View source for Stellar corona

{{Short description|Outermost layer of a star's atmosphere}}
{{refimprove|date=January 2024}}
[[File:Total Solar Eclipse 8-21-17.jpg|thumb|The [[solar corona]] with its [[coronal streamer]]s stretching out, as well as [[solar prominence]]s (in red) along the [[lunar limb|limb]] of the [[earthshine]] illuminated Moon during a total [[solar eclipse]] on [[Solar eclipse of August 21, 2017|August 21, 2017]]]]
A '''corona''' ({{plural form}}: '''coronas''' or '''coronae''') is the outermost layer of a [[star]]'s [[Stellar atmosphere|atmosphere]]. It is a hot but relatively [[luminosity|dim]] region of [[Plasma (physics)|plasma]] populated by intermittent '''coronal''' structures known as solar [[solar prominence|prominence]]s or filaments.
The [[Sun]]'s corona lies above the [[chromosphere]] and extends millions of kilometres into outer space. Coronal light is typically obscured by [[diffuse sky radiation]] and [[Glare (vision)|glare]] from the solar disk, but can be easily seen by the naked eye during a total [[solar eclipse]] or with a specialized [[coronagraph]].<ref>{{cite journal |last1=Liberatore |first1=Alessandro |last2=Capobianco |first2=Gerardo |last3=Fineschi |first3=Silvano |last4=Massone |first4=Giuseppe |last5=Zangrilli |first5=Luca |last6=Susino |first6=Roberto |last7=Nicolini |first7=Gianalfredo |title=Sky Brightness Evaluation at Concordia Station, Dome C, Antarctica, for Ground-Based Observations of the Solar Corona |journal=Solar Physics |date=March 2022 |volume=297 |issue=3 |page=29 |doi=10.1007/s11207-022-01958-x|pmid=35250102 |pmc=8889400 |arxiv=2201.00660 |bibcode=2022SoPh..297...29L }}</ref> [[Spectroscopic]] measurements indicate strong [[ionization]] in the corona and a plasma temperature in excess of {{gaps|1|000|000 [[kelvin]]s}},<ref name="Aschwanden">{{cite book|title=Physics of the Solar Corona: An Introduction with Problems and Solutions|last=Aschwanden|first=Markus J.|publisher=Praxis Publishing|year=2005|isbn=978-3-540-22321-4|location=Chichester, UK}}
</ref> much hotter than the surface of the Sun, known as the [[photosphere]].
{{langnf|la|Corona|crown}} is, in turn, derived {{etymology|grc|''{{wikt-lang|grc|κορώνη}}'' ({{grc-transl|κορώνη}})|garland, wreath}}.
== History ==
[[File:Solar eclipse 1806Jun16-Corona-Ferrer.png|thumb|Corona sketched by [[José Joaquín de Ferrer]] during the [[solar eclipse of June 16, 1806]] in [[Kinderhook (town), New York|Kinderhook, New York]].]]
In 1724, French-Italian astronomer [[Giacomo F. Maraldi]] recognized that the aura visible during a [[solar eclipse]] belongs to the Sun, not to the [[Moon]].<ref>{{cite encyclopedia |title=Maraldi, Giacomo Filippo |encyclopedia=Biographical Encyclopedia of Astronomers |year=2007 |publisher=Springer |location=New York |doi=10.1007/978-0-387-30400-7_899 |id= |url=https://link.springer.com/referenceworkentry/10.1007%2F978-0-387-30400-7_899 |access-date=31 October 2021 |last1=Hall |first1=Graham |last2=Elliott |first2=Ian |last3=Joeveer |first3=Mihkel |last4=Bònoli |first4=Fabrizio |last5=Langermann |first5=Y. Tzvi |last6=Casulleras |first6=Josep |last7=Sarma |first7=Ke Ve |last8=Bell |first8=Trudy E. |last9=Gurshtein |first9=Alexander A. |last10=Cunning |first10=David |last11=Wegner |first11=Gary A. |last12=Berggren |first12=Len |last13=Bònoli |first13=Fabrizio |last14=Hatch |first14=Robert Alan |last15=Jarrell |first15=Richard A. |last16=Durham |first16=Ian T. |last17=Durham |first17=Ian T. |last18=Snedegar |first18=Keith |last19=Trimble |first19=Virginia |last20=Dick |first20=Steven J. |last21=McCarthy |first21=Dennis D. |last22=Charette |first22=François |last23=Bolt |first23=Marvin |last24=Belenkiy |first24=Ari |last25=McFarland |first25=John |last26=Jackson |first26=Francine |last27=Green |first27=Daniel W. E. |last28=Ikeyama |first28=Setsuro |last29=Snedegar |first29=Keith |last30=Ogilvie |first30=Marilyn Bailey |page=736 |isbn=978-0-387-31022-0 |display-authors=1 }}</ref> In 1809, Spanish astronomer [[José Joaquín de Ferrer]] coined the term 'corona'.<ref>{{Cite journal|last=de Ferrer|first=José Joaquín|date=1809|title=Observations of the eclipse of the sun June 16th 1806 made at Kinderhook in the State of New York|url=https://books.google.com/books?id=DbkAAAAAYAAJ&pg=PA264|journal=Transactions of the American Philosophical Society|volume=6|pages=264–275|doi=10.2307/1004801|jstor=1004801}}</ref> Based on his own observations of the 1806 solar eclipse at Kinderhook (New York), de Ferrer also proposed that the corona was part of the Sun and not of the Moon. English astronomer [[Norman Lockyer]] identified the first element unknown on Earth in the Sun's chromosphere, which was called [[helium]] (from [[Greek language|Greek]] {{lang|grc-Latn|helios}} 'sun'). French astronomer [[Pierre Janssen|Jules Jenssen]] noted, after comparing his readings between the 1871 and 1878 eclipses, that the size and shape of the corona changes with the [[Solar cycle|sunspot cycle]].<ref>{{Cite web|last=Espenak|first=Fred|title=Chronology of Discoveries about the Sun|url=http://www.mreclipse.com/Totality2/TotalityApH.html|url-status=live|archive-url=https://web.archive.org/web/20201019152942/http://www.mreclipse.com/Totality2/TotalityApH.html|archive-date=19 October 2020|access-date=6 November 2020|website=Mr. Eclipse}}</ref> In 1930, [[Bernard Lyot]] invented the [[Coronagraph|"coronograph" (now "coronagraph")]], which allows viewing the corona without a total eclipse. In 1952, American astronomer [[Eugene Parker]] proposed that the solar corona might be heated by myriad tiny 'nanoflares', miniature brightenings resembling [[solar flares]] that would occur all over the surface of the Sun.
=== Historical theories ===
The high temperature of the Sun's corona gives it unusual [[spectroscopy|spectral]] features, which led some in the 19th century to suggest that it contained a previously unknown element, "[[coronium]]". Instead, these spectral features have since been explained by [[highly charged ion|highly ionized]] [[iron]] (Fe-XIV, or Fe<sup>13+</sup>).  [[Bengt Edlén]], following the work of [[Walter Grotrian]] in 1939, first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying [[metastable]] levels of the ground configuration of highly ionised metals (the green Fe-XIV line from Fe<sup>13+</sup> at {{gaps|5|303|[[Ångström|Å]]}}, but also the red Fe-X line from Fe<sup>9+</sup> at {{gaps|6|374|Å}}).<ref name="Aschwanden" />
== Observable components ==
The solar corona has three recognized, and distinct, sources of light that occupy the same volume: the "F-corona" (for "Fraunhofer"), the "K-corona" (for "Kontinuierlich"), and the "E-corona" (for "emission").<ref>Golub & Pasachoff (1997). "The Solar Corona", Cambridge University Press (London), ISBN 0 521 48082 5, p. 4</ref>
The "F-corona" is named for the [[Fraunhofer lines|Fraunhofer spectrum]] of absorption lines in ordinary sunlight, which are preserved by reflection off small material objects.  The F-corona is faint near the Sun itself, but drops in brightness only gradually far from the Sun, extending far across the sky and becoming the [[zodiacal light]].  The F-corona is recognized to arise from small dust grains orbiting the Sun; these form a tenuous cloud that extends through much of the solar system.
The "K-corona" is named for the fact that its spectrum is a continuum, with no major spectral features.  It is sunlight light that is [[Thomson scattering|Thomson-scattered]] by free electrons in the [[hot plasma]] of the Sun's outer atmosphere.  The continuum nature of the spectrum arises from [[Doppler broadening]] of the Sun's Fraunhofer absorption lines in the reference frame of the (hot and therefore fast-moving) electrons. Although the K-corona is a phenomenon of the electrons in the plasma, the term is frequently used to describe the plasma itself (as distinct from the dust that gives rise to the F-corona).
The "E-corona" is the component of the corona with an [[emission-line]] spectrum, either inside or outside the wavelength band of visible light.  It is a phenomenon of the ion component of the plasma, as individual ions are excited by collision with other ions or electrons, or by absorption of ultraviolet light from the Sun.
== Physical features ==
[[File:Twistedflux.png|thumb|Configuration of solar magnetic flux during the solar cycle]]
The Sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the corona's temperature is 1 to 3 million [[kelvin]] compared to the [[photosphere]]'s average temperature – around {{gaps|5|800|[[kelvin]]}}. The corona is far less dense than the photosphere, and produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow [[chromosphere]]. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include episodic energy releases from the pervasive [[magnetic field]] and [[Magnetohydrodynamics|magnetohydrodynamic waves]] from below. The outer edges of the Sun's corona are constantly being transported away, creating the "open" magnetic flux entrained in the [[solar wind]].
The corona is not always evenly distributed across the surface of the Sun. During periods of quiet, the corona is more or less confined to the [[equator]]ial regions, with [[coronal hole]]s covering the [[Geographical pole|polar]] regions. However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with [[sunspot]] activity. The [[solar cycle]] spans approximately 11 years, from one [[solar minimum]] to the following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun's equator ([[differential rotation]]), sunspot activity is more pronounced at [[solar maximum]] where the [[magnetic field]] is more twisted. Associated with sunspots are [[coronal loop]]s, loops of [[magnetic flux]], upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the relatively dark sun spots.
High-resolution X-ray images of the Sun's corona photographed by [[Skylab]] in 1973, by [[Yohkoh]] in 1991–2001, and by subsequent space-based instruments revealed the structure of the corona to be quite varied and complex, leading astronomers to classify various zones on the coronal disc.<ref>{{cite journal|doi = 10.1007/BF00152731|last1 = Vaiana|first1 = G. S.|last2 = Krieger|first2 = A. S.|last3 = Timothy|first3 = A. F.|title = Identification and analysis of structures in the corona from X-ray photography
| journal = Solar Physics | volume = 32|issue = 1| pages = 81–116| year = 1973| bibcode=1973SoPh...32...81V|s2cid = 121940724}}</ref><ref>{{cite book |last1=Vaiana |first1=G.S. |last2=Tucker |first2=W.H. |chapter=Solar X-Ray Emission |title=X-Ray Astronomy |editor=R. Giacconi |editor2=H. Gunsky |page=169 |year=1974}}</ref><ref>{{cite journal|doi = 10.1146/annurev.aa.16.090178.002141|last1 = Vaiana |first1 = G S|last2 = Rosner|first2 = R | title = Recent advances in Coronae Physics
| journal = Annual Review in Astronomy and Astrophysics | volume = 16| pages = 393–428| year = 1978 | bibcode=1978ARA&A..16..393V}}</ref>
Astronomers usually distinguish several regions,<ref name="Gibson">{{cite book
|last=Gibson
|first= E. G.
|year=1973
|title= The Quiet Sun
|publisher=National Aeronautics and Space Administration, Washington, D.C.
}}</ref> as described below.
===Active regions===
{{main|Active region}}
Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million kelvin, while the density goes from 10<sup>9</sup> to 10<sup>10</sup> particles per cubic centimetre.
[[File:Prominence (PSF).png|thumb|[[Solar prominences]] and sunspots]]
Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights above the Sun's surface:<ref name="Gibson"/> sunspots and [[Solar facula|facula]]e occur in the photosphere; [[Solar spicule|spicules]], [[Hα]] [[Solar prominence|filaments]] and [[Solar plage|plages]] in the chromosphere; prominences in the chromosphere and transition region; and [[Solar flare|flares]] and [[coronal mass ejection]]s (CME) happen in the corona and chromosphere. If flares are very violent, they can also perturb the photosphere and generate a [[Moreton wave]]. On the contrary, quiescent prominences are large, cool, dense structures which are observed as dark, "snake-like" Hα ribbons (appearing like filaments) on the solar disc. Their temperature is about {{gaps|5|000}}–{{gaps|8|000|K}}, and so they are usually considered as chromospheric features.
In 2013, images from the [[High Resolution Coronal Imager]] revealed never-before-seen "magnetic braids" of plasma within the outer layers of these active regions.<ref>{{cite web|url=http://www.space.com/19400-sun-corona-secrets-suborbital-telescope.html|title=How NASA Revealed Sun's Hottest Secret in 5-Minute Spaceflight|website=[[Space.com]]|date=23 January 2013|url-status=live|archive-url=https://web.archive.org/web/20130124115740/http://www.space.com/19400-sun-corona-secrets-suborbital-telescope.html|archive-date=2013-01-24}}</ref>
====Coronal loops====
{{main|Coronal loop}}
[[File:Traceimage.jpg|left|thumb|Image from [[TRACE]] at 171Å wavelength ([[extreme ultraviolet]]) showing coronal loops]]
Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of the open-magnetic flux that can be found in coronal holes and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma.<ref>{{cite journal|doi = 10.1086/427488|last1 = Katsukawa|first1 = Yukio|last2 = Tsuneta|first2 = Saku | title = Magnetic Properties at Footpoints of Hot and Cool Loops | journal = The Astrophysical Journal | volume = 621 |issue = 1| pages = 498–511 | year = 2005 | bibcode=2005ApJ...621..498K|doi-access = free}}</ref> Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and CMEs.
The solar plasma that feeds these structures is heated from under {{gaps|6|000|K}} to well over 10<sup>6</sup>&nbsp;K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points ([[siphon]] flow due to a pressure difference,<ref>{{cite journal|doi = 10.1023/A:1005182503751|last1 = Betta |first1 = Rita|last2 = Orlando|first2 = Salvatore|last3 = Peres|first3 = Giovanni|last4 = Serio|first4 = Salvatore | title = On the Stability of Siphon Flows in Coronal Loops | journal = Space Science Reviews | volume = 87 | pages = 133–136| year = 1999|bibcode = 1999SSRv...87..133B |s2cid = 117127214 }}</ref> or asymmetric flow due to some other driver).
When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation. There may also be [[symmetric]] flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), its dark filaments obvious against the solar disk or prominences off the [[limb darkening|Sun's limb]].
Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as ''[[steady state]]'' or ''[[wikt:quiescent|quiescent]]'' coronal loops ([[:File:Energyfig.png|example]]).
Coronal loops are very important to our understanding of the current ''coronal heating problem''. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as ''[[TRACE]]''. An explanation of the coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present (i.e., radiation contributions along the [[line-of-sight propagation]]). ''[[In-situ]]'' measurements are required before a definitive answer can be determined, but due to the high plasma temperatures in the corona, ''in-situ'' measurements are, at present, impossible. The next mission of the NASA [[Parker Solar Probe]] will approach the Sun very closely, allowing more direct observations.
====Large-scale structures====
Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions.
They were first detected in the June 8, 1968, flare observation during a rocket flight.<ref name = Giacconi>{{cite conference |last = Giacconi| first = Riccardo| title = G. S. Vaiana memorial lecture |book-title=Physics of Solar and Stellar Coronae: G.S. Vaiana Memorial Symposium: G.S. Vaiana Memorial Symposium : Proceedings of a Conference of the International Astronomical Union |editor-first1=J. F. |editor-last1=Linsky |editor-first2=S. |editor-last2=Serio| pages = 3–19 | year = 1992 | publisher = Kluwer Academic |location=Netherlands | isbn = 978-0-7923-2346-4}}</ref>
The large-scale structure of the corona changes over the 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component).
====Interconnections of active regions====
[[File:Parker Solar Probe Encounters Streamers on the Way to the Sun.webm|thumb|As [[Parker Solar Probe]] passed through the Sun's corona in early 2021, the spacecraft flew by structures called [[Helmet streamer|coronal streamers]].]]
The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare.<ref name=":2" />
Some other features of this kind are [[helmet streamer]]s – large, cap-like coronal structures with long, pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered to be sources of the slow solar wind.<ref name=":2">{{cite journal| doi= 10.1029/2000GL000097| last= Ofman | first= Leon | title= Source regions of the slow solar wind in coronal streamers | journal= Geophysical Research Letters | volume = 27 | issue= 18 | pages= 2885–2888 |year=2000 | bibcode=2000GeoRL..27.2885O| url= https://zenodo.org/record/1231279 | doi-access= free }}</ref>
====Filament cavities====
[[File:Crackling with Solar Flares.jpg|thumb|left|Image taken by the [[Solar Dynamics Observatory]] on October 16, 2010. A very long filament cavity is visible across the Sun's southern hemisphere.]]
Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected ''coronal holes''.<ref name=Giacconi />
Filament cavities are cooler clouds of plasma suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the [[magnetic pressure]] and plasma pressure must be constant everywhere on the [[heliosphere]] in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure <math>p</math> can be calculated by the [[state equation]] of a perfect gas: <math> p = n k_B T</math>, where <math>n</math> is the [[particle number density]], <math>k_B</math> the [[Boltzmann constant]] and <math>T</math> the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.{{Citation needed|date=February 2022}}
====Bright points====
Bright points are small active regions found on the solar disk. X-ray bright points were first detected on April 8, 1969, during a rocket flight.<ref name=Giacconi />
The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1&nbsp;MK to 3.4&nbsp;MK. The variations in temperature are often correlated with changes in the X-ray emission.<ref>{{cite journal |last1=Kariyappa |first1= R. |last2=Deluca |first2= E. E. |last3=Saar |first3= S. H. |last4=Golub |first4= L. |last5=Damé |first5= L. |last6=Pevtsov |first6= A. A. |last7=Varghese |first7= B. A. |title= Temperature variability in X-ray bright points observed with Hinode/XRT | journal= Astronomy & Astrophysics| year= 2011 | volume= 526 |pages= A78 | bibcode = 2011A&A...526A..78K | doi = 10.1051/0004-6361/201014878|doi-access=free }}</ref>
===Coronal holes===
{{main|Coronal hole}}
Coronal holes are unipolar regions which look dark in the X-rays since they do not emit much radiation.<ref>{{cite journal|doi = 10.1088/0004-637X/719/1/131|last1 = Ito |first1 = Hiroaki|last2 = Tsuneta|first2 = Saku|last3 = Shiota|first3 = Daikou|last4 = Tokumaru|first4 = Munetoshi|last5 = Fujiki|first5 = Ken'Ichi | title = Is the Polar Region Different from the Quiet Region of the Sun?| journal = The Astrophysical Journal | volume = 719 |issue = 1 | pages = 131–142| year = 2010 | bibcode=2010ApJ...719..131I|arxiv = 1005.3667 |s2cid = 118504417 }}</ref> These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions.
In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind. These are the coronal plumes. More precisely, they are long thin streamers that project outward from the Sun's north and south poles.<ref>{{cite journal| doi=10.1051/0004-6361:20021628| last1=Del Zanna | first1=G.| last2=Bromage| first2=B. J. I.| last3=Mason| first3=H. E.| title= Spectroscopic characteristics of polar plumes| journal= Astronomy & Astrophysics| year=2003| volume=398| issue=2 | pages= 743–761| bibcode=2003A&A...398..743D| doi-access=free}}</ref>
===The quiet Sun===
The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun.
The equatorial region has a faster rotation speed than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore, the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle. Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are coronal holes.
===Alfvén surface===
{{Main|Alfvén surface}}
[[File:Parker Solar Probe touches the Sun.webm|thumb|NASA animation of the [[Parker Solar Probe]] passing through the Sun's corona. Inside the corona's boundary, its [[Alfvén surface]], plasma waves travel back and forth to the Sun's surface.]]
The [[Alfvén surface]] is the boundary separating the corona from the [[solar wind]] defined as where the coronal plasma's [[Alfvén speed]] and the large-scale solar wind speed are equal.<ref>{{cite journal |last1=Adhikari |first1=L. |last2=Zank |first2=G. P. |last3=Zhao |first3=L.-L. |title=Does Turbulence Turn off at the Alfvén Critical Surface? |journal=The Astrophysical Journal |date=30 April 2019 |volume=876 |issue=1 |page=26 |doi=10.3847/1538-4357/ab141c|bibcode=2019ApJ...876...26A |s2cid=156048833 |doi-access=free }}</ref><ref>{{cite journal |last1=DeForest |first1=C. E. |last2=Howard |first2=T. A. |last3=McComas |first3=D. J. |title=Inbound waves in the solar corona: a direct indicator of Alfvén Surface location |journal=The Astrophysical Journal |date=12 May 2014 |volume=787 |issue=2 |page=124 |doi=10.1088/0004-637X/787/2/124|arxiv=1404.3235 |bibcode=2014ApJ...787..124D |s2cid=118371646 }}</ref>
Researchers were unsure exactly where the Alfvén critical surface of the Sun lay. Based on remote images of the corona, estimates had put it somewhere between 10 and 20 solar radii from the surface of the Sun. On April 28, 2021, during its eighth flyby of the Sun, NASA's [[Parker Solar Probe]] encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface.<ref>{{citation-attribution|1={{cite web |last1=Hatfield |first1=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}}}}</ref>
==Variability of the corona==
A portrait, as diversified as the one already pointed out for the coronal features, is emphasized by the analysis of the dynamics of the main structures of the corona,  which evolve at differential times. Studying coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table.
{| class="wikitable" | title="Typical length of observable coronal features"
|-
! '''Coronal event''' !! '''Typical time-scale''' !! '''Typical length-scale (Mm)'''
|-
| Active region flare || 10 to {{gaps|10|000|seconds}} || 10–100
|-
| X-ray bright point || minutes || 1–10
|-
| Transient in large-scale structures || from minutes to hours || ~100
|-
| Transient in interconnecting arcs || from minutes to hours || ~100
|-
| Quiet Sun || from hours to months || 100–{{gaps|1|000}}
|-
| Coronal hole || several rotations || 100–{{gaps|1|000}}
|}
===Flares===
{{main|Solar flares}}
[[File:Magnificent CME Erupts on the Sun - August 31.jpg|thumb|On August 31, 2012, a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted at 4:36 p.m. EDT]]
Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.
Flares are impulsive phenomena, of average duration of 15 minutes, and the most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature.
An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.
In the corona, the morphology of flares is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and is very complex. However, two kinds of basic structures can be distinguished:
<ref>{{cite journal|doi = 10.1086/155452|last1 = Pallavicini |first1 = R.|last2 = Serio|first2 = S.|last3 = Vaiana|first3 = G. S.|title = A survey of soft X-ray limb flare images – The relation between their structure in the corona and other physical parameters
| journal = The Astrophysical Journal  | volume = 216| page = 108| year = 1977 | bibcode=1977ApJ...216..108P}}</ref>
* ''Compact flares'', when each of the two arches where the event is happening maintains its morphology: only an increase of the emission is observed without significant structural variations. The emitted energy is of the order of 10<sup>22</sup>&nbsp;– 10<sup>23</sup> J.
* ''Flares of long duration'', associated with eruptions of prominences, transients in white light and ''two-ribbon flares'':<ref>{{cite journal|doi = 10.1038/344842a0|last1 = Golub |first1 = L.|last2 = Herant|first2 = M.|last3 = Kalata|first3 = K.|last4 = Lovas|first4 = I.|last5 = Nystrom|first5 = G.|last6 = Pardo|first6 = F.|last7 = Spiller|first7 = E.|last8 = Wilczynski|first8 = J. | title = Sub-arcsecond observations of the solar X-ray corona | journal = Nature | volume = 344 | pages = 842–844| year = 1990|bibcode = 1990Natur.344..842G | issue=6269|s2cid = 4346856 }}</ref> in this case the magnetic loops change their configuration during the event. The energies emitted during these flares are of such great proportion they can reach 10<sup>25</sup> J.
[[File:Solar-filament.gif|thumb|Filament erupting during a solar flare, seen at EUV wavelengths ([[TRACE]])]]
As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event:
* ''An initial impulsive phase'', whose duration is on the order of minutes, strong emissions of energy are often observed even in the microwaves, EUV wavelengths and in the hard X-ray frequencies.
* ''A maximum phase''
* ''A decay phase'', which can last several hours.
Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.
===Coronal mass ejections===
{{main|Coronal mass ejection}}
Often accompanying large solar flares and prominences are coronal mass ejections (CME). These are enormous emissions of coronal material and magnetic field that travel outward from the Sun at up to 3000 km/s,<ref name="NOAA / NWS Space Weather Prediction Center 2024 x766">{{cite web | title=Coronal Mass Ejections | website=NOAA / NWS Space Weather Prediction Center | date=April 3, 2024 | url=https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections | access-date=April 3, 2024}}</ref> containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger CMEs can propel hundreds of millions of tons of material into [[interplanetary space]] at roughly 1.5 million kilometers an hour.{{Citation needed|date=February 2022}}
==Stellar coronae==
Coronal stars are ubiquitous among the stars in the cool half of the [[Hertzsprung–Russell diagram]].<ref name=Gudel>{{cite journal |author=Güdel M |title=X-ray astronomy of stellar coronae |journal=The Astronomy and Astrophysics Review |year=2004 |volume=12 |issue=2–3 |pages=71–237 |doi=10.1007/s00159-004-0023-2 |url=http://astronomy.sci.ege.edu.tr/~rpekunlu/GBDG/papers/XRayfromStellarCoronae.pdf |arxiv=astro-ph/0406661 |bibcode=2004A&ARv..12...71G |s2cid=119509015 |archive-url=https://web.archive.org/web/20110811230835/http://astronomy.sci.ege.edu.tr/~rpekunlu/GBDG/papers/XRayfromStellarCoronae.pdf |archive-date=2011-08-11 }}</ref> These coronae can be detected using [[X-ray telescope]]s. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. For example, [[FK Comae Berenices]] is the prototype for the FK Com class of [[variable star]]. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (''L''<sub>x</sub> ≥ 10<sup>32</sup> erg·s<sup>−1</sup> or 10<sup>25</sup>W) and the hottest known with dominant temperatures up to 40 MK.<ref name=Gudel/>
The astronomical observations planned with the [[Einstein Observatory]] by Giuseppe Vaiana and his group<ref name=Vaianaetal81>{{cite journal |doi= 10.1086/158797 |last=Vaiana |first= G.S. |title= Results from an extensive Einstein stellar survey |journal= The Astrophysical Journal|year=1981 |volume=245 |page=163 |bibcode=1981ApJ...245..163V|display-authors=etal}}</ref> showed that F-, G-, K- and M-stars have chromospheres and often coronae much like the Sun.
The ''O-B stars'', which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs.
Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.
==Physics of the corona==
[[File:171879main LimbFlareJan12 lg.jpg|thumb|left|This image, taken by [[Hinode (satellite)|Hinode]] on 12 January 2007, reveals the filamentary nature of the corona.]]
The matter in the external part of the solar atmosphere is in the state of [[plasma (physics)|plasma]], at very high temperature (a few million kelvin) and at very low density (of the order of 10<sup>15</sup> particles/m<sup>3</sup>).
According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.
The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization of its heavier elements than that found in the photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the [[Saha equation]] in the lowest atmosphere, but by collisional equilibrium in the optically thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.
The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately a million times higher than on the solar surface. However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average
(for the [[equipartition theorem]]), electrons have a mass roughly {{gaps|1|800}} times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from the photospheric radiative processes), or on thermal conduction.
Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields.
Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma,<ref name="Jeffrey">{{cite book
|last = Jeffrey
|first = Alan
|year =1969
|title = Magneto-hydrodynamics
|publisher = UNIVERSITY MATHEMATICAL TEXTS
}}</ref> even though it is still not clear how they can be transmitted or generated in the corona.
===Radiation===
{{see also|Coronal radiative losses}}
Coronal plasma is optically thin and therefore transparent to the [[electromagnetic radiation]] that it emits and to that coming from lower layers. The plasma is very rarefied and the [[photon]] [[mean free path]] overcomes by far all the other length-scales, including the typical sizes of common coronal features.{{citation needed|date=January 2022}}
Electromagnetic radiation from the corona has been identified coming from three main sources, located in the same volume of space:
* The K-corona (K&nbsp;for {{lang|de|kontinuierlich}}, "continuous" in German) is created by sunlight [[Thomson scattering]] off free [[electron]]s; [[doppler broadening]] of the reflected photospheric [[absorption line]]s spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines.
* The F-corona (F&nbsp;for [[Joseph von Fraunhofer|Fraunhofer]]) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high [[elongation (astronomy)|elongation]] angles from the Sun, where it is called the [[zodiacal light]].
* The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or [[forbidden line|forbidden]] or hot [[spectral line|spectral emission lines]] and is the main source of information about the corona's composition.<ref name="Corfield">{{cite book
|last=Corfield
|first=Richard
|year=2007
|title=Lives of the Planets
|publisher=Basic Books
|isbn=978-0-465-01403-3
|url-access=registration
|url=https://archive.org/details/livesofplanetsna00corf
}}
</ref>
===Thermal conduction===
[[File:STEREO-A first images.jpg|thumb|A mosaic of the extreme ultraviolet images taken from [[STEREO]] on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C ({{gaps|195|Å—green}}), {{gaps|60|000}}–{{gaps|80|000|°C}} (304 Å—red), and 2.5 million °C (286 Å—yellow).]]
[[File:STEREO-A first images slow anim.gif|thumb|[[STEREO]]&nbsp;– First images as a slow animation]]
In the corona [[thermal conduction]] occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.
When there is a magnetic field the [[thermal conductivity]] of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction.<ref name="Spitzer">{{cite book
|last=Spitzer
|first= L.
|year=1962
|title= Physics of fully ionized gas
|publisher= Interscience tracts of physics and astronomy
}}</ref>
A charged particle moving in the direction perpendicular to the magnetic field line is subject to the [[Lorentz force]] which is normal to the plane individuated by the velocity and the magnetic field. This force bends the path of the particle. In general, since particles also have a velocity component along the magnetic field line, the Lorentz force constrains them to bend and move along spirals around the field lines at the [[cyclotron]] frequency.
If collisions between the particles are very frequent, they are scattered in every direction. This happens in the photosphere, where the plasma carries the magnetic field in its motion. In the corona, on the contrary, the mean free-path of the electrons is of the order of kilometres and even more, so each electron can do a helicoidal motion long before being scattered after a collision. Therefore, the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction.
In the direction longitudinal to the magnetic field, the thermal conductivity of the corona is<ref name="Spitzer" />
<math display="block">
k = 20 \left(\frac{2}{\pi}\right)^{3/2}\frac{\left(k_\text{B} T \right)^{5/2}k_\text{B}}{m_e^{1/2} e^4 \ln \Lambda} \approx \frac{T^{5/2}}{\ln \Lambda} \times 1.8 \times 10^{-10}~ \mathrm{W m^{-1}K^{-1}}
</math>
where <math>k_\text{B}</math> is the [[Boltzmann constant]], <math>T</math> is the temperature in kelvin, <math>m_e</math> is the electron mass, <math>e</math> is the electric charge of the electron,
<math display="block"> \ln \Lambda = \ln \left(12\pi n \lambda_D^3 \right) </math>
is the Coulomb logarithm, and
<math display="block">\lambda_D = \sqrt{ \frac{k_\text{B} T }{4 \pi n e^2 }}</math>
is the [[Debye length]] of the plasma with particle density  <math>n</math>. The Coulomb logarithm <math> \ln \Lambda </math> is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10<sup>15</sup> particles/m<sup>3</sup>, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 10<sup>18</sup> particles/m<sup>3</sup>, and in practice it can be assumed constant.
Thence, if we indicate with <math>q</math> the heat for a volume unit, expressed in J m<sup>−3</sup>, the Fourier equation of heat transfer, to be computed only along the direction <math>x</math> of the field line, becomes
<math display="block"> \frac{\partial q}{\partial t} = 0.9 \times 10^{-11}~ \frac{\partial^2  T^{7/2}}{\partial x ^2 }.</math>
Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.
===Coronal seismology===
{{main|Coronal seismology}}
Coronal seismology is a method of studying the plasma of the solar corona with the use of [[magnetohydrodynamic]] (MHD) waves. MHD studies the [[dynamics (mechanics)|dynamics]] of [[electrical conduction|electrically conducting]] [[fluid]]s – in this case, the fluid is the coronal plasma. Philosophically, coronal seismology is similar to the Earth's [[seismology]], the Sun's [[helioseismology]], and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kinds are used to probe a medium. The potential of coronal seismology in the estimation of the coronal magnetic field, density [[scale height]], [[fine structure]] and heating has been demonstrated by different research groups.
==Coronal heating problem==
{{unsolved|physics|Why is the Sun's corona so much hotter than the Sun's surface?}}
[[File:Van Gogh Sun.ogv|thumb|A new visualisation technique can provide clues to the coronal heating problem.]]
The coronal heating problem in [[solar physics]] relates to the question of why the temperature of the Sun's corona is millions of kelvins greater than the thousands of kelvins of the surface. Several theories have been proposed to explain this phenomenon, but it is still challenging to determine which is correct.<ref name=":0">{{Cite web|url=http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2004ESASP.575....2K&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES|title=2004ESASP.575....2K Page 2|website=adsbit.harvard.edu|access-date=2019-02-28}}</ref> The problem first emerged after the identification of unknown spectral lines in the solar spectrum with highly ionized iron and calcium atoms.<ref name=":1">{{Cite book|title=Physics of the Solar Corona: An Introduction with Problems and Solutions|url=https://link.springer.com/chapter/10.1007/3-540-30766-4_9|url-access=limited|last=Aschwanden|first=Markus|publisher=Springer Science & Business Media|year=2006|isbn=978-3-540-30765-5|location=Berlin|page=355|doi=10.1007/3-540-30766-4_9 }}</ref><ref name=":0" /> The comparison of the coronal and the photospheric temperatures of {{gaps|6|000|K}}, leads to the question of how the 200-times-hotter coronal temperature can be maintained.<ref name=":1" /> The problem is primarily concerned with how the energy is transported up into the corona and then converted into heat within a few solar radii.<ref>{{Cite book|title=Turbulence and Magnetic Fields in Astrophysics|url=https://archive.org/details/turbulencemagnet00falg_199|url-access=limited|last1=Falgarone|first1=Edith|last2=Passot|first2=Thierry|publisher=Springer Science & Business Media|year=2003|isbn=978-3-540-00274-1|location=Berlin|pages=[https://archive.org/details/turbulencemagnet00falg_199/page/n33 28]}}</ref>
The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the [[second law of thermodynamics]] prevents heat from flowing directly from the solar photosphere (surface), which is at about {{gaps|5|800|K}}, to the much hotter corona at about 1 to 3 [[SI prefix|MK]] (parts of the corona can even reach {{gaps|10|MK}}).
Between the photosphere and the corona, the thin region through which the temperature increases is known as the [[solar transition region|transition region]]. It ranges from only tens to hundreds of kilometers thick. Energy cannot be transferred from the cooler photosphere to the corona by conventional heat transfer as this would violate the second law of thermodynamics. An analogy of this would be a light bulb raising the temperature of the air surrounding it to something greater than its glass surface.  Hence, some other manner of energy transfer must be involved in the heating of the corona.
The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region. It is about 1 kilowatt for every square meter of surface area on the Sun's chromosphere, or 1/{{gaps|40|000}} of the amount of light energy that escapes the Sun.
Many coronal heating theories have been proposed,<ref>{{cite book |last = Ulmshneider |first= Peter |title = Heating of Chromospheres and Coronae in ''Space Solar Physics'', Proceedings, Orsay, France |editor = J.C. Vial |editor2 = K. Bocchialini |editor3 = P. Boumier| publisher = Springer | pages = 77–106| year = 1997| isbn= 978-3-540-64307-4}}</ref> but two theories have remained as the most likely candidates: wave heating and [[magnetic reconnection]] (or [[nanoflares]]).<ref>{{cite book |last1=Malara |first1= F. |last2=Velli |first2= M. |title = Observations and Models of Coronal Heating in ''Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and Other Space Missions'', Proceedings of IAU Symposium 203 |editor = Pål Brekke |editor2 = Bernhard Fleck |editor3 = Joseph B. Gurman| publisher = Astronomical Society of the Pacific | pages = 456–466| year = 2001| isbn= 978-1-58381-069-9}}</ref> Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.
In 2012, high resolution (<0.2″) [[soft X-ray]] imaging with the [[High Resolution Coronal Imager]] aboard a [[sounding rocket]] revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) is assumed to originate from MHD waves.<ref name=Cirtain2013>{{Cite journal | last1 = Cirtain | first1 = J. W. | last2 = Golub | first2 = L. | last3 = Winebarger | first3 = A. R. | last4 = De Pontieu | first4 = B. | last5 = Kobayashi | first5 = K. | last6 = Moore | first6 = R. L. | last7 = Walsh | first7 = R. W. | last8 = Korreck | first8 = K. E. | last9 = Weber | first9 = M. | doi = 10.1038/nature11772 | last10 = McCauley | first10 = P. | last11 = Title | first11 = A. | last12 = Kuzin | first12 = S. | last13 = Deforest | first13 = C. E. | title = Energy release in the solar corona from spatially resolved magnetic braids | journal = Nature | volume = 493 | issue = 7433 | pages = 501–503 | year = 2013 | pmid =  23344359|bibcode = 2013Natur.493..501C | s2cid = 205232074 }}</ref>
[[NASA]]'s [[Parker Solar Probe]] is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind. It was successfully launched on August 12, 2018<ref>{{cite web |url=http://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Journey-to-the-Sun |title= Parker Solar Probe: The Mission|website=parkersolarprobe.jhuapl.edu |archive-url=https://web.archive.org/web/20170822222954/http://parkersolarprobe.jhuapl.edu/The-Mission/index.php |archive-date=2017-08-22}}</ref> and as of fall 2022 has completed the first 13 of more than 20 planned close approaches to the Sun.<ref>{{Cite web|url=https://blogs.nasa.gov/parkersolarprobe/2019/09/03/parker-solar-probe-completes-third-close-approach-of-the-sun/|title=Parker Solar Probe Completes Third Close Approach of the Sun|website=blogs.nasa.gov|date=3 September 2019 |access-date=2019-12-06}}</ref>
{| class="wikitable" style="margin: 1em auto 1em auto"
|+Competing theories of heating mechanisms
|-
! colspan="3" |Heating models
|-
! Hydrodynamic
! colspan="2" |Magnetic
|-
| rowspan="2"  |
* No magnetic field
* Slow rotating stars
! [[Direct current|DC]] (''reconnection'')
! [[Alternating current|AC]] (''waves'')
|-
|
* B-field stresses
* Reconnection events
* Flares-nanoflares
* ''Uniform heating rates''
|
* Photospheric foot point ''shuffling''
* MHD wave propagation
* High Alfvén wave flux
* ''Non-uniform heating rates''
|}
===Wave heating theory===
The wave heating theory, proposed in 1949 by [[Évry Schatzman]], proposes that waves carry energy from the solar interior to the solar chromosphere and corona. The Sun is made of plasma rather than ordinary gas, so it supports several types of waves analogous to [[sound waves]] in air. The most important types of wave are [[magneto-acoustic wave]]s and [[Alfvén wave]]s.<ref>{{cite journal
| last = Alfvén
| first = Hannes
| title = Magneto hydrodynamic waves, and the heating of the solar corona
| journal = MNRAS
| volume = 107
| issue = 2
| pages = 211–219
| year = 1947
|bibcode = 1947MNRAS.107..211A
| doi=10.1093/mnras/107.2.211| doi-access = free
}}</ref> Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to [[ultra low frequency]] [[radio waves]] that have been modified by interaction with [[matter]] in the plasma. Both types of waves can be launched by the turbulence of [[Granule (solar physics)|granulation]] and [[super granulation]] at the solar photosphere, and both types of waves can carry energy for some distance through the solar atmosphere before turning into [[shock waves]] that dissipate their energy as heat.
One problem with wave heating is delivery of the heat to the appropriate place. Magneto-acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona, both because of the low pressure present in the chromosphere and because they tend to be [[reflection (physics)|reflected]] back to the photosphere. Alfvén waves can carry enough energy, but do not dissipate that energy rapidly enough once they enter the corona. Waves in plasmas are notoriously difficult to understand and describe analytically, but computer simulations, carried out by Thomas Bogdan and colleagues in 2003, seem to show that Alfvén waves can transmute into other wave modes at the base of the corona, providing a pathway that can carry large amounts of energy from the photosphere through the chromosphere and transition region and finally into the corona where it dissipates it as heat.
Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the [[Solar and Heliospheric Observatory]] space-borne solar observatory, the first platform capable of observing the Sun in the [[extreme ultraviolet]] (EUV) for long periods of time with stable [[Photometry (astronomy)|photometry]]. Those were magneto-acoustic waves with a frequency of about 1 [[hertz|millihertz]] (mHz, corresponding to a {{gaps|1|000|second}} wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.
It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the [[TRACE]] spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as {{gaps|100|mHz}} (10 second period). Measurements of the temperature of different [[ions]] in the solar wind with the UVCS instrument aboard [[Solar and Heliospheric Observatory|SOHO]] give strong indirect evidence that there are waves at frequencies as high as {{gaps|200|Hz}}, well into the range of human hearing. These waves are very difficult to detect under normal circumstances, but evidence collected during solar eclipses by teams from [[Williams College]] suggest the presences of such waves in the 1–{{gaps|10|Hz}} range.
Recently, Alfvénic motions have been found in the lower solar atmosphere<ref>{{cite web |url=http://www.science20.com/news_releases/alfven_waves_our_sun_doing_magnetic_twist |title=Alfven Waves – Our Sun Is Doing The Magnetic Twist |publisher=read on Jan 6 2011 |url-status=live |archive-url=https://web.archive.org/web/20110723053337/http://www.science20.com/news_releases/alfven_waves_our_sun_doing_magnetic_twist |archive-date=2011-07-23 }}</ref><ref>{{cite journal | doi = 10.1126/science.1168680 | last1 = Jess | first1 = D. B. | last2 = Mathioudakis | first2 = M. | last3 = Erdélyi | first3 = R. | last4 = Crockett | first4 = P. J. | last5 = Keenan | first5 = F. P. | last6 = Christian | first6 = D. J. | title = Alfvén Waves in the Lower Solar Atmosphere | journal = Science | volume = 323| issue =  5921 | pages = 1582–1585 | year = 2009 | pmid = 19299614|bibcode = 2009Sci...323.1582J |arxiv = 0903.3546 | hdl = 10211.3/172550 | s2cid = 14522616 }}</ref> and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the [[Solar Dynamics Observatory]].<ref>{{cite journal|last1=McIntosh |first1= S. W. |last2=de Pontieu |first2= B. |last3=Carlsson |first3= M. |last4=Hansteen |first4= V. H. |author5=The Sdo |author6=Aia Mission Team | title = Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and Active Region Corona
| journal = American Geophysical Union
| volume = abstract #SH14A-01
| date = Fall 2010
|pages=SH14A–01 |bibcode=2010AGUFMSH14A..01M }}</ref>
These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the [[Hinode (satellite)|Hinode]] spacecraft.<ref>{{cite web
|url         = http://www.space.com/scienceastronomy/080122-st-sunshine-hinode.html
|title       = Sun's Magnetic Secret Revealed
|website = [[Space.com]]
|date = 22 January 2008
|access-date = January 6, 2011<!--assume this is what's meant by "read on..."-->
|url-status     = live
|archive-url  = https://web.archive.org/web/20101224061437/http://www.space.com/scienceastronomy/080122-st-sunshine-hinode.html
|archive-date = 2010-12-24
}}</ref>
Solar wind observations with the [[Wind (spacecraft)|''Wind'']] spacecraft have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.<ref>{{cite journal|last=Kasper|first=J.C.|title=Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfven-Cyclotron Dissipation|journal=Physical Review Letters |date=December 2008|volume=101|pmid=19113766|issue=26|page=261103|doi=10.1103/PhysRevLett.101.261103|bibcode=2008PhRvL.101z1103K|display-authors=etal}}</ref>
===Magnetic reconnection theory===
{{main|Magnetic reconnection}}
[[File:Arcing Active Region.jpg|thumb|Arcing active region by [[Solar Dynamics Observatory]]]]
The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona.<ref>{{cite book
| last = Priest
| first = Eric
| title = Solar Magneto-hydrodynamics
| publisher = D.Reidel |location=Dordrecht, Holland
| year = 1982
| isbn = 978-90-277-1833-4
}}</ref> The currents then collapse suddenly, releasing energy as heat and wave energy in the corona. This process is called "reconnection" because of the peculiar way that magnetic fields behave in plasma (or any electrically conductive fluid such as [[Mercury (element)|mercury]] or [[seawater]]). In a plasma, [[magnetic field lines]] are normally tied to individual pieces of matter, so that the [[topology]] of the magnetic field remains the same: if a particular north and south [[Poles of astronomical bodies#Magnetic poles|magnetic pole]] are connected by a single field line, then even if the plasma is stirred or if the magnets are moved around, that field line will continue to connect those particular poles. The connection is maintained by electric currents that are induced in the plasma. Under certain conditions, the electric currents can collapse, allowing the magnetic field to "reconnect" to other magnetic poles and release heat and wave energy in the process.
Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in the Solar System. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–{{gaps|1|000|km}} across. These small magnetic poles are buffeted and churned by the constant granulation. The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this "magnetic carpet", so the energy released by the reconnection is a natural candidate for the coronal heat, perhaps as a series of "microflares" that individually provide very little energy but together account for the required energy.
The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial. In particular, [[ultraviolet]] telescopes such as [[TRACE]] and [[Solar and Heliospheric Observatory|SOHO]]/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light,<ref>{{cite journal
| doi = 10.1051/0004-6361:20020151
| last1 = Patsourakos
| first1 = S.
| last2 = Vial
| first2 = J.-C.
| title = Intermittent behavior in the transition region and the low corona of the quiet Sun
| journal = Astronomy and Astrophysics
| volume = 385
| issue = 3
| pages = 1073–1077
| year = 2002
| bibcode=2002A&A...385.1073P
| doi-access = free
}}</ref> but there seem to be too few of these small events to account for the energy released into the corona. The additional energy not accounted for could be made up by wave energy, or by gradual magnetic reconnection that releases energy more smoothly than micro-flares and therefore does not appear well in the TRACE data. Variations on the micro-flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.
===Spicules (type II)===
For decades, researchers believed spicules could send heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so the theory was discounted.
As per studies performed in 2010 at the ''National Center for Atmospheric Research'' in [[Colorado]], in collaboration with the ''Lockheed Martin's Solar and Astrophysics Laboratory'' (LMSAL) and the ''Institute of Theoretical Astrophysics'' of the [[University of Oslo]], a new class of spicules (TYPE II) discovered in 2007, which travel faster (up to 100&nbsp;km/s) and have shorter lifespans, can account for the problem.<ref>{{cite web |url=http://www.rediff.com/news/slide-show/slide-show-1-mystery-of-suns-hot-outer-atmosphere-solved/20110107.htm |title=Mystery of Sun's hot outer atmosphere 'solved' |work=Rediff |date=2011-01-07 |access-date=2012-05-21 |url-status=live |archive-url=https://web.archive.org/web/20120415111049/http://www.rediff.com/news/slide-show/slide-show-1-mystery-of-suns-hot-outer-atmosphere-solved/20110107.htm |archive-date=2012-04-15 }}</ref> These jets insert heated plasma into the Sun's outer atmosphere.
Thus, a much greater understanding of the corona and improvement in the knowledge of the Sun's subtle influence on the Earth's upper atmosphere can be expected henceforth. The Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite which was used to test this hypothesis. The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply.
These observations reveal a one-to-one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona.<ref>{{cite journal | doi = 10.1126/science.1197738 | last1 = De Pontieu | first1 = B. | last2 = McIntosh | first2 = S. W. | last3 = Carlsson | first3 = M. | last4 = Hansteen | first4 = V. H. | last5 = Tarbell | first5 = T.D. | last6 = Boerner | first6 = P. | last7 = Martinez-Sykora | first7 = J. | last8 = Schrijver | first8 = C. J. | last9 = Title | first9 = A. M. | title = The Origins of Hot Plasma in the Solar Corona | journal = Science | volume = 331| issue =  6013  | pages = 55–58| year = 2011 | pmid = 21212351|bibcode = 2011Sci...331...55D | s2cid = 42068767 }}</ref>
==See also==
{{col div|colwidth=20em}}
* [[Advanced Composition Explorer]]
* [[Geocorona]]
*[[Supernova]]
* [[Supra-arcade downflows]]
* [[X-ray astronomy]]
{{colend}}
==References==
{{Reflist|30em}}
==External links==
{{commons category|Solar corona}}
* [http://solarscience.msfc.nasa.gov/corona.shtml NASA description of the solar corona] {{Webarchive|url=https://web.archive.org/web/20100223073835/http://solarscience.msfc.nasa.gov/corona.shtml |date=2010-02-23 }}
* [http://www.innovations-report.com/html/reports/physics_astronomy/report-33153.html Coronal heating problem at Innovation Reports]
* [http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/corona.html NASA/GSFC description of the coronal heating problem]
* [http://solar-center.stanford.edu/FAQ/Qcorona.html FAQ about coronal heating]
* [http://sohowww.nascom.nasa.gov Solar and Heliospheric Observatory, including near-real-time images of the solar corona]
* [http://xrt.cfa.harvard.edu/ Coronal x-ray images from the Hinode XRT]
* [http://antwrp.gsfc.nasa.gov/apod/ap090726.html nasa.gov Astronomy Picture of the Day July 26,  2009]&nbsp;– a combination of thirty-three photographs of the Sun's corona that were digitally processed to highlight faint features of a total eclipse that occurred in March 2006
* [http://alienworlds.southwales.ac.uk/sunStructure.html#/corona Animated explanation of the core of the Sun] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/corona |date=2015-11-16 }} (University of South Wales)
* [https://web.archive.org/web/20110710155828/http://blogs.physicstoday.org/update/2009/03/alfven-waves-may-heat-the-suns.html Alfvén waves may heat the Sun's corona]
* [https://www.youtube.com/watch?v=o-v9kLLiK4s Solar Interface Region&nbsp;– Bart de Pontieu (SETI Talks) Video]
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