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. Diagnostic evidence strongly supports the idea that these sawtooth crashes are caused by impulsive reconnection.
The surprisingly short time scale of these events, which contradicted the traditional Sweet-Parker reconnection model, catalyzed an intense theory and modeling effort to understand fast magnetic reconnection and its explosive onset.
For many years, laboratory investigations of reconnection were limited to the collisional regime within which the predictions of slow Sweet-Parker reconnection were verified. However, Masaaki Yamada and his colleagues at the Princeton Plasma Physics Laboratory recently reported that fast reconnection could be driven when the ion skin depth was greater than the Sweet-Parker resistive layer. This result is another confirmation that the reconnection rate is determined by ion-scale dynamics; phenomena such as the out-of-plane magnetic field component predicted by Hall reconnection have now been confirmed in the laboratory as well as in space.
What are the next major steps in magnetic reconnection research? In addition to continuing the current aggressive program of astronomical observations, laboratory investigations and numerical modeling, NASA has initiated a carefully crafted experiment in space. The mission, named Magnetospheric Multiscale MMS , will begin with the launch of a four-spacecraft constellation currently scheduled for Figure 7. The Magnetospheric Multiscale mission will use a set of four satellites to probe magnetic reconnection at the boundary of the magnetosphere.
The tetrahedrally aligned satellites will gradually precess their orbit in each phase of the mission over a period of many months. In phase one, at a distance of 12 Earth radii, the group will be on the day side so that the crafts hover near the boundary between the magnetosphere and the solar wind. The significance of these two regions is that they provide the only accessible laboratories in space for a reconnection experiment. They are nearby, are not too hot and are known to contain explosive reconnection of the type that has been found to be important throughout the universe.
To be sure, the scaling between these regions of very tenuous plasma and the more extreme environments of the Sun, accretion disks, neutron stars and fusion machines extends over many orders of magnitude. Nonetheless, the next important goal is to determine what causes magnetic field lines to reconnect in a collisionless plasma. Then it will be up to a theoretical research program to bridge the gap to other environments and make predictions that can be tested by future observations.
First, the spacecraft must sample the most likely reconnection sites repeatedly and probe both those regions in which the magnetic fields are very nearly antiparallel which is usually the case in the tail and regions where a significant guide field exists which is generally true at the dayside boundary.
Next, since the reconnection regions are generally moving rapidly Sunward and Earthward on the dayside and tailward on the nightside , the spacecraft need to hover in these regions. This means their orbit apogees must be near the expected reconnection sites.
On the dayside, this distance is on the order of 10 Earth radii, whereas on the night side it is between 20 and 30 Earth radii one Earth radius is 6, kilometers. For this reason alone, two different orbits are needed. Once near a reconnection region, four identical spacecraft are needed in order to identify it by its two areas of inflow and two zones of outflow. Once reconnection regions have been found, the distances between satellites can be reduced in later encounters so that first the ion diffusion region and then the electron diffusion region can be sampled in detail.
The directions and intensities of currents, electric and magnetic fields, and plasmas and accelerated particles all need to be determined in unprecedented detail in order to identify the physical processes responsible for the non-classical resistivity and the resulting reconnection of the magnetic field.
From previous missions we know the speed with which reconnection layers move through space to be from tens to hundreds of kilometers per second. With computed electron skin depths of 5 to 10 kilometers, this means that the full three-dimensional electron population has to be sampled at rates greater than 10 per second.
How do x-rays help determine a celestial body's temperature? What range of temperatures can an x-ray distinguish when studying celestial bodies? If x-rays are normally absorbed by the earth's atmosphere, how do astronomers overcome this What types of objects in space emit x-rays? How are the x-rays detected? Impact of this question views around the world.
You can reuse this answer Creative Commons License. Reconnection describes the process in which magnetic field lines in a plasma break and reconnect, converting magnetic energy to heating and acceleration of particles. Magnetic reconnection contributes to events beyond the solar system.
Yet many of the details of reconnection are poorly understood, in part because no one had actually seen it in nature until a few years ago. Most of what physicists know about it comes from modeling and simulations. One problem is understanding how magnetic reconnection accelerates electrons and ions. Reconnection has drawn increasing interest in recent years as astrophysicists try to account for the electron acceleration detected in stellar flares and jets, magnetars, active galactic nuclei, and other objects and phenomena.
Dahlin has tackled the problem in a study published in Physics of Plasmas. The study reports that the scale of reconnection events plays a key role in acceleration, as do guide fields generated during such events. 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. The most efficient of the three appears to be Fermi reflection, which operates in plasmoids—a key middle ground in the scale of magnetic reconnection events.
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