Editing Solar flare (section)

== Physical description ==
[[File:X3.2 Solar flare on 2013-05-14 at four wavelengths.jpg|thumb|An X3.2-class solar flare observed in different wavelengths. Clockwise from top left: 304, 335, 131, and 193 Å]]
Solar flares are eruptions of [[electromagnetic radiation]] originating in the Sun's atmosphere.<ref name="NOAAflare">{{cite web |title=Solar Flares (Radio Blackouts) |url=https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts |publisher=NOAA/NWS Space Weather Prediction Center |access-date=11 November 2021}}</ref> They affect all layers of the solar atmosphere ([[Photosphere#The Sun|photosphere]], [[chromosphere]], and [[solar corona|corona]]).<ref name="woods09" /> The [[Plasma (physics)|plasma]] medium is heated to >10<sup>7</sup> [[kelvin]], while [[electron]]s, [[proton]]s, and heavier [[ion]]s are accelerated to near the [[speed of light]].<ref>{{Cite journal |last1=Ishikawa |first1=Shin-nosuke |last2=Glesener |first2=Lindsay |author2-link=Lindsay Glesener|last3=Krucker |first3=Säm |last4=Christe |first4=Steven |last5=Buitrago-Casas |first5=Juan Camilo |last6=Narukage |first6=Noriyuki |last7=Vievering |first7=Juliana |date=2017 |title=Detection of nanoflare-heated plasma in the solar corona by the FOXSI-2 sounding rocket |url=https://www.nature.com/articles/s41550-017-0269-z |journal=[[Nature Astronomy]] |language=en |volume=1 |issue=11 |pages=771–774 |doi=10.1038/s41550-017-0269-z |bibcode=2017NatAs...1..771I |issn=2397-3366}}</ref><ref>{{Cite journal |last1=Sigalotti |first1=Leonardo Di G. |last2=Cruz |first2=Fidel |date=2023 |title=Unveiling the mystery of solar-coronal heating |url=https://pubs.aip.org/physicstoday/article/76/4/34/2879433/Unveiling-the-mystery-of-solar-coronal |access-date=2024-05-17 |journal=[[Physics Today]] |volume=76 |issue=4 |pages=34–40 |doi=10.1063/pt.3.5217|bibcode=2023PhT....76d..34S }}</ref> Flares emit [[electromagnetic radiation]] across the [[electromagnetic spectrum]], from [[Radio frequency|radio waves]] to [[gamma ray]]s.<ref name="woods09">{{cite journal |last1=Woods |first1=Thomas N. |last2=Kopp |first2=Greg |last3=Chamberlin |first3=Phillip C. |title=Contributions of the solar ultraviolet irradiance to the total solar irradiance during large flares |journal=[[Journal of Geophysical Research]] |date=2006 |volume=111 |issue=A10 |doi=10.1029/2005JA011507 |doi-access=free |bibcode=2005AGUFMSA33A..07W |bibcode-access=free }}</ref>

Flares occur in [[active regions]], often around [[sunspot]]s, where intense [[Magnetic field|magnetic fields]] penetrate the photosphere to link the corona to the solar interior. Flares are [[#Cause|powered by]] the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may also produce [[coronal mass ejections]] (CMEs), although the relationship between CMEs and flares is not well understood.<ref>{{cite journal |last1=Fletcher |first1=L. |last2=Dennis |first2=B. R. |last3=Hudson |first3=H. S. |last4=Krucker |first4=S. |last5=Phillips |first5=K. |last6=Veronig |first6=A. |last7=Battaglia |first7=M. |last8=Bone |first8=L. |last9=Caspi |first9=A. |last10=Chen |first10=Q. |last11=Gallagher |first11=P. |last12=Grigis |first12=P. T. |last13=Ji |first13=H. |last14=Liu |first14=W. |last15=Milligan |first15=R. O. |last16=Temmer |first16=M.|author16-link=Manuela Temmer |title=An Observational Overview of Solar Flares |journal=[[Space Science Reviews]] |date=September 2011 |volume=159 |issue=1–4 |pages=19–106 |doi=10.1007/s11214-010-9701-8 |bibcode=2011SSRv..159...19F |arxiv=1109.5932 |s2cid=21203102 |url=https://hesperia.gsfc.nasa.gov/rhessi2/docs/monograph/fletcher.pdf}}</ref>

Associated with solar flares are flare sprays.<ref name="Moromoto">{{cite conference |url=http://wwwsoc.nii.ac.jp/jepsjmo/cd-rom/2002cd-rom/pdf/e021/e021-005_e.pdf |archive-url=https://web.archive.org/web/20110611232648/http://wwwsoc.nii.ac.jp/jepsjmo/cd-rom/2002cd-rom/pdf/e021/e021-005_e.pdf |url-status=dead |archive-date=11 June 2011 |title=Effects of Magnetic and Gravity forces on the Acceleration of Solar Filaments and Coronal Mass Ejections |first1=Tarou |last1=Morimoto |first2=Hiroki |last2=Kurokawa |conference=地球惑星科学関連学会2002年合同大会 2002 Joint Conference of Earth and Planetary Science Related Societies (in Japanese) |date=31 May 2002 |location=Tokyo |access-date=8 October 2009}}</ref> They involve faster ejections of material than eruptive [[Solar prominence|prominences]],<ref>{{Cite journal |last1=Tandberg-Hanssen |first1=E. |last2=Martin |first2=Sara F.|last3=Hansen |first3=Richard T. |date=March 1980 |title=Dynamics of flare sprays |url=https://link.springer.com/article/10.1007/BF00152799 |url-access=subscription |journal=[[Solar Physics (journal)|Solar Physics]] |language=en |volume=65 |issue=2 |pages=357–368 |doi=10.1007/BF00152799 |bibcode=1980SoPh...65..357T |bibcode-access=free |s2cid=122385884 |issn=0038-0938}}</ref> and reach velocities of 20 to 2000 kilometers per second.<ref>{{cite news |title=Biggest Solar Flare on Record |url=https://visibleearth.nasa.gov/images/55580/biggest-solar-flare-on-record |work=Visible Earth |publisher=NASA |date=15 May 2001 |language=en}}</ref>

=== Cause ===
Flares occur when accelerated charged particles, mainly electrons, interact with the [[Plasma (physics)|plasma]] medium. Evidence suggests that the phenomenon of [[magnetic reconnection]] leads to this extreme acceleration of charged particles.<ref>{{cite journal |last1=Zhu |first1=Chunming |last2=Liu |first2=Rui |last3=Alexander |first3=David |last4=McAteer |first4=R. T. James |author4-link=James McAteer |title=Observation of the Evolution of a Current Sheet in a Solar Flare |journal=[[The Astrophysical Journal]] |date=19 April 2016 |volume=821 |issue=2 |pages=L29 |doi=10.3847/2041-8205/821/2/L29 |doi-access=free |bibcode=2016ApJ...821L..29Z |bibcode-access=free |arxiv=1603.07062 }}</ref> On the Sun, magnetic reconnection may happen on solar arcades&nbsp;– a type of [[Solar prominence|prominence]] consisting of a series of closely occurring [[Coronal loop|loops]] following magnetic lines of force.<ref>{{cite journal |first1=E. R. |last1=Priest |author1-link=Eric Priest |first2=T. G. |last2=Forbes |title=The magnetic nature of solar flares |journal=[[The Astronomy and Astrophysics Review]] |volume=10 |pages=314–317 |date=2002 |issue=4 |doi=10.1007/s001590100013 |bibcode=2002A&ARv..10..313P }}</ref> These lines of force quickly reconnect into a lower arcade of loops leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection is the origin of the particle acceleration. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a coronal mass ejection.<ref name="sciam1">{{cite news |last1=Holman |first1=Gordon D. |author1-link=Gordon Dean Holman |title=The Mysterious Origins of Solar Flares |url=https://www.scientificamerican.com/article/the-mysterious-origins-of/ |access-date=17 October 2023 |work=[[Scientific American]] |date=1 April 2006 |language=en}}</ref> This also explains why solar flares typically erupt from active regions on the Sun where magnetic fields are much stronger.

Although there is a general agreement on the source of a flare's energy, the mechanisms involved are not well understood. It is not clear how the magnetic energy is transformed into the kinetic energy of the particles, nor is it known how some particles can be accelerated to the GeV range (10<sup>9</sup> [[electronvolt|electron volt]]) and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop.<ref>{{Cite journal |last1=Ryan |first1=James M. |last2=Lee |first2=Martin A. |date=1991-02-01 |title=On the Transport and Acceleration of Solar Flare Particles in a Coronal Loop |url=https://ui.adsabs.harvard.edu/abs/1991ApJ...368..316R |journal=The Astrophysical Journal |volume=368 |pages=316 |doi=10.1086/169695 |bibcode=1991ApJ...368..316R |issn=0004-637X}}</ref>

=== Post-eruption loops and arcades ===
{{see also|Coronal loop}}
[[File:Coronal arcade.png|thumb|A post-eruption arcade present after an X5.7-class solar flare during the [[Bastille Day solar storm]]<ref>{{cite web |last1=Handy |first1=Brian |last2=Hudson |first2=Hugh |title=Super Regions |url=https://solar.physics.montana.edu/nuggets/2000/000714/000714.html |publisher=[[Montana State University]] Solar Physics Group |access-date=23 December 2021 |date=14 July 2000}}</ref>]]
After the eruption of a solar flare, '''post-eruption loops''' made of hot plasma begin to form across the neutral line separating regions of opposite magnetic polarity near the flare's source. These loops extend from the photosphere up into the corona and form along the neutral line at increasingly greater distances from the source as time progresses.<ref name="Livshits11" /> The existence of these hot loops is thought to be continued by prolonged heating present after the eruption and during the flare's decay stage.<ref>{{cite journal |last1=Grechnev |first1=V. V. |last2=Kuzin |first2=S. V. |last3=Urnov |first3=A. M. |last4=Zhitnik |first4=I. A. |last5=Uralov |first5=A. M. |last6=Bogachev |first6=S. A. |last7=Livshits |first7=M. A. |last8=Bugaenko |first8=O. I. |last9=Zandanov |first9=V. G. |last10=Ignat’ev |first10=A. P. |last11=Krutov |first11=V. V. |last12=Oparin |first12=S. N. |last13=Pertsov |first13=A. A. |last14=Slemzin |first14=V. A. |last15=Chertok |first15=I. M. |last16=Stepanov |first16=A. I. |title=Long-lived hot coronal structures observed with CORONAS-F/SPIRIT in the Mg XII line |journal=[[Solar System Research]] |date=July 2006 |volume=40 |issue=4 |pages=286–293 |doi=10.1134/S0038094606040046 |bibcode=2006SoSyR..40..286G |s2cid=121291767 |url=https://link.springer.com/article/10.1134%2FS0038094606040046 |url-access=subscription |access-date=23 December 2021}}</ref>

In sufficiently powerful flares, typically of [[#Classification|C-class]] or higher, the loops may combine to form an elongated arch-like structure known as a '''post-eruption arcade'''. These structures may last anywhere from multiple hours to multiple days after the initial flare.<ref name="Livshits11">{{cite journal |last1=Livshits |first1=M. A. |last2=Urnov |first2=A. M. |last3=Goryaev |first3=F. F. |last4=Kashapova |first4=L. K. |last5=Grigor’eva |first5=I. Yu. |last6=Kal’tman |first6=T. I. |title=Physics of post-eruptive solar arcades: Interpretation of RATAN-600 and STEREO spacecraft observations |journal=[[Astronomy Reports]] |date=October 2011 |volume=55 |issue=10 |pages=918–927 |doi=10.1134/S1063772911100064 |bibcode=2011ARep...55..918L |s2cid=121487634 |url=https://link.springer.com/article/10.1134/S1063772911100064 |url-access=subscription |access-date=23 December 2021}}</ref> In some cases, dark sunward-traveling plasma voids known as [[supra-arcade downflows]] may form above these arcades.<ref>{{Cite journal|journal = The Astrophysical Journal |date=1 May 2011 |volume = 730 |issue=2 |pages=98 |doi=10.1088/0004-637x/730/2/98 |doi-access=free |first1=Sabrina L. |last1=Savage |first2=David E. |last2=McKenzie |arxiv=1101.1540 |bibcode=2011ApJ...730...98S |bibcode-access=free |title=Quantitative Examination of a Large Sample of Supra-Arcade Downflows in Eruptive Solar Flares |s2cid=119273860 |s2cid-access=free }}</ref>