The outward vertical motion of the plasma pulls in the magnetic field and plasma on either side of the reconnection region. The arrows indicate the direction of motion of the plasma and magnetic field. The new magnetic fields coming from the sides are now ready to reconnect and release their energy. The result is an explosive release of magnetic energy that is self-driven. What would cause the two magnetic fields to reconnect with each other?
Two oppositely directed magnetic fields must be separated by an electrical current directed inward in the boundary between them a. The plasma filling the region between the two magnetic fields must carry this current and provide a medium for energy transfer.
Another necessary ingredient is an electric field which also would be pointing into the page. In any electric circuit, including this one, an electric field parallel to the current flow acts as a load on the circuit, which dissipates electrical energy into heat or other forms of energy.
It is this dissipation that allows the two magnetic fields to interconnect. The box in b and c encloses the diffusion region, within which magnetic dissipation occurs. In the rather rarified environments where magnetic reconnection takes place, the classical particle collisions that produce dissipation in electrical circuits are negligible.
The shape of the diffusion region, specifically its aspect ratio the ratio of its length to its width , controls the rate of release of magnetic energy. In a high aspect-ratio diffusion region, the release of energy is slow—all of the plasma flowing into the diffusion region from the sides must flow out the ends, which act like a pair of nozzles. The resulting rate of inflow of magnetic field from the sides is low and so is the rate of energy dissipation.
Thus the diffusion region, a very narrow boundary layer of unknown structure and dynamics, controls the release of magnetic energy in a macroscopic system. What determines the aspect ratio of the dissipation region and the rate of release of magnetic energy is the second great mystery of reconnection.
Magnetic reconnection often occurs as an explosion, with solar and stellar flares being obvious examples. Particles reach kinetic energies that far exceed those expected from the fluid flows thought to be involved in magnetic reconnection as shown in c.
Therefore the acceleration mechanism cannot be described through classical fluid dynamics. What is the mechanism for such efficient conversion of magnetic energy into the kinetic energy of charged particles? This question is the fourth great mystery of reconnection. We now know that the mechanism for the fast release of magnetic energy requires that oppositely pointing magnetic fields be torn apart and reattached to their neighbors in a process called magnetic reconnection.
This idea was proposed back in the s but remains to date only partially understood, despite intense efforts of many scientists. In the following sections we discuss some of the history of magnetic reconnection, explain the basic concept, explain why the problem has been so challenging and discuss plans for addressing some of the outstanding issues with computer simulations, laboratory experiments, and both remote sensing and in situ measurements in space.
The past decade and a half has witnessed noteworthy advances in our understanding, but a breakthrough requires a highly sophisticated space experiment, the NASA Magnetospheric Multiscale mission, which is now in the implementation phase and currently scheduled for launch in The history of magnetic reconnection is generally traced back to the work of the Australian solar physicist Ronald Giovanelli, who noted in that solar flares often occur in locations where a neutral point in the magnetic field is expected.
Ten years later, Peter A. Sweet of the University of Glasgow and Eugene Parker of the University of Chicago independently proposed a reconnection model of solar flares based on electrical-resistance effects causing energy dissipation in a diffusion region. This model requires a diffusion region with a very large aspect ratio of length to width, an aspect ratio of 10,, in the case of the solar corona. However, it proceeds much too slowly to be able to explain explosive phenomena such as solar flares.
Figure 2. Computer simulations of electron populations in reconnection regions right have been key in deciphering more about how this event unfolds. Figure 3. The solar wind carries the interplanetary magnetic field yellow lines , here oriented southward, into the northward-facing geomagnetic field lines emanating from the Earth green lines at the dayside of the magnetopause.
The reconnection of these fields dotted box at left allows energy and charged particles from the solar wind to enter the magnetosphere. Open magnetic field lines purple lines are carried downstream in the solar wind and eventually reconnect in the distant tail of the magnetosphere dotted box at right. This figure shows only a noon-to-midnight cross-section of the three-dimensional magnetosphere.
The idea of a connection with the aurora has also turned out to be true, but for all types of auroras except one, the link is only indirect. For the most part, the widespread and violent auroral displays associated with severe space storms are caused by processes internal to the magnetosphere, which nonetheless rely on magnetic reconnection at much higher altitudes for their existence see figure 3 for an illustration of the process.
The idea here is that magnetic fields from the Sun and the Earth reconnect on the dayside of the Earth the side facing the Sun , after which the solar wind carries the reconnected magnetic flux along the magnetopause the boundary of the magnetosphere to the nightside, resulting in a build-up of magnetic energy in the tail of the magnetosphere.
A second reconnection event in the tail, which reconnects northern and southern magnetic flux and releases the solar field lines, is established after some time delay a half hour or so , and it is this event that leads to widespread magnetic and auroral activity known as the magnetospheric substorm, as well as to strong beams of high-energy particles.
The question of whether reconnection triggers the substorm or is a secondary effect was answered in by the five-spacecraft NASA THEMIS mission, launched specifically to study space storms, which showed conclusively that reconnection is in fact the trigger mechanism.
The southward fields were also associated with inward movement of the dayside boundary of the magnetosphere, as magnetic flux is stripped from the dayside and transferred to the nightside because it is connected to the flowing solar wind. Later on, predictions of reconnection theory, regarding plasma outflow from the reconnection region and magnetic penetration of the boundary, were both confirmed by spacecraft data. Figure 4. Auroras are usually created by electrons but this image shows a North Pole aurora emitted by protons, which glow brightly in the ultraviolet range.
The nearly circular white spot results from reconnection with a northward-directed interplanetary magnetic field. But sometimes the auroras result from collisions between the elements and protons, which are heavier and thus cause a more energetic display, particularly in the ultraviolet region. Although the motions of these spots, in concert with the changing east-west components of the generally northward field, were consistent with a reconnection source, the smoking gun came with a simultaneous observation of reconnecting magnetic fields above the spots by Cluster II, as reported by Tai D.
Phan of the University of California, Berkeley, and his colleagues in One of the reasons it took so long to confirm this simple prediction is that a sophisticated experiment had to be performed with simultaneous measurements from a spacecraft in the solar wind, a cluster of four spacecraft that straddled the reconnection point with high-resolution measurements, and a spacecraft capable of imaging the proton aurora.
Of course, our own Sun is strong evidence in favor of this possibility. The soft x-ray emission from the million-degree solar corona reveals loop-like structures that are effectively images of coronal magnetic field lines, because high-temperature particles in the solar corona rapidly spread along magnetic fields and therefore provide an effective image of the magnetic fields.
Similarly, throughout the universe explosive phenomena are observed through the energetic photons and charged particles they emit. For many years gravitational collapse and explosive shock waves were thought to explain most of what we observed. But as more exotic phenomena have been discovered, they often are found to contain strong magnetic fields and to undergo mysterious interactions that are sometimes best explained by magnetic reconnection.
An early insight into the role of magnetic fields in driving astrophysical dynamics came in from Albert Galeev, Robert Rosner and Giuseppe Vaiana of the Harvard-Smithsonian Center for Astrophysics. They described a gravitationally bound magnetized accretion disk—a collection of matter surrounding a gravitationally collapsed object in outer space—within which magnetic fields are both continuously created by a dynamo and annihilated by magnetic reconnection.
As happens in the solar corona, they discovered magnetic loops forming and reconnecting while producing the x-ray emissions that were observed. Numerous experimental and theoretical studies since that time have generally confirmed the validity of this model. Figure 5. The supermassive black hole Sagittarius A and its accretion disk have been observed to emit massive flare ejections, which may be caused by magnetic reconnection. Some of the most energetic phenomena in the universe are associated with the supernova explosions that are part of the death throes of stars.
After such an explosion, the star collapses into a neutron star and often into a black hole. Although accretion disks are usually connected to stars or protostars, they also surround neutron stars and black holes, with angular momentum and plasma being transferred between these three objects by turbulent magnetic fields. Any nearby stars also can be distorted and literally sucked into the black hole through its magnetically connected accretion disk.
In addition to localized accretion-disk x-ray emissions, massive flare activity has also been observed from black-hole accretion disks. A dramatic example of massive flare ejection was observed from the supermassive black hole Sagittarius A, located at the center of our galaxy.
It has been proposed that such massive flare ejections are caused by magnetic reconnection, as in solar flares. A neutron star can also evolve into a pulsar or, in extreme cases, into a magnetar, which exhibits very energetic flare-type emissions that also are very likely produced by magnetic reconnection. In general, astrophysicists consider reconnection as a possible mechanism for any phenomena exhibiting plasma heating, particle acceleration, magnetic field collapse or magnetic topology changes.
Remote sensing of these phenomena provides vast amounts of information on their scale, temporal development and energy transfer. However, the lack of in-situ measurements limits the information that can be gleaned about the processes that drive reconnection. The rapid increase in computer capacity over the past decade has facilitated numerical simulation of reconnection. Insights gained through these computations have dramatically advanced our understanding of magnetic reconnection and for the first time have enabled quantitative comparisons with observations.
The simulations are now able to treat single-particle motions of billions of electrons and protons in three dimensions and at time scales appropriate to the dynamical behavior of the plasma. With continued increases in computing power, this limitation too will gradually be overcome. As noted before, resistivity based on classical electron-ion collisions, as first proposed by Sweet and Parker, produces insufficient dissipation to explain the explosive release of magnetic energy seen in nature—most plasmas of interest are tenuous and as a result collisions are rare.
So the first mystery was what replaces classical resistivity in nearly collisionless plasma. In Galeev and Roald Sagdeev, now at the University of Maryland in College Park, proposed that the intense layers of current produced during reconnection generate turbulent electric-field fluctuations that scatter electrons.
The swirling electric-field vortices that develop in reconnection are similar to the gusty vortices of wind that develop during the passage of a strong weather front, which is a boundary layer of the neutral atmosphere of the Earth. With turbulence, the field lines would be strongly twisted so that multiple ones could reconnect simultaneously, vastly increasing the reconnection rate. Confirmation of the anomalous resistivity idea with numerical simulations had to wait nearly 20 years until computers were sufficiently powerful to explore reconnection and self- generated turbulence.
The bottom line from this modeling effort is that anomalous resistivity develops, but only when the current layer and associated diffusion region are sufficiently narrow. There is as yet no observational smoking-gun evidence, however, that this turbulence acts as an effective dissipation mechanism for magnetic energy. A laminar, or non-turbulent, mechanism for dissipating magnetic energy has been described by Michael Hesse and his colleagues at the Goddard Space Flight Center.
In this model, electrons take energy from the magnetic field in the diffusion region as they are accelerated by the reconnection electric field. Because of their high thermal mobility, they are able to rapidly transit through, and carry energy away from, the diffusion region.
The effect appears in the form of a pressure that is non-isotropic, or not the same in all directions. Therefore it cannot be described by conventional fluid dynamics, although it has been well documented in computer simulations.
Thus, the first great mystery of reconnection, which addresses how magnetic field lines break and magnetic energy is dissipated in a collisionless plasma, can be restated: Can the non-isotropic electron pressure explain the rapid reconnection over the vast scales of space and astrophysics, or is turbulence and its associated anomalous resistivity required? Unfortunately, because the breaking of magnetic field lines happens at very small spatial scales, comparable to the electron skin depth, the present fleet of heliospheric satellites is incapable of resolving the issue.
Figure 6. Massive stars die in type-2 supernova explosions; their stellar cores implode into a dense ball of subatomic particles a. If the newly formed neutron star is spinning fast enough, it will generate an intense magnetic field, and field lines inside the star will become twisted from the rapid movement b.
Over the first 10, years of its life, the star will settle down so that there are turbulent fields inside but smooth field lines on the surface c. At some point these internal stresses crack the solid surface, resulting in a quake that creates an electrical current burst and a flow of material that emits x rays d. The material dissipates in a matter of minutes. One of the major successes of reconnection research over the past two decades relates to the second great mystery of reconnection, which concerns what controls the rate of energy release.
Bengt Sonnerup in noted that ions and electrons, because of their large mass difference, would move differently at the small spatial scales of the diffusion region. Mark E. Mandt, Richard E. Denton and one of us Drake in showed that this differing motion completely changes the dynamics and structure of the diffusion region.
The ion motion can be neglected at very small scales; freed from the heavier ions, the electrons—together with the embedded magnetic field—can flow away at very high velocity. In the flurry of papers that followed, scientists showed that the rate of reconnection dramatically increased from the classical Sweet-Parker rate, the aspect ratio of the dissipation region was modest and the rate of reconnection was controlled by ions and not electrons.
The structure of the Hall magnetic field has been extensively documented in magnetospheric satellite observations, which brought positive closure to the idea of electron-ion decoupling within the small spatial scales of the dissipation region.
Moreover, for the first time since reconnection was first proposed in the s, the theoretical predictions for the rate of reconnection agree with astrophysical observations.
One of the major efforts in plasma physics has been the quest to sustain high enough temperatures to trigger nuclear fusion on a continuous basis. One approach, magnetic confinement fusion, has yielded very promising results with devices such as tokamaks, which produce a ring-shaped magnetic field to confine plasma inside. However, energy leakage caused by small-scale turbulence or larger-scale disruptive events driven by reconnection continues to be an issue in these devices.
Magnetic reconnection in tokamaks typically begins if the plasma pressure or current exceeds a threshold. One common result is a sawtooth crash —the core electron temperature slowly rises and then suddenly falls in a rapid crash of 50 to microseconds that repeats nearly periodically, causing a massive transfer of energy out of the plasma core. They reconnect at the interface where they do align, producing a guide field. In solar flares, however, the fields are less symmetrically aligned, so the guide field is expected to be strong.
Simulations and some tentative evidence show that the guide field weakens as reconnection proceeds, however, suggesting that it plays a significant role in electron acceleration. Magnetic reconnection was first observed directly by the Magnetospheric Multiscale MMS mission in The ongoing mission consists of four spacecraft flying in a tight formation. The craft study two main regions of the magnetosphere: one where the solar wind slams into the magnetosphere and one in the magnetotail.
They observe magnetic fields and events in far greater detail than any other space- or ground-based instruments. Each reconnection event that MMS observes will be different, and that can have a profound impact on how the dynamics play out.
Benningfield, D. Published on 24 November The authors. Any reuse without express permission from the copyright owner is prohibited. Iddris et al. Credit: NASA Magnetic reconnection plays a key role in energetic events across the solar system, from aurorae to solar flares. Finding a Middle Ground Reconnection can release magnetic energy and accelerate electrons through several mechanisms, which Dahlin distilled into three main categories: parallel electric fields, betatron acceleration, and Fermi reflection.
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