Coronal hole

File:Helmet streamers at min.jpg (pictured), coronal structures not otherwise visible can be observed above the limb.]]

Coronal holes were first observed during total solar eclipses. They appeared as dark regions surrounded by much brighter helmet streamers above the Sun's limb.

In the 1960s, coronal holes appeared in X-ray images taken by sounding rockets and in observations at radio wavelengths by the Sydney Chris Cross radio telescope. At the time, what they were was unclear. Their true nature was recognized in the 1970s, when X-ray telescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona.{{cite web|title=Massive Coronal Hole on the Sun|url=http://www.nasa.gov/mission_pages/sunearth/news/gallery/20130618-coronalhole.html|website=NASA|date=24 June 2013|accessdate=31 October 2014|archive-date=19 December 2020|archive-url=https://web.archive.org/web/20201219082116/https://www.nasa.gov/mission_pages/sunearth/news/gallery/20130618-coronalhole.html|url-status=dead}}

{{Further|Solar cycle}}

File:Sun in X-Ray.png]]

Coronal hole size and population correspond with the solar cycle. As the Sun heads toward solar maximum, the coronal holes move closer and closer to the Sun's poles. During solar maxima, the number of coronal holes decreases until the magnetic fields on the Sun reverse. Afterwards, new coronal holes appear near the new poles. The coronal holes then increase in size and number, extending farther from the poles as the Sun moves toward a solar minimum again.{{cite web|last1=Fox|first1=Karen|title=Large Coronal Hole Near the Sun's North Pole|url=http://www.nasa.gov/content/goddard/large-coronal-hole-near-sun-north-pole/|website=NASA|date=19 July 2013|accessdate=31 October 2014|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112021409/http://www.nasa.gov/content/goddard/large-coronal-hole-near-sun-north-pole/|url-status=dead}}

{{Further|Solar wind|Space weather}}

The solar wind exists primarily in two alternating states referred to as the slow solar wind and the fast solar wind. The latter originates in coronal holes and has radial flow speeds of 450–800 km/s compared to speeds of 250–450 km/s for the slow solar wind.{{cite journal |last1=Geiss |first1=J. |last2=Gloeckler |first2=G. |last3=Von Steiger |first3=R. |title=Origin of the solar wind from composition data |journal=Space Science Reviews |date=April 1995 |volume=72 |issue=1–2 |pages=49–60 |doi=10.1007/BF00768753|bibcode=1995SSRv...72...49G }}{{cite journal |last1=Cranmer |first1=Steven R. |last2=Gibson |first2=Sarah E. |last3=Riley |first3=Pete |title=Origins of the Ambient Solar Wind: Implications for Space Weather |journal=Space Science Reviews |date=November 2017 |volume=212 |issue=3–4 |pages=1345–1384 |doi=10.1007/s11214-017-0416-y|arxiv=1708.07169 |bibcode=2017SSRv..212.1345C }} Interactions between fast and slow solar wind streams produce stream interaction regions which, if present after a solar rotation, are referred to as co-rotating interaction regions (CIRs).{{cite journal |last1=Tsurutani |first1=Bruce T. |last2=Gonzalez |first2=Walter D. |last3=Gonzalez |first3=Alicia L. C. |last4=Guarnieri |first4=Fernando L. |last5=Gopalswamy |first5=Nat |last6=Grande |first6=Manuel |last7=Kamide |first7=Yohsuke |last8=Kasahara |first8=Yoshiya |last9=Lu |first9=Gang |last10=Mann |first10=Ian |last11=McPherron |first11=Robert |last12=Soraas |first12=Finn |last13=Vasyliunas |first13=Vytenis |title=Corotating solar wind streams and recurrent geomagnetic activity: A review |journal=Journal of Geophysical Research: Space Physics |date=July 2006 |volume=111 |issue=A7 |doi=10.1029/2005JA011273|bibcode=2006JGRA..111.7S01T |url=http://urlib.net/sid.inpe.br/mtc-m16@80/2006/08.02.14.43 }}{{cite journal |last1=Temmer |first1=Manuela |title=Space weather: the solar perspective: An update to Schwenn (2006) |journal=Living Reviews in Solar Physics |date=December 2021 |volume=18 |issue=1 |doi=10.1007/s41116-021-00030-3|arxiv=2104.04261 }}

CIRs can interact with Earth's magnetosphere, producing minor- to moderate-intensity geomagnetic storms. The majority of moderate-intensity geomagnetic storms originate from CIRs. Typically, geomagnetic storms originating from CIRs have a gradual commencement (over hours) and are not as severe as storms caused by coronal mass ejections (CMEs), which usually have a sudden onset. Because coronal holes and associated CIRs can last for several solar rotations (i.e., several months), predicting the recurrence of this type of disturbance is often possible significantly farther in advance than for CME-related disturbances.{{cite web|title=Fast Solar Wind Causes Aurora Light Shows|url= https://www.nasa.gov/image-feature/goddard/fast-solar-wind-causes-aurora-light-shows |website=NASA|date= 9 October 2015 |accessdate=11 April 2022}}{{cite journal |last1=Cranmer |first1=Steven R. |title=Coronal Holes |journal=Living Reviews in Solar Physics |date=2009 |volume=6 |issue=1 |page=3 |doi=10.12942/lrsp-2009-3|doi-access=free |pmid=27194961 |arxiv=0909.2847 |bibcode=2009LRSP....6....3C }}

• {{slink|Coronal mass ejection|Coronal signatures}} – includes coronal dimmings, sometimes referred to as transient coronal holes
• Sunspot – dark spots on the Sun's photosphere
• List of solar storms

{{reflist|2}}

1. {{cite book |last= Gombosi|first= Tamas|title= Physics of the Space Environment|year= 1998|publisher= Cambridge University Press|location= New York|isbn= 0-521-59264-X}}
2. Jiang, Y., Chen, H., Shen, Y., Yang, L., & Li, K. (2007, January). Hα dimming associated with the eruption of a coronal sigmoid in the quiet Sun. Solar Physics, 240(1), 77–87.

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Category:Solar phenomena