Coronal mass ejection
A coronal mass ejection is a significant ejection of plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
If a CME enters interplanetary space, it is sometimes referred to as an interplanetary coronal mass ejection. ICMEs are capable of reaching and colliding with Earth's magnetosphere, where they can cause geomagnetic storms, aurorae, and in rare cases damage to electrical power grids. The largest recorded geomagnetic perturbation, resulting presumably from a CME, was the solar storm of 1859. Also known as the Carrington Event, it disabled parts of the newly created United States telegraph network, starting fires and electrically shocking some telegraph operators.
Near the time in the Sun's eleven-year solar cycle known as solar maxima, the Sun produces about three CMEs daily, whereas near solar minima, there is about one CME every five days.
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 consisting primarily of electrons and protons embedded within its magnetic field. This magnetic field is commonly in the form of a flux rope, a helical magnetic field with changing pitch angles.The average mass ejected is. 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 structures in the corona that are kept in equilibrium by overlying magnetic fields.
Origin
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 regions. At the photosphere, active region magnetic flux is often distributed in a 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. 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, 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.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 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. Some pre-eruption structures, referred to as, take on an S or reverse-S shape as shear accumulates. This has been observed in active region coronal loops and filaments with forward-S sigmoids more common in the southern hemisphere and reverse-S sigmoids more common in the northern hemisphere.
Magnetic flux ropes—twisted and sheared magnetic 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 is subject to ongoing debate.
Some pre-eruption structures have been observed to support prominences, also known as filaments, composed of 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.
Early evolution
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.It is unknown whether a magnetic flux rope exists prior to initiation, in which case either ideal or non-ideal magnetohydrodynamic processes drive the expulsion of this flux rope, or whether a flux rope is created during the eruption by non-ideal process. Under ideal MHD circumstances, initiation may involve ideal instabilities or catastrophic loss of equilibrium along an existing flux rope:
- 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.
- The catastrophe model involves a catastrophic loss of equilibrium.
- 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 an electrical current sheet and the core flux system reconnects with the overlying magnetic field.
Initial acceleration
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 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.
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.
Coronal signatures
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.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 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.
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 waves when observed in the chromosphere, which are fast-mode MHD wave fronts that emanate from the site of the CME.
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. Coronal dimming was first reported in 1974, and, due to their appearance resembling that of coronal holes, they were sometimes referred to as transient coronal holes.