Magnetic reconnection is a process that happens in electrically charged gases called plasmas. During this process, the arrangement of magnetic fields changes, and magnetic energy is transformed into energy that moves objects (kinetic energy), heat (thermal energy), and the speeding up of particles. This process involves the movement of plasma at speeds that are a large part of the Alfvén wave speed, which is the main speed at which information moves in a plasma that is influenced by magnetic fields.
The idea of magnetic reconnection was developed at the same time by scientists studying the Sun and those examining how the solar wind interacts with planets that have magnetic fields. This shows that reconnection can either separate magnetic fields that were once connected or connect magnetic fields that were once separate, depending on the situation.
Ron Giovanelli was the first person to publish a paper that suggested magnetic energy release could explain how particles are accelerated during solar flares. In 1946, he proposed that solar flares happen because charged particles gain energy from electric fields near sunspots. Between 1947 and 1948, he wrote more papers that expanded his model of reconnection in solar flares. In these papers, he suggested that this process occurs at points where the magnetic field is very weak or absent within structured magnetic fields.
James Dungey was the first to use the term "magnetic reconnection" in his 1950 PhD thesis. He used the term to describe how mass, energy, and movement from the solar wind are transferred into Earth's magnetic field. The concept was first published in an important paper in 1961. Dungey named the process "reconnection" because he imagined magnetic field lines and plasma moving together toward a point where the magnetic field is weak, breaking apart and then reconnecting with different field lines and plasma moving away from that point.
At the same time, the first theory explaining magnetic reconnection was created by Peter Sweet and Eugene Parker during a conference in 1956. Sweet explained that when two plasmas with opposite magnetic fields are pushed together, a process called resistive diffusion can happen over a much shorter distance than usual. Parker, who was at the conference, developed equations to describe this model while traveling home.
Fundamental principles
Magnetic reconnection happens when the "ideal-magnetohydrodynamics" model, which includes "Alfvén's theorem" (also called the "frozen-in flux theorem"), no longer applies. This model describes how magnetic fields move with plasma in regions where the Magnetic Reynolds Number is very large, meaning the movement of the plasma (convective term) dominates in the induction equation. Alfvén's theorem states that in these regions, magnetic fields move with the plasma's average velocity (a mix of ion and electron speeds based on their mass). However, reconnection occurs in areas with strong magnetic shear, which create thin current sheets. In these sheets, the Magnetic Reynolds Number becomes small enough for the diffusion term in the induction equation to dominate, allowing magnetic fields to spread from high-field regions to low-field regions. During reconnection, inflow and outflow regions still follow Alfvén's theorem, but the central diffusion region is a small area where magnetic field lines merge and reconfigure, changing their structure from the inflow pattern (along the current sheet) to the outflow pattern (through the current sheet). The speed of this magnetic flux transfer is called the "reconnection rate."
One of Maxwell's equations, ∇ × B = μ J + μ ε ∂ E ∂ t, connects magnetic shear and current. In plasmas, the displacement current (the second term on the right) is usually ignored except for very high-frequency events, reducing the equation to Ampère's law for free charges. This simplification is used in theories like Parker-Sweet and Petschek reconnection, as well as in ideal MHD and Alfvén's theorem, which apply outside the small diffusion region.
The resistivity in current layers allows magnetic flux from both sides to spread through the layer, canceling flux from the opposite side. The thin size of current sheets reduces the Magnetic Reynolds Number, making the diffusion term dominant even without increased resistivity. When diffusing field lines from both sides meet, they form separatrices, which have the structure of both inflow regions (along the current sheet) and outflow regions (through the current sheet). Magnetic field lines change from the inflow pattern through the separatrices to the outflow pattern. Magnetic tension forces then pull plasma along the current sheet, creating a self-sustaining process. Dungey's concept of localized ideal-MHD breakdown is important because outflow along the current sheet prevents plasma pressure from building up and blocking inflow. In Parker-Sweet reconnection, outflow occurs only in a thin central layer, limiting the reconnection rate. In Petschek reconnection, outflow occurs in a broader region between shock fronts (now thought to be Alfvén waves), allowing faster plasma escape and higher reconnection rates.
Dungey named the process "reconnection" because he imagined field lines breaking and reconnecting. However, this would temporarily create magnetic monopoles, violating Maxwell's equations. By considering the separatrices, the need for monopoles is avoided. Global MHD models of the magnetosphere still simulate reconnection even though it breaks ideal-MHD rules. This happens because each time step in the simulation solves ideal-MHD equations, which can create errors in thin current sheets. These errors cause field lines to reconnect through the current sheet, a process called "numerical resistivity." Simulations remain useful because errors spread according to a diffusion equation.
A major challenge in plasma physics is that observed reconnection occurs much faster than predicted by MHD in high Lundquist number plasmas (plasmas with very high conductivity). For example, solar flares happen 13–14 times faster than simple calculations suggest and faster than models including turbulence and kinetic effects. One possible explanation is electromagnetic turbulence in boundary layers, which scatters electrons and increases local resistivity, allowing magnetic flux to diffuse more quickly.
Properties
Magnetic reconnection is a process where magnetic field lines from different areas (called domains) connect to each other, changing how they link to their sources. This process breaks a rule in plasma physics called Alfvén's theorem, which states that magnetic field lines usually stay connected to the same sources. Reconnection can gather energy in specific places and times. Solar flares, the biggest explosions in the Solar System, may happen when large magnetic fields on the Sun reconnect, quickly releasing energy stored over hours or days. On Earth, magnetic reconnection in the magnetosphere helps create auroras. It is also important for nuclear fusion research because it can prevent magnetic fields from keeping fusion fuel contained.
In a plasma, which is a type of electrically charged gas, magnetic field lines are grouped into domains. These domains are bundles of lines that connect specific points and are structurally different from nearby lines. Even when the magnetic field is strongly changed by moving charges or currents, the structure of these domains usually stays the same. This happens because changes in the field often create small electrical currents in the plasma, which cancel out the changes in structure.
In two dimensions, the most common type of magnetic reconnection is called separator reconnection. This happens when four different magnetic domains exchange field lines. These domains are separated by curved surfaces called separatrices. Field lines on one side of a separatrix end at one magnetic pole, while lines on the other side end at a different pole. In simple systems, four domains are divided by two separatrices. One separatrix splits the field into two bundles that share a south pole, and the other splits it into two bundles that share a north pole. Where the separatrices cross is a line called a separator, which marks the boundary of the four domains. During separator reconnection, field lines from two domains meet at the separator, connect, and then leave the separator into the other two domains.
In three dimensions, magnetic field lines form more complex shapes than in two dimensions. Reconnection can occur in areas without a separator, where field lines are connected by sharp changes in direction. These areas are called quasi-separatrix layers (QSLs) and have been seen in theoretical models and solar flares.
Theoretical descriptions
In 1956, Peter Sweet and Eugene Parker introduced the first theory explaining magnetic reconnection during a scientific meeting. Sweet explained that when two plasmas with opposite magnetic fields are forced together, a process called resistive diffusion can occur over a much shorter distance than usual. Parker attended the meeting and later developed mathematical relationships to describe this process during his return journey.
The Sweet–Parker model describes a type of magnetic reconnection that happens slowly and steadily in plasmas. This model applies when magnetic fields point in opposite directions and when effects like viscosity and compressibility are not important. The initial movement of plasma is determined by the E × B velocity, where E represents the electric field and B represents the magnetic field.
Using a version of Ampere’s law that ignores certain effects, scientists found a relationship between the electric current in a thin magnetic layer and the strength of the magnetic field. This relationship depends on the thickness of the magnetic layer. By comparing the electric field inside the layer to the field outside, scientists calculated the inflow velocity, which depends on the magnetic diffusivity (a measure of how quickly magnetic fields change in a plasma) and the thickness of the layer.
When the amount of plasma flowing into the reconnection region is similar to the amount flowing out, the conservation of mass gives a relationship between the inflow and outflow velocities. This relationship connects the size of the magnetic layer to the speed of the plasma moving away from the reconnection site. By balancing the magnetic pressure with the pressure from moving plasma, scientists found that the outflow speed depends on the strength of the magnetic field and the density of the plasma. This speed is called the Alfvén velocity.
Using these relationships, scientists calculated the reconnection rate, which describes how quickly magnetic fields change. This rate can be expressed in two ways: one involving the magnetic diffusivity, the Alfvén velocity, and the thickness of the magnetic layer, and another involving the size of the magnetic layer and its thickness. These two expressions are connected through a number called the Lundquist number, which relates the size of the magnetic layer, the Alfvén velocity, and the magnetic diffusivity. The reconnection rate is inversely proportional to the square root of the Lundquist number.
The Sweet–Parker model explains reconnection rates faster than a process called global diffusion but cannot fully explain the very fast reconnection rates seen in solar flares, Earth’s magnetosphere, or laboratory plasmas. This model also ignores effects like three-dimensional structures, particle collisions, time changes, and pressure differences. Computer simulations of two-dimensional reconnection often match this model. Experiments with plasmas that include effects like pressure differences and unusual resistivity show agreement with a modified version of the Sweet–Parker model.
Petschek’s model of reconnection is faster than the Sweet–Parker model because it spreads out the region where plasma flows away from the reconnection site, reducing pressure buildup. In 1964, Harry Petschek proposed that slow magnetic shocks separate the inflow and outflow regions. This allows the reconnection rate to depend less on the Lundquist number. Simulations and theories suggest that these shocks can be created by magnetic waves called rotational discontinuities.
Experiments using uniform resistivity in plasmas often produce long, thin magnetic layers that match the Sweet–Parker model. However, when resistivity is unusually high in certain areas, simulations can show faster reconnection similar to Petschek’s model. This is only possible under specific conditions where particle movement is not limited by collisions.
In the Sweet–Parker model, scientists often assume that magnetic diffusivity is constant. This can be estimated by studying the motion of electrons in a plasma. When the movement of electrons becomes too fast compared to the thermal motion of the plasma, a steady state cannot be reached, and magnetic diffusivity increases significantly. This is called anomalous resistivity, which can speed up reconnection in the Sweet–Parker model.
Another proposed mechanism involves a process called Bohm diffusion, which describes how particles move across magnetic fields. This mechanism replaces…
Observations
Magnetic reconnection happens during solar flares, coronal mass ejections, and other events in the solar atmosphere. Observations of solar flares include evidence of inflows and outflows, loops that move downward, and changes in the shape of magnetic fields. In the past, scientists studied the solar atmosphere using remote imaging, which meant they had to guess or calculate the magnetic fields instead of seeing them directly. However, the first direct observations of magnetic reconnection on the Sun were collected in 2012 (and shared in 2013) by the High Resolution Coronal Imager.
Magnetic reconnection in Earth’s magnetosphere, such as near the dayside magnetopause and in the magnetotail, was long studied by guessing because it explained many behaviors of the magnetosphere and how it depends on the direction of the near-Earth interplanetary magnetic field. Later, spacecraft like Cluster II and the Magnetospheric Multiscale Mission made observations with enough detail and from multiple locations to see the process directly. Cluster II uses four spacecraft arranged in a tetrahedron shape to separate changes in space and time as they move. It has observed events where Earth’s magnetic field reconnects with the Sun’s magnetic field (the interplanetary magnetic field). These include "reverse reconnection," which causes movement of charged particles toward Earth near the polar cusps; "dayside reconnection," which allows particles and energy to reach Earth; and "tail reconnection," which causes auroral substorms by sending particles deep into Earth’s magnetosphere and releasing stored energy. The Magnetospheric Multiscale Mission, launched on March 13, 2015, improved the detail and timing of Cluster II’s results by using spacecraft positioned closer together. This helped scientists better understand the behavior of electrical currents in the electron diffusion region.
On February 26, 2008, the THEMIS probes identified the event that starts magnetospheric substorms. Two of the five probes, located about one-third the distance to the Moon, measured signs of a magnetic reconnection event 96 seconds before auroras intensified. Vassilis Angelopoulos, the principal investigator for the THEMIS mission at the University of California, Los Angeles, stated, "Our data show clearly and for the first time that magnetic reconnection is the trigger."
Magnetic reconnection has also been observed in laboratory experiments. For example, studies on the Large Plasma Device (LAPD) at UCLA have mapped areas near magnetic reconnection in a system with two magnetic flux ropes. Experiments on the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory confirmed many aspects of magnetic reconnection, including the Sweet–Parker model in situations where it applies. Analysis of how energy is injected into plasma in the NSTX spherical tokamak led Fatima Ebrahimi to design a plasma thruster that uses fast magnetic reconnection to accelerate plasma for space propulsion.
Sawtooth oscillations are regular mixing events in the core of tokamak plasma. The Kadomtsev model explains sawtooth oscillations as a result of magnetic reconnection caused by movement of the central region of the plasma where the safety factor q is less than 1 due to the internal kink mode.