Editing Solar flare (section)

== Classification ==
=== Soft X-ray ===
[[File:GOES-16 X-ray flux (1-minute data) on 2023-12-14 with flares labeled.svg|thumb|upright=1.6|An M5.8, M2.3, and X2.8 flare were recorded by [[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 [[flux]] in watts per square metre (W/m<sup>2</sup>) of [[soft X-ray]]s with [[wavelength]]s {{convert|0.1|to|0.8|nm|angstrom|abbr=off|lk=on|sigfig=1}}, as measured by [[GOES]] satellites in [[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. M-class flares are a tenth the size of X-class flares with the same numeric suffix.<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 |page=375 |year=2010 |editor1-last=Schrijver |editor1-first=Carolus J. |url=https://books.google.com/books?id=OukfAwAAQBAJ&pg=PA375 |publisher=Cambridge University Press |isbn=978-1107049048 |editor2-last=Siscoe |editor2-first=George L. |editor2-link=George Siscoe}}.</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 were added in the 1990s as instruments became more sensitive to weaker flares. Around the same time, the [[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 |place=Stockholm, Sweden}}</ref>

=== Importance ===
An earlier classification system, sometimes referred to as the ''flare importance'', was based on [[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 is then 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
 | first1 = Einar|author-link=Einar Tandberg-Hanssen
 | last2 = Emslie
 | first2 = A. Gordon
 | date = 1988
 | title = The Physics of Solar Flares
 | publisher = Cambridge University Press
| bibcode = 1988psf..book.....T}}</ref>

=== Duration ===
A common measure of flare duration is the [[full width at half maximum]] (FWHM) time of flux in the soft X-ray bands {{val|0.05|to|0.4|and|0.1|to|0.8|u=nm}} measured by GOES. 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 {{val|0.05|to|0.4|and|0.1|to|0.8|u=nm}} 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>

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>