1722006 |title=Mars Express Finds Auroras on Mars |work=Universe Today |accessdate=5 August 2010}}</ref> |
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| − | The first ever [[Extra-solar object|extra-solar]] auroras were discovered in July 2015 over the [[brown dwarf]] star [[LSR J1835+3259]].<ref>{{cite web |url=http://news.discovery.com/space/alien-life-exoplanets/monstrous-aurora-detected-beyond-our-solar-system-150729.htm |title=Monstrous Aurora Detected Beyond our Solar System |last1=O'Neill |first1=Ian |date=29 July 2015 |publisher=Discovery |access-date=29 July 2015}}</ref> The mainly red aurora was found to be a million times brighter than the Northern Lights, a result of the charged particles interacting with hydrogen in the atmosphere. It has been speculated that stellar winds may be stripping off material from the surface of the brown dwarf to produce their own electrons. Another possible explanation for the auroras is that an as-yet-undetected body around the dwarf star is throwing off material, as is the case with Jupiter and its moon Io.<ref>{{cite web |url=http://www.space.com/30087-alien-auroras-found-beyond-solar-system.html |title=First Alien Auroras Found, Are 1 Million Times Brighter Than Any on Earth |last1=Q. Choi |first1=Charles |date=29 July 2015 |publisher=space.com |access-date=29 July 2015}}</ref> |
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| − | |||
| − | ==See also== |
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| − | {{Portal|Astronomy|Weather}} |
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| − | * [[Airglow]] |
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| − | * [[Aurora (heraldry)]] |
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| − | * [[Heliophysics]] |
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| − | * [[List of plasma physics articles]] |
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| − | * [[List of solar storms]] |
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| − | * [[Paschen's law]] |
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| − | * [[Space tornado]] |
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| − | * [[Space weather]] |
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| − | |||
| − | ==Notes== |
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| − | {{notelist}} |
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| − | |||
| − | ==References== |
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| − | {{reflist}} |
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| − | |||
| − | ==Further reading== |
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| − | * {{cite journal |first=David P. |last=Stern |title=A Brief History of Magnetospheric Physics During the Space Age |journal=Reviews of Geophysics |volume=34 |issue=1 |date=1996 |pages=1–31 |doi=10.1029/95rg03508 |bibcode=1996RvGeo..34....1S|url=https://zenodo.org/record/1231372 }} |
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| − | * {{cite web |last1=Stern |first1=David P. |last2=Peredo |first2=Mauricio |title=The Exploration of the Earth's Magnetosphere |website=phy6.org |url=http://www.phy6.org/Education/Intro.html }} |
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| − | * {{cite book |first=Robert H. |last=Eather |title=Majestic Lights: The Aurora in Science, History, and The Arts |publisher=American Geophysical Union |location=Washington, DC |isbn=978-0-87590-215-9 |date=1980 }} |
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| − | * {{cite journal |last=Akasofu |first=Syun-Ichi |title=Secrets of the Aurora Borealis |journal=Alaska Geographic Series |volume=29 |issue=1 |date=April 2002 }} |
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| − | * {{Cite journal|last1=Daglis|first1=Ioannis|title=Aurora – The magnificent northern lights|journal=Recorder|volume=29|issue=9|pages=45–48|archive-url=https://web.archive.org/web/20200614024652/http://proteus.space.noa.gr/~daglis/images/pdf_files/other_pubs/recorder.pdf|archive-date=14 June 2020|url-status=|last2=Akasofu|first2=Syun-Ichi|date=November 2004|url=http://proteus.space.noa.gr/~daglis/images/pdf_files/other_pubs/recorder.pdf}} [https://csegrecorder.com/articles/view/aurora-the-magnificent-northern-lights Alt URL] |
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| − | * {{cite book |first=Candace Sherk |last=Savage |title=Aurora: The Mysterious Northern Lights |location=San Francisco |publisher=[[Sierra Club Books]] / Firefly Books |date=1994 |isbn=978-0-87156-419-1 |url-access=registration |url=https://archive.org/details/aurora00cand }} |
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| − | * {{cite book |first=Bengt |last=Hultqvist |title=Handbook of the Solar-Terrestrial Environment |chapter=The Aurora |location=Berlin Heidelberg |publisher=Springer-Verlag |date=2007 |editor-last==Kamide |editor-first=Y. |editor2-last=Chian |editor2-first=A |isbn=978-3-540-46314-6 |doi=10.1007/978-3-540-46315-3_13 |pages=331–354 }} |
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| − | * {{cite book |last1=Sandholt |first1=Even |last2=Carlson |first2=Herbert C. |last3=Egeland |first3=Alv |title=Dayside and Polar Cap Aurora |chapter=Optical Aurora |location=Netherlands |publisher=Springer Netherlands |date=2002 |isbn=978-0-306-47969-4 |doi=10.1007/0-306-47969-9_3 |pages=33–51 }} |
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| − | * {{cite web |url=https://science.nasa.gov/headlines/y2001/ast26oct_1.htm |title='tis the Season for Auroras |date=21 October 2001 |first=Tony |last=Phillips |publisher=NASA |accessdate=15 May 2006 |archiveurl=https://web.archive.org/web/20060411100954/https://science.nasa.gov/headlines/y2001/ast26oct_1.htm |archivedate=11 April 2006 |url-status=dead}} |
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| − | * {{Cite EB1911|wstitle=Aurora Polaris|volume=2|pages=927–934}} This includes a highly detailed description of historical observations and descriptions. |
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| − | |||
| − | ==External links== |
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| − | {{Commons|Aurora}} |
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| − | {{Wikiquote|Aurora}} |
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| − | {{Wikivoyage|Northern Lights}} |
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| − | * [https://www.nordlysvarsel.com/ Aurora forecast – Will there be northern lights?] |
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| − | * [https://earth.nullschool.net/#current/space/surface/level/anim=off/overlay=aurora/winkel3/ Current global map showing the probability of visible aurora] |
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| − | * [https://web.archive.org/web/20161124084503/http://www.gi.alaska.edu/AuroraForecast Aurora – Forecasting] |
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| − | * [http://www.northernlightsiceland.com/northern-lights-forecast/ Official MET aurora forecasting in Iceland] |
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| − | * [http://www.aurorahunter.com/aurora-prediction.php Aurora Borealis – Predicting] |
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| − | * [http://www.hamqsl.com/solar1.html#converters Solar Terrestrial Data] – Online Converter – ''Northern Lights'' Latitude. |
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| − | * [https://web.archive.org/web/20190311081225/http://www.aurora-service.eu/ Aurora Service Europe] – Aurora forecasts for Europe. |
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| − | * [https://news.avclub.com/bask-in-natures-majesty-without-getting-your-tootsies-c-1841696490 Live Northern Lights webstream] |
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| − | === Multimedia=== |
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| − | * [http://vimeo.com/user10702000/fireinthesky Amazing time-lapse video of Aurora Borealis] – Shot in Iceland over the winter of 2013/2014. |
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| − | * [http://nrk.no/nyheter/distrikt/troms_og_finnmark/1.7467857 Popular video of Aurora Borealis] – Taken in Norway in 2011. |
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| − | * [https://web.archive.org/web/20111004061641/http://www.aurora-northern-lights.com/ Aurora Photo Gallery] – Views taken 2009–2011. |
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| − | * [http://apod.nasa.gov/apod/ap120103.html Aurora Photo Gallery] – "Full-Sky Aurora" over Eastern [[Norway]]. December 2011. |
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| − | * [https://web.archive.org/web/20100902122923/http://www.twanight.org/newTWAN/gallery.asp?Gallery=Aurora&page=1 Videos and Photos – Auroras at Night]. |
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| − | * [https://www.youtube.com/watch?v=lT3J6a9p_o8 Video (04:49)] – Aurora Borealis – How The ''Northern Lights'' Are Created. |
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| − | * [http://www.nfb.ca/film/northern_lights Video (47:40)] – ''Northern Lights'' – Documentary. |
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| − | * [https://vimeo.com/62602652 Video (5:00)] – Northern lights video in real time |
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| − | * [https://web.archive.org/web/20110817082341/http://vimeo.com/27315234 Video (01:42)] – ''Northern Lights'' – Story of [[Geomagnetic storm|Geomagnetc Storm]] ([[Terschelling|Terschelling Island]] – 6/7 April 2000). |
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| − | * [https://www.youtube.com/watch?v=Lc3FxNXjBs0 Video (01:56)] (Time-Lapse) − Auroras – Ground-Level View from [[Finnish Lapland]] 2011. |
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| − | * [https://www.youtube.com/watch?v=Vq3o3sYpk78 Video (02:43)] (Time-Lapse) − Auroras – Ground-Level View from [[Tromsø, Norway]]. 24 November 2010. |
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| − | * [https://www.youtube.com/watch?v=l6ahFFFQBZY Video (00:27)] (Time-Lapse) – [[Earth]] and Auroras – Viewed from [[International Space Station|The International Space Station]]. |
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| − | {{Magnetospherics|state=collapsed}} |
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| − | {{Authority control}} |
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| − | {{DEFAULTSORT:Aurora}} |
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| − | [[Category:Space plasmas]] |
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| − | [[Category:Polar regions of the Earth]] |
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| − | [[Category:Atmospheric optical phenomena]] |
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| − | [[Category:Earth phenomena]] |
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| − | [[Category:Electrical phenomena]] |
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| − | [[Category:Light sources]] |
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| − | [[Category:Plasma physics]] |
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| − | [[Category:Planetary science]] |
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| − | [[Category:Sources of electromagnetic interference]] |
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| − | [[Category:Articles containing video clips]] |
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Latest revision as of 21:55, 27 October 2020
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This page may contain some of the same content as the Marvel Database article. A more complete and current article is at the Marvel Comics Database Aurora. |
History
History of character is unknown.
Links and References
- 2 Appearances of Aurora
- Minor Appearances of Aurora
- Media Aurora was Mentioned in
- Images featuring Aurora
- Quotations by or about Aurora
- Character Gallery: Aurora
Footnotes
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