Editing Sun (section)

== Structure ==
{{Also|Standard solar model}}
[[File:Sun poster.svg|thumb|upright=2.25|left|Illustration of the Sun's structure, in false colour for contrast|alt=See caption]]
{{Clear}}

=== Core ===
{{Main|Solar core}}
The core of the Sun extends from the centre to about 20–25% of the solar radius.<ref name="Garcia2007">{{Cite journal |last=García |first=R. |date=2007 |title=Tracking solar gravity modes: the dynamics of the solar core |journal=[[Science (journal)|Science]] |volume=316 |issue=5831 |pages=1591–1593 |bibcode=2007Sci...316.1591G |doi=10.1126/science.1140598 |pmid=17478682 |s2cid=35285705 |display-authors=etal}}</ref> It has a density of up to {{val|150|u=g|up=cm3}}<ref name="Basu">{{Cite journal |last1=Basu |first1=Sarbani |last2=Chaplin |first2=William J. |last3=Elsworth |first3=Yvonne |last4=New |first4=Roger |last5=Serenelli |first5=Aldo M. |year=2009 |title=Fresh insights on the structure of the solar core |journal=[[The Astrophysical Journal]] |volume=699 |issue=2 |pages=1403–1417 |arxiv=0905.0651 |bibcode=2009ApJ...699.1403B |doi=10.1088/0004-637X/699/2/1403 |s2cid=11044272}}</ref><ref name="NASA1">{{Cite web |date=18 January 2007 |title=NASA/Marshall Solar Physics |url=http://solarscience.msfc.nasa.gov/interior.shtml |url-status=live |archive-url=https://web.archive.org/web/20190329081742/https://solarscience.msfc.nasa.gov/interior.shtml |archive-date=29 March 2019 |access-date=11 July 2009 |publisher=[[Marshall Space Flight Center]]}}</ref> (about 150 times the density of water) and a temperature of close to 15.7&nbsp;million [[kelvin]] (K).<ref name="NASA1" /> By contrast, the Sun's surface temperature is about {{val|5,800|u=K}}. Recent analysis of [[Solar and Heliospheric Observatory|SOHO]] mission data favours the idea that the core is rotating faster than the radiative zone outside it.<ref name="Garcia2007" /> Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the [[proton–proton chain]]; this process converts hydrogen into helium.<ref>{{Cite conference |last=Broggini |first=C. |date=2003 |title=Physics in Collision, Proceedings of the XXIII International Conference: Nuclear Processes at Solar Energy |url=http://www.slac.stanford.edu/econf/C030626 |conference=XXIII Physics in Collisions Conference |location=Zeuthen, Germany |page=21 |arxiv=astro-ph/0308537 |bibcode=2003phco.conf...21B |archive-url=https://web.archive.org/web/20170421113407/http://www.slac.stanford.edu/econf/C030626/ |archive-date=21 April 2017 |access-date=12 August 2013 |url-status=live}}</ref> Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the [[CNO cycle]]; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.<ref name="jpcs271_1_012031">{{Cite journal |last1=Goupil |first1=M. J. |last2=Lebreton |first2=Y. |last3=Marques |first3=J. P. |last4=Samadi |first4=R. |last5=Baudin |first5=F. |date=2011 |title=Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns |journal=[[Journal of Physics: Conference Series]] |volume=271 |issue=1 |page=012031 |arxiv=1102.0247 |bibcode=2011JPhCS.271a2031G |doi=10.1088/1742-6596/271/1/012031 |s2cid=4776237}}</ref><ref>{{Cite journal |last=The Borexino Collaboration |date=2020 |title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun |url=https://www.nature.com/articles/s41586-020-2934-0 |journal=[[Nature (journal)|Nature]] |volume=587 |issue=? |pages=577–582 |arxiv=2006.15115 |bibcode=2020Natur.587..577B |doi=10.1038/s41586-020-2934-0 |pmid=33239797 |s2cid=227174644 |access-date=26 November 2020 |archive-date=27 November 2020 |archive-url=https://web.archive.org/web/20201127093809/https://www.nature.com/articles/s41586-020-2934-0 |url-status=live}}</ref>

The core is the only region of the Sun that produces an appreciable amount of [[thermal energy]] through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).<ref name="Phillips1995-47">{{Cite book |last=Phillips |first=K. J. H. |title=Guide to the Sun |year=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |url=https://books.google.com/books?id=idwBChjVP0gC&pg=PA47 |pages=47–53}}</ref><ref name=Zirker2002-15>{{Cite book |last=Zirker |first=J. B. |date=2002 |title=Journey from the Center of the Sun |pages=[https://archive.org/details/journeyfromcente0000zirk/page/15 15–34] |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |url=https://archive.org/details/journeyfromcente0000zirk/page/15}}</ref>
[[File:Proton-proton reaction chain.svg|thumb|Illustration of a proton-proton reaction chain, from hydrogen forming [[deuterium]], [[helium-3]], and regular [[helium-4]]|alt=circles and arrows showing protons combining in a series of fusion reactions yielding helium-3 which breaks down tow helium-4]]
The proton–proton chain occurs around {{val|9.2|e=37}} times each second in the core, converting about 3.7{{e|38}} protons into [[alpha particle]]s (helium nuclei) every second (out of a total of ~8.9{{e|56}} free protons in the Sun), or about {{val|6.2|e=11|u=kg|up=s}}. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain.<ref name="Phillips1995-47" /> Fusing four free [[proton]]s (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,<ref>{{Cite book |last=Shu |first=F. H. |url=https://archive.org/details/physicaluniverse00shuf/page/102 |title=The Physical Universe: An Introduction to Astronomy |year=1982 |publisher=University Science Books |isbn=978-0-935702-05-7 |page=[https://archive.org/details/physicaluniverse00shuf/page/102 102]}}</ref> so the Sun releases energy at the mass–energy conversion rate of 4.26&nbsp;billion kg/s (which requires 600 billion kg of hydrogen<ref>{{Cite web |year=2012 |title=Ask Us: Sun |url=https://helios.gsfc.nasa.gov/qa_sun.html |url-status=dead |archive-url=https://web.archive.org/web/20180903223810/https://helios.gsfc.nasa.gov/qa_sun.html |archive-date=3 September 2018 |access-date=13 July 2017 |website=Cosmicopia |publisher=NASA}}</ref>), for 384.6&nbsp;[[Yotta-|yottawatts]] ({{val|3.846|e=26|u=W}}),<ref name="nssdc" /> or 9.192{{e|10}}&nbsp;[[TNT equivalent|megatons of TNT]] per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per [[cubic metre]]. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 [[watt]]s per cubic metre at the centre of the core,<ref>{{Cite web |last=Cohen |first=H. |date=9 November 1998 |title=Table of temperatures, power densities, luminosities by radius in the Sun |url=http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasmas/Sunlayers.html |archive-url=http://webarchive.loc.gov/all/20011129122524/http%3A//fusedweb%2Ellnl%2Egov/cpep/chart_pages/5%2Eplasmas/sunlayers%2Ehtml |archive-date=29 November 2001 |access-date=30 August 2011 |publisher=Contemporary Physics Education Project}}</ref> which, according to [[Karl Kruszelnicki]], is about the same power density inside a [[compost pile]].<ref>{{Cite web |date=17 April 2012 |title=Lazy Sun is less energetic than compost |url=http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |url-status=live |archive-url=https://web.archive.org/web/20140306123113/http://www.abc.net.au/science/articles/2012/04/17/3478276.htm |archive-date=6 March 2014 |access-date=25 February 2014 |publisher=Australian Broadcasting Corporation}}</ref>

The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and [[thermal expansion|expand]] slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the [[Perturbation (astronomy)|perturbation]]; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.<ref>{{Cite journal |last1=Haubold |first1=H. J. |last2=Mathai |first2=A. M. |date=1994 |title=Solar Nuclear Energy Generation & The Chlorine Solar Neutrino Experiment |volume=320 |issue=1994 |pages=102–116 |journal=[[AIP Conference Proceedings]] |arxiv=astro-ph/9405040 |bibcode=1995AIPC..320..102H |doi=10.1063/1.47009 |citeseerx=10.1.1.254.6033 |s2cid=14622069}}</ref><ref>{{Cite web |last=Myers |first=S. T. |date=18 February 1999 |title=Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium |url=http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |url-status=live |archive-url=https://web.archive.org/web/20110512180052/http://www.aoc.nrao.edu/~smyers/courses/astro12/L11.html |archive-date=12 May 2011 |access-date=15 July 2009 |website=Introduction to Astrophysics II}}</ref>

=== Radiative zone ===
{{Main|Radiative zone}}
[[File:Heat Transfer in Stars.svg|thumb|300x300px|Illustration of different stars' internal structure based on mass. The Sun in the middle has an inner radiating zone and an outer convective zone.|alt=See caption]]
The radiative zone is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 [[Solar radius|solar radii]], [[thermal radiation]] is the primary means of energy transfer.<ref name="autogenerated1">{{cite web |url=http://mynasa.nasa.gov/worldbook/sun_worldbook.html |publisher=NASA |title=Sun |website=World Book at NASA |access-date=10 October 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130510142009/http://mynasa.nasa.gov/worldbook/sun_worldbook.html |archive-date=10 May 2013}}</ref> The temperature drops from approximately 7&nbsp;million to 2&nbsp;million kelvins with increasing distance from the core.<ref name="NASA1" /> This [[temperature gradient]] is less than the value of the [[adiabatic lapse rate]] and hence cannot drive convection, which explains why the transfer of energy through this zone is by [[radiation]] instead of thermal convection.<ref name="NASA1" /> [[Ions]] of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.<ref name="autogenerated1" /> The density drops a hundredfold (from 20,000&nbsp;kg/m<sup>3</sup> to 200&nbsp;kg/m<sup>3</sup>) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.<ref name="autogenerated1" /><!-- http://adsabs.harvard.edu/abs/2008SoPh..251..101M -->

=== Tachocline ===
{{Main|Tachocline}}

The radiative zone and the convective zone are separated by a transition layer, the [[tachocline]]. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the [[convection zone]] results in a large [[shear (fluid)|shear]] between the two—a condition where successive horizontal layers slide past one another.<ref>{{Cite book |last=Tobias |first=S. M. |title=Fluid Dynamics and Dynamos in Astrophysics and Geophysics |date=2005 |publisher=[[CRC Press]] |isbn=978-0-8493-3355-2 |editor-first=A. M. |editor-last=Soward |pages=193–235 |chapter=The solar tachocline: Formation, stability and its role in the solar dynamo |access-date=22 August 2020 |display-editors=etal |chapter-url=https://books.google.com/books?id=PLNwoJ6qFoEC&pg=PA193 |archive-url=https://web.archive.org/web/20201029102001/https://books.google.com/books?id=PLNwoJ6qFoEC&pg=PA193 |archive-date=29 October 2020 |url-status=live}}</ref> Presently, it is hypothesised that a magnetic dynamo, or [[solar dynamo]], within this layer generates the Sun's [[magnetic field]].<ref name=NASA1 />

=== Convective zone ===
{{Main|Convection zone}}
The Sun's convection zone extends from 0.7 solar radii (500,000&nbsp;km) to near the surface. In this layer, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped 350-fold to {{convert|5,700|K|F}} and the density to only 0.2&nbsp;g/m<sup>3</sup> (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone).<ref name=NASA1 />

The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the [[solar granulation]] at the smallest scale and [[supergranulation]] at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun.<ref name=NASA1 /> The Sun's thermal columns are [[Bénard cells]] and take the shape of roughly hexagonal prisms.<ref>{{Cite book |last=Mullan |first=D. J. |title=From the Sun to the Great Attractor |year=2000 |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-3-540-41064-5 |editor-last=Page |editor-first=D. |page=22 |chapter=Solar Physics: From the Deep Interior to the Hot Corona |access-date=22 August 2020 |editor-last2=Hirsch |editor-first2=J. G. |chapter-url=https://books.google.com/books?id=rk5fxs55_OkC&pg=PA22 |archive-url=https://web.archive.org/web/20210417080656/https://books.google.com/books?id=rk5fxs55_OkC&pg=PA22 |archive-date=17 April 2021 |url-status=live}}</ref>

=== Atmosphere ===
{{Main|Stellar atmosphere}}
The solar atmosphere is the region of the Sun that extends from the top of the convection zone to the inner boundary of the [[heliosphere]]. It is often divided into three primary layers: the photosphere, the [[chromosphere]], and the [[Stellar corona|corona]].<ref>{{Cite book |url=http://link.springer.com/10.1007/978-3-540-46315-3_3 |title=Handbook of the Solar-Terrestrial Environment |date=2007 |publisher=Springer Berlin Heidelberg |isbn=978-3-540-46314-6 |editor-last=Kamide |editor-first=Y. |location=Berlin, Heidelberg |pages=55–93 |language=en |doi=10.1007/978-3-540-46315-3_3 |editor-last2=Chian |editor-first2=A.}}</ref> The chromosphere and corona are separated by a thin [[Solar transition region|transition region]] that is frequently considered as an additional distinct layer.<ref>{{cite book |last1=Cravens |first1=Thomas E. |title=Physics of Solar System Plasmas |date=1997 |publisher=Cambridge University Press |location=Cambridge |isbn=9780511529467 |doi=10.1017/CBO9780511529467}}</ref>{{rp|173–174}} Some sources consider the heliosphere to be the ''outer'' or ''extended solar atmosphere''.<ref>{{cite web |title=Components of the Heliosphere |url=https://www.nasa.gov/image-article/components-of-heliosphere/ |publisher=NASA |access-date=8 April 2025 |date=25 January 2013}}</ref><ref>{{cite journal |last1=Solanki |first1=Sami K |last2=Inhester |first2=Bernd |last3=Schüssler |first3=Manfred |title=The Solar Magnetic Field |journal=Reports on Progress in Physics |date=1 March 2006 |volume=69 |issue=3 |pages=563–668 |doi=10.1088/0034-4885/69/3/R02 |bibcode=2006RPPh...69..563S |arxiv=1008.0771}}</ref>

==== Photosphere ====
{{Main|Photosphere}}
[[File:Highest resolution photo of Sun (NSF) as of January 20, 2020.jpg|thumb|alt=A false-colour image of the solar photosphere|The photosphere is structured by convection cells referred to as ''[[Solar granule|granules]]''.]]

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes [[opacity (optics)|opaque]] to visible light.<ref name=Abhyankar1977 /> Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of [[Hydrogen anion|H<sup>−</sup> ions]], which absorb visible light easily.<ref name=Abhyankar1977 /> Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H<sup>−</sup> ions.<ref name="Gibson">{{Cite book |last=Gibson |first=Edward G. |date=1973 |title=The Quiet Sun (NASA SP-303) |publisher=NASA |asin=B0006C7RS0}}</ref><ref name="Shu">{{Cite book |last=Shu |first=F. H. |title=The Physics of Astrophysics |volume=1 |publisher=University Science Books |year=1991 |isbn=978-0-935702-64-4}}</ref>

The photosphere is tens to hundreds of kilometres thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the centre than on the edge or ''limb'' of the solar disk, in a phenomenon known as ''[[limb darkening]]''.<ref name="Abhyankar1977" /> The spectrum of sunlight has approximately the spectrum of a [[black-body]] radiating at {{convert|5,772|K|F}},<ref name="IAU2015resB3"/> interspersed with atomic [[absorption line]]s from the tenuous layers above the photosphere. The photosphere has a particle density of ~10<sup>23</sup>&nbsp;m<sup>−3</sup> (about 0.37% of the particle number per volume of [[Earth's atmosphere]] at sea level). The photosphere is not fully ionised—the extent of ionisation is about 3%, leaving almost all of the hydrogen in atomic form.<ref>{{cite journal |last1=Rast |first1=M. |last2=Nordlund |first2=Å. |last3=Stein |first3=R. |last4=Toomre |first4=J. |date=1993 |title=Ionization Effects in Three-Dimensional Solar Granulation Simulations |journal=[[The Astrophysical Journal Letters]] |volume=408 |issue=1 |page=L53–L56 |bibcode=1993ApJ...408L..53R |doi=10.1086/186829 |doi-access=free}}</ref>

The coolest layer of the Sun is a temperature minimum region extending to about {{val|500|u=km}} above the photosphere, and has a temperature of about {{val|4100|u=K|fmt=commas}}.<ref name="Abhyankar1977">{{Cite journal |last=Abhyankar |first=K. D. |date=1977 |title=A Survey of the Solar Atmospheric Models |url=http://prints.iiap.res.in/handle/2248/510 |url-status=live |journal=[[Bulletin of the Astronomical Society of India]] |volume=5 |pages=40–44 |bibcode=1977BASI....5...40A |archive-url=https://web.archive.org/web/20200512151641/http://prints.iiap.res.in/handle/2248/510 |archive-date=12 May 2020 |access-date=12 July 2009}}</ref> This part of the Sun is cool enough to allow for the existence of simple molecules such as [[carbon monoxide]] and water.<ref name="Solanki1994">{{Cite journal |last1=Solanki |first1=S. K. |last2=Livingston |first2=W. |last3=Ayres |first3=T. |date=1994 |title=New Light on the Heart of Darkness of the Solar Chromosphere |journal=[[Science (journal)|Science]] |pmid=17748350 |volume=263 |issue=5143 |pages=64–66 |bibcode=1994Sci...263...64S |doi=10.1126/science.263.5143.64 |s2cid=27696504}}</ref>

==== Chromosphere ====
{{Main|Chromosphere}}
Above the temperature minimum layer is a layer about {{val|2000|u=km|fmt=commas}} thick, dominated by a spectrum of emission and absorption lines.<ref name="Abhyankar1977" /> It is called the ''chromosphere'' from the Greek root ''chroma'', meaning colour, because the chromosphere is visible as a coloured flash at the beginning and end of total solar eclipses.<ref name="autogenerated1" /> The temperature of the chromosphere increases gradually with altitude, ranging up to around {{val|20000|u=K|fmt=commas}} near the top.<ref name="Abhyankar1977" /> In the upper part of the chromosphere helium becomes partially [[ionization|ionised]].<ref name="Hansteen1997">{{Cite journal |last1=Hansteen |first1=V. H. |last2=Leer |first2=E. |last3=Holzer |first3=T. E. |date=1997 |title=The role of helium in the outer solar atmosphere |journal=[[The Astrophysical Journal]] |volume=482 |issue=1 |pages=498–509 |bibcode=1997ApJ...482..498H |doi=10.1086/304111 |doi-access=free}}</ref>

[[File:171879main LimbFlareJan12 lg.jpg|thumb|The Sun's transition region taken by [[Hinode (satellite)|Hinode]]'s Solar Optical Telescope|left|alt=A photograph of the surface of the sun, with flares terminating from the surface on the left.]]
The chromosphere and overlying corona are separated by a thin (about {{val|200|u=km}}) transition region where the temperature rises rapidly from around {{val|20000|u=K|fmt=commas}} in the upper chromosphere to coronal temperatures closer to {{val|1000000|u=K|fmt=commas}}.<ref name="Erdelyi2007">{{Cite journal |last1=Erdèlyi |first1=R. |last2=Ballai |first2=I. |date=2007 |title=Heating of the solar and stellar coronae: a review |journal=Astron. Nachr. |volume=328 |issue=8 |pages=726–733 |bibcode=2007AN....328..726E |doi=10.1002/asna.200710803 |doi-access=free}}</ref> The temperature increase is facilitated by the full ionisation of helium in the transition region, which significantly reduces radiative cooling of the plasma.<ref name="Hansteen1997" /> The transition region does not occur at a well-defined altitude, but forms a kind of nimbus around chromospheric features such as [[Solar spicule|spicules]] and [[Solar filament|filaments]], and is in constant, chaotic motion.<ref name="autogenerated1" /> The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to [[extreme ultraviolet]].<ref name="Dwivedi2006">{{Cite journal |last=Dwivedi |first=B. N. |date=2006 |title=Our ultraviolet Sun |url=http://www.iisc.ernet.in/currsci/sep102006/587.pdf |url-status=live |journal=[[Current Science]] |volume=91 |issue=5 |pages=587–595 |archive-url=https://web.archive.org/web/20201025001339/http://www.iisc.ernet.in/currsci/sep102006/587.pdf |archive-date=25 October 2020 |access-date=22 March 2015}}</ref>

====Corona====
{{Main|Stellar corona}}
[[File:2017 Total Solar Eclipse (35909952653).jpg|thumb|During a [[solar eclipse]] the solar corona can be seen with the naked eye during totality.|alt=A photograph of a solar eclipse]]

The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 10<sup>15</sup>&nbsp;m<sup>−3</sup> to 10<sup>16</sup>&nbsp;m<sup>−3</sup>.<ref name=Hansteen1997 />{{efn|name=particle density}} The average temperature of the corona and solar wind is about 1,000,000–2,000,000&nbsp;K; however, in the hottest regions it is 8,000,000–20,000,000 K.<ref name=Erdelyi2007 /> Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from [[magnetic reconnection]].<ref name=Erdelyi2007 /><ref name="Russell2001">{{Cite book |last=Russell |first=C. T. |title=Space Weather (Geophysical Monograph) |date=2001 |publisher=[[American Geophysical Union]] |isbn=978-0-87590-984-4 |editor-last=Song |editor-first=Paul |pages=73–88 |chapter=Solar wind and interplanetary magnetic field: A tutorial |access-date=11 July 2009 |editor-last2=Singer |editor-first2=Howard J. |editor-last3=Siscoe |editor-first3=George L. |editor-link3=George Siscoe |chapter-url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-url=https://web.archive.org/web/20181001131951/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-date=1 October 2018 |url-status=live}}</ref>

The outer boundary of the corona is located where the radially increasing, large-scale [[solar wind]] speed is equal to the radially decreasing [[Alfvén wave|Alfvén wave phase speed]]. This defines a closed, nonspherical surface, referred to as the ''[[Alfvén critical surface]]'', below which coronal flows are [[Alfvén Mach number|sub-Alfvénic]] and above which the solar wind is super-Alfvénic.<ref>{{cite journal |last1=Cranmer |first1=Steven R. |last2=Chhiber |first2=Rohit |last3=Gilly |first3=Chris R. |last4=Cairns |first4=Iver H. |last5=Colaninno |first5=Robin C. |last6=McComas |first6=David J. |last7=Raouafi |first7=Nour E. |last8=Usmanov |first8=Arcadi V. |last9=Gibson |first9=Sarah E. |last10=DeForest |first10=Craig E. |title=The Sun's Alfvén Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH) |journal=Solar Physics |date=November 2023 |volume=298 |issue=11 |page=126 |doi=10.1007/s11207-023-02218-2 |bibcode=2023SoPh..298..126C |arxiv=2310.05887}}</ref> The height at which this transition occurs varies across space and with solar activity, reaching its lowest near solar minimum and its highest near solar maximum. In April 2021 the surface was crossed for the first time at heliocentric distances ranging from 16 to 20&nbsp;solar radii by the [[Parker Solar Probe]].<ref>{{cite journal |last1=Kasper |first1=J. C. |last2=Klein |first2=K. G. |last3=Lichko |first3=E. |last4=Huang |first4=Jia |last5=Chen |first5=C. H. K. |last6=Badman |first6=S. T. |last7=Bonnell |first7=J. |last8=Whittlesey |first8=P. L. |last9=Livi |first9=R. |last10=Larson |first10=D. |last11=Pulupa |first11=M. |last12=Rahmati |first12=A. |last13=Stansby |first13=D. |last14=Korreck |first14=K. E. |last15=Stevens |first15=M. |last16=Case |first16=A. W. |last17=Bale |first17=S. D. |last18=Maksimovic |first18=M. |last19=Moncuquet |first19=M. |last20=Goetz |first20=K. |last21=Halekas |first21=J. S. |last22=Malaspina |first22=D. |last23=Raouafi |first23=Nour E. |last24=Szabo |first24=A. |last25=MacDowall |first25=R. |last26=Velli |first26=Marco |last27=Dudok de Wit |first27=Thierry |last28=Zank |first28=G. P. |title=Parker Solar Probe Enters the Magnetically Dominated Solar Corona |journal=Physical Review Letters |date=14 December 2021 |volume=127 |issue=25 |page=255101 |doi=10.1103/PhysRevLett.127.255101 |pmid=35029449 |bibcode=2021PhRvL.127y5101K}}</ref><ref name="touching">{{cite web |last=Hatfield |first=Miles |title=NASA Enters the Solar Atmosphere for the First Time |url=https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries |website=NASA |date=13 December 2021 |access-date=30 July 2022 |archive-date=27 December 2021 |archive-url=https://web.archive.org/web/20211227093247/https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries/ |url-status=live}}{{PD-notice}}</ref> Predictions of its full possible extent have placed its full range within 8 to 30&nbsp;solar radii.<ref>{{cite journal |last1=Liu |first1=Ying D. |last2=Chen |first2=Chong |last3=Stevens |first3=Michael L. |last4=Liu |first4=Mingzhe |title=Determination of Solar Wind Angular Momentum and Alfvén Radius from Parker Solar Probe Observations |journal=The Astrophysical Journal Letters |date=1 February 2021 |volume=908 |issue=2 |pages=L41 |doi=10.3847/2041-8213/abe38e |bibcode=2021ApJ...908L..41L |arxiv=2102.03376 |doi-access=free}}</ref><ref>{{cite journal |last1=Katsikas |first1=Valadis |last2=Exarhos |first2=George |last3=Moussas |first3=Xenophon |title=Study of the Solar Slow Sonic, Alfvén and Fast Magnetosonic Transition Surfaces |journal=Advances in Space Research |date=August 2010 |volume=46 |issue=4 |pages=382–390 |doi=10.1016/j.asr.2010.05.003 |bibcode=2010AdSpR..46..382K}}</ref><ref>{{cite journal |last1=Wexler |first1=David B. |last2=Stevens |first2=Michael L. |last3=Case |first3=Anthony W. |last4=Song |first4=Paul |title=Alfvén Speed Transition Zone in the Solar Corona |journal=The Astrophysical Journal Letters |date=1 October 2021 |volume=919 |issue=2 |pages=L33 |doi=10.3847/2041-8213/ac25fa |bibcode=2021ApJ...919L..33W |doi-access=free}}</ref>

=== Heliosphere===
{{Main|Heliosphere}}
[[File:PIA22835-VoyagerProgram&Heliosphere-Chart-20181210.png|thumb|left|Depiction of the [[heliosphere]]]]
The heliosphere is defined as the region of space where the solar wind dominates over the interstellar medium.<ref>{{cite book |last1=Parker |first1=E. N. |author1-link=Eugene Parker |editor1-last=Kamide |editor1-first=Yohsuke |editor2-last=Chian |editor2-first=Abraham C.-L. |title=Handbook of the Solar-Terrestrial Environment |date=2007 |publisher=Springer |location=Berlin |isbn=978-3-540-46315-3 |chapter=Solar Wind |doi=10.1007/978-3-540-46315-3 |bibcode=2007hste.book.....K |url=https://archive.org/details/handbookofsolart0000unse |url-access=registration}}</ref> Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere,<ref>{{Cite web |date=22 April 2003 |title=A Star with two North Poles |url=https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm |url-status=dead |archive-url=https://web.archive.org/web/20090718014855/https://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm |archive-date=18 July 2009 |website=Science @ NASA |publisher=NASA}}</ref><ref>{{Cite journal |last1=Riley |first1=P. |last2=Linker |first2=J. A. |last3=Mikić |first3=Z. |date=2002 |title=Modeling the heliospheric current sheet: Solar cycle variations |journal=[[Journal of Geophysical Research]] |volume=107 |issue=A7 |pages=SSH 8–1 |bibcode=2002JGRA..107.1136R |doi=10.1029/2001JA000299 |id=CiteID 1136 |doi-access=free}}</ref> forming the solar magnetic field into a [[Parker spiral|spiral]] shape,<ref name=Russell2001 /> until it impacts the [[Heliopause (astronomy)|heliopause]] more than {{val|50|u=AU}} from the Sun. In December 2004, the ''[[Voyager 1]]'' probe passed through a shock front that is thought to be part of the heliopause.<ref>{{Cite press release |title=The Distortion of the Heliosphere: Our Interstellar Magnetic Compass |date=2005 |publisher=[[European Space Agency]] |url=http://www.spaceref.com/news/viewpr.html?pid=16394 |access-date=22 March 2006 |url-status=live |archive-url=https://archive.today/20120604110953/http://www.spaceref.com/news/viewpr.html?pid=16394 |archive-date=4 June 2012}}</ref> In late 2012, ''Voyager 1'' recorded a marked increase in [[cosmic ray]] collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered the [[interstellar medium]],<ref>{{Cite press release |last=Landau |first=Elizabeth |url=https://voyager.jpl.nasa.gov/news/details.php?article_id=44 |title=Voyager 1 Helps Solve Interstellar Medium Mystery |publisher=[[Jet Propulsion Laboratory]] |date=29 October 2015 |url-status=live |archive-url=https://web.archive.org/web/20230803125531/https://voyager.jpl.nasa.gov/news/details.php?article_id=44 |archive-date=3 August 2023}}</ref> and indeed did so on 25 August 2012, at approximately 122 astronomical units (18&nbsp;Tm) from the Sun.<ref>{{Cite web |url=https://voyager.jpl.nasa.gov/mission/interstellar-mission/ |title=Interstellar Mission |publisher=[[Jet Propulsion Laboratory]] |access-date=14 May 2021 |archive-date=14 September 2017 |archive-url=https://web.archive.org/web/20170914060928/https://voyager.jpl.nasa.gov/mission/interstellar-mission/#:~:text=On%20Aug.,billion%20kilometers)%20from%20the%20sun. |url-status=live}}</ref> The heliosphere has a [[Heliosphere#Heliotail|heliotail]] which stretches out behind it due to the Sun's [[peculiar motion]] through the galaxy.<ref>{{cite web |last1=Dunbar |first1=Brian |title=Components of the Heliosphere |url=https://www.nasa.gov/mission_pages/sunearth/science/heliosphere-components.html |website=NASA |date=2 March 2015 |access-date=20 March 2021 |archive-date=8 August 2021 |archive-url=https://web.archive.org/web/20210808183941/https://www.nasa.gov/mission_pages/sunearth/science/heliosphere-components.html |url-status=live}}</ref>