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Active region

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In solar physics and observation, an active region is a temporary feature in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar flares.[1] The number and location of active regions on the solar disk at any given time is dependent on the solar cycle.[2][3][4][5][6]

Region numbers

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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:[7]

  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 needed]
  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.[8][9]

Magnetic field

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A highly simplified diagram of the magnetic field of an active region illustrating its bipolar nature.

Mount Wilson magnetic classification

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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.[10] 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.[11][9]

Classification Description[12][9][13]
α An active region containing a single sunspot or group of sunspots all having the same magnetic polarity. An opposite polarity counterpart is still present, but is weak or not concentrated enough to form sunspots.
β An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity. A simple neutral line between the two polarities is also present.
γ An active region with sunspots having completely intermixed magnetic polarity.
β-γ An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity (hence β) but no well-defined neutral line dividing the opposite polarities (hence γ).
δ A qualifier to the other classes indicating the presence of opposite polarity umbrae within a single penumbra separated by at most 2° in heliographic distance.
β-δ An active region with a β magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).
β-γ-δ An active region with a β-γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).
γ-δ An active region with a γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).

Sunspots

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Main article: Sunspot
 
An active region seen in visible light showing a group of sunspots.
The evolution of a group of sunspots in time.

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.[14] However, not all active regions possess sunspots.[8]

Magnetic flux emergence

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Active regions form through the process of magnetic flux emergence, during which magnetic fields generated by the solar dynamo emerge from the solar interior.[15][16][17]: 118 

See also

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References

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  1. ^ Zell, Holly (20 April 2015). "Active Regions on the Sun". NASA. Retrieved 18 July 2021.
  2. ^ Warren, Harry P.; Winebarger, Amy R.; Brooks, David H. (10 November 2012). "A Systematic Survey of High-Temperature Emission in Solar Active Regions". The Astrophysical Journal. 759 (2): 141. arXiv:1204.3220. Bibcode:2012ApJ...759..141W. doi:10.1088/0004-637X/759/2/141. S2CID 119117669.
  3. ^ Del Zanna, G. (October 2013). "The multi-thermal emission in solar active regions". Astronomy & Astrophysics. 558: A73. Bibcode:2013A&A...558A..73D. doi:10.1051/0004-6361/201321653.
  4. ^ Basu, Sarbani; Antia, H. M.; Bogart, Richard S. (August 2004). "Ring-Diagram Analysis of the Structure of Solar Active Regions". The Astrophysical Journal. 610 (2): 1157–1168. Bibcode:2004ApJ...610.1157B. doi:10.1086/421843.
  5. ^ Hagino, Masaoki; Sakurai, Takashi (25 October 2004). "Latitude Variation of Helicity in Solar Active Regions". Publications of the Astronomical Society of Japan. 56 (5): 831–843. doi:10.1093/pasj/56.5.831.
  6. ^ Zhang, Jie; Wang, Yuming; Liu, Yang (10 November 2010). "Statistical Properties of Solar Active Regions Obtained from an Automatic Detection System and the Computational Biases". The Astrophysical Journal. 723 (2): 1006–1018. Bibcode:2010ApJ...723.1006Z. doi:10.1088/0004-637X/723/2/1006. S2CID 122852367.
  7. ^ Pietrow, A.G.M. (2022). Physical properties of chromospheric features: Plage, peacock jets, and calibrating it all (PhD). Stockholm University. doi:10.13140/RG.2.2.36047.76968.
  8. ^ a b "Solar Region Summary | NOAA / NWS Space Weather Prediction Center". www.swpc.noaa.gov. Retrieved 4 November 2021.
  9. ^ a b c Jaeggli, S. A.; Norton, A. A. (16 March 2016). "THE MAGNETIC CLASSIFICATION OF SOLAR ACTIVE REGIONS 1992–2015". The Astrophysical Journal. 820 (1): L11. arXiv:1603.02552. Bibcode:2016ApJ...820L..11J. doi:10.3847/2041-8205/820/1/L11. S2CID 15138687.
  10. ^ Hale, George E.; Ellerman, Ferdinand; Nicholson, S. B.; Joy, A. H. (April 1919). "The Magnetic Polarity of Sun-Spots". The Astrophysical Journal. 49: 153. Bibcode:1919ApJ....49..153H. doi:10.1086/142452. Retrieved 29 December 2021.
  11. ^ Künzel, H. (December 1965). "Zur Klassifikation von Sonnenfleckengruppen". Astronomische Nachrichten. 288: 177. Bibcode:1965AN....288..177K. Retrieved 29 December 2021.
  12. ^ Space Environmental Observations, Solar Optical Observing Techniques, Manual AFWAMAN 15-1 (PDF). Air Force Weather Agency. 2013. Retrieved 28 December 2021.
  13. ^ "The magnetic classification of sunspots". SpaceWeatherLive. Parsec vzw. Retrieved 29 December 2021.
  14. ^ "SECEF Sunspot Resource". image.gsfc.nasa.gov. Archived from the original on 2021-11-22. Retrieved 2021-08-24.
  15. ^ van Driel-Gesztelyi, Lidia; Green, Lucie May (December 2015). "Evolution of Active Regions". Living Reviews in Solar Physics. 12 (1). Bibcode:2015LRSP...12....1V. doi:10.1007/lrsp-2015-1. S2CID 118831968.
  16. ^ Cheung, Mark C. M.; Isobe, Hiroaki (2014). "Flux Emergence (Theory)". Living Reviews in Solar Physics. 11 (3). Bibcode:2014LRSP...11....3C. doi:10.12942/lrsp-2014-3. S2CID 122762353.
  17. ^ Aschwanden, Markus J. (2019). New Millennium Solar Physics. Astrophysics and Space Science Library. Vol. 458. Cham, Switzerland. doi:10.1007/978-3-030-13956-8. ISBN 978-3-030-13956-8. S2CID 181739975.{{cite book}}: CS1 maint: location missing publisher (link)
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