Editing Stellar corona (section)

===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.