Editing Magnetohydrodynamics (section)

=== Astrophysics ===
MHD applies to [[astrophysics]], including stars, the [[interplanetary medium]] (space between the planets), and possibly within the [[interstellar medium]] (space between the stars) and [[Relativistic jet|jets]].<ref>{{cite book|last1=Kennel|first1=C.F. |last2=Arons|first2=J.|last3=Blandford|first3=R.|last4=Coroniti|first4=F.|last5=Israel |first5=M.|last6=Lanzerotti|first6=L.|last7=Lightman|first7=A.|title=Unstable Current Systems and Plasma Instabilities in Astrophysics |chapter=Perspectives on Space and Astrophysical Plasma Physics |chapter-url=https://authors.library.caltech.edu/96134/2/1985IAUS__107__537K.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://authors.library.caltech.edu/96134/2/1985IAUS__107__537K.pdf |archive-date=2022-10-09 |url-status=live|date=1985|volume=107 |doi=10.1007/978-94-009-6520-1_63|pages=537–552 |access-date=2019-07-22|isbn=978-90-277-1887-7 |bibcode=1985IAUS..107..537K |s2cid=117512943 }}</ref>  Most astrophysical systems are not in local thermal equilibrium, and therefore require an additional kinematic treatment to describe all the phenomena within the system (see [[Astrophysical plasma]]).<ref>{{cite journal | url=https://link.springer.com/article/10.1007/s41114-021-00031-6 | doi=10.1007/s41114-021-00031-6 | title=Relativistic fluid dynamics: Physics for many different scales | year=2021 | last1=Andersson | first1=Nils | last2=Comer | first2=Gregory L. | journal=Living Reviews in Relativity | volume=24 | issue=1 | page=3 | arxiv=2008.12069 | bibcode=2021LRR....24....3A | s2cid=235631174 }}</ref><ref>{{cite web |url=https://www.astro.princeton.edu/~kunz/Site/AST521/AST521_lecture_notes_Kunz.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.astro.princeton.edu/~kunz/Site/AST521/AST521_lecture_notes_Kunz.pdf |archive-date=2022-10-09 |url-status=live |title=Lecture Notes on Introduction to Plasma Astrophysics (Draft) |first=Matthew W. |last=Kunz |date=9 November 2020 |website=astro.princeton.edu}}</ref>

[[Sunspot]]s are caused by the Sun's magnetic fields, as [[Joseph Larmor]] theorized in 1919. The [[solar wind]] is also governed by MHD. The differential [[solar rotation]] may be the long-term effect of magnetic drag at the poles of the Sun, an MHD phenomenon due to the [[Parker spiral]] shape assumed by the extended magnetic field of the Sun.

Previously, theories describing the formation of the Sun and planets could not explain how the Sun has 99.87% of the mass, yet only 0.54% of the [[angular momentum]] in the [[Solar System]]. In a [[closed system]] such as the cloud of gas and dust from which the Sun was formed, mass and angular momentum are both [[Conservation law|conserved]]. That conservation would imply that as the mass concentrated in the center of the cloud to form the Sun, it would spin faster, much like a skater pulling their arms in. The high speed of rotation predicted by early theories would have flung the [[Protostar|proto-Sun]] apart before it could have formed. However, magnetohydrodynamic effects transfer the Sun's angular momentum into the outer solar system, slowing its rotation.

Breakdown of ideal MHD (in the form of magnetic reconnection) is known to be the likely cause of [[solar flare]]s. The magnetic field in a solar [[active region]] over a sunspot can store energy that is released suddenly as a burst of motion, [[X-ray]]s, and [[radiation]] when the main current sheet collapses, reconnecting the field.<ref>{{cite web | url=https://link.springer.com/collections/djhjfaffha | title=Solar Activity }}</ref><ref>{{cite journal | doi=10.12942/lrsp-2011-6 | title=Solar Flares: Magnetohydrodynamic Processes | year=2011 | last1=Shibata | first1=Kazunari | last2=Magara | first2=Tetsuya | journal=Living Reviews in Solar Physics | volume=8 | issue=1 | page=6 | doi-access=free | bibcode=2011LRSP....8....6S | s2cid=122217405 | hdl=2433/153022 | hdl-access=free }}</ref>