active region

Newly observed active regions on the solar disk are assigned 4-digit region numbers by the Space Weather Prediction Center (SWPC) on the day following the initial observation. The region number assigned to a particular active region is one added to the previously assigned number. For example, the first observation of active region 8090, or AR8090, was followed by AR8091.

According to the SWPC, a number is assigned to a region if it meets at least one of the following criteria:{{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}}

1. It contains a sunspot group of class C or larger based on the Modified Zurich Class sunspot classification system.
2. It contains a sunspot group of class A or B confirmed by at least two observers, preferably with observations more than one hour apart.
3. It has produced a solar flare with an X-ray burst.{{Clarification|date=June 2022}}
4. It contains plage with a white-light brightness of at least 2.5 (on a linear scale 1-5, 5=flare) and has an extent of at least five heliographic degrees.
5. It contains plage that is bright near the west limb and is suspected of growing.

The region numbers reached 10,000 in July 2002. However, the SWPC continued using 4-digits, with the inclusion of leading zeros.{{cite web |title=Solar Region Summary {{!}} NOAA / NWS Space Weather Prediction Center |url=https://www.swpc.noaa.gov/products/solar-region-summary |website=www.swpc.noaa.gov |access-date=4 November 2021}}

File:Sunspot diagram.svg

The Mount Wilson magnetic classification system, also known as the Hale magnetic classification system, is a method of classifying the magnetic field of active regions. It was first introduced in 1919 by George Ellery Hale and coworkers working at the Mount Wilson Observatory.{{cite journal |last1=Hale |first1=George E. |last2=Ellerman |first2=Ferdinand |last3=Nicholson |first3=S. B. |last4=Joy |first4=A. H. |title=The Magnetic Polarity of Sun-Spots |journal=The Astrophysical Journal |date=April 1919 |volume=49 |pages=153 |doi=10.1086/142452 |bibcode=1919ApJ....49..153H |url=https://articles.adsabs.harvard.edu/full/1919ApJ....49..153H |access-date=29 December 2021}} It originally included only the α, β, and γ magnetic classifications, but it was later modified by H. Künzel in 1965 to include the δ qualifier.{{cite journal |last1=Künzel |first1=H. |title=Zur Klassifikation von Sonnenfleckengruppen |journal=Astronomische Nachrichten |date=December 1965 |volume=288 |page=177 |bibcode=1965AN....288..177K |url=https://articles.adsabs.harvard.edu/full/1965AN....288..177K |access-date=29 December 2021}}

{{main|Sunspot}}

File:Solar Archipelago - Flickr - NASA Goddard Photo and Video.jpg

File:Growing Sunspots Tracking Closeup - February 2011.ogv

The strong magnetic flux found in active regions is often strong enough to inhibit convection. Without convection transporting energy from the Sun's interior to the photosphere, surface temperature decreases along with the intensity of emitted black body radiation. These areas of cooler plasma are known as sunspots, and often appear in groups.{{cite web |title=SECEF Sunspot Resource |url=https://image.gsfc.nasa.gov/poetry/workbook/sunspot.html |website=image.gsfc.nasa.gov |access-date=2021-08-24 |archive-date=2021-11-22 |archive-url=https://web.archive.org/web/20211122225451/https://image.gsfc.nasa.gov/poetry/workbook/sunspot.html |url-status=dead }} However, not all active regions possess sunspots.

Active regions form through the process of magnetic flux emergence, during which magnetic fields generated by the solar dynamo emerge from the solar interior.{{cite journal |last1=van Driel-Gesztelyi |first1=Lidia |last2=Green |first2=Lucie May |title=Evolution of Active Regions |journal=Living Reviews in Solar Physics |date=December 2015 |volume=12 |issue=1 |doi=10.1007/lrsp-2015-1 |s2cid=118831968 |doi-access=free |bibcode=2015LRSP...12....1V }}{{cite journal |last1=Cheung |first1=Mark C. M. |last2=Isobe |first2=Hiroaki |title=Flux Emergence (Theory) |journal=Living Reviews in Solar Physics |date=2014 |volume=11 |issue=3 |doi=10.12942/lrsp-2014-3 |s2cid=122762353 |doi-access=free |bibcode=2014LRSP...11....3C }}{{cite book |last1=Aschwanden |first1=Markus J. |title=New Millennium Solar Physics |series=Astrophysics and Space Science Library |date=2019 |volume=458 |location=Cham, Switzerland |doi=10.1007/978-3-030-13956-8 |isbn=978-3-030-13956-8 |s2cid=181739975 |url=https://doi.org/10.1007/978-3-030-13956-8}}{{rp|118}}

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