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== Occurrence == [[File:Antarctic aurora ESA313457.jpg|thumb|upright=1.3|Earth's night-side upper atmosphere appears from the bottom as bands of [[afterglow]] illuminating the [[troposphere]] in orange with silhouettes of clouds at the top, and the [[stratosphere]] in white and blue at the top of the middle. 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 auroras stretching over several hundred kilometres.]] Auroras are most commonly observed in 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> a band approximately 6° (~660 km) wide in latitude centered on 67° north and south.<ref name="Bruzek" /> The region that currently displays an aurora is called the "auroral oval". The oval is displaced by the solar wind, pushing it about 15° away from the geomagnetic pole (not the geographic pole) in the noon direction and 23° away in the midnight direction.<ref name=Bruzek>{{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> The peak equatorward extent of the oval is displaced slightly from geographic midnight. It is 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]], which is called [[magnetic midnight]]. 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]], the [[Falkland Islands]], 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]]. A [[geomagnetic storm]] causes the auroral ovals (north and south) to expand, bringing the aurora to lower latitudes or higher in the south. On rare occasions, the aurora borealis can be seen as far south as the Mediterranean and the southern states of the US while the aurora australis can be seen as far north as [[New Caledonia]] and the [[Pilbara]] region in [[Western Australia]]. During the [[Carrington Event]], the greatest geomagnetic storm ever observed, auroras were seen even in the [[tropics]]. 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 on 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 on 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 on 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 Kp-index and estimated Kp-index|''K''<sub>p</sub>]]={{hsp}}3''' corresponds to relatively low levels of geomagnetic activity, while '''[[K-index#The Kp-index and estimated 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) is 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 [[Triangulation|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 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> === Colours and wavelengths of auroral light === * Red: At its highest altitudes, [[Excited state|excited]] atomic oxygen emits at 630 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this colour 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.<ref>{{Cite web |title=The Colors of the Aurora (U.S. National Park Service) |url=https://www.nps.gov/articles/-articles-aps-v8-i1-c9.htm |access-date=2025-05-19 |website=www.nps.gov |language=en}}</ref> * Green: At lower altitudes, the more frequent collisions suppress the 630 nm (red) mode: rather the 557.7 nm emission (green) dominates. A fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to the high stability of the N<sub>2</sub> molecule) plays a role here, as it can transfer energy by collision with 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 in concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to [[forbidden transition]]s of atomic oxygen, a slow mechanism responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading.<ref>{{Cite web |title=The Colors of the Aurora (U.S. National Park Service) |url=https://www.nps.gov/articles/-articles-aps-v8-i1-c9.htm |access-date=2025-05-19 |website=www.nps.gov |language=en}}</ref> [[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 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 the human eye) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars, Jupiter, and Saturn.<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 | 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> * 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. Yellow and pink auroras are relatively rare and are associated with high solar activity.<ref>{{Cite web |last=updated |first=Daisy Dobrijevic last |date=23 June 2023 |title=Aurora colors: What causes them and why do they vary? |url=https://www.space.com/aurora-colors-explained |access-date=31 March 2025 |website=Space.com |language=en}}</ref> Other shades of red, as well as orange and gold, also may be seen on rare occasions. 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 auroras 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 were 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 dark side 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 km for blue and green emissions and above-average solar wind speeds ({{circa|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 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> === Abnormal 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 the 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 gives 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>
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