Editing Sun (section)

== Magnetic activity ==

The Sun has a [[stellar magnetic field]] that varies across its surface. Its polar field is {{convert|1|-|2|G|sigfig=1|lk=on}}, whereas the field is typically {{convert|3000|G|sigfig=1}} in features on the Sun called ''sunspots'' and {{convert|10|-|100|G|sigfig=1}} in [[solar prominence]]s.<ref name="nssdc" /> The magnetic field varies in time and location. The quasi-periodic 11-year [[solar cycle]] is the most prominent variation in which the number and size of sunspots waxes and wanes.<ref name="doi10.1146/annurev-astro-081913-040012">{{Cite journal |doi=10.1146/annurev-astro-081913-040012 |title=Solar Dynamo Theory |journal=Annual Review of Astronomy and Astrophysics |volume=52 |pages=251–290 |year=2014 |last1=Charbonneau |first1=P. |bibcode=2014ARA&A..52..251C |s2cid=17829477 |doi-access=free}}</ref><ref name="Zirker2002-119">{{Cite book |last=Zirker |first=J. B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/119 119–120] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/119}}</ref><ref name="Lang">{{Cite book |last=Lang |first=Kenneth R. |date=2008 |title=The Sun from Space |page=75 |publisher=[[Springer-Verlag]] |isbn=978-3-540-76952-1}}</ref>

The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the [[interplanetary magnetic field]].<ref name="Russell2001" /> In an approximation known as ideal [[magnetohydrodynamics]], plasma particles only move along magnetic field lines. As a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin [[heliospheric current sheet|current sheet]] is formed in the solar wind. At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an [[Archimedean spiral]] structure called the ''Parker spiral''.<ref name="Russell2001" />

=== Sunspot ===
{{Main|Sunspot}}
[[File:The Largest Sunspot Group (noao0114).tiff|thumb|A large sunspot group observed in white light|alt=A black-and-white photograph of a group of sunspots.]]

Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field where convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, so they appear dark. At a typical [[solar minimum]], few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses toward its [[Solar maximum|maximum]], sunspots tend to form closer to the solar equator, a phenomenon known as ''[[Spörer's law]]''. The largest sunspots can be tens of thousands of kilometres across.<ref name="Sunspot2001">{{cite web |date=30 March 2001 |title=The Largest Sunspot in Ten Years |url=http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/sunspot.htm |publisher=[[Goddard Space Flight Center]] |access-date=10 July 2009 |url-status=dead |archive-url=https://web.archive.org/web/20070823050403/http://www.gsfc.nasa.gov/gsfc/spacesci/solarexp/sunspot.htm |archive-date=23 August 2007}}</ref>

An 11-year sunspot cycle is half of a 22-year [[Babcock Model|Babcock]]–Leighton [[solar dynamo|dynamo]] cycle, which corresponds to an oscillatory exchange of energy between [[toroidal and poloidal]] solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength; but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of the toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon described by [[Hale's law]].<ref>{{Cite journal |last1=Hale |first1=G. E. |last2=Ellerman |first2=F. |last3=Nicholson |first3=S. B. |last4=Joy |first4=A. H. |title=The Magnetic Polarity of Sun-Spots |journal=The Astrophysical Journal |volume=49 |page=153 |year=1919 |doi=10.1086/142452 |bibcode=1919ApJ....49..153H |doi-access=free}}</ref><ref name="solarcycle">{{cite web |date=4 January 2008 |title=NASA Satellites Capture Start of New Solar Cycle |publisher=[[PhysOrg]] |url=http://www.physorg.com/news119271347.html |access-date=10 July 2009 |archive-date=6 April 2008 |archive-url=https://web.archive.org/web/20080406132839/http://www.physorg.com/news119271347.html |url-status=live}}</ref>

During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealised, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.<ref>{{Cite news |date=16 February 2001 |title=Sun flips magnetic field |url=http://edition.cnn.com/2001/TECH/space/02/16/sun.flips/ |publisher=CNN|access-date=11 July 2009 |archive-date=21 January 2015 |archive-url=https://web.archive.org/web/20150121063331/http://edition.cnn.com/2001/TECH/space/02/16/sun.flips/ |url-status=live}}</ref><ref>{{cite web |last=Phillips |first=T. |date=15 February 2001 |title=The Sun Does a Flip |url=https://science.nasa.gov/headlines/y2001/ast15feb_1.htm |publisher=NASA |access-date=11 July 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090512121817/https://science.nasa.gov/headlines/y2001/ast15feb_1.htm |archive-date=12 May 2009}}</ref>

=== Solar activity ===
{{Main||Solar cycle}}
[[File:Solar-cycle-data.png|thumb|Measurements from 2005 of solar cycle variation during the previous 30 years|left|alt=See caption]]

The Sun's magnetic field leads to many effects that are collectively called [[solar variation|solar activity]]. [[Solar flares]] and [[coronal mass ejections]] tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from [[coronal holes]] at the photospheric surface. Both coronal mass ejections and high-speed streams of solar wind carry plasma and the interplanetary magnetic field outward into the Solar System.<ref name=Zirker2002>{{Cite book |last=Zirker |first=J. B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/120 120–127] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/120}}</ref> The effects of solar activity on Earth include [[aurora (astronomy)|auroras]] at moderate to high latitudes and the disruption of radio communications and [[electric power]]. Solar activity is thought to have played a large role in the [[formation and evolution of the Solar System]].<ref>{{cite journal |last1=Nandy |first1=Dibyendu |last2=Martens |first2=Petrus C. H. |last3=Obridko |first3=Vladimir |last4=Dash |first4=Soumyaranjan |last5=Georgieva |first5=Katya |title=Solar evolution and extrema: current state of understanding of long-term solar variability and its planetary impacts |journal=Progress in Earth and Planetary Science |date=5 July 2021 |volume=8 |issue=1 |pages=40 |doi=10.1186/s40645-021-00430-x |doi-access=free |bibcode=2021PEPS....8...40N |issn=2197-4284}}</ref>

Changes in solar irradiance over the 11-year solar cycle have been correlated with changes in sunspot number.<ref>{{cite journal |last1=Willson |first1=R. C. |last2=Hudson |first2=H. S. |date=1991 |title=The Sun's luminosity over a complete solar cycle |journal=[[Nature (journal)|Nature]] |volume=351 |issue=6321 |pages=42–44 |doi=10.1038/351042a0 |bibcode=1991Natur.351...42W |s2cid=4273483}}</ref> The solar cycle influences [[space weather]] conditions, including those surrounding Earth. For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during a period known as the [[Maunder minimum]]. This coincided in time with the era of the [[Little Ice Age]], when Europe experienced unusually cold temperatures.<ref>{{cite journal |author-link=John A. Eddy |last=Eddy |first=John A. |title=The Maunder Minimum |journal=[[Science (journal)|Science]] |volume=192 |issue=4245 |pages=1189–1202 |date=June 1976 |pmid=17771739 |doi=10.1126/science.192.4245.1189 |jstor=1742583 |bibcode=1976Sci...192.1189E |s2cid=33896851}}</ref><ref name="Lean">{{Cite journal |last1=Lean |first1=J. |author-link=Judith Lean |last2=Skumanich |first2=A. |last3=White |first3=O. |date=1992 |title=Estimating the Sun's radiative output during the Maunder Minimum |journal=[[Geophysical Research Letters]] |volume=19 |issue=15 |pages=1591–1594 |doi=10.1029/92GL01578 |bibcode=1992GeoRL..19.1591L |url=https://zenodo.org/record/1231321 |access-date=16 December 2019 |archive-date=11 May 2020 |archive-url=https://web.archive.org/web/20200511052658/https://zenodo.org/record/1231321 |url-status=live}}</ref> Earlier extended minima have been discovered through analysis of [[tree ring]]s and appear to have coincided with lower-than-average global temperatures.<ref>{{Cite book |last1=Mackay |first1=R. M. |last2=Khalil |first2=M. A. K. |chapter=Greenhouse gases and global warming |chapter-url=https://books.google.com/books?id=tQBS3bAX8fUC&q=solar+minimum+dendochronology&pg=PA1 |editor-last=Singh |editor-first=S. N. |date=2000 |title=Trace Gas Emissions and Plants |pages=1–28 |publisher=[[Springer (publisher)|Springer]] |isbn=978-0-7923-6545-7 |access-date=3 November 2020 |archive-date=17 April 2021 |archive-url=https://web.archive.org/web/20210417054703/https://books.google.com/books?id=tQBS3bAX8fUC&q=solar+minimum+dendochronology&pg=PA1 |url-status=live}}</ref>

=== Coronal heating ===
{{Main|Stellar corona}}
{{unsolved|astronomy|Why is the Sun's corona so much hotter than the Sun's surface?}}

The temperature of the photosphere is approximately 6,000&nbsp;K, whereas the temperature of the corona reaches {{val|1000000|-|2000000|u=K|fmt=commas}}.<ref name="Erdelyi2007" /> The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere.<ref name="Russell2001" />

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.<ref name="Erdelyi2007" /> The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.<ref name="Erdelyi2007" /> These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.<ref name="Alfven">{{Cite journal |last=Alfvén |first=H. |date=1947 |title=Magneto-hydrodynamic waves, and the heating of the solar corona |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=107 |issue=2 |pages=211–219 |bibcode=1947MNRAS.107..211A |doi=10.1093/mnras/107.2.211 |doi-access=free}}</ref> The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large solar flares and myriad similar but smaller events—[[nanoflares]].<ref name="Parker2">{{Cite journal |last=Parker |first=E. N. |date=1988 |title=Nanoflares and the solar X-ray corona |journal=[[The Astrophysical Journal]] |volume=330 |issue=1 |page=474 |bibcode=1988ApJ...330..474P |doi=10.1086/166485}}</ref>

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.<ref name="Sturrock">{{Cite journal |last1=Sturrock |first1=P. A. |last2=Uchida |first2=Y. |year=1981 |title=Coronal heating by stochastic magnetic pumping |journal=[[The Astrophysical Journal]] |volume=246 |issue=1 |page=331 |bibcode=1981ApJ...246..331S |doi=10.1086/158926 |hdl-access=free |hdl=2060/19800019786}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. The current research focus has therefore shifted toward flare heating mechanisms.<ref name="Erdelyi2007" />