Editing Stellar corona (section)

== 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,<ref>{{cite web |title=Corona |url=https://astronomy.swin.edu.au/cosmos/C/Corona |website=COSMOS - The SAO Encyclopedia of Astronomy |publisher=Swinburne University of Technology |access-date=30 May 2025}}</ref> and produces about one-millionth as much visible light.<ref>{{cite book |last1=Ambastha |first1=Ashok |title=Physics of the invisible Sun: instrumentation, observations, and inferences |date=2020 |publisher=CRC Press |location=Boca Raton, FL |isbn=9781138197442 |page=48 |url=https://doi.org/10.1201/9781003005674 |access-date=30 May 2025}}</ref> 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 of 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>