Editing Coronal mass ejection (section)

===Early evolution===
{{heliophysics}}
The early evolution of a CME involves its initiation from a pre-eruption structure in the corona and the acceleration that follows. The processes involved in the early evolution of CMEs are poorly understood due to a lack of observational evidence.

====Initiation====
CME initiation occurs when a pre-eruption structure in an equilibrium state enters a nonequilibrium or [[metastable]] state where energy can be released to drive an eruption. The specific processes involved in CME initiation are debated, and various models have been proposed to explain this phenomenon based on physical speculation. Furthermore, different CMEs may be initiated by different processes.<ref name="howard11" />{{rp|175}}<ref name="vial15" />{{rp|303}}

It is unknown whether a magnetic flux rope exists prior to initiation, in which case either [[Magnetohydrodynamics#Ideal MHD|ideal]] or non-ideal magnetohydrodynamic (MHD) processes drive the expulsion of this flux rope, or whether a flux rope is created during the eruption by non-ideal process.<ref>{{cite journal |last1=Chen |first1=Bin |last2=Bastian |first2=T. S. |last3=Gary |first3=D. E. |title=Direct Evidence of an Eruptive, Filament-Hosting Magnetic Flux Rope Leading to a Fast Solar Coronal Mass Ejection |journal=The Astrophysical Journal |date=6 October 2014 |volume=794 |issue=2 |page=149 |doi=10.1088/0004-637X/794/2/149 |arxiv=1408.6473 |bibcode=2014ApJ...794..149C |s2cid=119207956 |url=https://iopscience.iop.org/article/10.1088/0004-637X/794/2/149/meta}}</ref><ref name="aschwanden19" />{{rp|555}} Under ideal MHD, initiation may involve ideal instabilities or [[Catastrophe theory|catastrophic]] loss of equilibrium along an existing flux rope:<ref name="chen11">{{cite journal |last1=Chen |first1=P. F. |title=Coronal Mass Ejections: Models and Their Observational Basis |journal=Living Reviews in Solar Physics |date=2011 |volume=8 |issue=1 |page=1 |doi=10.12942/lrsp-2011-1 |bibcode=2011LRSP....8....1C |s2cid=119386112 |doi-access=free }}</ref>
* The '''kink instability''' occurs when a magnetic flux rope is twisted to a critical point, whereupon the flux rope is unstable to further twisting.
* The '''torus instability''' occurs when the magnetic field strength of an arcade overlying a flux rope decreases rapidly with height. When this decrease is sufficiently rapid, the flux rope is unstable to further expansion.<ref>{{cite journal |last1=Titov |first1=V. S. |last2=Démoulin |first2=P. |title=Basic topology of twisted magnetic configurations in solar flares |journal=Astronomy and Astrophysics |date=October 1999 |volume=351 |issue=2 |pages=707–720 |bibcode=1999A&A...351..707T |url=https://www.researchgate.net/publication/234530273}}</ref>
* The '''catastrophe model'''  involves a catastrophic loss of equilibrium.
Under non-ideal MHD, initiations mechanisms may involve resistive instabilities or [[magnetic reconnection]]:
* '''Tether-cutting''', or '''flux cancellation''', occurs in strongly sheared arcades when nearly antiparallel field lines on opposite sides of the arcade form a current sheet and reconnect with each other. This can form a helical flux rope or cause a flux rope already present to grow and its axis to rise.
* The '''magnetic breakout model''' consists of an initial quadrupolar [[magnetic topology]] with a null point above a central flux system. As shearing motions cause this central flux system to rise, the null point forms a current sheet and the core flux system reconnects with the overlying magnetic field.<ref name="aschwanden19">{{cite book |last1=Aschwanden |first1=Markus J. |title=New Millennium Solar Physics |series=Astrophysics and Space Science Library |date=2019 |volume=458 |location=Cham, Switzerland | publisher=Springer International Publishing |doi=10.1007/978-3-030-13956-8 |isbn=978-3-030-13956-8 |s2cid=181739975 }}</ref>

[[File:Close-up on launching filament (SDO-AIA, 304 Å).ogv|thumb|Video of a [[solar prominence|solar filament]] being launched]]

====Initial acceleration====
Following initiation, CMEs are subject to different forces that either assist or inhibit their rise through the lower corona. Downward [[magnetic tension]] force exerted by the strapping magnetic field as it is stretched and, to a lesser extent, the gravitational pull of the Sun oppose movement of the core CME structure. In order for sufficient acceleration to be provided, past models have involved magnetic reconnection below the core field or an ideal MHD process, such as instability or acceleration from the solar wind.

In the majority of CME events, acceleration is provided by magnetic reconnection cutting the strapping field's connections to the photosphere from below the core and outflow from this reconnection pushing the core upward. When the initial rise occurs, the opposite sides of the strapping field below the rising core are oriented nearly [[Euclidean vector#Opposite, parallel, and antiparallel vectors|antiparallel]] to one another and are brought together to form a [[current sheet]] above the PIL. Fast magnetic reconnection can be excited along the current sheet by microscopic instabilities, resulting in the rapid release of stored magnetic energy as kinetic, thermal, and nonthermal energy. The restructuring of the magnetic field cuts the strapping field's connections to the photosphere thereby decreasing the downward magnetic tension force while the upward reconnection outflow pushes the CME structure upwards. A [[positive feedback loop]] results as the core is pushed upwards and the sides of the strapping field are brought in closer and closer contact to produce additional magnetic reconnection and rise. While upward reconnection outflow accelerates the core, simultaneous downward outflow is sometimes responsible for other phenomena associated with CMEs (see {{slink||Coronal signatures}}).

In cases where significant magnetic reconnection does not occur, ideal MHD instabilities or the dragging force from the solar wind can theoretically accelerate a CME. However, if sufficient acceleration is not provided, the CME structure may fall back in what is referred to as a ''failed'' or ''confined eruption''.<ref name=aschwanden19 /><ref name=chen11 />

====Coronal signatures====
{{Expand section|with=information about EUV waves and other coronal signatures|small=no|date=April 2023}}
The early evolution of CMEs is frequently associated with other [[solar phenomena]] observed in the low corona, such as eruptive prominences and solar flares. CMEs that have no observed signatures are sometimes referred to as ''stealth CMEs''.<ref>{{cite journal |last1=Nitta |first1=Nariaki V. |last2=Mulligan |first2=Tamitha |last3=Kilpua |first3=Emilia K. J. |last4=Lynch |first4=Benjamin J. |last5=Mierla |first5=Marilena |last6=O'Kane |first6=Jennifer |last7=Pagano |first7=Paolo |last8=Palmerio |first8=Erika |last9=Pomoell |first9=Jens |last10=Richardson |first10=Ian G. |last11=Rodriguez |first11=Luciano |last12=Rouillard |first12=Alexis P. |last13=Sinha |first13=Suvadip |last14=Srivastava |first14=Nandita |last15=Talpeanu |first15=Dana-Camelia |last16=Yardley |first16=Stephanie L. |last17=Zhukov |first17=Andrei N. |title=Understanding the Origins of Problem Geomagnetic Storms Associated with 'Stealth' Coronal Mass Ejections |journal=Space Science Reviews |date=December 2021 |volume=217 |issue=8 |page=82 |doi=10.1007/s11214-021-00857-0 |pmid=34789949 |pmc=8566663 |arxiv=2110.08408 |bibcode=2021SSRv..217...82N }}</ref><ref>{{cite journal |last1=Howard |first1=Timothy A. |last2=Harrison |first2=Richard A. |title=Stealth Coronal Mass Ejections: A Perspective |journal=Solar Physics |date=July 2013 |volume=285 |issue=1–2 |pages=269–280 |doi=10.1007/s11207-012-0217-0 |bibcode=2013SoPh..285..269H |s2cid=255067586 |url=https://link.springer.com/article/10.1007/s11207-012-0217-0|url-access=subscription }}</ref>

Prominences embedded in some CME pre-eruption structures may erupt with the CME as eruptive prominences. Eruptive prominences are associated with at least 70% of all CMEs<ref>{{cite journal |last1=Gopalswamy |first1=N. |last2=Shimojo |first2=M. |last3=Lu |first3=W. |last4=Yashiro |first4=S. |last5=Shibasaki |first5=K. |last6=Howard |first6=R. A. |title=Prominence Eruptions and Coronal Mass Ejection: A Statistical Study Using Microwave Observations |journal=The Astrophysical Journal |date=20 March 2003 |volume=586 |issue=1 |pages=562–578 |doi=10.1086/367614|bibcode=2003ApJ...586..562G |s2cid=119654267 |doi-access=free }}</ref> and are often embedded within the bases of CME flux ropes. When observed in white-light coronagraphs, the eruptive prominence material, if present, corresponds to the observed bright core of dense material.<ref name="vial15">{{cite book |editor1-last=Vial |editor1-first=Jean-Claude |editor2-last=Engvold |editor2-first=Oddbjørn |title=Solar Prominences |series=Astrophysics and Space Science Library |date=2015 |volume=415 |doi=10.1007/978-3-319-10416-4 |isbn=978-3-319-10416-4 |s2cid=241566003 |url=https://link.springer.com/book/10.1007/978-3-319-10416-4}}</ref>

When magnetic reconnection is excited along a current sheet of a rising CME core structure, the downward reconnection outflows can collide with loops below to form a cusp-shaped, two-ribbon solar flare.

CME eruptions can also produce EUV waves, also known as ''EIT waves'' after the [[Extreme ultraviolet Imaging Telescope]] or as ''[[Moreton wave]]s'' when observed in the chromosphere, which are fast-mode MHD wave fronts that emanate from the site of the CME.<ref name="howard11" /><ref name="chen11" />

A coronal dimming is a localized decrease in [[extreme ultraviolet]] and [[soft X-ray]] emissions in the lower corona. When associated with a CME, coronal dimmings are thought to occur predominantly due to a decrease in plasma density caused by mass outflows during the expansion of the associated CME. They often occur either in pairs located within regions of opposite magnetic polarity, a core dimming, or in a more widespread area, a secondary dimming. Core dimmings are interpreted as the footpoint locations of the erupting flux rope; secondary dimmings are interpreted as the result of the expansion of the overall CME structure and are generally more diffuse and shallow.<ref>{{cite journal |last1=Cheng |first1=J. X. |last2=Qiu |first2=J. |title=The Nature of CME-Flare-Associated Coronal Dimming |journal=The Astrophysical Journal |date=2016 |volume=825 |issue=1 |page=37 |doi=10.3847/0004-637X/825/1/37 |arxiv=1604.05443 |bibcode=2016ApJ...825...37C |s2cid=119240929 |doi-access=free }}</ref> Coronal dimming was first reported in 1974,<ref>{{cite journal |last1=Hansen |first1=Richard T. |last2=Garcia |first2=Charles J. |last3=Hansen |first3=Shirley F. |last4=Yasukawa |first4=Eric |title=Abrupt Depletions of the Inner Corona |journal=Publications of the Astronomical Society of the Pacific |date=April 1974 |volume=86 |issue=512 |page=300 |doi=10.1086/129638 |bibcode=1974PASP...86..500H |s2cid=123151593 |doi-access=free }}</ref> and, due to their appearance resembling that of [[coronal hole]]s, they were sometimes referred to as ''transient coronal holes''.<ref>{{cite journal |last1=Vanninathan |first1=Kamalam |last2=Veronig |first2=Astrid M. |last3=Dissauer |first3=Karin |last4=Temmer |first4=Manuela |author-link4=Manuela Temmer |date=2018 |title=Plasma Diagnostics of Coronal Dimming Events |journal=The Astrophysical Journal |volume=857 |issue=1 |page=62 |arxiv=1802.06152 |bibcode=2018ApJ...857...62V |doi=10.3847/1538-4357/aab09a |s2cid=118864203 |doi-access=free }}</ref>