Editing Magnetic reconnection (section)

{{Short description|Process in plasma physics}}
[[Image:reconnection.gif|right|thumb|380px|Magnetic reconnection: This view is a cross-section through four magnetic domains undergoing separator Parker-Sweet reconnection. Two separatrices (see text) divide space into four magnetic domains with a separator at the center of the figure. Field lines (and associated plasma) flow inward from above and below the separator, reconnect, and spring outward along the current sheet. In-situ spacecraft measurements in the magnetosphere <ref name=":1">{{Cite journal|last=Øieroset|first=M.|display-authors=et al.|date=2001-07-26|title=In situ detection of collisionless reconnection in the Earth's magnetotail|url=https://www.nature.com/articles/35086520| journal=Nature| volume=412| issue=6845|pages=414–417|doi=10.1038/35086520| pmid=11473310|bibcode=2001Natur.412..414O|s2cid=4412119|url-access=subscription}}</ref> and laboratory plasma experiments <ref name=":2">{{Cite journal |last1=Boozer |first1=Allen H. |date=2020-05-18 |title=Flattening of the tokamak current profile by a fast magnetic reconnection with implications for the solar corona |url=https://aip.scitation.org/doi/abs/10.1063/5.0014107 |journal=Physics of Plasmas |volume=27 |issue=10 |page=102305 | doi=10.1063/5.0014107 |arxiv=2005.02285 |bibcode=2020PhPl...27j2305B |s2cid=218502561}}</ref> mean that this process is increasingly well understood: once started, it proceeds many orders of magnitude faster than predicted by the Parker-Sweet theory.]]

[[File:Quad-flare.ogv|thumb|380px|The evolution of magnetic reconnection during a [[solar flare]].<ref>{{Cite journal|doi = 10.3847/2041-8205/821/2/L29|title = Observation of the Evolution of a Current Sheet in a Solar Flare|year = 2016|last1 = Zhu|first1 = Chunming|last2 = Liu|first2 = Rui|last3 = Alexander|first3 = David|last4 = McAteer|first4 = R. T. James | journal = The Astrophysical Journal|volume = 821|issue = 2|pages = L29|arxiv = 1603.07062|bibcode = 2016ApJ...821L..29Z | s2cid = 119188103 | doi-access=free }}</ref>]]

'''Magnetic reconnection''' is a physical process occurring in electrically conducting [[Plasma (physics)|plasmas]], in which the [[magnetic topology]] is rearranged and magnetic energy is converted to [[kinetic energy]], [[thermal energy]], and [[particle acceleration]].<ref>{{Cite journal |last1=Mozer |first1=Forrest S. |last2=Pritchett |first2=Philip L. |date=2010-06-01 |title=Magnetic field reconnection: A first-principles perspective |url=https://pubs.aip.org/physicstoday/article/63/6/34/598740/Magnetic-field-reconnection-A-first-principles |journal=Physics Today |language=en |volume=63 |issue=6 |pages=34–39 |doi=10.1063/1.3455250 |bibcode=2010PhT....63f..34M |issn=0031-9228|url-access=subscription }}</ref> Magnetic reconnection involves plasma flows at a substantial fraction of the [[Alfvén wave]] speed, which is the fundamental speed for mechanical information flow in a magnetized plasma.

The concept of magnetic reconnection was developed in parallel by researchers working in solar physics and in the interaction between the solar wind and magnetized planets. This reflects the bidirectional nature of reconnection, which can either disconnect formerly connected magnetic fields or connect formerly disconnected magnetic fields, depending on the circumstances.

[[Ron Giovanelli]] is credited with the first publication invoking magnetic energy release as a potential mechanism for [[particle acceleration]] in [[Solar flare|solar flares]].<ref>{{Cite journal |last=Giovanelli |first=R. G. |date=July 1946 |title=A Theory of Chromospheric Flares |url=https://www.nature.com/articles/158081a0 |journal=Nature |language=en |volume=158 |issue=4003 |pages=81–82 |doi=10.1038/158081a0 |bibcode=1946Natur.158...81G |s2cid=4139384 |issn=0028-0836|url-access=subscription }}</ref> Giovanelli proposed in 1946 that solar flares stem from the energy obtained by charged particles influenced by induced [[Electric field|electric fields]] within close proximity of [[Sunspot|sunspots]].<ref>Giovanelli, R.G. (1947). "Magnetic and Electric Phenomena in the Sun's Atmosphere associated with Sunspots". ''Monthly Notices of the Royal Astronomical Society''. '''107''' (4): 338–355. </ref> In the years 1947-1948, he published more papers further developing the reconnection model of solar flares.<ref>{{Cite journal |last=Giovanelli |first=R. G. |date=1947-11-01 |title=Magnetic and Electric Phenomena in the Sun's Atmosphere associated with Sunspots |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=107 |issue=4 |pages=338–355 |doi=10.1093/mnras/107.4.338 |issn=0035-8711|doi-access=free }}</ref> In these works, he proposed that the mechanism occurs at points of neutrality (weak or null magnetic field) within structured magnetic fields.

[[James Dungey]] is credited with first use of the term “magnetic reconnection” in his 1950 PhD thesis, to explain the coupling of mass, energy and momentum from the [[solar wind]] into Earth's [[magnetosphere]]. The concept was published for the first time in a seminal paper in 1961.<ref>{{Cite journal |last=Dungey |first=J. W. |date=1961-01-15 |title=Interplanetary Magnetic Field and the Auroral Zones |url=https://link.aps.org/doi/10.1103/PhysRevLett.6.47 |journal=Physical Review Letters |language=en |volume=6 |issue=2 |pages=47–48 |doi=10.1103/PhysRevLett.6.47 |bibcode=1961PhRvL...6...47D |issn=0031-9007|url-access=subscription }}</ref> [[James Dungey|Dungey]] coined the term "reconnection" because he envisaged field lines and plasma moving together in an inflow toward a magnetic neutral point (2D) or line (3D), breaking apart and then rejoining again but with different magnetic field lines and plasma, in an outflow away from the magnetic neutral point or line. 

In the meantime, the first theoretical framework of magnetic reconnection was established by [[Peter Alan Sweet|Peter Sweet]] and [[Eugene Parker]] at a conference in 1956. Sweet pointed out that by pushing two plasmas with oppositely directed magnetic fields together, resistive diffusion is able to occur on a length scale much shorter than a typical equilibrium length scale.<ref>Sweet, P. A., The Neutral Point Theory of Solar Flares, in IAU Symposium 6, Electromagnetic Phenomena in Cosmical Physics, ed. B. Lehnert (Dordrecht: Kluwer), 123, 1958</ref> Parker was in attendance at this conference and developed scaling relations for this model during his return travel.<ref>{{Cite journal |last=Parker |first=E. N. |date=December 1957 |title=Sweet's mechanism for merging magnetic fields in conducting fluids |url=http://doi.wiley.com/10.1029/JZ062i004p00509 |journal=Journal of Geophysical Research |language=en |volume=62 |issue=4 |pages=509–520 |doi=10.1029/JZ062i004p00509|bibcode=1957JGR....62..509P |url-access=subscription }}</ref>