Editing Stellar corona (section)

===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>