Editing Stellar corona (section)

====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.