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Coronal mass ejection
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==Physical description== CMEs release large quantities of matter from the Sun's atmosphere into the [[solar wind]] and [[interplanetary space]]. The ejected matter is a [[Plasma (physics)|plasma]] consisting primarily of [[electron]]s and [[proton]]s embedded within its magnetic field. This magnetic field is commonly in the form of a flux rope, a [[Helix|helical]] magnetic field with changing [[Pitch angle (particle motion)|pitch angle]]s. The average mass ejected is {{convert|1.6e12|kg|lb|abbr=on}}. However, the estimated mass values for CMEs are only lower limits, because coronagraph measurements provide only two-dimensional data. CMEs erupt from strongly twisted or sheared, large-scale magnetic field structures in the corona that are kept in equilibrium by overlying magnetic fields. ===Origin=== {{Expand section|with=information about precursors and observations thereof|small=no|date=April 2023}} [[File:Sunspot diagram.svg|left|thumb|Simplified model of magnetic fields emerging from the photosphere]] CMEs erupt from the lower corona, where processes associated with the local magnetic field dominate over other processes. As a result, the coronal magnetic field plays an important role in the formation and eruption of CMEs. Pre-eruption structures originate from magnetic fields that are initially generated in the Sun's interior by the [[solar dynamo]]. These magnetic fields rise to the Sun's surface—the [[photosphere]]—where they may form localized areas of highly concentrated magnetic flux and expand into the lower solar atmosphere forming [[active region]]s. At the photosphere, active region magnetic flux is often distributed in a [[Magnetic dipole|dipole configuration]], that is, with two adjacent areas of opposite magnetic polarity across which the magnetic field arches. Over time, the concentrated magnetic flux cancels and disperses across the Sun's surface, merging with the remnants of past active regions to become a part of the [[Quiet-Sun region|quiet Sun]]. Pre-eruption CME structures can be present at different stages of the growth and decay of these regions, but they always lie above polarity inversion lines (PIL), or boundaries across which the sign of the vertical component of the magnetic field reverses. PILs may exist in, around, and between active regions or form in the quiet Sun between active region remnants. More complex magnetic flux configurations, such as quadrupolar fields, can also host pre-eruption structures.<ref>{{cite journal |last1=van Driel-Gesztelyi |first1=Lidia |last2=Green |first2=Lucie May |title=Evolution of Active Regions |journal=Living Reviews in Solar Physics |date=December 2015 |volume=12 |issue=1 |page=1 |doi=10.1007/lrsp-2015-1 |bibcode=2015LRSP...12....1V |s2cid=118831968 |doi-access=free }}</ref><ref>{{cite journal |last1=Martin |first1=Sara F. |title=Conditions for the Formation and Maintenance of Filaments – (Invited Review) |journal=Solar Physics |date=1998 |volume=182 |issue=1 |pages=107–137 |doi=10.1023/A:1005026814076 |bibcode=1998SoPh..182..107M |s2cid=118346113 |url=https://link.springer.com/article/10.1023/A:1005026814076|url-access=subscription }}</ref> In order for pre-eruption CME structures to develop, large amounts of energy must be stored and be readily available to be released. As a result of the dominance of magnetic field processes in the lower corona, the majority of the energy must be stored as [[magnetic energy]]. The magnetic energy that is freely available to be released from a pre-eruption structure, referred to as the ''magnetic free energy'' or ''nonpotential energy'' of the structure, is the excess magnetic energy stored by the structure's magnetic configuration relative to that stored by the lowest-energy magnetic configuration the underlying photospheric magnetic flux distribution could theoretically take, a [[Force-free magnetic field#Zero current density|potential field]] state. Emerging magnetic flux and photospheric motions continuously shifting the footpoints of a structure can result in magnetic free energy building up in the coronal magnetic field as twist or shear.<ref name="chen11" /> Some pre-eruption structures, referred to as {{wikt-lang|en|sigmoids}}, take on an ''S'' or reverse-''S'' shape as shear accumulates. This has been observed in active region [[coronal loop]]s and [[Solar prominence|filaments]] with forward-''S'' sigmoids more common in the southern hemisphere and reverse-''S'' sigmoids more common in the northern hemisphere.<ref>{{cite journal |last1=Rust |first1=D. M. |last2=Kumar |first2=A. |title=Evidence for Helically Kinked Magnetic Flux Ropes in Solar Eruptions |journal=The Astrophysical Journal |date=1996 |volume=464 |issue=2 |pages=L199–L202 |doi=10.1086/310118 |bibcode=1996ApJ...464L.199R |s2cid=122151729 |url=https://iopscience.iop.org/article/10.1086/310118/meta|url-access=subscription }}</ref><ref>{{cite journal |last1=Canfield |first1=Richard C. |last2=Hudson |first2=Hugh S. |last3=McKenzie |first3=David E. |title=Sigmoidal morphology and eruptive solar activity |journal=Geophysical Research Letters |date=1999 |volume=26 |issue=6 |pages=627–630 |doi=10.1029/1999GL900105 |bibcode=1999GeoRL..26..627C |s2cid=129937738 |doi-access=free }}</ref> Magnetic flux ropes—twisted and sheared [[magnetic flux tube]]s that can carry electric current and magnetic free energy—are an integral part of the post-eruption CME structure; however, whether flux ropes are always present in the pre-eruption structure or whether they are created during the eruption from a strongly sheared core field (see {{slink||Initiation}}) is subject to ongoing debate.<ref name="chen11" /><ref name="howard11">{{cite book |last1=Howard |first1=Timothy |title=Coronal Mass Ejections: An Introduction |series=Astrophysics and Space Science Library |date=2011 |volume=376 |publisher=Springer |location=New York |doi=10.1007/978-1-4419-8789-1 |isbn=978-1-4419-8789-1 }}</ref> Some pre-eruption structures have been observed to support [[Solar prominence|prominences]], also known as filaments, composed of much cooler material than the surrounding coronal plasma. Prominences are embedded in magnetic field structures referred to as prominence cavities, or filament channels, which may constitute part of a pre-eruption structure (see {{slink||Coronal signatures}}). ===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> ===Propagation=== {{Expand section|small=no|date=April 2023}} Observations of CMEs are typically through white-light [[coronagraph]]s which measure the [[Thomson scattering]] of sunlight off of free electrons within the CME plasma.<ref>{{cite journal |last1=Howard |first1=T. A. |last2=DeForest |first2=C. E. |title=The Thomson Surface. I. Reality and Myth |journal=The Astrophysical Journal |date=20 June 2012 |volume=752 |issue=2 |page=130 |doi=10.1088/0004-637X/752/2/130 |bibcode=2012ApJ...752..130H |s2cid=122654351 |url=https://www.boulder.swri.edu/~deforest/ewExternalFiles/The%20Astrophysical%20Journal%202012%20Howard.pdf |access-date=9 December 2021}}</ref> An observed CME may have any or all of three distinctive features: a bright core, a dark surrounding cavity, and a bright leading edge.<ref>{{cite journal |last1=Gopalswamy |first1=N. |title=Coronal mass ejections: Initiation and detection |journal=Advances in Space Research |date=January 2003 |volume=31 |issue=4 |pages=869–881 |doi=10.1016/S0273-1177(02)00888-8 |bibcode=2003AdSpR..31..869G |url=https://cdaw.gsfc.nasa.gov/publications/gopal/gopal2003AdvSpRes31_869.pdf |access-date=27 August 2021}}</ref> The bright core is usually interpreted as a prominence embedded in the CME (see {{slink||Origin}}) with the leading edge as an area of compressed plasma ahead of the CME flux rope. However, some CMEs exhibit more complex geometry.<ref name="vial15" /> From white-light coronagraph observations, CMEs have been measured to reach speeds in the plane-of-sky ranging from {{convert|20|to|3200|km/s|abbr=on|sigfig=2}} with an average speed of {{convert|489|km/s|abbr=on}}.<ref>{{cite journal |last1=Yashiro |first1=S. |last2=Gopalswamy |first2=N. |last3=Michalek |first3=G. |last4=Cyr |first4=O. C. St. |last5=Plunkett |first5=S. P. |last6=Rish |first6=N. B. |last7=Howard |first7=R. A. |title=A catalog of white light coronal mass ejections observed by the SOHO spacecraft |journal=Journal of Geophysical Research: Atmospheres |date=July 2004 |volume=109 |issue=A7 |doi=10.1029/2003JA010282 |bibcode=2004JGRA..109.7105Y |url=https://www.researchgate.net/publication/260073213 |access-date=16 February 2022|doi-access=free }}</ref> Observations of CME speeds indicate that CMEs tend to accelerate or decelerate until they reach the speed of the solar wind ({{slink||Interactions in the heliosphere}}). When observed in interplanetary space at distances greater than about {{convert|50|solar radius|AU}} away from the Sun, CMEs are sometimes referred to as ''interplanetary CMEs'', or ''ICMEs''.<ref name="howard11" />{{rp|4}} ====Interactions in the heliosphere==== As CMEs propagate through the heliosphere, they may interact with the surrounding solar wind, the interplanetary magnetic field, and other CMEs and celestial bodies. CMEs can experience aerodynamic drag forces that act to bring them to kinematic equilibrium with the solar wind. As a consequence, CMEs faster than the solar wind tend to slow down whereas CMEs slower than the solar wind tend to speed up until their speed matches that of the solar wind.<ref name="Manoharan2006">{{cite journal |title=Evolution of Coronal Mass Ejections in the Inner Heliosphere: A Study Using White-Light and Scintillation Images |journal=Solar Physics |first=P. K. |last=Manoharan |volume=235 |issue=1–2 |pages=345–368 |date=May 2006 |doi=10.1007/s11207-006-0100-y |bibcode=2006SoPh..235..345M|s2cid=122757011 }}</ref> How CMEs evolve as they propagate through the heliosphere is poorly understood. Models of their evolution have been proposed that are accurate to some CMEs but not others. Aerodynamic drag and snowplow models assume that ICME evolution is governed by its interactions with the solar wind. Aerodynamic drag alone may be able to account for the evolution of some ICMEs, but not all of them.<ref name=howard11 />{{rp|199}} [[File:Dynamic Earth-A New Beginning.webm|thumb|Follow a CME as it passes Venus then Earth, and explore how the Sun drives Earth's winds and oceans]] CMEs typically reach Earth one to five days after leaving the Sun. The strongest deceleration or acceleration occurs close to the Sun, but it can continue even beyond Earth orbit (1 [[Astronomical unit|AU]]), which was observed using measurements at [[Mars]]<ref>{{cite journal |title=Using Forbush Decreases to Derive the Transit Time of ICMEs Propagating from 1 AU to Mars |journal=Journal of Geophysical Research: Space Physics |last1=Freiherr von Forstner |first1=Johan L. |last2=Guo |first2=Jingnan |last3=Wimmer-Schweingruber |first3=Robert F. |last4=Hassler |first4=Donald M. |last5=Temmer |first5=Manuela |last6=Dumbović |first6=Mateja |last7=Jian |first7=Lan K. |last8=Appel |first8=Jan K. |last9=Čalogović |first9=Jaša |display-authors=3 |volume=123 |issue=1 |pages=39–56 |date=January 2018 |doi=10.1002/2017JA024700 |bibcode=2018JGRA..123...39F |arxiv=1712.07301|s2cid=119249104 }}</ref> and by the [[Ulysses (spacecraft)|''Ulysses'' spacecraft]].<ref>{{cite journal |title=Identification of Interplanetary Coronal Mass Ejections at Ulysses Using Multiple Solar Wind Signatures |journal=Solar Physics |first=I. G. |last=Richardson |volume=289 |issue=10 |pages=3843–3894 |date=October 2014 |doi=10.1007/s11207-014-0540-8 |bibcode=2014SoPh..289.3843R|s2cid=124355552 }}</ref> ICMEs faster than about {{convert|500|km/s|abbr=on}} eventually drive a [[shock wave]].<ref name="Wilkinson2012">{{cite book |title=New Eyes on the Sun: A Guide to Satellite Images and Amateur Observation |publisher=Springer |first=John |last=Wilkinson |page=98 |date=2012 |isbn=978-3-642-22838-4}}</ref> This happens when the speed of the ICME in the [[frame of reference]] moving with the solar wind is faster than the local fast [[Magnetosonic wave|magnetosonic]] speed. Such shocks have been observed directly by coronagraphs<ref>{{cite journal |title=Direct Detection of a Coronal Mass Ejection-Associated Shock in Large Angle and Spectrometric Coronagraph Experiment White-Light Images |journal=[[The Astrophysical Journal]] |first1=A. |last1=Vourlidas |first2=S. T. |last2=Wu |first3=A. H. |last3=Wang |first4=P. |last4=Subramanian |first5=R. A. |last5=Howard |volume=598 |issue=2 |pages=1392–1402 |date=December 2003 |doi=10.1086/379098 |bibcode=2003ApJ...598.1392V |arxiv=astro-ph/0308367|s2cid=122760120 }}</ref> in the corona, and are related to type II radio bursts. They are thought to form sometimes as low as {{solar radius|2}} ([[solar radius|solar radii]]). They are also closely linked with the acceleration of [[solar energetic particles]].<ref>{{cite journal |url=http://csem.engin.umich.edu/CSEM/Publications/ManchesterApJ_2005.pdf |title=Coronal Mass Ejection Shock and Sheath Structures Relevant to Particle Acceleration |journal=[[The Astrophysical Journal]] |first1=W. B. IV |last1=Manchester |first2=T. I. |last2=Gombosi |first3=D. L. |last3=De Zeeuw |first4=I. V. |last4=Sokolov |first5=I. I. |last5=Roussev |first6=K. G. |last6=Powell |first7=J. |last7=Kóta |first8=G. |last8=Tóth |first9=T. H. |last9=Zurbuchen |display-authors=5 |volume=622 |issue=2 |pages=1225–1239 |date=April 2005 |doi=10.1086/427768 |bibcode=2005ApJ...622.1225M |s2cid=67802388 |archive-url=https://web.archive.org/web/20070205194952/http://csem.engin.umich.edu/csem/publications/ManchesterApJ_2005.pdf |archive-date=5 February 2007}}</ref> As ICMEs propagate through the interplanetary medium, they may collide with other ICMEs in what is referred to as ''CME–CME interaction'' or ''CME cannibalism''.<ref name=aschwanden19 />{{rp|599}} During such CME-CME interactions, the first CME may clear the way for the second one<ref name="Liu, Y., Luhmann, J., Kajdič, P. et al.">{{cite journal |last1=Liu |first1=Ying D. |last2=Luhmann |first2=Janet G. |last3=Kajdič |first3=Primož |last4=Kilpua |first4=Emilia K. J. |last5=Lugaz |first5=Noé |last6=Nitta |first6=Nariaki V. |last7=Möstl |first7=Christian |last8=Lavraud |first8=Benoit |last9=Bale |first9=Stuart D. |last10=Farrugia |first10=Charles J. |last11=Galvin |first11=Antoinette B. |title=Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections |journal=Nature Communications |date=18 March 2014 |volume=5 |issue=1 |page=3481 |doi=10.1038/ncomms4481 |pmid=24642508 |arxiv=1405.6088 |bibcode=2014NatCo...5.3481L |s2cid=11999567 |language=en |issn=2041-1723}}</ref><ref>{{cite journal |last1=Temmer |first1=M. |last2=Nitta |first2=N. V. |title=Interplanetary Propagation Behavior of the Fast Coronal Mass Ejection on 23 July 2012 |journal=Solar Physics |date=1 March 2015 |volume=290 |issue=3 |pages=919–932 |doi=10.1007/s11207-014-0642-3 |arxiv=1411.6559 |bibcode=2015SoPh..290..919T |s2cid=255063438 |language=en |issn=1573-093X}}</ref><ref>{{cite journal |last1=Desai |first1=Ravindra T. |last2=Zhang |first2=Han |last3=Davies |first3=Emma E. |last4=Stawarz |first4=Julia E. |last5=Mico-Gomez |first5=Joan |last6=Iváñez-Ballesteros |first6=Pilar |title=Three-Dimensional Simulations of Solar Wind Preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection |journal=Solar Physics |date=29 September 2020 |volume=295 |issue=9 |page=130 |doi=10.1007/s11207-020-01700-5 |arxiv=2009.02392 |bibcode=2020SoPh..295..130D |s2cid=221516966 |language=en |issn=1573-093X}}</ref> and/or when two CMEs collide<ref>{{cite journal |last1=Shiota |first1=D. |last2=Kataoka |first2=R. |title=Magnetohydrodynamic simulation of interplanetary propagation of multiple coronal mass ejections with internal magnetic flux rope (Susanoo-CME)|journal=Space Weather |date=February 2016 |volume=14 |issue=2 |pages=56–75 |doi=10.1002/2015SW001308 |s2cid=124227937 |language=en|doi-access=free |bibcode=2016SpWea..14...56S }}</ref><ref>{{cite journal |last1=Scolini |first1=Camilla |last2=Chané |first2=Emmanuel |last3=Temmer |first3=Manuela |last4=Kilpua |first4=Emilia K. J. |last5=Dissauer |first5=Karin |last6=Veronig |first6=Astrid M. |last7=Palmerio |first7=Erika |last8=Pomoell |first8=Jens |last9=Dumbović |first9=Mateja |last10=Guo |first10=Jingnan |last11=Rodriguez |first11=Luciano |last12=Poedts |first12=Stefaan |title=CME–CME Interactions as Sources of CME Geoeffectiveness: The Formation of the Complex Ejecta and Intense Geomagnetic Storm in 2017 Early September |journal=The Astrophysical Journal Supplement Series |date=24 February 2020 |volume=247 |issue=1 |page=21 |doi=10.3847/1538-4365/ab6216|arxiv=1911.10817 |bibcode=2020ApJS..247...21S |s2cid=208268241 |doi-access=free }}</ref> it can lead to more severe impacts on Earth. Historical records show that the most extreme space weather events involved multiple successive CMEs. For example, the famous [[Carrington Event|Carrington event]] in 1859 had several eruptions and caused auroras to be visible at low latitudes for four nights.<ref>{{cite journal |last1=Tsurutani |first1=B. T. |title=The extreme magnetic storm of 1–2 September 1859 |journal=Journal of Geophysical Research |date=2003 |volume=108 |issue=A7 |page=1268 |doi=10.1029/2002JA009504 |bibcode=2003JGRA..108.1268T |language=en |issn=0148-0227|url=https://zenodo.org/record/1000695 }}</ref> Similarly, the [[solar storm of September 1770]] lasted for nearly nine days, and caused repeated low-latitude auroras.<ref>{{cite journal |last1=Hayakawa |first1=Hisashi |last2=Iwahashi |first2=Kiyomi |last3=Ebihara |first3=Yusuke |last4=Tamazawa |first4=Harufumi |last5=Shibata |first5=Kazunari |last6=Knipp |first6=Delores J. |last7=Kawamura |first7=Akito D. |last8=Hattori |first8=Kentaro |last9=Mase |first9=Kumiko |last10=Nakanishi |first10=Ichiro |last11=Isobe |first11=Hiroaki |title=Long-lasting Extreme Magnetic Storm Activities in 1770 Found in Historical Documents |journal=The Astrophysical Journal |date=29 November 2017 |volume=850 |issue=2 |pages=L31 |doi=10.3847/2041-8213/aa9661|arxiv=1711.00690 |bibcode=2017ApJ...850L..31H |s2cid=119098402 |doi-access=free }}</ref> The interaction between two moderate CMEs between the Sun and Earth can create extreme conditions on Earth. Recent studies have shown that the magnetic structure in particular its [[chirality]]/handedness, of a CME can greatly affect how it interacts with Earth's magnetic field. This interaction can result in the conservation or loss of magnetic flux, particularly its southward magnetic field component, through [[magnetic reconnection]] with the [[interplanetary magnetic field]].<ref>{{cite journal |last1=Koehn |first1=G. J. |last2=Desai |first2=R. T. |last3=Davies |first3=E. E. |last4=Forsyth |first4=R. J. |last5=Eastwood |first5=J. P. |last6=Poedts |first6=S. |title=Successive Interacting Coronal Mass Ejections: How to Create a Perfect Storm |journal=The Astrophysical Journal |date=1 December 2022 |volume=941 |issue=2 |page=139 |doi=10.3847/1538-4357/aca28c |arxiv=2211.05899 |bibcode=2022ApJ...941..139K |s2cid=253498895 |issn=0004-637X |doi-access=free }}</ref> ===Morphology=== {{Expand section|with=information about CME sheaths and other aspects of CME morphology|small=no|date=April 2023}} In the solar wind, CMEs manifest as '''magnetic clouds'''. They have been defined as regions of enhanced magnetic field strength, smooth rotation of the magnetic field vector, and low [[proton]] temperature.<ref>Burlaga, L. F., E. Sittler, F. Mariani, and R. Schwenn, "Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations" in ''Journal of Geophysical Research'', 86, 6673, 1981</ref> The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed by [[Helios probes|Helios-1]] two days after being observed by the [[Solar Maximum Mission]] (SMM).<ref>Burlaga, L. F. et al., "A magnetic cloud and a coronal mass ejection" in ''Geophysical Research Letters'', 9, 1317–1320, 1982</ref> However, because observations near Earth are usually done by a single spacecraft, many CMEs are not seen as being associated with magnetic clouds. The typical structure observed for a fast CME by a satellite such as the [[Advanced Composition Explorer]] (ACE) is a fast-mode [[shock wave]] followed by a dense (and hot) sheath of plasma (the downstream region of the shock) and a magnetic cloud. Other signatures of magnetic clouds are now used in addition to the one described above: among other, bidirectional superthermal [[electron]]s, unusual charge state or abundance of [[iron]], [[helium]], [[carbon]], and/or [[oxygen]]. The typical time for a magnetic cloud to move past a satellite at the [[Lagrange Point]] (L1 point) is 1 day corresponding to a [[Earth radius|radius]] of 0.15 [[Astronomical Unit|AU]] with a typical speed of {{convert|450|km/s|mi/s|abbr=on}} and magnetic field strength of 20 [[Tesla (unit)|nT]].<ref>Lepping, R. P. et al. "Magnetic field structure of interplanetary magnetic clouds at 1 AU" in ''Journal of Geophysical Research'', 95, 11957–11965, 1990</ref>
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