Editing Sun (section)

=== 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>