Acoronal mass ejection (CME) is a significant ejection ofplasma mass from theSun's corona into theheliosphere. CMEs are often associated withsolar flares and other forms ofsolar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
If a CME entersinterplanetary space, it is referred to as aninterplanetary coronal mass ejection (ICME). ICMEs are capable of reaching and colliding withEarth's magnetosphere, where they can causegeomagnetic storms,aurorae, and in rare cases damage toelectrical power grids. The largest recorded geomagnetic perturbation, resulting presumably from a CME, was thesolar storm of 1859. Also known as theCarrington Event, it disabled parts of the newly created United Statestelegraph network, starting fires and electrically shocking some telegraph operators.
Nearsolar maxima, the Sun produces about three CMEs every day, whereas nearsolar minima, there is about one CME every five days.
CMEs release large quantities of matter from the Sun's atmosphere into thesolar wind andinterplanetary space. The ejected matter is aplasma consisting primarily ofelectrons andprotons embedded within its magnetic field. This magnetic field is commonly in the form of a flux rope, ahelical magnetic field with changingpitch angles.
The average mass ejected is 1.6×1012 kg (3.5×1012 lb). 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.
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg) | This sectionneeds expansion with: information about precursors and observations thereof. You can help byadding to it.(April 2023) |
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 thesolar dynamo. These magnetic fields rise to the Sun's surface—thephotosphere—where they may form localized areas of highly concentrated magnetic flux and expand into the lower solar atmosphere formingactive regions. At the photosphere, active region magnetic flux is often distributed in adipole 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 thequiet 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.[1][2]
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 asmagnetic energy. The magnetic energy that is freely available to be released from a pre-eruption structure, referred to as themagnetic free energy ornonpotential 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, apotential 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.[3] Some pre-eruption structures, referred to assigmoids, take on anS or reverse-S shape as shear accumulates. This has been observed in active regioncoronal loops andfilaments with forward-S sigmoids more common in the southern hemisphere and reverse-S sigmoids more common in the northern hemisphere.[4][5]
Magnetic flux ropes—twisted and shearedmagnetic flux tubes 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§ Initiation) is subject to ongoing debate.[3][6]
Some pre-eruption structures have been observed to supportprominences, 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§ Coronal signatures).
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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.
CME initiation occurs when a pre-eruption structure in an equilibrium state enters a nonequilibrium ormetastable 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.[6]: 175 [7]: 303
It is unknown whether a magnetic flux rope exists prior to initiation, in which case eitherideal 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.[8][9]: 555 Under ideal MHD, initiation may involve ideal instabilities orcatastrophic loss of equilibrium along an existing flux rope:[3]
Under non-ideal MHD, initiations mechanisms may involve resistive instabilities ormagnetic reconnection:
Following initiation, CMEs are subject to different forces that either assist or inhibit their rise through the lower corona. Downwardmagnetic 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 nearlyantiparallel to one another and are brought together to form acurrent 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. Apositive 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§ 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 afailed orconfined eruption.[9][3]
| This sectionneeds expansion with: information about EUV waves and other coronal signatures. You can help byadding to it.(April 2023) |
The early evolution of CMEs is frequently associated with othersolar phenomena observed in the low corona, such as eruptive prominences and solar flares. CMEs that have no observed signatures are sometimes referred to asstealth CMEs.[11][12]
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[13] 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.[7]
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 asEIT waves after theExtreme ultraviolet Imaging Telescope or asMoreton waves when observed in the chromosphere, which are fast-mode MHD wave fronts that emanate from the site of the CME.[6][3]
A coronal dimming is a localized decrease inextreme ultraviolet andsoft 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.[14] Coronal dimming was first reported in 1974,[15] and, due to their appearance resembling that ofcoronal holes, they were sometimes referred to astransient coronal holes.[16]
| This sectionneeds expansion. You can help byadding to it.(April 2023) |
Observations of CMEs are typically through white-lightcoronagraphs which measure theThomson scattering of sunlight off of free electrons within the CME plasma.[17] An observed CME may have any or all of three distinctive features: a bright core, a dark surrounding cavity, and a bright leading edge.[18] The bright core is usually interpreted as a prominence embedded in the CME (see§ Origin) with the leading edge as an area of compressed plasma ahead of the CME flux rope. However, some CMEs exhibit more complex geometry.[7]
From white-light coronagraph observations, CMEs have been measured to reach speeds in the plane-of-sky ranging from 20 to 3,200 km/s (12 to 2,000 mi/s) with an average speed of 489 km/s (304 mi/s).[19] Observations of CME speeds indicate that CMEs tend to accelerate or decelerate until they reach the speed of the solar wind (§ Interactions in the heliosphere).
When observed in interplanetary space at distances greater than about 50 solar radii (0.23 AU) away from the Sun, CMEs are sometimes referred to asinterplanetary CMEs, orICMEs.[6]: 4
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.[20]
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.[6]: 199
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 (1AU), which was observed using measurements atMars[21] and by theUlysses spacecraft.[22] ICMEs faster than about 500 km/s (310 mi/s) eventually drive ashock wave.[23] This happens when the speed of the ICME in theframe of reference moving with the solar wind is faster than the local fastmagnetosonic speed. Such shocks have been observed directly by coronagraphs[24] in the corona, and are related to type II radio bursts. They are thought to form sometimes as low as 2 R☉ (solar radii). They are also closely linked with the acceleration ofsolar energetic particles.[25]
As ICMEs propagate through the interplanetary medium, they may collide with other ICMEs in what is referred to asCME–CME interaction orCME cannibalism.[9]: 599
During such CME-CME interactions, the first CME may clear the way for the second one[26][27][28] and/or when two CMEs collide[29][30] 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 famousCarrington event in 1859 had several eruptions and caused auroras to be visible at low latitudes for four nights.[31] Similarly, thesolar storm of September 1770 lasted for nearly nine days, and caused repeated low-latitude auroras.[32] 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 itschirality/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, throughmagnetic reconnection with theinterplanetary magnetic field.[33]
| This sectionneeds expansion with: information about CME sheaths and other aspects of CME morphology. You can help byadding to it.(April 2023) |
In the solar wind, CMEs manifest asmagnetic clouds. They have been defined as regions of enhanced magnetic field strength, smooth rotation of the magnetic field vector, and lowproton temperature.[34] The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed byHelios-1 two days after being observed bySMM.[35] 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 asACE is a fast-modeshock 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 superthermalelectrons, unusual charge state or abundance ofiron,helium,carbon, and/oroxygen.
The typical time for a magnetic cloud to move past a satellite at theL1 point is 1 day corresponding to aradius of 0.15AU with a typical speed of 450 km/s (280 mi/s) and magnetic field strength of 20nT.[36]
The frequency of ejections depends on the phase of thesolar cycle: from about 0.2 per day near thesolar minimum to 3.5 per day near thesolar maximum.[37] However, the peak CME occurrence rate is often 6–12 months after sunspot number reaches its maximum.[3]
Only a very small fraction of CMEs are directed toward, and reach, the Earth. A CME arriving at Earth results in ashock wave causing ageomagnetic storm that may disrupt Earth'smagnetosphere, compressing it on the day side and extending the night-sidemagnetic tail. When the magnetospherereconnects on the nightside, it releasespower on the order ofterawatts directed back toward Earth'supper atmosphere.[citation needed] This can result in events such as theMarch 1989 geomagnetic storm.
CMEs, along withsolar flares, can disruptradio transmissions and cause damage tosatellites andelectrical transmission line facilities, resulting in potentially massive and long-lastingpower outages.[38][39]
Shocks in the upper corona driven by CMEs can also acceleratesolar energetic particles toward the Earth resulting ingradual solar particle events. Interactions between these energetic particles and the Earth can cause an increase in the number of free electrons in theionosphere, especially in the high-latitude polar regions, enhancing radio wave absorption, especially within the D-region of the ionosphere, leading to polar cap absorption events.[40]
The interaction of CMEs with the Earth'smagnetosphere leads to dramatic changes in the outerradiation belt, with either a decrease or an increase of relativistic particle fluxes by orders of magnitude.[quantify][41] The changes in radiation belt particle fluxes are caused by acceleration, scattering and radialdiffusion of relativistic electrons, due to the interactions with variousplasma waves.[42]
Ahalo coronal mass ejection is a CME which appears in white-light coronagraph observations as an expanding ring completely surrounding the occulting disk of the coronagraph. Halo CMEs are interpreted as CMEs directed toward or away from the observing coronagraph. When the expanding ring does not completely surround the occulting disk, but has anangular width of more than 120 degrees around the disk, the CME is referred to as apartial halo coronal mass ejection. Partial and full halo CMEs have been found to make up about 10% of all CMEs with about 4% of all CMEs being full halo CMEs.[43] Frontside, or Earth-direct, halo CMEs are often associated with Earth-impacting CMEs; however, not all frontside halo CMEs impact Earth.[44]
In 2019, researchers used an alternative method (Weibull distribution) and estimated the chance of Earth being hit by a Carrington-class storm in the next decade to be between 0.46% and 1.88%.[45]
| This sectionneeds expansion with: information about the history of indirect observations made prior to 1973 discovery. You can help byadding to it.(May 2023) |
CMEs have been observed indirectly for thousands of years via aurora. Other indirect observations that predated the discovery of CMEs were through measurements of geomagnetic perturbations, radioheliograph measurements of solar radio bursts, and in-situ measurements of interplanetary shocks.[6]
The largest recorded geomagnetic perturbation, resulting presumably from a CME, coincided with the first-observedsolar flare on 1 September 1859. The resulting solar storm of 1859 is referred to as theCarrington Event. The flare and the associated sunspots were visible to the naked eye, and the flare was independently observed by English astronomersR. C. Carrington andR. Hodgson. At around the same time as the flare, a magnetometer atKew Gardens recorded what would become known as amagnetic crochet, a magnetic field detected by ground-based magnetometers induced by a perturbation of Earth's ionosphere by ionizingsoft X-rays. This could not easily be understood at the time because it predated the discovery of X-rays in 1895 and the recognition of theionosphere in 1902.
About 18 hours after the flare, further geomagnetic perturbations were recorded by multiple magnetometers as a part of ageomagnetic storm. The storm disabled parts of the recently created US telegraph network, starting fires and shocking some telegraph operators.[39]
The first optical observation of a CME was made on 14 December 1971 using the coronagraph ofOrbiting Solar Observatory 7 (OSO-7). It was first described by R. Tousey of theNaval Research Laboratory in a research paper published in 1973.[46] The discovery image (256 × 256 pixels) was collected on a Secondary Electron Conduction (SEC)vidicon tube, transferred to the instrument computer after being digitized to 7bits. Then it was compressed using a simple run-length encoding scheme and sent down to the ground at 200 bit/s. A full, uncompressed image would take 44 minutes to send down to the ground. Thetelemetry was sent to ground support equipment (GSE) which built up the image ontoPolaroid print. David Roberts, an electronics technician working for NRL who had been responsible for the testing of the SEC-vidicon camera, was in charge of day-to-day operations. He thought that his camera had failed because certain areas of the image were much brighter than normal. But on the next image the bright area had moved away from the Sun and he immediately recognized this as being unusual and took it to his supervisor, Dr.Guenter Brueckner,[47] and then to the solar physics branch head, Dr. Tousey. Earlier observations ofcoronal transients or even phenomena observed visually duringsolar eclipses are now understood as essentially the same thing.
On 1 November 1994,NASA launched theWind spacecraft as a solar wind monitor to orbit Earth'sL1 Lagrange point as the interplanetary component of theGlobal Geospace Science (GGS) Program within the International Solar Terrestrial Physics (ISTP) program. The spacecraft is a spin axis-stabilized satellite that carries eight instruments measuring solar wind particles from thermal to greater thanMeV energies, electromagnetic radiation from DC to 13 MHz radio waves, and gamma-rays.[citation needed]
On 25 October 2006, NASA launchedSTEREO, two near-identical spacecraft which, from widely separated points in their orbits, are able to produce the firststereoscopic images of CMEs and other solar activity measurements. The spacecraft orbit the Sun at distances similar to that of Earth, with one slightly ahead of Earth and the other trailing. Their separation gradually increased so that after four years they were almost diametrically opposite each other in orbit.[48][49]
On 9 March 1989, aCME occurred, which struck Earth four days later on 13 March. It caused power failures in Quebec, Canada and short-wave radio interference.
On 23 July 2012, a massive, and potentially damaging,solar superstorm (solar flare, CME,solar EMP) occurred but missed Earth,[50][51] an event that many scientists consider to be aCarrington-class event.
On 14 October 2014, an ICME was photographed by the Sun-watching spacecraftPROBA2 (ESA),Solar and Heliospheric Observatory (ESA/NASA), andSolar Dynamics Observatory (NASA) as it left the Sun, andSTEREO-A observed its effects directly at1 AU. ESA'sVenus Express gathered data. The CME reachedMars on 17 October and was observed by theMars Express,MAVEN,Mars Odyssey, andMars Science Laboratory missions. On 22 October, at3.1 AU, it reached comet67P/Churyumov–Gerasimenko, perfectly aligned with the Sun and Mars, and was observed byRosetta. On 12 November, at9.9 AU, it was observed byCassini atSaturn. TheNew Horizons spacecraft was at31.6 AU approachingPluto when the CME passed three months after the initial eruption, and it may be detectable in the data.Voyager 2 has data that can be interpreted as the passing of the CME, 17 months after. TheCuriosity rover'sRAD instrument,Mars Odyssey,Rosetta andCassini showed a sudden decrease in galactic cosmic rays (Forbush decrease) as the CME's protective bubble passed by.[52][53]
There have been a small number of CMEs observed on other stars, all of which as of 2016[update] have been found onred dwarfs.[54] These have been detected mainly by spectroscopy, most often by studyingBalmer lines: the material ejected toward the observer causes asymmetry in the blue wing of the line profiles due toDoppler shift.[55] This enhancement can be seen in absorption when it occurs on the stellar disc (the material is cooler than its surroundings), and in emission when it is outside the disc. The observed projected velocities of CMEs range from ≈84 to 5,800 km/s (52 to 3,600 mi/s).[56][57] There are few stellar CME candidates in shorter wavelengths inUV orX-ray data.[58][59][60][61] Compared to activity on the Sun, CME activity on other stars seems to be far less common.[55][62] The low number of stellar CME detections can be caused by lower intrinsic CME rates compared to the models (e.g. due tomagnetic suppression), projection effects, or overestimated Balmer signatures because of the unknown plasma parameters of the stellar CMEs.[63]
These assessments indicate that severe geomagnetic storms pose a risk for long-term outages to major portions of the North American grid. John Kappenman remarked that the analysis shows 'not only the potential for large-scale blackouts but, more troubling, ... the potential for permanent damage that could lead to extraordinarily long restoration times.'
But the most serious potential for damage rests with the transformers that maintain the proper voltage for efficient transmission of electricity through the grid.
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