View source for Astronomy:Solar flare

{{Short description|Eruption of electromagnetic radiation}}
[[File:X Class Solar Flare Sends ‘Shockwaves’ on The Sun (6819094556).jpg|thumb|An X5.4-class solar flare causing blooming, vertical streaking, and diffraction patterns to form in the image taken by the {{cvt|131|Å|nm|lk=on}} sensor aboard the Solar Dynamics Observatory on 6 March 2012]]
{{heliophysics}}
A '''solar flare''' is a relatively intense, localized emission of [[Physics:Electromagnetic radiation|electromagnetic radiation]] in the [[Astronomy:Sun|Sun]]'s [[Astronomy:Stellar atmosphere|atmosphere]]. Flares occur in [[Astronomy:Active region|active region]]s and are often, but not always, accompanied by [[Astronomy:Coronal mass ejection|coronal mass ejection]]s, [[Astronomy:Solar particle event|solar particle event]]s, and other eruptive [[Astronomy:Solar phenomena|solar phenomena]]. The occurrence of solar flares varies with the 11-year [[Astronomy:Solar cycle|solar cycle]].

Solar flares are thought to occur when stored [[Physics:Magnetic energy|magnetic energy]] in the Sun's atmosphere accelerates [[Physics:Charged particle|charged particle]]s in the surrounding [[Physics:Plasma|plasma]]. This results in the emission of electromagnetic radiation across the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]].

High-energy electromagnetic radiation from solar flares is absorbed by the [[Physics:Daylight|daylight]] side of Earth's upper atmosphere, in particular the [[Astronomy:Ionosphere|ionosphere]], and does not reach the surface. This absorption can temporarily increase the ionization of the ionosphere which may interfere with short-wave radio communication. The prediction of solar flares is an active area of research.

Flares also occur on other stars, where the term ''[[Astronomy:Flare star|stellar flare]]'' applies.

== 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 [[Physics:Electromagnetic radiation|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 ([[Astronomy:Photosphere#The Sun|photosphere]], [[Astronomy:Chromosphere|chromosphere]], and [[Physics:Solar corona|corona]]).<ref name="woods09" /> The [[Physics:Plasma|plasma]] medium is heated to >10<sup>7</sup> [[Kelvin|kelvin]], while [[Physics:Electron|electron]]s, [[Physics:Proton|proton]]s, and heavier [[Physics:Ion|ion]]s are accelerated to near the [[Physics:Speed of light|speed of light]].{{citation needed|date=July 2023}} Flares emit [[Physics:Electromagnetic radiation|electromagnetic radiation]] across the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]] at all [[Wavelength|wavelength]]s, from [[Engineering:Radio frequency|radio waves]] to [[Physics:Gamma ray|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 [[Astronomy:Sunspot|sunspot]]s, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are 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 still 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. |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 |date=31 May 2002 |location=Tokyo |access-date=8 October 2009}}</ref> They involve faster ejections of material than eruptive 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 |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>

=== Frequency ===
The frequency of occurrence of solar flares varies with the 11-year [[Astronomy:Solar cycle|solar cycle]]. It can range from several per day during [[Astronomy:Solar maximum|solar maximum]] to less than one every week during [[Astronomy:Solar minimum|solar minimum]]. Additionally, more powerful flares are less frequent than weaker ones. For example, X10-class (severe) flares occur on average about eight times per cycle, whereas M1-class (minor) flares occur on average about 2000 times per cycle.<ref name="SWPCscales">{{cite web |title=NOAA Space Weather Scales |url=https://www.swpc.noaa.gov/noaa-scales-explanation |publisher=NOAA/NWS Space Weather Prediction Center |access-date=20 November 2021}}</ref>

[[Biography:Erich Rieger|Erich Rieger]] discovered with coworkers in 1984 an approximately 154 day period in the occurrence of gamma-ray emitting solar flares at least since the [[Astronomy:Solar cycle 19|solar cycle 19]].<ref>{{Cite journal|title=A 154-day periodicity in the occurrence of hard solar flares?|year=1984|last1=Rieger|first1=E.|last2=Share|first2=G. H.|last3=Forrest|first3=D. J.|last4=Kanbach|first4=G.|last5=Reppin|first5=C.|last6=Chupp|first6=E. L.|journal=Nature|volume=312|issue=5995|pages=623–625|doi=10.1038/312623a0 |bibcode=1984Natur.312..623R|s2cid=4348672}}</ref> The period has since been confirmed in most heliophysics data and the [[Physics:Interplanetary magnetic field|interplanetary magnetic field]] and is commonly known as the [[Biography:Erich Rieger#Rieger period|''Rieger period'']].  The period's [[Physics:Resonance|resonance]] harmonics also have been reported from most data types in the [[Astronomy:Heliosphere|heliosphere]].

The [[Frequency distribution|frequency distribution]]s of various flare phenomena can be characterized by power-law distributions. For example, the peak fluxes of radio, extreme ultraviolet, and hard and soft X-ray emissions; total energies; and flare durations (see {{slink|#Duration}}) have been found to follow power-law distributions.<ref>{{cite journal |last1=Kurochka |first1=L. N. |title=Energy distribution of 15,000 solar flares |journal=Astronomicheskii Zhurnal |date=April 1987 |volume=64 |page=443 |bibcode=1987AZh....64..443K}}</ref><ref>{{cite journal |last1=Crosby |first1=Norma B. |last2=Aschwanden |first2=Markus J. |last3=Dennis |first3=Brian R. |title=Frequency distributions and correlations of solar X-ray flare parameters |journal=Solar Physics |date=February 1993 |volume=143 |issue=2 |pages=275–299 |doi=10.1007/BF00646488 |bibcode=1993SoPh..143..275C}}</ref><ref>{{cite journal |last1=Li |first1=Y. P. |last2=Gan |first2=W. Q. |last3=Feng |first3=L. |title=Statistical analyses on thermal aspects of solar flares |journal=The Astrophysical Journal |date=March 2012 |volume=747 |issue=2 |pages=133 |doi=10.1088/0004-637X/747/2/133 |bibcode=2012ApJ...747..133L}}</ref><ref>{{cite book |last1=Aschwanden |first1=Markus J. |title=Self-Organized Criticality in Astrophysics: The Statistics of Nonlinear Processes in the Universe |date=2011 |publisher=Springer |location=Berlin, Heidelberg |isbn=978-3-642-15001-2 |url=https://link.springer.com/book/10.1007/978-3-642-15001-2 |url-access=subscription}}</ref>{{rp|23-28}}

===Duration===
[[File:SDO EVE Late Phase Flares.webm|thumb|Observations of a solar flare by different instruments aboard the Solar Dynamics Observatory show varying durations]]
The duration of a solar flare depends heavily on the wavelength of the electromagnetic radiation used in its calculation. This is due to different wavelengths being emitted through different processes and at different heights in the Sun's atmosphere.

A common measure of flare duration is the [[Full width at half maximum|full width at half maximum]] (FWHM) time of soft X-ray flux within the wavelength bands {{convert|0.05|to|0.4|and|0.1|to|0.8|nm|angstrom|abbr=off|lk=on|sigfig=1}} measured by the GOES spacecraft in [[Astronomy:Geosynchronous orbit|geosynchronous orbit]]. The FWHM time spans from when a flare's flux first reaches halfway between its maximum flux and the background flux and when it again reaches this value as the flare decays. Using this measure, the duration of a flare ranges from approximately tens of seconds to several hours with a median duration of approximately 6 and 11 minutes in the 0.05 to 0.4 and 0.1 to 0.8 nanometre bands, respectively.<ref>{{cite journal |last1=Reep |first1=Jeffrey W. |last2=Knizhnik |first2=Kalman J. |title=What Determines the X-Ray Intensity and Duration of a Solar Flare? |journal=The Astrophysical Journal |date=3 April 2019  |volume=874 |issue=2 |pages=157 |doi=10.3847/1538-4357/ab0ae7 |doi-access=free |arxiv=1903.10564 |bibcode=2019ApJ...874..157R |bibcode-access=free |s2cid=85517195 |s2cid-access=free }}</ref><ref>{{cite journal |last1=Reep |first1=Jeffrey W. |last2=Barnes |first2=Will T. |title=Forecasting the Remaining Duration of an Ongoing Solar Flare |journal=Space Weather |date=October 2021 |volume=19 |issue=10 |doi=10.1029/2021SW002754 |doi-access=free |arxiv=2103.03957 |bibcode=2021SpWea..1902754R |bibcode-access=free |s2cid=237709521 |s2cid-access=free }}</ref>

===Post-eruption loops and arcades===
[[File:Coronal arcade.png|thumb|A post-eruption arcade present after an X5.7-class solar flare during the [[Astronomy:Bastille Day solar storm|Bastille Day solar storm]].<ref>{{cite web |last1=Brian |first1=Handy |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 up 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 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 [[Astronomy:Supra-arcade downflows|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>

== Cause ==
Flares occur when accelerated charged particles, mainly electrons, interact with the [[Physics:Plasma|plasma]] medium. Evidence suggests that the phenomenon of [[Astronomy:Magnetic reconnection|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 |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 series of closely occurring loops following magnetic lines of force. 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. |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 still 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> [[Physics: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. Scientists are unable to forecast flares.{{citation needed|date=August 2013}}

== Classification ==

===Soft X-ray===
[[File:GOES-16 X-ray flux (1-minute data) on 2023-12-14 with flares labeled.svg|thumb|An M5.8, M2.3, and X2.8 flare were recorded by [[Engineering:GOES-16|GOES-16]] on 14 December 2023. Their corresponding peak fluxes in the 0.1 to 0.8&nbsp;nm channel were 5.8×10<sup>−5</sup>, 2.3×10<sup>−5</sup>, and 2.8×10<sup>−4</sup> W/m<sup>2</sup>, respectively.]]
The modern classification system for solar flares uses the letters A, B, C, M, or X, according to the peak [[Physics:Flux|flux]] in watts per square metre (W/m<sup>2</sup>) of soft X-rays with [[Wavelength|wavelength]]s {{convert|0.1|to|0.8|nm|angstrom|abbr=off|lk=on|sigfig=1}}, as measured by GOES satellites in [[Astronomy:Geosynchronous orbit|geosynchronous orbit]].

{| class="wikitable"
|-
! Classification !! Peak flux range (W/m<sup>2</sup>)
|-
| A || < 10<sup>−7</sup>
|-
| B || 10<sup>−7</sup> – 10<sup>−6</sup>
|-
| C || 10<sup>−6</sup> – 10<sup>−5</sup>
|-
| M || 10<sup>−5</sup> – 10<sup>−4</sup>
|-
| X || > 10<sup>−4</sup>
|}

The strength of an event within a class is noted by a numerical suffix ranging from 1 up to, but excluding, 10, which is also the factor for that event within the class. Hence, an X2 flare is twice the strength of an X1 flare, an X3 flare is three times as powerful as an X1, and only 50% more powerful than an X2.<ref>{{cite web|url=http://www.nasa.gov/feature/goddard/2017/active-region-on-sun-continues-to-emit-solar-flares|title=Sun Erupts With Significant Flare|first=Rob|last=Garner|date=6 September 2017|website=NASA|access-date=2 June 2019}}</ref> An X2 is four times more powerful than an M5 flare.<ref>{{citation
 | title=Heliophysics: Space Storms and Radiation: Causes and Effects
 | editor1-first=Carolus J. | editor1-last=Schrijver
 | publisher=Cambridge University Press
 | year=2010 | isbn=978-1107049048 | page=375
 | url=https://books.google.com/books?id=OukfAwAAQBAJ&pg=PA375 }}</ref> X-class flares with a peak flux that exceeds 10<sup>−3</sup> W/m<sup>2</sup> may be noted with a numerical suffix equal to or greater than 10.

This system was originally devised in 1970 and included only the letters C, M, and X. These letters were chosen to avoid confusion with other optical classification systems. The A and B classes would later be added in the 1990s as instruments became more sensitive to weaker flares. Around the same time, the [[Social:Backronym|backronym]] ''moderate'' for M-class flares and ''extreme'' for X-class flares began to be used.<ref name="pietrow22">{{cite thesis |type=PhD |last=Pietrow |first=A.G.M. |date=2022 |title=Physical properties of chromospheric features: Plage, peacock jets, and calibrating it all. |publisher=Stockholm University| url=https://www.diva-portal.org/smash/record.jsf?aq2=%5B%5B%5D%5D&c=10&af=%5B%5D&searchType=LIST_LATEST&sortOrder2=title_sort_asc&query=&language=en&pid=diva2%3A1651858&aq=%5B%5B%5D%5D&sf=all&aqe=%5B%5D&sortOrder=author_sort_asc&onlyFullText=false&noOfRows=50&dswid=5451 |doi=10.13140/RG.2.2.36047.76968 |doi-access=free}}</ref>

=== Importance ===
An earlier classification system, what is sometimes referred to as the ''flare importance'', was based on [[Physics:H-alpha|H-alpha]] spectral observations. The scheme uses both the intensity and emitting surface. The classification in intensity is qualitative, referring to the flares as: faint (f), normal (n), or brilliant (b). The emitting surface is measured in terms of millionths of the hemisphere and is described below. (The total hemisphere area ''A<sub>H</sub>'' = 15.5 × 10<sup>12</sup> km<sup>2</sup>.)
{| class="wikitable"
|-
! Classification !! Corrected area<br />(millionths of hemisphere)
|-
| S || < 100
|-
| 1 || 100–250
|-
| 2 || 250–600
|-
| 3 || 600–1200
|-
| 4 || > 1200
|}
A flare then is classified taking S or a number that represents its size and a letter that represents its peak intensity, v.g.: Sn is a normal sunflare.<ref>{{cite book
 | last1 = Tandberg-Hanssen
 | last2 = Emslie
 | first2 = A. Gordon
 | date = 1988
 | title = The Physics of Solar Flares
 | publisher = Cambridge University Press
}}</ref>

=== Duration ===
Solar flares can also be classified based on their duration as either '''impulsive''' or '''long duration events''' ('''LDE'''). The time threshold separating the two is not well defined. The SWPC regards events requiring 30 minutes or more to decay to half maximum as LDEs, whereas Belgium's Solar-Terrestrial Centre of Excellence regards events with duration greater than 60 minutes as LDEs.<ref>{{cite web |title=Space Weather Glossary |url=https://www.swpc.noaa.gov/content/space-weather-glossary#longduration |publisher=NOAA/NWS Space Weather Prediction Center |access-date=18 April 2022}}</ref><ref>{{cite web |title=The duration of solar flares |url=https://www.stce.be/news/332/welcome.html |publisher=Solar-Terrestrial Centre of Excellence |access-date=18 April 2022}}</ref>

== Effects ==

=== Terrestrial ===
X-rays and extreme ultraviolet radiation emitted by solar flares are absorbed by the [[Physics:Daylight|daylight]] side of Earth's atmosphere and do not reach the Earth's surface. Therefore, solar flares pose no direct danger to humans on Earth. However, this absorption of high-energy electromagnetic radiation can temporarily increase the [[Physics:Ionization|ionization]] of the upper atmosphere, which can interfere with short-wave radio communication, and can temporarily heat and expand the Earth's outer atmosphere. This expansion can increase drag on satellites in [[Astronomy:Low Earth orbit|low Earth orbit]], which can lead to [[Astronomy:Orbital decay|orbital decay]] over time.<ref>{{cite web |title=The Impact of Flares |url=https://hesperia.gsfc.nasa.gov/rhessi3/mission/science/the-impact-of-flares/index.html |website=RHESSI Web Site |publisher=NASA |access-date=23 December 2021}}</ref>

==== Radio blackouts ====
{{Further|Astronomy:Sudden ionospheric disturbance}}
The temporary increase in ionization of the daylight side of Earth's atmosphere, in particular the D layer of the [[Astronomy:Ionosphere|ionosphere]], can interfere with short-wave radio communications that rely on its level of ionization for [[Astronomy:Skywave|skywave]] propagation. Skywave, or skip, refers to the propagation of radio waves reflected or refracted off of the ionized ionosphere. When ionization is higher than normal, radio waves get degraded or completely absorbed by losing energy from the more frequent collisions with free electrons.<ref name="NOAAflare" />

The level of ionization of the atmosphere correlates with the strength of the associated solar flare in soft X-ray radiation. The NOAA classifies radio blackouts by the peak soft X-ray intensity of the associated flare.
{| class="wikitable"
|-
! Classification !! Associated solar flare !! Description<ref name="SWPCscales" />
|-
| R1 || M1 || Minor radio blackout
|-
| R2 || M5 || Moderate radio blackout
|-
| R3 || X1 || Strong radio blackout
|-
| R4 || X10 || Severe radio blackout
|-
| R5 || X20 || Extreme radio blackout
|}

==== Magnetic crochet ====
The increased ionization of the D and E layers of the ionosphere caused by large solar flares increases the [[Physics:Electrical conductivity|electrical conductivity]] of these layers allowing for the flow of [[Physics:Electric current|electric current]]s. These ionospheric currents induce a magnetic field which can be measured by ground-based magnetometers. This phenomenon is known as a '''magnetic crochet''' or '''solar flare effect''' ('''SFE''').<ref>{{cite web |last1=Thompson |first1=Richard |title=A Solar Flare Effect |url=https://www.sws.bom.gov.au/Educational/3/1/1 |publisher=Australian Bureau of Meteorology Space Weather Forecasting Centre |access-date=12 May 2022}}</ref> The former name derives from its appearance on magnetometers resembling a crochet hook.{{Citation needed|date=May 2022}} These disturbances are on the order of a few [[Tesla (unit)|nanoteslas]], which is relatively minor compared to those induced by geomagnetic storms.<ref>{{cite journal |last1=Curto |first1=Juan José |title=Geomagnetic solar flare effects: a review |journal=Journal of Space Weather and Space Climate |date=2020 |volume=10 |pages=27 |doi=10.1051/swsc/2020027 |bibcode=2020JSWSC..10...27C |s2cid=226442270 |url=https://www.swsc-journal.org/articles/swsc/abs/2020/01/swsc190079/swsc190079.html|doi-access=free }}</ref>

=== In space ===
For astronauts in low earth orbit an expected radiation dose from the electromagnetic radiation emitted during a solar flare is about 0.05 [[Physics:Gray (unit)|gray]], which is not immediately lethal on its own. Of much more concern for astronauts is the [[Physics:Particle radiation|particle radiation]] associated with solar particle events.<ref>{{cite journal |last1=Whittaker |first1=Ian |title=The invisible space killers - The dangers of space radiation from both inside and outside the solar system |journal=Physiology News Magazine |doi=10.36866/pn.117.36 |s2cid=214067105 |url=https://www.physoc.org/magazine-articles/the-invisible-space-killers/ |access-date=14 June 2022}}</ref>{{Better source needed|date=June 2022}}

== Observations ==
{{Further|Astronomy:Solar observation}}
Flares produce radiation across the electromagnetic spectrum, although with different intensity. They are not very intense in visible light, but they can be very bright at particular [[Physics:Spectral line|spectral line]]s. They normally produce [[Physics:Bremsstrahlung|bremsstrahlung]] in X-rays and [[Physics:Synchrotron radiation|synchrotron radiation]] in radio.{{Citation needed|date=May 2022}}

=== History ===
==== Optical observations ====
[[File:Carrington Richard sunspots 1859.jpg|thumb|Richard Carrington's sketch of the first recorded solar flare (A and B mark the initial bright points which moved over the course of five minutes to C and D before disappearing)<ref name="Carrington1859" />]]
Solar flares were first observed by Richard Carrington and Richard Hodgson independently on [[Astronomy:Solar storm of 1859|1 September 1859]] by projecting the image of the solar disk produced by an optical telescope through a broad-band filter.<ref>{{cite journal |last=Carrington |first=R.C. |year=1859 |title=Description of a singular appearance seen in the Sun on September&nbsp;1, 1859 |journal=Monthly Notices of the Royal Astronomical Society |volume=20 |issue=1 |pages=13–15 |doi=10.1093/mnras/20.1.13  |doi-access=free |bibcode=1859MNRAS..20...13C |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101081655332&view=1up&seq=357}}</ref><ref>{{cite journal |last1=Hodgson |first1=R. |title=On a curious Appearance seen in the Sun |journal=Monthly Notices of the Royal Astronomical Society |date=November 1859 |volume=20 |issue=1 |pages=15–16 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101081655332&view=1up&seq=359}}</ref> It was an extraordinarily intense ''white light flare'', a flare emitting a high amount of light in the visual spectrum.<ref name="Carrington1859">{{cite journal |last1=Carrington |first1=R. C. |title=Description of a Singular Appearance seen in the Sun on September 1, 1859 |journal=Monthly Notices of the Royal Astronomical Society |date=November 1859 |volume=20 |pages=13–15 |doi=10.1093/mnras/20.1.13 |doi-access=free |url=https://academic.oup.com/mnras/article/20/1/13/983482 |bibcode=1859MNRAS..20...13C |bibcode-access=free}}</ref>

Since flares produce copious amounts of radiation at [[Physics:H-alpha|H-alpha]],{{citation needed|date=November 2021}} adding a narrow (≈1 Å) passband filter centered at this wavelength to the optical telescope allows the observation of not very bright flares with small telescopes. For years Hα was the main, if not the only, source of information about solar flares. Other passband filters are also used.

==== Radio observations ====
{{Further|Astronomy:Solar radio emission}}
During [[Social:World War II|World War II]], on February 25 and 26, 1942, British radar operators observed radiation that [[Biography:James Stanley Hey|Stanley Hey]] interpreted as solar emission. Their discovery did not go public until the end of the conflict. The same year [[Biography:George Clark Southworth|Southworth]] also observed the Sun in radio, but as with Hey, his observations were only known after 1945. In 1943, [[Biography:Grote Reber|Grote Reber]] was the first to report radioastronomical observations of the Sun at 160&nbsp;MHz. The fast development of radioastronomy revealed new peculiarities of the solar activity like ''storms'' and ''bursts'' related to the flares. Today ground-based radiotelescopes observe the Sun from c. 15&nbsp;MHz up to 400&nbsp;GHz.

==== Space telescopes ====
Because the Earth's atmosphere [[Physics:Absorption (electromagnetic radiation)|absorb]]s much of the electromagnetic radiation emitted by the Sun with wavelengths shorter than 300&nbsp;nm, space-based telescopes allowed for the observation of solar flares in previously unobserved high-energy spectral lines. Since the 1970s, the GOES series of satellites have been continuously observing the Sun in soft X-rays, and their observations have become the standard measure of flares, diminishing the importance of the H-alpha classification. Additionally, space-based telescopes allow for the observation of extremely long wavelengths—as long as a few kilometres—which cannot propagate through the ionosphere.

=== Examples of large solar flares ===
[[File:ExtremeEvent 20120304-00h 20120317-24h.jpg|thumb|200px|Space weather—March 2012.<ref>{{cite web | title=Extreme Space Weather Events | publisher=National Geophysical Data Center | url=http://sxi.ngdc.noaa.gov/sxi_greatest.html | access-date=May 21, 2012 | archive-date=May 22, 2012 | archive-url=https://web.archive.org/web/20120522031032/http://sxi.ngdc.noaa.gov/sxi_greatest.html | url-status=dead }}</ref>]]

The most powerful flare ever observed is thought to be the flare associated with the 1859 Carrington Event.<ref>{{cite web |last1=Bell |first1=Trudy E. |last2=Phillips |first2=Tony |title=A Super Solar Flare |url=https://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare/ |website=Science News |publisher=NASA Science |access-date=22 December 2012 |archive-url=https://web.archive.org/web/20100412084331/https://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare/ |archive-date=12 April 2010 |date=6 May 2008 |url-status=dead}}</ref> While no soft X-ray measurements were made at the time, the magnetic crochet associated with the flare was recorded by ground-based magnetometers allowing the flare's strength to be estimated after the event. Using these magnetometer readings, its soft X-ray class has been estimated to be greater than X10.<ref>{{cite journal |last1=Cliver |first1=E. W. |last2=Svalgaard |first2=L. |title=The 1859 Solar–Terrestrial Disturbance And the Current Limits of Extreme Space Weather Activity |journal=Solar Physics |date=October 2004 |volume=224 |issue=1–2 |pages=407–422 |doi=10.1007/s11207-005-4980-z |url=https://archive.org/details/DTIC_ADA434906/page/n1/mode/2up |bibcode=2004SoPh..224..407C|s2cid=120093108 }}</ref> The soft X-ray class of the flare has also been estimated to be around X50.<ref>{{cite web |last1=Woods |first1=Tom |title=Solar Flares |url=http://lasp.colorado.edu/media/education/reu/2011/docs/talks/Woods_Flare_Lecture_Jun11.pdf |url-status=live |archive-url=https://web.archive.org/web/20151023060752/http://lasp.colorado.edu/media/education/reu/2011/docs/talks/Woods_Flare_Lecture_Jun11.pdf |archive-date=23 October 2015 |access-date=24 November 2019}}</ref><ref>{{cite web |last1=Cliver |first1=Edward W. |last2=Dietrich |first2=William F. |title=The 1859 space weather event revisited: limits of extreme activity |url=https://www.swsc-journal.org/articles/swsc/pdf/2013/01/swsc130015.pdf |date=4 April 2013 |journal=J. Space Weather Space Clim. |doi=10.1051/swsc/2013053 |access-date=31 December 2023}}</ref>

In modern times, the largest solar flare measured with instruments occurred on [[Astronomy:Halloween solar storms, 2003|4 November 2003]]. This event saturated the GOES detectors, and because of this its classification is only approximate. Initially, extrapolating the GOES curve, it was estimated to be X28.<ref>{{cite web |title=X-Whatever Flare! (X 28) |url=https://soho.nascom.nasa.gov/hotshots/2003_11_04/ |website=SOHO Hotshots |publisher=ESA/NASA |access-date=21 May 2012 |date=4 November 2003}}</ref> Later analysis of the ionospheric effects suggested increasing this estimate to X45.<ref>{{cite web |url=http://www.spaceref.com/news/viewpr.html?pid=13844 |title=Biggest ever solar flare was even bigger than thought {{!}} SpaceRef – Your Space Reference |date=2004-03-15 |publisher=SpaceRef |access-date=May 21, 2012 |archive-date=2012-09-10 |archive-url=https://archive.today/20120910055509/http://www.spaceref.com/news/viewpr.html?pid=13844 |url-status=dead }}</ref> This event produced the first clear evidence of a new spectral component above 100&nbsp;GHz.<ref>{{cite journal |last1=Kaufmann |first1=Pierre |last2=Raulin |first2=Jean-Pierre |last3=de Castro |first3=C. G. Gimnez |last4=Levato |first4=Hugo |last5=Gary |first5=Dale E. |last6=Costa |first6=Joaquim E. R. |last7=Marun |first7=Adolfo |last8=Pereyra |first8=Pablo |last9=Silva |first9=Adriana V. R. |last10=Correia |first10=Emilia |title=A New Solar Burst Spectral Component Emitting Only in the Terahertz Range |journal=The Astrophysical Journal |date=10 March 2004 |volume=603 |issue=2 |pages=L121–L124 |doi=10.1086/383186 |doi-access=free |bibcode=2004ApJ...603L.121K |bibcode-access=free|s2cid=54878789 |s2cid-access=free}}</ref>

<!-- To be considered a "large" solar flare, it should be above X5-class. See the SWPC's average frequency for radio blackouts: https://www.swpc.noaa.gov/noaa-scales-explanation -->Other large solar flares also occurred on 2 April 2001 (X20+),<ref name=Soho1>{{cite web |title=Biggest Solar X-Ray Flare on Record - X20 |url=https://soho.nascom.nasa.gov/hotshots/X17/ |website=SOHO Hotshots |publisher=ESA/NASA |access-date=21 May 2012}}</ref> 28 October 2003 (X17.2+ and 10),<ref>{{cite web |title=X 17.2 and 10.0 Flares! (October 28, 2003) |url=https://soho.nascom.nasa.gov/hotshots/2003_10_28/ |website=SOHO Hotshots |publisher=ESA/NASA |access-date=21 May 2012 |date=28 October 2008}}</ref> 7 September 2005 (X17),<ref name=Soho1 /> 9 August 2011 (X6.9),<ref name="NASA1">{{cite web |last1=Fox |first1=Karen C. |last2=Hendrix |first2=Susan |title=Sun Unleashes X6.9 Class Flare |url=https://www.nasa.gov/mission_pages/sunearth/news/News080911-xclass.html |publisher=NASA |archive-url=https://web.archive.org/web/20111022215151/https://www.nasa.gov/mission_pages/sunearth/news/News080911-xclass.html |archive-date=22 October 2011 |date=9 August 2011 |url-status=dead}}</ref> 7 March 2012 (X5.4),<ref>{{cite web |last1=Fox |first1=Karen C. |title=Geomagnetic Storm Strength Increases |url=https://www.nasa.gov/mission_pages/sunearth/news/News030712-X5-4.html |publisher=NASA |access-date=9 July 2012 |archive-url=https://web.archive.org/web/20230503221713/https://www.nasa.gov/mission_pages/sunearth/news/News030712-X5-4.html |archive-date=3 May 2023 |date=9 March 2012 |url-status=dead}}</ref> and 6 September 2017 (X9.3).<ref>{{cite news |title=Two significant solar flares imaged by NASA's SDO |url=https://phys.org/news/2017-09-significant-solar-flares-imaged-nasa.html |access-date=17 October 2023 |work=Phys.org |date=6 September 2017}}</ref><ref>{{cite web|url=https://www.nasa.gov/feature/goddard/2017/two-significant-solar-flares-imaged-by-nasas-sdo|title=Two Significant Solar Flares Imaged by NASA's SDO|date=6 September 2017|access-date=6 September 2017|archive-date=7 September 2017|archive-url=https://web.archive.org/web/20170907012229/https://www.nasa.gov/feature/goddard/2017/two-significant-solar-flares-imaged-by-nasas-sdo/|url-status=dead}}</ref>

== Prediction ==
Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions (based on line-of-sight magnetic field) called delta spots produce the largest flares. A simple scheme of sunspot classification due to McIntosh, or related to fractal complexity<ref>{{cite journal |last1= McAteer|first1= James|date= 2005|title= Statistics of Active Region Complexy |journal= The Astrophysical Journal|volume= 631 |issue= 2|page=638|bibcode= 2005ApJ...631..628M |bibcode-access=free |doi = 10.1086/432412 |doi-access= free}}</ref> is commonly used as a starting point for flare prediction.<ref>{{cite journal |last1= Wheatland|first1= M. S.|date= 2008|title= A Bayesian approach to solar flare prediction |journal= The Astrophysical Journal|volume= 609|issue= 2|pages= 1134–1139|doi= 10.1086/421261 |doi-access=free |arxiv = astro-ph/0403613 |bibcode = 2004ApJ...609.1134W |bibcode-access=free |s2cid= 10273389 |s2cid-access=free}}</ref> Predictions are usually stated in terms of probabilities for occurrence of flares above M- or X-class within 24 or 48 hours. The [[Organization:National Oceanic and Atmospheric Administration|U.S. National Oceanic and Atmospheric Administration (NOAA)]] issues forecasts of this kind.<ref>{{cite web |title=Forecasts |url=https://www.swpc.noaa.gov/forecasts |publisher=NOAA/NWS Space Weather Prediction Center |access-date=17 October 2023}}</ref>
[https://www.uah.edu/cspar/research/mag4-page MAG4] was developed at the University of Alabama in Huntsville with support from the Space Radiation
Analysis Group at Johnson Space Flight Center (NASA/SRAG) for forecasting M- and X-class flares, CMEs, fast
CME, and Solar Energetic Particle events.<ref>{{cite journal |last1=Falconer |first1=David |last2=Barghouty |first2=Abdulnasser F. |last3=Khazanov |first3=Igor |last4=Moore |first4=Ron |title=A tool for empirical forecasting of major flares, coronal mass ejections, and solar particle events from a proxy of active-region free magnetic energy |journal=Space Weather |date=April 2011 |volume=9 |issue=4 |doi=10.1029/2009SW000537 |doi-access=free |bibcode=2011SpWea...9.4003F |bibcode-access=free|hdl=2060/20100032971 |hdl-access=free }}</ref>
A physics-based method that can predict imminent large solar flares was proposed by Institute for Space-Earth Environmental Research (ISEE), Nagoya University.<ref>{{cite journal|last1=Kusano|first1=Kanya|last2=Iju|first2=Tomoya|last3=Bamba |first3=Yumi |last4=Inoue|first4=Satoshi|date=July 31, 2020|title=A physics-based method that can predict imminent large solar flares|journal=Science|volume=369|issue=6503|pages=587–591|bibcode= 2020Sci...369..587K |bibcode-access=free |doi= 10.1126/science.aaz2511 |pmid=32732427 |doi-access=free}}</ref>

==See also==
{{Columns-list|
* [[Astronomy:Aurora|Aurora]]
* [[Astronomy:Gamma-ray burst|Gamma-ray burst]]
* [[Astronomy:Hyder flare|Hyder flare]]
* [[Astronomy:Moreton wave|Moreton wave]]
* [[Physics:Neupert effect|Neupert effect]]
* Sun in culture
* Sun in fiction
* [[Astronomy:Superflare|Superflare]]
}}

== References ==
{{Reflist|30em}}

== External links ==
* NOAA [[Astronomy:Space Weather Prediction Center|Space Weather Prediction Center]]'s near real-time solar flare data and resources:
** [https://www.swpc.noaa.gov/products/goes-x-ray-flux GOES X-Ray Flux (1-minute data)]
** [https://www.swpc.noaa.gov/products/goes-solar-ultraviolet-imager-suvi GOES Solar Ultraviolet Imager (SUVI)]
** [https://www.swpc.noaa.gov/products/d-region-absorption-predictions-d-rap D Region Absorption Predictions (D-RAP)]
** [https://www.swpc.noaa.gov/products/3-day-forecast 3-Day Forecast]
** [https://www.swpc.noaa.gov/products/forecast-discussion Forecast Discussion]
* {{APOD |date=6 November 2007 |title=An X Class Flare Region on the Sun}}

{{The Sun}}
{{Magnetospherics}}
{{Solar storms}}
{{Natural disasters}}
{{Doomsday}}

[[Category:Solar phenomena|Flare]]
[[Category:Space plasmas]]

{{Sourceattribution|Solar flare}}