Editing Aurora

{{Short description|Atmospheric effect caused by the solar wind}}
{{Redirect-several|Aurora|Aurora Borealis|Aurora Australis|Northern Lights|Southern Lights}}
{{Use Canadian English|date=September 2024}}
{{Use dmy dates|date=August 2024}}
{{multiple image
| total_width              = 320px
| footer                   = Images of auroras from across the world, including those with rarer red and blue lights
| perrow                   = 2/2
| image1                   = Northern Lights 02.jpg
| alt1                     = Northern Lights with very rare blue light emitted by nitrogen
| image2                   = Aurora borealis over Eielson Air Force Base, Alaska.jpg
| alt2                     = Aurora corealis shines above Bear Lake near Eielson Air Force Base, Alaska
| image3                   = Aurore australe - Aurora australis.jpg
| alt3                     = Aurora australis in Antarctica
| image4                   = Red and green aurora.jpg
| alt4                     = Red and green Aurora in Fairbanks, Alaska
}}
[[File:Aurora australis ISS.jpg|thumb|Aurora australis seen from the [[International Space Station|ISS]], 2017<ref>{{cite web |url=https://eol.jsc.nasa.gov/BeyondThePhotography/CrewEarthObservationsVideos/videos/slights_iss_20170817/slights_iss_20170817.mp4 |title=Southern Lights over the Australian Bight |publisher=NASA |access-date=12 September 2022 |archive-date=21 October 2022 |archive-url=https://web.archive.org/web/20221021182327/https://eol.jsc.nasa.gov/BeyondThePhotography/CrewEarthObservationsVideos/videos/slights_iss_20170817/slights_iss_20170817.mp4 |url-status=live }}</ref>]]

An '''aurora'''{{efn|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|access-date=5 August 2010|archive-url=https://web.archive.org/web/20100722101345/http://www1.umn.edu/urelate/style/sciterminology.html#Anchor-37516|archive-date=22 July 2010|url-status=dead}}</ref>}} ({{plural form}} '''aurorae''' or '''auroras'''),{{efn|The name "auroras" is now the more common plural in the US;{{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".}}
also commonly known as the '''northern lights''' ('''aurora borealis''') or '''southern lights''' ('''aurora australis'''),{{efn|The aurorae seen in northern latitudes, around the Arctic, can be referred to as the '''northern lights''' or '''aurora borealis''', while those seen in southern latitudes, around the Antarctic, are known as the '''southern lights''' or '''aurora australis'''. '''Polar lights''' and '''aurora polaris''' are the more general equivalents of these terms.}} is a natural light display in [[Earth]]'s sky, predominantly seen in [[polar regions of Earth|high-latitude regions]] (around the [[Arctic]] and [[Antarctic]]). Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals, or dynamic flickers covering the entire sky.<ref>Lui, A., 2019. Imaging global auroras in space. Light: Science & Applications, 8(1).</ref>

Auroras are the result of disturbances in the Earth's [[magnetosphere]] caused by the [[solar wind]]. Major disturbances result from enhancements in the speed of the solar wind from [[coronal holes]] and [[coronal mass ejections]]. These disturbances alter the trajectories of [[charged particle]]s in the magnetospheric [[Plasma (physics)|plasma]]. These particles, mainly [[electron]]s and [[proton]]s, [[Electron precipitation|precipitate]] into the upper atmosphere ([[thermosphere]]/[[exosphere]]). The resulting [[ionization]] and excitation of atmospheric constituents emit light of varying colour 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.

Most of the [[planet]]s in the [[Solar System]], some [[natural satellite]]s, [[brown dwarf]]s, and even [[comet]]s also host auroras.

== Etymology ==
The term ''aurora borealis'' was coined by [[Galileo]] in 1619, from the [[Ancient Rome|Roman]] [[Aurora (mythology)|Aurora, goddess of the dawn]], and the [[Greek language|Greek]] [[Boreas (god)|Boreas, god of the cold 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 |pages=11–14|year=1986|author-link1=George Siscoe|last1=Siscoe|first1=G. L.|isbn=978-0-87590-276-0|bibcode = 1986HGeo....2...11S  | issn = 8755-1217 }}</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=it|access-date=31 July 2019|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165109/https://books.google.com/books?id=_EtbAAAAcAAJ&pg=PA39#v=onepage&q&f=false|url-status=live}} On p. 39, Galileo explains that auroras are due to sunlight reflecting from thin, high clouds. From p. 39: {{lang|it|"...&nbsp;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."}} ("...&nbsp;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 expected, 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 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|archive-date=2 January 2019|archive-url=https://web.archive.org/web/20190102234318/https://www.etymonline.com/word/aurora|url-status=live}}</ref> Ancient Greek poets used the corresponding name [[Eos]] metaphorically to refer to dawn, often mentioning its play of colours across the otherwise dark sky (e.g., "rosy-fingered dawn").<!-- If the citation I have provided is sufficient, may this previous note be deleted?
{{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}}
--><ref>{{cite web|url=http://classics.mit.edu/Homer/odyssey.mb.txt|title=The Odyssey ca. 500 B.C. by Homer (translated by Samuel Butler 1900); online at Internet Classics Archive (Retrieved 15 February 2021)|year=1993|access-date=16 February 2021|archive-date=22 April 2021|archive-url=https://web.archive.org/web/20210422135151/http://classics.mit.edu/Homer/odyssey.mb.txt|url-status=live}}</ref>

The words ''borealis'' and ''australis'' are derived from the names of the ancient gods of the north wind ([[Boreas (god)|Boreas]]) and the south wind ([[Anemoi#Notus (Auster)|Auster]]) in [[Greco-Roman mythology]].

== Occurrence ==
[[File:Antarctic aurora ESA313457.jpg|thumb|upright=1.3|Earth's night-side upper atmosphere appearing from the bottom as bands of [[afterglow]] illuminating the [[troposphere]] in orange with silhouettes of clouds, and the [[stratosphere]] in white and blue. Next the [[mesosphere]] (pink area) extends to the orange and faintly green line of the lowest [[airglow]], at about one hundred kilometres at the [[Outer space#Boundary|edge of space]] and the lower edge of the [[thermosphere]] (invisible). Continuing with green and red bands of aurorae stretching over several hundred kilometres.]]
Most auroras occur in a band known as the "auroral zone",<ref name="feldstein-2011">{{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° (approximately 330–660&nbsp;km) 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 Earth. Auroras at the [[North Pole]] itself are rare due to it being on the [[Arctic Ocean]], while auroras at the [[South Pole]] itself are very common and guaranteed to be visible.<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|access-date=30 August 2017|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165153/https://books.google.com/books?id=9gLwCAAAQBAJ&pg=PA190#v=onepage&q&f=false|url-status=live}}</ref> Early evidence for a geomagnetic connection comes from the statistics of auroral observations. [[Elias Loomis]] (1860),<ref name="loomis-1859" /> and later Hermann Fritz (1881)<ref>{{cite book|last1=Fritz|first1=Hermann|title=Das Polarlicht|series=Internationale wissenschaftliche Bibliothek|volume=49|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=de|access-date=31 July 2019|archive-date=28 August 2021|archive-url=https://web.archive.org/web/20210828192807/https://babel.hathitrust.org/cgi/pt?id=nnc1.cu50485466&view=1up&seq=11|url-status=live}}</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=da, fr|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.

In northern [[latitude]]s, the effect is known as the aurora borealis or the northern lights. 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|page=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]], the [[Southern Cone]], [[South Africa]], [[Australasia]] and under exceptional circumstances as far north as [[Uruguay]].<ref>{{cite news |title=Aurora austral en Uruguay: fotógrafos registran un hecho "histórico" y astrónomos explican por qué pasó |url=https://www.elobservador.com.uy/nacional/aurora-austral-uruguay-fotografos-registran-un-hecho-historico-y-astronomos-explican-que-paso-n5939403 |access-date=13 May 2024 |work=El Observador (Uruguay)}}</ref> The aurora borealis is visible from areas around the Arctic such as [[Alaska]], [[Canada]], [[Iceland]], [[Greenland]], the [[Faroe Islands]], [[Scandinavia]], [[Finland]], [[Scotland]], and [[Russia]]. On rare occasions, the aurora borealis can be seen as far south as the Mediterranean and the southern states of the US. During the [[Carrington Event]], the greatest geomagnetic storm ever observed, auroras were seen even in the tropics.

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="feldstein-2011" /> 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]].

Auroras seen within the auroral oval may be directly overhead. 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=Reviews of Geophysics|volume=45|issue=1|pages=RG1003|bibcode=2007RvGeo..45.1003F|doi-access=free }}</ref> which can be subvisual.<div style="overflow:auto;">

{{multiple image
| title = Videos of the aurora australis taken by the crew of [[Expedition 28]] on board the International Space Station
| align = center
| direction = horizontal
| image1 = Aurora Australis from ISS 2011 - 1.ogv
| width1 = 300
| alt1 =
| caption1 = This sequence of shots was taken 17 September 2011 from 17:22:27 to 17:45:12 GMT, on an ascending pass from south of [[Madagascar]] to just north of [[Australia]] over the [[Indian Ocean]].
| image2 = Aurora Australis over Indian Ocean.ogv
| width2 = 300
| alt2 =
| caption2 = This sequence of shots was taken 7 September 2011 from 17:38:03 to 17:49:15 GMT, from the [[French Southern and Antarctic Lands]] in the South Indian Ocean to southern Australia.
| image3 = Aurora Australis south of Australia.ogv
| width3 = 300
| alt3 =
| 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]].
}}
{{multiple image
| title = [[National Oceanic and Atmospheric Administration|NOAA]] maps of North America and Eurasia
| align = center
| caption_align = center
| direction = horizontal
| width = 450
| footer = These maps show the local midnight equatorward boundary of the aurora at different levels of geomagnetic activity as of 28 October 2011 – these maps change as the [[North magnetic pole|location of the geomagnetic poles]] change. A [[K-index|''K''-index]] of '''[[K-index#The&nbsp;Kp-index and estimated&nbsp;Kp-index|''K''<sub>p</sub>]]={{hsp}}3''' corresponds to relatively low levels of geomagnetic activity, while '''[[K-index#The&nbsp;Kp-index and estimated&nbsp;Kp-index|''K''<sub>p</sub>]]={{hsp}}9''' represents high levels.
| image1 = Aurora Kp Map North America.gif
| alt1 = Kp map of North America
| caption1 =North America
| image2 = Aurora Kp Map Eurasia.gif
| alt2 = Kp map of Eurasia
| caption2 =Eurasia
 }}
</div>
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]] 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=Journal of Geophysical Research|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|doi-access=free|access-date=7 December 2019|archive-date=30 April 2019|archive-url=https://web.archive.org/web/20190430080959/http://centaur.reading.ac.uk/38740/1/180_Stamperetal_1999JA900311.pdf|url-status=live}}</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=Geophysical Research Letters|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|doi-access=free|access-date=20 April 2018|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165313/https://deepblue.lib.umich.edu/bitstream/2027.42/94796/1/grl13462.pdf|url-status=live}}</ref> 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. Other types of auroras have been observed from space; for example, "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|page=2125|year=2003|last1=Østgaard|first1=N.|bibcode=2003GeoRL..30.2125O|doi-access=free }}</ref> and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as pulsating aurora, "black aurora" and their rarer companion "anti-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 Earth from those that are swept into the tail and close remotely.

=== Images ===
[[File:Spacecraft View of Aurora Australis from Space.webm|thumb|Video of the complete aurora australis by [[IMAGE (spacecraft)|IMAGE]], superimposed over a digital image of Earth]]

Early work on the imaging of the auroras was done in 1949 by the [[University of Saskatchewan]] using the [[SCR-270]] radar.<ref>{{Cite web |title=Northern Lights |url=https://www.geirangerguide.no/northern-lights |access-date=1 March 2024 |website=Geiranger Guide |language=en-US |archive-date=1 March 2024 |archive-url=https://web.archive.org/web/20240301060356/https://www.geirangerguide.no/northern-lights |url-status=live }}</ref> 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 {{convert|90|and|150|km|mi|abbr=on}} above the ground, while extending at times to more than {{convert|1000|km|mi|abbr=on}}.

=== Forms ===
According to Clark (2007), there are five main forms that can be seen from the ground, from least to most visible:<ref>{{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>

[[File:Aurora shapes.jpg|thumb|Different forms]]
[[File:Magenta G5 aurora over Tuntorp, Lysekil Municipality 11.jpg|thumb|Divergence point of a coronal aurora]]
* 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|page=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.
* ''Patches'' or ''surfaces'' that look like clouds.
* ''Arcs'' curve across the [[sky]].
* ''Rays'' are light and dark stripes across arcs, reaching upwards by various amounts.
* ''Coronas'' cover much of the sky and diverge from one point on it.

Brekke (1994) also described some auroras as "curtains".<ref name="a-1994">{{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="yahnin-1997">{{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|page=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>

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>{{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|doi-access=free}}</ref>

=== Colors and wavelengths of auroral light ===

* Red: At its highest altitudes, excited atomic oxygen emits at 630&nbsp;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.
* Green: At lower altitudes, the more frequent collisions suppress the 630&nbsp;nm (red) mode: rather the 557.7&nbsp;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&nbsp;km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0&nbsp;nm wavelengths correspond to [[forbidden transition]]s of atomic oxygen, a slow mechanism responsible for the graduality (0.7&nbsp;s and 107&nbsp;s respectively) of flaring and fading.
[[File:AuroraBorealisOkeford20240510-01.jpg|thumb|2024 appearance seen in England radiating blue through red aurora]] 
* 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&nbsp;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|work=Windows to the Universe|title=Auroral colors and spectra|url=http://www.windows2universe.org/earth/Magnetosphere/tour/tour_earth_magnetosphere_09.html|access-date=13 January 2014|archive-date=19 December 2014|archive-url=https://web.archive.org/web/20141219143402/http://www.windows2universe.org/earth/Magnetosphere/tour/tour_earth_magnetosphere_09.html|url-status=live}}</ref> The molecular nitrogen transitions are much faster than the atomic oxygen ones.
* 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="scinewscom">{{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 &#124; Space Exploration|publisher=Sci-News.com|access-date=16 August 2015|archive-date=25 July 2015|archive-url=https://web.archive.org/web/20150725140509/http://www.sci-news.com/space/science-nasas-maven-ultraviolet-aurora-mars-02614.html|url-status=live}}</ref> Jupiter, and Saturn.
* Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras.<ref name="scinewscom" /><ref>{{cite web|url=http://www.dapep.org/DAPT/EM-Wiki/aurora-borealis.html|title=Aurora Borealis|publisher=dapep.org|access-date=16 August 2015|archive-date=19 April 2015|archive-url=https://web.archive.org/web/20150419140349/http://www.dapep.org/DAPT/EM-Wiki/aurora-borealis.html|url-status=dead}}{{clarify|reason=dapep.org is down. Cite by name, not domain name. What was it – possibly Denver Area Physics Education Project?|date=November 2023}}</ref>
* Yellow and pink are [[Additive colour|a mix]] of red and green or blue. Other shades of red, as well as orange and gold, may be seen on rare occasions; yellow-green is moderately common.{{Clarify|reason=vague|date=July 2020}} As red, green, and blue are linearly independent colours, additive synthesis could, in theory, produce most human-perceived colours, but the ones mentioned in this article comprise a virtually exhaustive list.

=== Changes with time ===
[[File:Keogram explainer.gif|thumb|Construction of a [[keogram]] from one night's recording by an all-sky camera, 6/7 September 2021. Keograms are commonly used to visualize changes in aurorae over time.]]
Auroras change with time, over the night they begin with glows and progress toward coronas, although they may not reach them. They tend to fade in the opposite order.<ref name="a-1994" /> Until about 1963, it was thought that these changes are due to the rotation of the Earth under a pattern fixed with respect to the Sun. Later, it was found by comparing all-sky films of auroras from different places (collected during the [[International Geophysical Year]]) that they often undergo global changes in a process called [[auroral substorm]]. They change in a few minutes from quiet arcs all along the auroral oval to active displays along the darkside and after 1 – 3 hours they gradually change back.<ref>{{cite book|last1=T.|first1=Potemra|last2=S.-I.|first2=Akasofu|title=Magnetospheric Substorms|date=1991|publisher=American Geophysical Union |location=Washington, D.C.|isbn=0-87590-030-5|page=5}}</ref> Changes in auroras over time are commonly visualized using [[keogram]]s.<ref>{{cite web|url=http://blog.aurorasaurus.org/?p=1229|title=Eyes on the Aurora, Part 2: What is a Keogram?|website=Aurorasaurus|date=9 September 2020|accessdate=26 February 2022|archive-date=24 February 2022|archive-url=https://web.archive.org/web/20220224164140/http://blog.aurorasaurus.org/?p=1229|url-status=live}}</ref>

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="yahnin-1997" /> 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&nbsp;km for blue and green emissions and above average solar wind speeds (c.&nbsp;500&nbsp;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|s2cid=38394431|issn=2169-9402|url=http://urn.fi/urn:nbn:fi-fe2019092429533|access-date=7 December 2019|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165123/https://oulurepo.oulu.fi/handle/10024/24149|url-status=live}}</ref>

=== Other auroral radiation ===
In addition, the aurora and associated currents produce a strong radio emission around 150&nbsp;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>

=== Noise ===
Aurora [[noise]], similar to a crackling noise, begins about {{convert|70|m|ft|abbr=on}} above 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|archive-url=https://web.archive.org/web/20160627153140/http://news.nationalgeographic.com/2016/06/auroras-sounds-noises-explained-earth-space-astronomy/|url-status=dead|archive-date=27 June 2016|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|archive-date=1 July 2016|archive-url=https://web.archive.org/web/20160701115415/http://elec.aalto.fi/en/current/news/2016-06-22/|url-status=live}}</ref>

=== Unusual types ===

==== STEVE ====
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". 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|3000|C|K 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>{{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=Phys.org|access-date=21 August 2018|archive-date=30 March 2022|archive-url=https://web.archive.org/web/20220330051224/https://phys.org/news/2018-08-kind-aurora.html|url-status=live}}</ref>

==== Picket-fence aurora ====

The processes that cause STEVE are also 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/|archive-url=https://web.archive.org/web/20190504000133/https://www.nationalgeographic.com/science/2019/05/odd-aurora-named-steve-revealed-to-be-two-different-sky-shows-in-one/|url-status=dead|archive-date=4 May 2019|website=National Geographic|access-date=4 May 2019|date=3 May 2019}}</ref><ref name="nishimura-2019">{{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|access-date=4 May 2019|archive-date=4 May 2019|archive-url=https://web.archive.org/web/20190504103127/https://news.agu.org/press-release/scientists-discover-what-powers-celestial-phenomenon-steve/|url-status=live}}</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|archive-date=22 March 2018|archive-url=https://web.archive.org/web/20180322013359/https://www.theguardian.com/science/shortcuts/2018/mar/19/steve-mystery-purple-aura-rivals-northern-lights-alberta-canada-nasa|url-status=live}}</ref> When the picket-fence aurora appears with STEVE, it is below.<ref name="nishimura-2019" />

==== Dune aurora ====

First reported in 2020,<ref>{{Cite journal|title=Citizen Scientists Discover a New Auroral Form: Dunes Provide Insight Into the Upper Atmosphere|url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019AV000133|journal=AGU Advances|year=2020|doi=10.1029/2019AV000133|last1=Palmroth|first1=M.|last2=Grandin|first2=M.|last3=Helin|first3=M.|last4=Koski|first4=P.|last5=Oksanen|first5=A.|last6=Glad|first6=M. A.|last7=Valonen|first7=R.|last8=Saari|first8=K.|last9=Bruus|first9=E.|last10=Norberg|first10=J.|last11=Viljanen|first11=A.|last12=Kauristie|first12=K.|last13=Verronen|first13=P. T.|volume=1|hdl=10138/322003|s2cid=213839228|hdl-access=free|access-date=22 May 2021|archive-date=22 May 2021|archive-url=https://web.archive.org/web/20210522031541/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019AV000133|url-status=live}}</ref><ref>{{Cite journal|title=Citizen scientists discover a new form of the Northern Lights|url=https://phys.org/news/2020-01-citizen-scientists-northern.html|website=phys.org|access-date=22 May 2021|archive-date=22 May 2021|archive-url=https://web.archive.org/web/20210522031539/https://phys.org/news/2020-01-citizen-scientists-northern.html|url-status=live}}</ref> and confirmed in 2021,<ref name="grandin-2021">{{Cite journal|title=Large-Scale Dune Aurora Event Investigation Combining Citizen Scientists' Photographs and Spacecraft Observations|journal=AGU Advances|year=2021|doi=10.1029/2020AV000338|last1=Grandin|first1=Maxime|last2=Palmroth|first2=Minna|last3=Whipps|first3=Graeme|last4=Kalliokoski|first4=Milla|last5=Ferrier|first5=Mark|last6=Paxton|first6=Larry J.|last7=Mlynczak|first7=Martin G.|last8=Hilska|first8=Jukka|last9=Holmseth|first9=Knut|last10=Vinorum|first10=Kjetil|last11=Whenman|first11=Barry|volume=2|issue=2|pages=EGU21-5986|bibcode=2021EGUGA..23.5986G|doi-access=free}}</ref><ref>{{Cite news|title=Confirmation of an auroral phenomenon|url=https://phys.org/news/2021-05-auroral-phenomenon.html|website=phys.org|access-date=22 May 2021|archive-date=22 May 2021|archive-url=https://web.archive.org/web/20210522031540/https://phys.org/news/2021-05-auroral-phenomenon.html|url-status=live}}</ref> the dune aurora phenomenon was discovered<ref>{{cite web|url=http://blog.aurorasaurus.org/?p=1461|title=The discovery of the auroral dunes: How one thing led to another|website=Aurorasaurus|access-date=22 May 2021|archive-date=13 May 2021|archive-url=https://web.archive.org/web/20210513151346/http://blog.aurorasaurus.org/?p=1461|url-status=live}}</ref> by Finnish [[Citizen science|citizen scientists]]. It consists of regularly-spaced, parallel stripes of brighter emission in the green diffuse aurora which give the impression of sand dunes.<ref>{{cite web|url=https://www.youtube.com/watch?v=F6xM-XY6NYg| archive-url=https://ghostarchive.org/varchive/youtube/20211211/F6xM-XY6NYg| archive-date=11 December 2021| url-status=live|title=Revontulien 'dyynit', uusia löydöksiä – Aurora 'dunes' revisited|website=YouTube| date=4 May 2021}}{{cbignore}}</ref> The phenomenon is believed to be caused by the modulation of atomic oxygen density by a large-scale atmospheric wave travelling horizontally in a waveguide through an [[Inversion (meteorology)|inversion]] layer in the [[mesosphere]] in presence of [[electron precipitation]].<ref name="grandin-2021" />

==== Horse-collar aurora ====

Horse-collar auroras (HCA) are auroral features in which the auroral ellipse shifts poleward during the dawn and dusk portions and the polar cap becomes teardrop-shaped. They form during periods when the interplanetary magnetic field (IMF) is permanently northward, when the IMF clock angle is small. Their formation is associated with the closure of the magnetic flux at the top of the dayside magnetosphere by the double lobe reconnection (DLR). There are approximately 8 HCA events per month, with no seasonal dependence, and that the IMF must be within 30 degrees of northwards.<ref>{{Cite journal |last1=Bower |first1=G. E. |last2=Milan |first2=S. E. |last3=Paxton |first3=L. J. |last4=Anderson |first4=B. J. |date=May 2022 |title=Occurrence Statistics of Horse Collar Aurora |url=https://onlinelibrary.wiley.com/doi/10.1029/2022JA030385 |journal=Journal of Geophysical Research: Space Physics |language=en |volume=127 |issue=5 |doi=10.1029/2022JA030385 |bibcode=2022JGRA..12730385B |s2cid=248842161 |issn=2169-9380 |hdl=11250/3055028 |hdl-access=free |access-date=1 December 2022 |archive-date=12 May 2024 |archive-url=https://web.archive.org/web/20240512165217/https://agupubs.onlinelibrary.wiley.com/action/ajaxShowRecommended?widgetId=5cf4c79f-0ae9-4dc5-96ce-77f62de7ada9&ajax=true&doi=10.1029%2F2022JA030385&pbContext=%3BrequestedJournal%3Ajournal%3A21699402%3Bjournal%3Ajournal%3A21562202a%3Bpage%3Astring%3AArticle%2FChapter+View%3Bctype%3Astring%3AJournal+Content%3Bwebsite%3Awebsite%3Aagupubs%3Bissue%3Aissue%3Adoi%5C%3A10.1002%2Fjgra.v127.5%3Barticle%3Aarticle%3Adoi%5C%3A10.1029%2F2022JA030385%3Bwgroup%3Astring%3APublication+Websites%3BpageGroup%3Astring%3APublication+Pages%3BsubPage%3Astring%3AFull+Text&widgetKey=ux3-publicationContent-widget_5cf4c79f-0ae9-4dc5-96ce-77f62de7ada9_3067_144859_en&accordionHeadingWrapper=h2 |url-status=live }}</ref>

==== Conjugate auroras ====
Conjugate auroras are nearly exact mirror-image auroras found at [[conjugate points]] in the northern and southern hemispheres on the same geomagnetic field lines. These generally happen at the time of the [[equinox]]es, when there is little difference in the orientation of the north and south geomagnetic poles to the sun. Attempts were made to image conjugate auroras by aircraft from Alaska and New Zealand in 1967, 1968, 1970, and 1971, with some success.<ref>{{cite book |title=The Aurora Watcher's Handbook |pages=117–124 |first=Neil |last=Davis |publisher=University of Alaska Press |date=1992 |isbn=0-912006-60-9 }}</ref>

== Causes ==
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 [[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.
# A quiescent solar wind flowing past 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 [[second law of thermodynamics]]. The complete process, which also generates an electric ring current around Earth, is uncertain.
# Geomagnetic disturbance from an enhanced [[solar wind]] causes distortions of the [[magnetosphere|magnetotail]] ("magnetic substorms"). These 'substorms' tend to occur after prolonged spells (on the order of 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 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.
#: [[File:Moon and Aurora.jpg|thumb|[[Moon]] and aurora]]
# 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.
[[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.

[[:File:Aurora Australis.gif|An animation]] created using the same satellite data is also available.]]
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>

=== Auroral particles ===
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&nbsp;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|page=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.

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|page=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.

=== Atmosphere ===
Auroras result from emissions of [[photon]]s in Earth's upper [[Earth's atmosphere|atmosphere]], above {{convert|80|km|mi|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|archive-url=https://web.archive.org/web/20110127004149/http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|archive-date=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:
;[[Oxygen]] emissions: green or orange-red, depending on the amount of energy absorbed.
;[[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.

Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7&nbsp;nm green light and up to two minutes for the red 630.0&nbsp;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.

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 colour. 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.

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|access-date=5 August 2015|archive-date=5 August 2015|archive-url=https://web.archive.org/web/20150805154623/http://auspace.athabascau.ca/handle/2149/518|url-status=live}}</ref>

=== Ionosphere ===
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&nbsp;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.

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>{{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). Ionosphere can contribute to the formation of auroral arcs via the [[feedback]] instability under high ionospheric resistance conditions, observed at night time and in dark Winter hemisphere.<ref>{{cite journal|last1=Pokhotelov|first1=D.|last2=Lotko|first2=W. |last3=Streltsov|first3=A.V.|title= Effects of the seasonal asymmetry in ionospheric Pedersen conductance on the appearance of discrete aurora | journal=Geophys. Res. Lett. |date=2002|volume=29|issue=10|pages=79-1-79-4|doi=10.1029/2001GL014010|bibcode=2002GeoRL..29.1437P |s2cid=123637108 |doi-access=free}}</ref>

== Interaction of the solar wind with Earth ==
Earth is constantly immersed in the [[solar wind]], a 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 solar wind reaches Earth with a velocity typically around 400&nbsp;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|access-date=10 November 2017|archive-date=4 November 2016|archive-url=https://web.archive.org/web/20161104205820/https://ntrs.nasa.gov/search.jsp?R=19770051690|url-status=live}}</ref> Her work resulted from data collected by the [[Explorer 33]] spacecraft.

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.

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>

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]].

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|access-date=26 July 2011}}</ref>

=== Magnetosphere ===
[[File:Structure of the magnetosphere LanguageSwitch.svg|lang=en|thumb|upright=1.4|Schematic of Earth's [[magnetosphere]]]]

Earth's [[magnetosphere]] is shaped by the impact of the solar wind on Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000&nbsp;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&nbsp;km to 15,000&nbsp;km (1.9 to 2.4 Re) further upstream. The width of the magnetosphere abreast of Earth is typically 190,000&nbsp;km (30 Re), and on the night side a long "magnetotail" of stretched field lines extends to great distances (> 200 Re).

The high latitude magnetosphere is filled with plasma as the solar wind passes Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favoured 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|doi-access=free }}</ref> The flow pattern of magnetospheric plasma is mainly from the magnetotail toward Earth, around Earth and back into the solar wind through the [[magnetopause]] on the day-side. In addition to moving perpendicular to Earth's magnetic field, some magnetospheric plasma travels down along 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 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.

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|access-date=26 July 2011| archive-url= https://web.archive.org/web/20110629043044/http://www.nasa.gov/mission_pages/themis/auroras/themis_power.html| archive-date= 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 |doi-access=free}}</ref>

[[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 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 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{{Colloquialism|date=June 2021}} through which energy from the solar wind can reach Earth's inner magnetosphere and thereby enhance auroras. Recent evidence in 2021 has shown that individual separate substorms may in fact be correlated networked communities.<ref>{{cite journal|title=Network community structure of substorms using SuperMAG magnetometers, L. Orr, S. C. Chapman, J. W. Gjerloev & W. Guo|issue=1|page=1842|journal=Nature Communications|date=23 March 2021|volume=12|doi=10.1038/s41467-021-22112-4|last1=Orr|first1=L.|last2=Chapman|first2=S. C.|last3=Gjerloev|first3=J. W.|last4=Guo|first4=W.|pmid=33758181|pmc=7988152 }}</ref>

== Auroral particle acceleration ==

Just as there are many types of aurora, there are many different mechanisms that accelerate auroral particles into the atmosphere. Electron aurora in Earth's auroral zone (i.e. commonly visible aurora) can be split into two main categories with different immediate causes: diffuse and discrete aurora. Diffuse aurora appear relatively structureless to an observer on the ground, with indistinct edges and amorphous forms. Discrete aurora are structured into distinct features with well-defined edges such as arcs, rays and coronas; they also tend to be much brighter than the diffuse aurora.

In both cases, the electrons that eventually cause the aurora start out as electrons trapped by the magnetic field in Earth's [[magnetosphere]]. These [[L-shell#Charged particle motions in a dipole field|trapped particles]] bounce back and forth along [[magnetic field lines]] and are prevented from hitting the atmosphere by the [[magnetic mirror]] formed by the increasing magnetic field strength closer to Earth. The magnetic mirror's ability to trap a particle depends on the particle's [[Pitch angle (particle motion)|pitch angle]]: the angle between its direction of motion and the local magnetic field. An aurora is created by processes that decrease the pitch angle of many individual electrons, freeing them from the magnetic trap and causing them to hit the atmosphere.

In the case of diffuse auroras, the electron pitch angles are altered by their interaction with various [[Waves in plasmas|plasma waves]]. Each interaction is essentially wave-particle [[scattering]]; the electron energy after interacting with the wave is similar to its energy before interaction, but the direction of motion is altered. If the final direction of motion after scattering is close to the field line (specifically, if it falls within the [[Magnetic mirror#Mirror ratios|loss cone]]) then the electron will hit the atmosphere. Diffuse auroras are caused by the collective effect of many such scattered electrons hitting the atmosphere. The process is mediated by the plasma waves, which become stronger during periods of high [[Geomagnetic storm|geomagnetic activity]], leading to increased diffuse aurora at those times.

In the case of discrete auroras, the trapped electrons are accelerated toward Earth by electric fields that form at an altitude of about 4000–12000&nbsp;km in the "auroral acceleration region". The electric fields point away from Earth (i.e. upward) along the magnetic field line.<ref>The theory of acceleration by parallel electric fields is reviewed in detail by {{cite journal|vauthors=Lysak R, Echim M, Karlsson T, Marghitu O, Rankin R, Song Y, Watanabe TH|date=2020|title=Quiet, Discrete Auroral Arcs: Acceleration Mechanisms|url=https://link.springer.com/content/pdf/10.1007/s11214-020-00715-5.pdf|journal=Space Science Reviews|volume=216|issue=92|page=92|doi=10.1007/s11214-020-00715-5|bibcode=2020SSRv..216...92L|s2cid=220509575|access-date=1 June 2021|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165316/https://link.springer.com/content/pdf/10.1007/s11214-020-00715-5.pdf|url-status=live}}</ref> Electrons moving downward through these fields gain a substantial amount of energy (on the order of a few [[electronvolt|keV]]) in the direction along the magnetic field line toward Earth. This field-aligned acceleration decreases the pitch angle for all of the electrons passing through the region, causing many of them to hit the upper atmosphere. In contrast to the scattering process leading to diffuse auroras, the electric field increases the kinetic energy of all of the electrons transiting downward through the acceleration region by the same amount. This accelerates electrons starting from the magnetosphere with initially low energies (tens of eV or less) to energies required to create an aurora (100s of eV or greater), allowing that large source of particles to contribute to creating auroral light.

The accelerated electrons carry an electric current along the magnetic field lines (a [[Birkeland current]]). Since the electric field points in the same direction as the current, there is a net conversion of electromagnetic energy into particle energy in the auroral acceleration region (an [[Electric power#Passive devices (loads)|electric load]]). The energy to power this load is eventually supplied by the magnetized solar wind flowing around the obstacle of Earth's magnetic field, although exactly how that power flows through the magnetosphere is still an active area of research.<ref>A discussion of 8 theories in use in 2020 as well as several no longer in common use can be found in: {{cite journal|vauthors=Borovsky JE, Birn J, Echim MM, Fujita S, Lysak RL, Knudsen DJ, Marghitu O, Otto A, Watanabe TH, Tanaka T|date=2020|title=Quiescent Discrete Auroral Arcs: A Review of Magnetospheric Generator Mechanisms|url=https://link.springer.com/content/pdf/10.1007%2Fs11214-019-0619-5.pdf|journal=Space Science Reviews|volume=216|issue=92|doi=10.1007/s11214-019-0619-5|s2cid=214002762|access-date=1 June 2021|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165327/https://link.springer.com/content/pdf/10.1007%2Fs11214-019-0619-5.pdf|url-status=live}}</ref> While the energy to power the aurora is ultimately derived from the solar wind, the electrons themselves do not travel directly from the solar wind into Earth's auroral zone; magnetic field lines from these regions do not connect to the solar wind, so there is no direct access for solar wind electrons.

Some auroral features are also created by electrons accelerated by dispersive [[Alfvén wave]]s. At small wavelengths transverse to the background magnetic field (comparable to the [[Plasma parameters|electron inertial length]] or [[Plasma parameters|ion gyroradius]]), Alfvén waves develop a significant electric field parallel to the background magnetic field. This electric field can accelerate electrons to [[keV]] energies, significant to produce auroral arcs.<ref>{{cite thesis |type=PhD Thesis |last=Pokhotelov |first=D. |date=2002 |title=Effects of the active auroral ionosphere on magnetosphere-ionosphere coupling. |publisher=Dartmouth College |doi=10.1349/ddlp.3332}}</ref> If the electrons have a speed close to that of the wave's phase velocity, they are accelerated in a manner analogous to a surfer catching an ocean wave.<ref>{{cite web|url=https://now.uiowa.edu/2021/06/physicists-determine-how-auroras-are-created|title=Physicists determine how auroras are created|author=Richard Lewis|website=IOWA university|date=7 June 2021|access-date=8 June 2021|archive-date=8 June 2021|archive-url=https://web.archive.org/web/20210608061856/https://now.uiowa.edu/2021/06/physicists-determine-how-auroras-are-created|url-status=live}}</ref><ref>{{cite journal|vauthors=Schroeder JW, Howes GG, Kletzing CA et al|date=7 June 2021|title=Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves|journal=Nature Communications|volume=12|issue=1|page=3103|doi=10.1038/s41467-021-23377-5|pmid=34099653|pmc=8184961|bibcode=2021NatCo..12.3103S }}</ref> This constantly-changing wave electric field can accelerate electrons along the field line, causing some of them to hit the atmosphere. Electrons accelerated by this mechanism tend to have a broad energy spectrum, in contrast to the sharply-peaked energy spectrum typical of electrons accelerated by quasi-static electric fields.

In addition to the discrete and diffuse electron aurora, proton aurora is caused when magnetospheric protons collide with the upper atmosphere. The proton gains an electron in the interaction, and the resulting neutral hydrogen atom emits photons. The resulting light is too dim to be seen with the naked eye. Other aurora not covered by the above discussion include transpolar arcs (formed poleward of the auroral zone), cusp aurora (formed in two small high-latitude areas on the dayside) and some non-terrestrial auroras.

== Historically significant events ==
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>{{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|archive-date=13 October 2017|archive-url=https://web.archive.org/web/20171013225201/http://www.atlasobscura.com/articles/aurora-kyoto-1770-painting-science-magnetic-storm|url-status=live}}</ref><ref>{{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|doi-access=free }}</ref>

The auroras that resulted from the Carrington event on both 28 August and 2 September 1859, are thought to be the most spectacular in recent history. In a paper to the [[Royal Society]] on 21 November 1861, Balfour Stewart described both auroral events as documented by a self-recording [[Magnetometer#Survey magnetometers|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 [428]|url=https://babel.hathitrust.org/cgi/pt?id=pst.000054593107&view=1up&seq=461|doi=10.1098/rstl.1861.0023|doi-access=free|access-date=30 July 2019|archive-date=28 August 2021|archive-url=https://web.archive.org/web/20210828193110/https://babel.hathitrust.org/cgi/pt?id=pst.000054593107&view=1up&seq=461|url-status=live}} </ref> The second auroral event, which occurred on 2 September 1859, was a result of the (unseen) coronal mass ejection associated with the exceptionally intense Carrington–Hodgson white light [[solar flare]] on 1 September 1859. This event 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="green-2006">{{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=145–154|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; 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-1859">See:
* {{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|access-date=30 July 2019|archive-date=13 May 2021|archive-url=https://web.archive.org/web/20210513073306/https://babel.hathitrust.org/cgi/pt?id=uva.x001679510&view=1up&seq=403|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514192319/https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=112|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=15 May 2021|archive-url=https://web.archive.org/web/20210515012519/https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=269|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=13 May 2021|archive-url=https://web.archive.org/web/20210513073309/https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=406|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514050130/https://babel.hathitrust.org/cgi/pt?id=uva.x001679512&view=1up&seq=93|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=13 May 2021|archive-url=https://web.archive.org/web/20210513233251/https://babel.hathitrust.org/cgi/pt?id=uva.x001679512&view=1up&seq=363|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514003736/https://babel.hathitrust.org/cgi/pt?id=uva.x001679513&view=1up&seq=85|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514105930/https://babel.hathitrust.org/cgi/pt?id=uva.x001679513&view=1up&seq=334|url-status=live}}
* {{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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514193441/https://babel.hathitrust.org/cgi/pt?id=uva.x001679515&view=1up&seq=62|url-status=live}}</ref>

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|access-date=30 July 2019|archive-date=14 May 2021|archive-url=https://web.archive.org/web/20210514192319/https://babel.hathitrust.org/cgi/pt?id=uva.x001679511&view=1up&seq=112|url-status=live}}</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 Traveller'':

{{Blockquote|
''Boston operator (to Portland operator):'' "Please cut off your battery [power source] entirely for fifteen minutes."<br />
''Portland operator:'' "Will do so. It is now disconnected."<br />
''Boston:'' "Mine is disconnected, and we are working with the auroral current. How do you receive my writing?"<br />
''Portland:'' "Better than with our batteries on. – Current comes and goes gradually."<br />
''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 />
''Portland:'' "Very well. Shall I go ahead with business?"<br />
''Boston:'' "Yes. Go ahead."
}}

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="green-2006" /> Such events led to the general conclusion that

{{Blockquote|The effect of the Aurora 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>{{cite news |title=Aurora Borealis and the Telegraph |newspaper=The British Colonist |volume=2 |issue=56 |date=19 October 1859 |publisher=Amor De Cosmos |publication-place=Victoria, V.I. [Vancouver Island, B.C.] |issn=0839-4229 |oclc=1115103262  |page=1, col. 2 |url=https://archive.org/details/dailycolonist18591019uvic/mode/1up |via=Internet Archive}}</ref>}}In May 2024, a [[May 2024 solar storms|series of solar storms]] caused the aurora borealis to be observed from as far south as [[Ferdows]], [[Iran]].<ref>{{Cite web |date=13 May 2024 |title=وقتی طوفان خورشیدی، آسمان ایران و جهان را رنگ‌آمیزی کرد |url=https://www.zoomit.ir/shutter/420985-aurora-borealis-solar-storm/ |access-date=20 July 2024 |website=زومیت |language=fa}}</ref><ref>{{Cite web |title=چطور شد که شفق قطبی در ایران هم دیده شد؟ +عکس |url=https://borna.news/fa/news/2093435/%DA%86%D8%B7%D9%88%D8%B1-%D8%B4%D8%AF-%DA%A9%D9%87-%D8%B4%D9%81%D9%82-%D9%82%D8%B7%D8%A8%DB%8C-%D8%AF%D8%B1-%D8%A7%DB%8C%D8%B1%D8%A7%D9%86-%D9%87%D9%85-%D8%AF%DB%8C%D8%AF%D9%87-%D8%B4%D8%AF-%D8%B9%DA%A9%D8%B3 |access-date=20 July 2024 |website=fa |language=fa}}</ref><ref>{{Cite web |date=12 May 2024 |title=شفق قطبی در آسمان کویر ایران |url=https://www.bbc.com/persian/articles/c72p05277ero |access-date=20 July 2024 |website=BBC News فارسی |language=fa}}</ref>

== Historical views and folklore ==
{{More citations needed section|date=May 2024|find=Aurora|find2=Historical views and folklore}}
The earliest datable record of an aurora was recorded in the ''[[Bamboo Annals]]'', a historical chronicle of the history of ancient China, in 977 or 957 BC.<ref>{{Cite web|url= http://www.sci-news.com/space/bamboo-annals-aurora-10703.html|title= Earliest Known Report of Aurora Found in Ancient Chinese Chronicle|date= 12 April 2022|work= SCI News|access-date= 5 June 2022|archive-date= 5 June 2022|archive-url= https://web.archive.org/web/20220605152345/http://www.sci-news.com/space/bamboo-annals-aurora-10703.html|url-status= live}}</ref>
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 {{lang|grc-Latn|pithaei}} ('barrel-like'); {{lang|grc-Latn|chasmata}} ('chasm'); {{lang|grc-Latn|pogoniae}} ('bearded'); {{lang|grc-Latn|cyparissae}} ('like [[cypress]] trees'); and describing their manifold colours. 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. (1910), [https://archive.org/stream/physicalsciencei00seneiala#page/38/mode/2up ''Physical Science in the time of Nero''], pp. 39–41, London: Macmillan, accessed 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 {{lang|grc-Latn|trabes}}, {{lang|grc-Latn|chasma}}, "falling red flames", and "daylight in the night".<ref>Bostock, J. and Riley, H. T. (1855), [https://archive.org/stream/naturalhistoryof11855plin#page/62/mode/2up/search/aurora ''The Natural History of Pliny''], Vol. II, London: Bohn, accessed 1 January 2017.</ref>

The earliest depiction of the aurora may have been in [[Cro-Magnon]] [[cave paintings]] of northern Spain dating to 30,000 BC.<ref>{{cite book |last1=Peratt |first1=Anthony L. |title=Physics of the Plasma Universe |date=2014 |publisher=Springer |location=New York|isbn=978-1-4614-7819-5 |edition=2nd |page=357 |doi=10.1007/978-1-4614-7819-5 |url=https://doi.org/10.1007/978-1-4614-7819-5 |url-access=subscription |language=en |access-date=18 March 2024 |archive-date=12 May 2024 |archive-url=https://web.archive.org/web/20240512201045/https://link.springer.com/book/10.1007/978-1-4614-7819-5 |url-status=live }}</ref>

The oldest known written record of the aurora was in a Chinese legend written around 2600 BC. On an autumn around 2000 BC,<ref>{{Cite web|last=Administrator|first=NASA|date=7 June 2013|title=The History of Auroras|url=http://www.nasa.gov/mission_pages/themis/auroras/aurora_history.html|access-date=22 May 2022|website=NASA|language=en|archive-date=29 March 2023|archive-url=https://web.archive.org/web/20230329082101/https://www.nasa.gov/mission_pages/themis/auroras/aurora_history.html|url-status=dead}}</ref> according to a legend, a young woman named Fubao was sitting alone in the wilderness by a bay, when suddenly a "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 son, the Emperor [[Xuanyuan]], known legendarily as the initiator of [[Chinese culture]] and the ancestor of all Chinese people.{{citation needed|date=June 2021}} In the {{lang|zh-Latn|[[Shanhaijing]]}}, a creature named {{lang|zh-Latn|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" ({{lang|zh|天狗}}), "Sword/Knife Star" ({{lang|zh|刀星}}), "Chiyou banner" ({{lang|zh|蚩尤旗}}), "Sky's Open Eyes" ({{lang|zh|天开眼}}), and "Stars like Rain" ({{lang|zh|星陨如雨}}).{{citation needed|date=June 2021}}

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|access-date=3 April 2020|archive-date=1 April 2020|archive-url=https://web.archive.org/web/20200401084459/https://phys.org/news/2020-03-modern-science-reveals-ancient-secret.html|url-status=live}}</ref>

[[File:Aurora australis.jpg|thumb|The Aboriginal Australians associated auroras (which are mainly low on the horizon and predominantly red) with fire.]]
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 {{lang|la|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 {{lang|dif|kootchee}}, an evil spirit creating a large fire. Similarly, the [[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{{which|date=January 2023}} 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|doi=10.3724/SP.J.1440-2807.2013.02.05 |url=http://www.narit.or.th/en/files/2013JAHHvol16/2013JAHH...16..207H.pdf|arxiv=1309.3367|bibcode=2013JAHH...16..207H|s2cid=118102443 |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>

Among the [[Māori people]] of [[New Zealand]], aurora australis or {{lang|mi|Tahunui-a-rangi}} ("great torches in the sky")<!-- based on https://maoridictionary.co.nz/search?idiom=&phrase=&proverb=&loan=&histLoanWords=&keywords=tahu https://maoridictionary.co.nz/search?idiom=&phrase=&proverb=&loan=&histLoanWords=&keywords=nui https://maoridictionary.co.nz/search?idiom=&phrase=&proverb=&loan=&histLoanWords=&keywords=rangi --> were lit by ancestors who sailed south to a "land of ice" (or their descendants);<ref name="steel-2018">{{Cite book|last1=Steel|first1=Frances|url=https://books.google.com/books?id=wluwDwAAQBAJ&pg=PA46|title=New Zealand and the Sea: Historical Perspectives|last2=Anderson|first2=Atholl|author2-link=Atholl Anderson|last3=Ballantyne|first3=Tony|last4=Benjamin|first4=Julie|last5=Booth|first5=Douglas|last6=Brickell|first6=Chris|last7=Gilderdale|first7=Peter|last8=Haines|first8=David|last9=Liebich|first9=Susan|date=2018|publisher=Bridget Williams Books|isbn=978-0-947518-71-4|page=46|language=en|access-date=1 June 2022|archive-date=18 April 2024|archive-url=https://web.archive.org/web/20240418135330/https://books.google.com/books?id=wluwDwAAQBAJ&pg=PA46#v=onepage&q&f=false|url-status=live}}</ref><ref>{{Cite book|last=Best|first=Elsdon|url=http://nzetc.victoria.ac.nz/tm/scholarly/tei-BesAstro-t1-body-d1-d9.html|title=The Astronomical Knowledge of the Maori, Genuine and Empirical|publisher=Dominion Museum|year=1922|location=Wellington|page=58|via=Victoria University of Wellington|access-date=13 September 2021|archive-date=13 September 2021|archive-url=https://web.archive.org/web/20210913001358/http://nzetc.victoria.ac.nz/tm/scholarly/tei-BesAstro-t1-body-d1-d9.html|url-status=live}}</ref> these people were said to be [[Ui-te-Rangiora]]'s expedition party who had reached the [[Southern Ocean]].<ref name="steel-2018" /> around the 7th century.<ref>{{cite journal|last1=Wehi|first1=Priscilla M.|author-link1=Priscilla Wehi|last2=Scott|first2=Nigel J.|last3=Beckwith|first3=Jacinta|last4=Pryor Rodgers|first4=Rata|last5=Gillies|first5=Tasman|last6=Van Uitregt|first6=Vincent|last7=Krushil|first7=Watene|year=2021|title=A short scan of Māori journeys to Antarctica|journal=Journal of the Royal Society of New Zealand|volume=52|issue=5 |pages=587–598|doi=10.1080/03036758.2021.1917633|doi-access=free}}</ref>

[[File:Utsjoki.vaakuna.svg|thumb|upright=0.7|Aurora pictured as wreath of rays in the coat of arms of [[Utsjoki]]]]
In Scandinavia, the first mention of {{lang|non|norðrljós}} (the northern lights) is found in the Norwegian chronicle {{lang|non|[[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|access-date=26 July 2011|archive-url=https://web.archive.org/web/20110721215920/http://www.irf.se/norrsken/Norrsken_history.html|archive-date=21 July 2011|url-status=dead}}</ref>

Walter William Bryant wrote in his book [[s: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>

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] {{Webarchive|url=https://web.archive.org/web/20190731005858/https://founders.archives.gov/documents/Franklin/01-28-02-0150 |date=31 July 2019 }}</ref><ref>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=fr|access-date=31 July 2019|archive-date=27 April 2021|archive-url=https://web.archive.org/web/20210427100326/https://babel.hathitrust.org/cgi/pt?id=mdp.39015077781162&view=1up&seq=439|url-status=live}}</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>

{{blockquote|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=|source=}}

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 fluctuation to the observation of an  aurora 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>

[[File:Aurora Borealis by Frederic Edwin Church.jpg|thumb|[[Frederic Edwin Church]]'s 1865 painting ''[[Aurora Borealis (painting)|Aurora Borealis]]'']]
A variety of [[Indigenous peoples of the Americas|Native American]] myths surround the spectacle. The European explorer [[Samuel Hearne]] travelled with [[Chipewyan]] Dene in 1771 and recorded their views on the {{lang|chp|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>

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=https://americanart.si.edu/artwork/aurora-borealis-4806|title=Aurora Borealis &#124; Smithsonian American Art Museum|website=americanart.si.edu|access-date=18 April 2024|archive-date=27 February 2024|archive-url=https://web.archive.org/web/20240227070127/https://americanart.si.edu/artwork/aurora-borealis-4806|url-status=live}}</ref>

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 and 1715 corresponds to the [[Maunder minimum]] in sunspot activity.

In [[Robert W. Service]]'s satirical poem "[[wikisource:The Ballad of the Northern Lights|The Ballad of the Northern Lights]]" (1908), a Yukon prospector discovers that the aurora is the glow from a [[radium]] mine. He stakes his claim, then goes to town looking for investors.

In the early 1900s, the Norwegian scientist [[Kristian Birkeland]] laid the foundation{{Colloquialism|date=June 2021}} for the current understanding of geomagnetism and polar auroras.

In [[Sami people|Sami]] mythology, the northern lights are caused by the deceased who bled to death cutting themselves, their blood spilling on the sky. Many aboriginal peoples of northern Eurasia and North America share similar beliefs of northern lights being the blood of the deceased, some believing they are caused by dead warriors' blood spraying on the sky as they engage in playing games, riding horses or having fun in some other way.{{citation needed|date=January 2023}}

== Extraterrestrial aurorae ==
{{See also|Magnetosphere of Jupiter#Aurorae}}
[[File: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]].]]
[[File:Saturns Northern Aurora still.jpg|thumb|An aurora high above the northern part of Saturn; image taken by the [[Cassini spacecraft]].

[[:File:Saturns Northern Aurora in Motion.gif|A movie]] shows images from 81 hours of observations of Saturn's aurora.]]

Both [[Jupiter]] and [[Saturn]] have magnetic fields that are stronger than Earth's (Jupiter's equatorial field strength is 4.3 [[Gauss (unit)|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="european space agency-2004">{{cite web|url=http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html|title=ESA Portal – Mars Express discovers auroras on Mars|publisher=European Space Agency|date=11 August 2004|access-date=5 August 2010|archive-date=19 October 2012|archive-url=https://web.archive.org/web/20121019183824/http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html|url-status=live}}</ref>

The aurorae on Saturn seem, like Earth's, to be powered by the solar wind. However, Jupiter's aurorae are more complex. 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.

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.<ref>{{Cite journal|last1=Phillips|first1=J. L.|last2=Stewart|first2=A. I. F.|last3=Luhmann|first3=J. G.|date=1986|title=The Venus ultraviolet aurora: Observations at 130.4 nm|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GL013i010p01047|journal=Geophysical Research Letters|language=en|volume=13|issue=10|pages=1047–1050|doi=10.1029/GL013i010p01047|bibcode=1986GeoRL..13.1047P|issn=1944-8007|access-date=17 January 2021|archive-date=22 January 2021|archive-url=https://web.archive.org/web/20210122212248/https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GL013i010p01047|url-status=live}}</ref> A Venusian aurora originates when electrons from the solar wind collide with the night-side atmosphere.

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&nbsp;km across, and possibly about 8&nbsp;km high. By analysing 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="european space agency-2004" /><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|access-date=5 August 2010|archive-date=10 February 2007|archive-url=https://web.archive.org/web/20070210031424/http://www.universetoday.com/am/publish/mars_express_aurorae.html?1722006|url-status=live}}</ref>

Between 2014 and 2016, cometary auroras were observed on comet [[67P/Churyumov–Gerasimenko]] by multiple instruments on the [[Rosetta (spacecraft)|Rosetta]] spacecraft.<ref>{{cite web|date=21 September 2020|title=Comet Chury's ultraviolet aurora|url=https://www.unibe.ch/news/media_news/media_relations_e/media_releases/2020/media_releases_2020/comet_chury_s_ultraviolet_aurora/index_eng.html|access-date=17 January 2021|website=Portal|archive-date=16 January 2021|archive-url=https://web.archive.org/web/20210116034054/https://www.unibe.ch/news/media_news/media_relations_e/media_releases/2020/media_releases_2020/comet_chury_s_ultraviolet_aurora/index_eng.html|url-status=live}}</ref><ref name="galand-2020">{{Cite journal|last1=Galand|first1=M.|last2=Feldman|first2=P. D.|last3=Bockelée-Morvan|first3=D.|author3-link=Dominique Bockelée-Morvan|last4=Biver|first4=N.|last5=Cheng|first5=Y.-C.|last6=Rinaldi|first6=G.|last7=Rubin|first7=M.|last8=Altwegg|first8=K.|author8-link=Kathrin Altwegg|last9=Deca|first9=J.|last10=Beth|first10=A.|last11=Stephenson|first11=P.|date=21 September 2020|title=Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko|url=https://www.nature.com/articles/s41550-020-1171-7.epdf?sharing_token=D757kcyUX_56njDTcvpSzdRgN0jAjWel9jnR3ZoTv0ODS5LjlN2IKlQgz2jQGPEcFej9C7svyQPjp54CQ50dBx_tkOS3bq-oMWB16Ux3zdWmIZbNCQBAGA1gFAIyXN43TupVPuTwe_oI8bjAahWq18wR7m9QXx7Yhz7zESpivB3btrTi5Qt3trSYVr1aO5yYHu6Hfcnq6u_UVzU3rpWFxvMwCy3aj-2263pTY4ThIYuLO3VW51M44nPr7Ff1Y5vP5tsgJekLXnza9PmvSWJF1Q==&tracking_referrer=www.space.com|journal=Nature Astronomy|language=en|volume=4|issue=11|pages=1084–1091|doi=10.1038/s41550-020-1171-7|bibcode=2020NatAs...4.1084G|issn=2397-3366|hdl=10044/1/82183|s2cid=221884342|hdl-access=free|access-date=17 January 2021|archive-date=9 April 2022|archive-url=https://web.archive.org/web/20220409035009/https://www.nature.com/articles/s41550-020-1171-7.epdf?sharing_token=D757kcyUX_56njDTcvpSzdRgN0jAjWel9jnR3ZoTv0ODS5LjlN2IKlQgz2jQGPEcFej9C7svyQPjp54CQ50dBx_tkOS3bq-oMWB16Ux3zdWmIZbNCQBAGA1gFAIyXN43TupVPuTwe_oI8bjAahWq18wR7m9QXx7Yhz7zESpivB3btrTi5Qt3trSYVr1aO5yYHu6Hfcnq6u_UVzU3rpWFxvMwCy3aj-2263pTY4ThIYuLO3VW51M44nPr7Ff1Y5vP5tsgJekLXnza9PmvSWJF1Q==&tracking_referrer=www.space.com|url-status=live}}</ref> The auroras were observed at [[Far ultraviolet|far-ultraviolet]] wavelengths. [[Coma (cometary)|Coma]] observations revealed atomic emissions of hydrogen and oxygen caused by the [[photodissociation]] (not [[photoionization]], like in terrestrial auroras) of water molecules in the comet's coma.<ref name="galand-2020" /> The interaction of accelerated electrons from the solar wind with gas particles in the coma is responsible for the aurora.<ref name="galand-2020" /> Since comet 67P has no magnetic field, the aurora is diffusely spread around the comet.<ref name="galand-2020" />

[[Exoplanet]]s, such as [[hot Jupiter]]s, have been suggested to experience ionization in their upper atmospheres and generate an aurora modified by [[weather]] in their turbulent [[troposphere]]s.<ref>{{Cite journal|last1=Helling|first1=Christiane|last2=Rimmer|first2=Paul B.|date=23 September 2019|title=Lightning and charge processes in brown dwarf and exoplanet atmospheres|url=|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=377|issue=2154|page=20180398|doi=10.1098/rsta.2018.0398|arxiv=1903.04565|pmid=31378171|pmc=6710897|bibcode=2019RSPTA.37780398H}}</ref> However, there is no current detection of an exoplanet aurora.

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|archive-date=31 July 2015|archive-url=https://web.archive.org/web/20150731022645/http://news.discovery.com/space/alien-life-exoplanets/monstrous-aurora-detected-beyond-our-solar-system-150729.htm|url-status=dead}}</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|archive-date=30 July 2015|archive-url=https://web.archive.org/web/20150730212125/http://www.space.com/30087-alien-auroras-found-beyond-solar-system.html|url-status=live}}</ref>

== See also ==
* [[Airglow]]
* [[Aurora (heraldry)]]
* [[Heliophysics]]
* [[List of solar storms]]
* [[Paschen's law]]
* [[Space tornado]]
* [[Space weather]]

== Explanatory notes ==
{{notelist}}

== References ==
{{reflist}}

== Further reading ==
* {{cite EB9 |wstitle= Aurora Polaris |volume= III |last= Procter |first= Henry Richardson |pages= 90-99 |short=1}}
* {{Cite EB1911|wstitle=Aurora Polaris|volume=2|last= Chree|first= Charles |author-link= Charles Chree|pages=927–934|short=1}} These two both include  detailed descriptions of historical observations and descriptions.
* {{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 }}
* {{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 }}
* {{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 }}
* {{cite journal|last=Akasofu|first=Syun-Ichi|title=Secrets of the Aurora Borealis|journal=Alaska Geographic Series|volume=29|issue=1|date=April 2002 }}
* {{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|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]
* {{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 }}
* {{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 }}
* {{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 }}
* {{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|access-date=15 May 2006|archive-url=https://web.archive.org/web/20060411100954/https://science.nasa.gov/headlines/y2001/ast26oct_1.htm|archive-date=11 April 2006|url-status=dead}}
* {{cite book |title=The Aurora Watcher's Handbook |first=Neil |last=Davis |publisher=University of Alaska Press |date=1992 |isbn=0-912006-60-9 }}

== External links ==
{{Commons}}
{{Wikiquote|Aurora}}
{{Wikivoyage|Northern Lights}}
{{Wikisource|Aurora}}
* [https://www.nordlysvarsel.com/ Aurora forecast – Will there be northern lights?]
* [https://earth.nullschool.net/#current/space/surface/level/anim=off/overlay=aurora/winkel3/ Current global map showing the probability of visible aurora]
* [https://web.archive.org/web/20161124084503/http://www.gi.alaska.edu/AuroraForecast Aurora – Forecasting] (archived 24 November 2016)
* [http://www.northernlightsiceland.com/northern-lights-forecast/ Official MET aurora forecasting in Iceland]
* [http://www.aurorahunter.com/aurora-prediction.php Aurora Borealis – Predicting]
* [http://www.hamqsl.com/solar1.html#converters Solar Terrestrial Data] – Online Converter – ''Northern Lights'' Latitude
* [https://web.archive.org/web/20190311081225/http://www.aurora-service.eu/ Aurora Service Europe] – Aurora forecasts for Europe (archived 11 March 2019)
* [https://news.avclub.com/bask-in-natures-majesty-without-getting-your-tootsies-c-1841696490 Live Northern Lights webstream] {{Webarchive|url=https://web.archive.org/web/20210427091351/https://news.avclub.com/bask-in-natures-majesty-without-getting-your-tootsies-c-1841696490 |date=27 April 2021 }}
* [https://spectacularnwt.com/what-to-do/aurora World's Best Aurora]  – The Northwest Territories is the world's Northern Lights mecca.

=== Multimedia ===
* [http://vimeo.com/user10702000/fireinthesky Amazing time-lapse video of Aurora Borealis] – Shot in Iceland over the winter of 2013/2014
* [http://nrk.no/nyheter/distrikt/troms_og_finnmark/1.7467857 Popular video of Aurora Borealis] – Taken in Norway in 2011
* [https://web.archive.org/web/20111004061641/http://www.aurora-northern-lights.com/ Aurora Photo Gallery] – Views taken 2009–2011 (archived 4 October 2011)
* [http://apod.nasa.gov/apod/ap120103.html Aurora Photo Gallery] – "Full-Sky Aurora" over Eastern [[Norway]]. December 2011
* [https://web.archive.org/web/20100902122923/http://www.twanight.org/newTWAN/gallery.asp?Gallery=Aurora&page=1 Videos and Photos – Auroras at Night] (archived 2 September 2010)
* [https://www.youtube.com/watch?v=lT3J6a9p_o8 Video (04:49)] – Aurora Borealis – How The ''Northern Lights'' Are Created (video on [[YouTube]])
* [http://www.nfb.ca/film/northern_lights Video (47:40)] – ''Northern Lights'' – Documentary
* [https://vimeo.com/62602652 Video (5:00)] – Northern lights video in real time
* [https://web.archive.org/web/20110817082341/http://vimeo.com/27315234 Video (01:42)] – ''Northern Light – Story of [[Geomagnetic storm|Geomagnetc Storm]]'' ([[Terschelling]] Island – 6/7 April 2000) (archived 17 August 2011)
* [https://www.youtube.com/watch?v=Lc3FxNXjBs0 Video (01:56)] (time-lapse) − Auroras – Ground-Level View from [[Finnish Lapland]] 2011 (video on [[YouTube]])
* [https://www.youtube.com/watch?v=Vq3o3sYpk78 Video (02:43)] (time-lapse) − Auroras – Ground-Level View from [[Tromsø]], [[Norway]], 24 November 2010 (video on [[YouTube]])
* [https://www.youtube.com/watch?v=l6ahFFFQBZY Video (00:27)] (time-lapse) – [[Earth]] and Auroras – Viewed from the [[International Space Station]] (video on [[YouTube]])

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