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Abstract
Magnetic reconnection is reviewed, covering theory; astronomical objects; earth magnetopause; earth magnetotail; computer models; laboratory plasmas; and future directions.
... The range from astrophysical observation and theory [17][18][19] seen indirectly in the solar corona using UV and X ray emission [20,21], solar flares [22,23], the Earth's magnetotail [14], as well as magnetospheres of other planets [24,25] thermonuclear fusion experiments [26,27], and basic plasma physics laboratory experiments [28][29][30][31][32]. We present several examples but point out that excellent review papers [12,[33][34][35] books [36][37][38] and compendiums of articles [39] are out there. A review of all the observations and theoretical analysis over the broad range of venues in which magnetic field-line reconnection occur is well beyond the scope of this paper. ...
In two-dimensional magnetic reconnection, involving neutral sheets and magnetic islands it is generally a straightforward task to recognize reconnection sites when detailed data sets or simulations are available. In fully three-dimensional reconnection, their analogues can be challenging to identify. In this study, we demonstrate how locations of high reconnective activity can be detected in highly complex turbulent plasmas. We use a recently developed topological measure of reconnection based on the magnetic winding number, which measures the entanglement of pairs of field lines, to identify sub-regions of magnetic field lines which are reconnecting. This diagnostic is combined with established measures of magnetic field complexity, such as quasi-separatrix layers and regions of high magnetic twisting, to characterize the spatial and temporal distributions of reconnective activity of the field. It is demonstrated that the regions with the highest reconnective activity do not always coincide with the largest QSL signatures are, thus indicating this is a more complete methodology for quantifying reconnective activity than standard methods. This framework can serve as a model for reconnection analysis in future studies, in combination with established methods for identifying the specific form of reconnection once its location is established.
... Discontinuous plasma flows in a magnetic field are present in various kinds of technical facilities and devices of practical significance123 , in laboratory and numerical experiments456, and in astrophysical conditionsespecially in connection with the magnetic reconnection effect7891011. The question about plasma heating to very high temperatures is generally important in this case [12]. ...
The possibility that the type of discontinuous flow changes as the conditions
gradually (continuously) change is investigated in connection with the problems
arising when the results of numerical simulations of magnetic reconnection in
plasma are interpreted. The conservation laws at a discontinuity surface in
magnetohydrodynamics admit such transitions, but the socalled transition
solutions for the boundary conditions that simultaneously satisfy two types of
discontinuities should exist in this case. The specific form of such solutions
has been found, and a generalized scheme of permitted transitions has been
constructed on their basis. An expression for the jump in internal energy at
discontinuity is derived. The dependence of the plasma heating efficiency on
the type of discontinuity is considered.
Magnetic reconnection is the key process that produces the dynamics of the Earth's magnetosphere, by efficiently converting the magnetic field energy into plasma kinetic energy and thermal energy. The understanding of magnetic reconnection is being revolutionized with in situ spacecraft observations and large‐scale computer simulations with particle code. Magnetic reconnection occurs in the microscale region, but has large‐scale consequences. The physical processes in magnetic reconnection should be explored on various scales. This chapter summarizes a macroscopic picture of magnetic reconnection and examines the structure of magnetic reconnection on the kinetic level from an observational point of view.
The Earth's magnetopause separates the plasma regions of the shocked solar wind and the magnetosphere. There are a variety of transfer processes at this plasma boundary. These processes include finite gyroradius scattering, diffusion, wave‐induced diffusion, impulsive penetration, and magnetic reconnection. Of these processes, magnetic reconnection is the dominant process that transfers mass, energy, and momentum across the magnetopause. Magnetic reconnection is a microscale process that has global consequences for the Earth's magnetosphere and solar wind–magnetosphere interactions. The global transfer of plasma occurs because reconnection reconfigures the Earth's magnetic field lines. This reconfiguration and the plasma transfer across the magnetopause are strongly dependent on solar wind conditions.
Dayside magnetospheric physics has an early history that is closely related to our understanding of the magnetosphere as a whole. The early years of magnetospheric physics are somewhat reminiscent of the gold rush era or the exploration of the American west. Moving into the satellite era, our field had, for the first time, the opportunity to examine in‐situ dayside plasma processes to confirm or reject theories, something that neither solar nor astrophysics can do. Since the late 1970s, with better and faster instrumentation, we have been able to develop a detailed understanding of magnetopause and bow shock plasma physics, where transient phenomena play a critical role. This article provides a brief history of these periods of time and how these led into a modern understanding of dayside physics and transient events.
Magnetic reconnection, topological changes in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence, magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the paper done by Lazarian and Vishniac [Astrophys. J. 517, 700–718 (1999)] proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence, the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies a magnetohydrodynamiclike treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss the numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D, and unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g., star formation, turbulent dynamo, and acceleration of cosmic rays. We provide criticism of the concept of “reconnection-mediated turbulence” and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity and why the latter have fatal conceptual flaws.
Our previous study on the generation and signatures of kinetic Alfvén waves (KAWs) associated with magnetic reconnection in a current sheet revealed that KAWs are a common feature during reconnection [Liang et al. J. Geophys. Res.: Space Phys. 121, 6526 (2016)]. In this paper, ion acceleration and heating by the KAWs generated during magnetic reconnection are investigated with a three-dimensional (3-D) hybrid model. It is found that in the outflow region, a fraction of inflow ions are accelerated by the KAWs generated in the leading bulge region of reconnection, and their parallel velocities gradually increase up to slightly super-Alfvénic. As a result of wave-particle interactions, an accelerated ion beam forms in the direction of the anti-parallel magnetic field, in addition to the core ion population, leading to the development of non-Maxwellian velocity distributions, which include a trapped population with parallel velocities consistent with the wave speed. The ions are heated in both parallel and perpendicular directions. In the parallel direction, the heating results from nonlinear Landau resonance of trapped ions. In the perpendicular direction, however, evidence of stochastic heating by the KAWs is found during the acceleration stage, with an increase of magnetic moment μ. The coherence in the perpendicular ion temperature T and the perpendicular electric and magnetic fields of KAWs also provides evidence for perpendicular heating by KAWs. The parallel and perpendicular heating of the accelerated beam occur simultaneously, leading to the development of temperature anisotropy with T>T. The heating rate agrees with the damping rate of the KAWs, and the heating is dominated by the accelerated ion beam. In the later stage, with the increase of the fraction of the accelerated ions, interaction between the accelerated beam and the core population also contributes to the ion heating, ultimately leading to overlap of the beams and an overall anisotropy with T>T.
Fast particles, the thermal ones escaping from a super-hot source of flare energy and the non-thermal ones accelerated in a flare, move with velocities larger than the mean thermal velocity in plasma in the solar atmosphere and heat it. These particles emit different radiations including the hard X-ray(with wavelength λ < 1 Å) bremsstrahlung. There exist some well-known models to describe these processes that provide the observational manifestations of a solar flare. These classical models are discussed and illustrated in this chapter.
The physics of flares on the Sun is now ‘an étalon’ for contemporary astrophysics, in particular for gamma and X-ray astronomy. In contrast to the flares on other stars and to many analogous phenomena in the Universe, solar flares are accessible to a broad variety of observational methods to see and investigate the magnetic reconnectioneffect in high-temperature strongly-magnetized plasma of the corona as well as in low-temperature weakly-ionized plasma in the photosphere.
The inductive electric field is directed along the current inside a collisionless reconnecting current layer (RCL). This strong field does positive work on charged particles, thus increasing their energy impulsively, for example, in solar flares or flares in the accretion disk coronae of compact astrophysical objects.
Energy of a solar flare, how can it be estimated in a frame of magnetic reconnection theory? We would like to know also the characteristic time of energy accumulation before a flare as well as the characteristic time of energy release during a flare. Another important question is how to relate the dynamical characteristics of a flare with observed changes of magnetic field in the photosphere. These and some other fundamental properties of solar flares, including the particle acceleration process, are considered in this chapter on the basis of the reconnection theory without invoking many detail assumptions.
Reconnection in astrophysical plasma serves as a highly efficient engine to convert magnetic energy into thermal and kinetic energies of plasma flows and accelerated particles. Stationary models of the reconnection in current layers are considered in this Chapter. Properties of a stationary current layer strongly depends on a state of plasma turbulence inside it. The super-hot turbulent-current layer(SHTCL) model fits well for solar flares with different properties: impulsive and gradual, compact and large-scale, thermal and non-thermal.
Reconnection in super-hot turbulent-current layers (SHTCLs) creates collapsingmagnetic traps in the solar corona. In this Chapter, we discuss the possibility that coronal HXR emission is generated as bremsstrahlung of the fast electrons accelerated in the collapsing traps due to joint action of the Fermi-type first-order mechanism and betatron acceleration.
When two oppositely directed magnetic fields are pressed together, the conductive plasma is squeezed out from between them, causing the field gradient to steepen until a reconnecting current layer(RCL) appears and becomes so thin that the collisional or collisionless dissipation determines the magnetic reconnection rate and dynamics of this layer. In this Chapter, the basic magnetohydrodynamic (MHD) properties of the process of a current layer formation are considered in the approximation of a strong magnetic field.
The famous ‘Bastille day 2000’ flare is still one of the best examples of a solar flare which was well observed by several space- and ground-based observatories and studied in detail extensively by many researchers. The modern observations of the Sun in multiple wavelengths demonstrate, in fact, that the Bastille day flare has the same behavior as many large solar flares. In this chapter, the flare is considered from observational and topological points of view in terms of three-dimensional magnetic reconnection.
Magnetic field line reconnection is a process which results in a change in the field topology, release of magnetic field energy, and associated acceleration and heating of plasma (see, e.g., [1], [2], [3]). This energy conversion process occurs in astrophysical, solar, space, and laboratory plasmas [4]. In space physics, reconnection has been investigated in analytical studies based on generalizations of the Petschek model ([5], [1], [6], [7]); in numerical simulations ([8], [9]); and in the study of in-situ data (e.g., [10]).
Most of the baryonic matter in the Universe is in the plasma state. This comes about when the temperature of the matter becomes so hot that the atoms spontaneously dissociate into positively charged ions and electrons. Because of charge conservation in this dissociation process, plasmas are usually quasineutral with equal numbers of negative and positive charges. In other words, the number densities, n
e,i, of the oppositely charged particles are equal, n
e
= Z
i
n
i
, where the indices e, i indicate electrons and ions, and Z
i
is the nuclear charge number. Since the overwhelming majority of baryonic matter is hydrogen, one usually has equal numbers of electrons and protons in the volume, n
e
= n
i
. Even when the fraction of the ionized component is small the ionized component shows a totally different behaviour than neutral matter. It is governed by the laws of electrodynamics and many particle theory rather than by gravitation. It is these interactions which lead to the particular properties of a plasma and distinguish it from other matter states.
Our current understanding of the MHD of the flare process is summarised, with some emphasis on processes which produce strong impulsive electric fields and current filamentation. As an introduction, a description of the two main types of flare (i.e., simple-loop and two-ribbon) is given, together with an account of the two branches of reconnection theory (tearing modes and the Petschek-Sonnerup mechanism). Modern numerical experiments of reconnection suggest impulsive bursty acceleration of particles in many small regions of width a hundred kilometres or less. This is followed by a discussion of the eruptive instability thought to initiate a large flare and of the reconnection process of energy release. Finally, the role of emerging flux and horizontally moving satellite sunspots is discussed briefly.
Magnetic reconnection is a fundamental feature of astrophysical and laboratory plasmas, which takes place under definite but quit general conditions and creates a sudden release of magnetic energy, an original electrodynamical explosion or flare . Surprisingly, the simplest approximation – a single particle in given force fields – gives us clear approach to several facets of reconnection and particle acceleration.
Magnetic reconnection is a phenomenon of considerable importance in solar system plasmas. In the solar corona it results in the rapid release to the plasma of energy stored in large-scale magnetic configurations which become unstable, resulting in solar flares, while small-scale reconnection may play a role in heating the coronal plasma which leads to the outflow of the solar wind. Reconnection also results in the formation of magnetically “open” planetary magnetospheric field structures, leading to efficient coupling of solar wind momentum into the magnetospheres via magnetic stresses, as well as plasma mass exchange along the “open” flux tubes. In the extended magnetic tails formed by the solar wind interaction with solar system bodies, the on-set of rapid reconnection between the tail lobes can produce large-scale dynamical plasma-field reconfigurations which are associated with auroral substorms on Earth and structure in the plasma tails of comets. Major comet tail disconnection events have also been suggested to result from dayside reconnection following changes in the direction of the solar wind magnetic field.
The interrelation between the stability and the structure of reconnecting current layers governs their nonlinear evolution and determines a reconnection regime. In this chapter we study the structural instability of the reconnecting current layer, i.e. its evolutionarity.
The open issues focused on in this chapter presumably will determine the nearest future as well as the most interesting perspectives of plasma astrophysics. Magnetic energy release due to reconnection in the very complex systems of magnetic flux tubes (for example, a cosmic MHD turbulent plasma) is one of these issues.
Following the results of numerical experiments on magnetic reconnection, we consider in this Chapter two-dimensional stationary reconnection models that include a thin Syrovatskii-type current layer and four discontinuous magnetohydrodynamic (MHD) flows of finite length attached to its endpoints. The flow pattern is not specified but is determined from a self-consistent solution of the problem in the approximation of a strong magnetic field. This solution allows to study the global structure of the magnetic field and its local properties near the reconnecting current layer and attached discontinuities.
Magnetic reconnection, while being well established in the solar corona and solar wind, is also successfully invoked for explanation of many phenomena in the low-temperature weakly-ionized plasma in the solar atmosphere.
The super-hot turbulent-current layer (SHTCL) theory offers an attractive opportunity for laboratory and astrophysical applications of the magnetic reconnection. New data on the mechanism of magnetic energy transformation into kinetic and thermal energies of a super-hot plasma at the Sun require new models of reconnection under conditions of anomalous resistivity, which are similar to that ones investigated in toroidal devices performed to study turbulent heating of a collisionless plasma.
The topological model of a flare, with a reasonable accuracy, predicts the location of a flare energy source in the corona. In order to clarify an origin of this energy, we have to consider the non-potential part of magnetic field in an active region. In this Chapter, we discuss the main electric currents related to magnetic reconnection in a large solar flare. More specifically, we continue a study of the Bastille day 2000 flare which topological model was considered in a previous Chapter.
We review the current status of the theory of discontinuous magnetohydrodynamic (MHD) flows and its application to the physics of magnetic reconnection in astrophysical plasmas and in laboratory and numerical simulation studies. The emphasis is on the study of continuous transitions occurring between different types of discontinuities under gradual and continuous variation of the plasma flow parameters. The properties of the Syrovatskii reconnecting current sheet are described, and the possibility of the splitting of the current sheet into a system of MHD discontinuities is demonstrated. A simplified analytic model of magnetic reconnection is used to study the system of shock waves associated with the current sheet. With this system as an example, some implications of the conditions of continuous transitions and the possibility of additional plasma heating by a shock wave are considered.
The fast rates of magnetic reconnection found in both nature and experiments
are important to understand theoretically. Recently, it was demonstrated that
two-fluid magnetic reconnection remains fast in the strong guide field regime,
regardless of the presence of fast-dispersive waves. This conclusion is in
agreement with recent results from kinetic simulations, and is in contradiction
to the findings in an earlier two-fluid study, where it was suggested that
fast-dispersive waves are necessary for fast reconnection. In this paper, we
give a more detailed derivation of the analytic model presented in a recent
letter, and present additional simulation results to support the conclusions
that the magnetic reconnection rate in this regime is independent of both
collisional dissipation and system-size. In particular, we present a detailed
comparison between fluid and kinetic simulations, finding good agreement in
both the reconnection rate and overall length of the current layer. Finally, we
revisit the earlier two-fluid study, which arrived at different conclusions,
and suggest an alternative interpretation for the numerical results presented
therein.
We have explored the moving structures in a coronal bright point (CBP) observed by the Solar Dynamic Observatory Atmospheric Imaging Assembly (AIA) on 2011 March 5. This CBP event has a lifetime of ~20 minutes and is bright with a curved shape along a magnetic loop connecting a pair of negative and positive fields. AIA imaging observations show that a lot of bright structures are moving intermittently along the loop legs toward the two footpoints from the CBP brightness core. Such moving bright structures are clearly seen at AIA 304 Å. In order to analyze their features, the CBP is cut along the motion direction with a curved slit which is wide enough to cover the bulk of the CBP. After integrating the flux along the slit width, we get the spacetime slices at nine AIA wavelengths. The oblique streaks starting from the edge of the CBP brightness core are identified as moving bright structures, especially on the derivative images of the brightness spacetime slices. They seem to originate from the same position near the loop top. We find that these oblique streaks are bi-directional, simultaneous, symmetrical, and periodic. The average speed is about 380 km s–1, and the period is typically between 80 and 100 s. Nonlinear force-free field extrapolation shows the possibility that magnetic reconnection takes place during the CBP, and our findings indicate that these moving bright structures could be the observational outflows after magnetic reconnection in the CBP.
The article summarises some of the basic properties of the sun, reminds
the reader of the magnetohydrodynamic equations and outlines some of the
major problems in solar activity.
We have identified an anti-sunward-directed, Petschek-type, reconnection
exhaust during a crossing of the heliospheric current sheet, HCS, by the
ACE spacecraft. The exhaust in this relatively rare HCS event was a
region of accelerated plasma flow filling the field reversal region and
was bounded by Alfven waves propagating in opposite directions along
recently merged field lines. Heliospheric magnetic field lines
disconnected from the Sun by the reconnection process were identified in
and surrounding the exhaust by the disappearance of the solar wind
electron strahl there and by the interpenetration along the field of
sunward-streaming halo electrons from opposite sides of the exhaust.
These observations demonstrate that strahl dropouts observed in the
vicinity of the HCS at least at times are signatures of magnetic
disconnection from the Sun.
We have performed a statistical study investigating how substorm
triggering and unloading is affected by the heavy ion content of the
magnetotail plasma sheet. During the substorm growth phase, magnetic
flux is accumulated in the tail lobes until the magnetotail reaches an
unstable state. A near-Earth neutral line then forms, and this excess
flux is reconnected. The increased lobe magnetic flux during the
substorm growth phase increases the magnetopause flaring angle. As a
result, a greater fraction of the solar-wind dynamic pressure is
observed in the tail lobes and plasma sheet. Therefore, the increase and
decrease of the lobe magnetic flux can be monitored by observing the
increase and decrease in the magnetotail pressure. Using Cluster data
from 2001 to 2004, we have determined how the maximum pressure (or
flaring angle) and the rate of change of pressure (or flaring angle)
during substorms depend on the O+ content of the plasma
sheet. In addition, we have estimated the maximum magnetic flux, and
rate of change of the magnetic flux. Our results show that both the
maximum tail pressure and the rate of change in the pressure are
positively correlated with the amount of O+ in the plasma
sheet. When the measurements are normalized to account for the external
solar-wind pressure and the different Cluster locations in the tail, the
maximum accumulated flux and the unloading rate still correlate
positively with the O+ density and
O+/H+ ratio. This suggests that the additional
O+ makes it more difficult to trigger the substorm onset, but
once it is triggered, the unloading is faster. This could either
indicate that the presence of O+ increases the reconnection
rate, or that it initiates reconnection over a broader width of the
tail.
For a collisionless plasma, the magnetic field B_ enables fluidlike behavior in the directions perpendicular to B; however, fluid behavior along B_ may fail. The magnetic field also introduces an Alfven-wave nature to flows perpendicular to B_. All Alfven waves are subject to Landau damping, which introduces a flow dissipation (viscosity) in collisionless plasmas. For three magnetized plasmas (the solar wind, the Earth’s magnetosheath, and the Earth’s plasma sheet), shear viscosity by Landau damping, Bohm diffusion, and by Coulomb collisions are investigated. For magnetohydrodynamic turbulence in those three plasmas, integral-scale Reynolds numbers are estimated, Kolmogorov dissipation scales are calculated, and Reynolds-number scaling is discussed. Strongly anisotropic Kolmogorov k−5∕3 and mildly anisotropic Kraichnan k−3∕2 turbulences are both considered and the effect of the degree of wavevector anisotropy on quantities such as Reynolds numbers and spectral-transfer rates are calculated. For all three plasmas, Braginskii shear viscosity is much weaker than shear viscosity due to Landau damping, which is somewhat weaker than Bohm diffusion.
In my article entitled ``Explosive Magnetic Reconnection: Puzzle to be Solved as the Energy Supply Process for Magnetospheric Substorms?,'' I suggested that magnetic reconnection occurs as the system involved is being driven, rather than as an explosive and spontaneous process after magnetic energy accumulation has been completed, as is supposed hypothetically. In their comments, T. W. Hill and G. L. Siscoe contend that explosive magnetic reconnection ``is strongly supported by relevant theoretical arguments. . .,'' while C. T. Russell states that ``existing models do not require'' explosive magnetic reconnection. It is worth mentioning that both refer to the same near-earth X line models! Hill and Siscoe consider that the book edited by Hones [1984] presents evidence for their view. On the other hand, in the same book, Axford [1984] noted that ``. . .it is fair to say that Akasofu might be right, in a certain sense, in claiming that the magnetosphere is basically a driven system.''
After 1958, when scientific satellites began exploring the Earth magnetic environment, many puzzling phenomena could be directly examined, especially the polar aurora and disturbances of the Earth's magnetic field [see Stern, 1989a]. The notion of the solar wind, also introduced in 1958, helped clarify the role of the Sun in driving such phenomena. The large-scale structure of the magnetosphere, the space region dominated by the Earth's magnetic field, was gradually revealed within the next decade: its trapped particles, its boundary, and its long magnetic tail on the nightside. Inevitably, however, at a more fundamental level, the new discoveries led to new questions about the transfer of energy, the flow patterns of plasmas and electric currents, the acceleration of the aurora, and transient events such as magnetic substorms and storms, which energized ions and electrons. Though significant progress has occurred in some of these areas, many unresolved issues still remain. This review outlines the history of magnetospheric research, draws some general conclusions, and provides an extensive bibliography.
We discuss the structure of the reconnection layer as recorded by ISEE 2 during an outbound crossing of the dayside, northern magnetopause on October 29, 1979. Besides a rotational discontinuity, this crossing shows slow mode shock structure. We interpret this event theoretically in terms of magnetic field line reconnection based on a pulse-like Petschek-type model, rather than in terms of steady state reconnection on a moving magnetopause. Our model is generalized through the introduction of a space- and time-varying reconnection rate. Furthermore, the magnetic fields and velocities on either side of the magnetopause current sheet may have arbitrary strength and orientation with respect to each other; additionally, the densities on either side of the current sheet may be different in general. Using boundary/initial conditions from the spacecraft data, we calculate the temporal/spatial behavior of all reconnection-associated structures. In particular, having chosen a suitable trajectory through the magnetopause, we consider the temporal behavior of the magnetic field and plasma parameters as these reconnection-associated events pass by. The results reproduce the behavior of the data reasonably well.
Reconnection is the most efficient way to release the energy accumulated in the tense astrophysical magnetoplasmas. As such it is a basic paradigm of energy conversion in the universe. Astrophysical reconnection is supposed to heat plasmas to high temperatures, it drives fast flows, winds and jets, it accelerates particles and leads to structure formation. Reconnection can take place only after a local breakdown of the plasma ideality, enabling a change of the magnetic connection between plasma elements. After Giovanelli first suggested magnetoplasma discharges in 1946, reconnection has usually been identified with vanishing magnetic field regions. However, for the last ten years a discussion has been going on about the structure of 3 D reconnection, e.g., whether in 3 D it is possible also without magnetic nulls or not. We first shortly review the relevant magnetostatic and kinematic fluid theory results to argue than that a kinetic approach is necessary to reveal the generic three-dimensional structure and dynamics of reconnection in collisionless astrophysical plasmas. We present results about the 3 D structure of kinetic reconnection in initially antiparallel magnetic fields. They were obtained by selfconsistently considering ion and electron inertia as well as dissipative wave-particle resonances. In this approach reconnection is a natural consequence of the instability of thin current sheets. We present the results of a nonlocal linear dispersion theory and describe the nonlinear evolution of the instability using numerical particle code simulations. The decay of thin current sheets directly leads to a configurational instability and three-dimensional dynamic reconnection. We report the resulting generic magnetic field structure. It contains pairs of magnetic nulls, connected by separating magnetic flux surfaces through which the plasma flows and along which reconnection induces large parallel electric fields. Our results are illustrated by virtual reality views and movies, both stored on the attached CD-ROM and also being available from the Internet.
There is in magnetospheric physics today a controversy of sorts, involving the nature of the process by which the kinetic energy of the solar wind is tapped to power magnetospheric phenomena and related geomagnetic disturbances. The controversy is conveniently described in the language of signal processing [Akasofu, 1985]: an input signal composed of a suitable combination of solar wind variables is processed by the magnetosphere to produce an output signal consisting of a suitable combination of geomagnetic activity indices and/or magnetospheric variables. Akasofu advocates the view that the output signal, when properly measured, is a linear function of the input signal, and that the magnetosphere is therefore ``directly driven'' (i.e., it responds passively, after a fixed time lag, to a given solar wind energy input). In the alternative view the magnetosphere plays a more active role: the system response depends not only on incident solar wind variables (allowing a suitable time lag) but also on the recent dynamical history of the system. This second view is described by Akasofu as the ``unloading'' scenario because it corresponds to a physical model in which solar wind kinetic energy is collected and stored temporarily in the magnetic field of the magnetospheric tail and is subsequently--- in a manner determined at least in part by internal magnetospheric processes---`` unloaded'' to the ionosphere and inner magnetosphere to produce readily observable geomagnetic effects.
We illustrate the implications of a generalized Petschek-type reconnection model for conditions prevailing at the magnetopause. Petschek's model is generalized through the introduction of a space- and time-varying reconnection rate. Furthermore, the model can incorporate skewed magnetic fields with different magnitudes, a velocity shear, and different densities on either side of the current sheet. Here we study a situation typical for the dayside magnetopause, in which all these features are present. On a qualitative level, it is shown that the physical manifestations of reconnection include phenomena which have been observed at the magnetopause, such as accelerated plasma flows, flux transfer events, and surface waves. To aid a quantitative and more detailed comparison with experimental data, we present the results in a format which is similar to that used to view the plasma bulk parameters and magnetic field data obtained from spacecraft measurements. In particular, at several fixed positions in space we analyze the time behavior of the magnetic field and plasma parameters resulting from a localized pulse of reconnection. This exercise reveals that although there is broad agreement between the observations and expectations based on the model, several discrepancies and unexplained features still remain, such as the increase in the total pressure associated with some observations of flux transfer events. These are presumably the result of various simplifying assumptions made in the model. We also reach the new conclusion that application of the stress balance relation does not necessarily guarantee the correct identification of a rotational discontinuity traversal, and we suggest an additional identifying criterion.
Using resistive magnetohydrodynamic simulations, we investigate the influence of various parameters on the reconnection rate in two scenarios of magnetic reconnection. The first scenario consists of the ``Newton Challenge'' problem [Birn et al., Geophys. Res. Lett. 32, L06105 (2005)]. In this scenario, reconnection is initiated in a plane Harris-type current sheet by temporally limited, spatially varying, inflow of magnetic flux. The second scenario consists of the well-studied island coalescence problem. This scenario starts from an equilibrium containing periodic magnetic islands with parallel current filaments. Due to the attraction between parallel currents, pairs of islands may move toward each other, forming a current sheet in between. This leads to reconnection and ultimately the merging of islands. In either scenario, magnetic reconnection may be considered as being driven by external or internal forcing. Consistent with that interpretation we find that in either case the maximum reconnection rate (electric field) depends approximately linearly on the maximum driving electric field, when other parameters remain unchanged. However, this can be understood mostly from the change of characteristic background parameters; particularly, the increase of the magnetic field strength in the inflow region due to the added magnetic flux. This interpretation is consistent with the result that the maximum of the reconnection electric field is assumed significantly later (tens of Alfvén times) than the maximum driving and typically does not match the instantaneous driving electric field. Furthermore, the reconnection rate also depends on the resistivity and the time scale of the driving.
We have obtained direct evidence for local magnetic reconnection in the solar wind using solar wind plasma and magnetic field data obtained by the Advanced Composition Explorer (ACE). The prime evidence consists of accelerated ion flow observed within magnetic field reversal regions in the solar wind. Here we report such observations obtained in the interior of an interplanetary coronal mass ejection (ICME) or at the interface between two ICMEs on 23 November 1997 at a time when the magnetic field was stronger than usual. The observed plasma acceleration was consistent with the Walen relationship, which relates changes in flow velocity to density-weighted changes in the magnetic field vector. Pairs of proton beams having comparable densities and counterstreaming relative to one another along the magnetic field at a speed of ∼1.4VA, where VA was the local Alfven speed, were observed near the center of the accelerated flow event. We infer from the observations that quasi-stationary reconnection occurred sunward of the spacecraft and that the accelerated flow occurred within a Petschek-type reconnection exhaust region bounded by Alfven waves and having a cross section width of ∼4 × 105 km as it swept over ACE. The counterstreaming ion beams resulted from solar wind plasma entering the exhaust region from opposite directions along the reconnected magnetic field lines. We have identified a limited number (five) of other accelerated flow events in the ACE data that are remarkably similar to the 23 November 1997 event. All such events identified occurred at thin current sheets associated with moderate to large changes in magnetic field orientation (98°–162°) in plasmas characterized by low proton beta (0.01–0.15) and high Alfven speed (51–204 km/s). They also were all associated with ICMEs.
Reconnection between the magnetosphere and magnetosheath at the dayside magnetopause is studied for southward IMF using high-resolution 3-D MHD simulations. The BATSRUS code is run at CCMC with a resistive spot added on the magnetopause to ensure that fast reconnection occurs and to control the reconnection physics. A large range of Mach numbers (1.9-15) are run. The reconnection rate at the nose of the magnetosphere is measured and compared with local plasma parameters and with upstream-solar wind parameters. It is found that the reconnection rate is controlled by four local plasma parameters: Bs (the magnetic field strength in the magnetosheath), Bm (the magnetic field strength in the magnetosphere), rho s (the plasma mass density in the magnetosheath), and rho m (the plasma mass density in the magnetosphere). The Cassak-Shay formula for fast reconnection was tested and found to successfully describe the reconnection rate on the dayside magnetopause. It was found that reconnection itself does not significantly modify the local plasma parameters that control dayside reconnection and argued that reconnection does not significantly alter the flow pattern of the magnetosheath. This means that dayside reconnection is not ``driven'' in the sense that plasma pileup occurs to change the local parameters to adjust the reconnection rate to balance the driving. A ``plasmasphere effect'' was observed in the simulations wherein high-density magnetospheric plasma flows into the magnetopause reconnection site and mass loads the reconnection: a spatially localized plume of plasma was observed to locally reduce the reconnection rate by about a factor of 2.
A model for time-varying, localized reconnection in a current sheet with skewed magnetic field orientations at opposite sides is described and analyzed. The analysis is restricted to an incompressible plasma, in which case the Alfven wave and the slow shock merge to form shocks bounding the field reversal or outflow region, and to the case of weak reconnection, which implies that the reconnection electric field is much smaller than the product of the characteristic values of the external field strength and Alfven speed. This model can be applied to the earth's magnetopause, where reconnection is considered to be the dominant process coupling the solar wind and the magnetosphere. The results can be used to interpret different manifestations of reconnection such as accelerated plasma flows along the magnetopause and flux transfer events.
The process of driven magnetic reconnection induced by perturbing the magnetopause boundary of the Earth's magnetotail is analytically studied in the framework of kinetic theory. An explicit expression for the reconnection flux is obtained. The driven reconnection can either be exponential or bursty (i.e., short lived) type. The exponential type reconnection can occur only when the trapped electron population is less than 30%. The reconnection rates for the exponential type reconnection are either smaller than or at the most equal to the ion tearing mode instability growth rates. On the other hand, the bursty type reconnection generally occurs at rates much faster than the growth rates of the ion tearing instability. The bursty type reconnection, however, lasts typically for a period equal to the inverse of its reconnection rate. The size of the magnetic islands formed, and the magnitude and duration of the plasma flows induced along the tail axis are much larger during the exponential type than during the bursty type reconnection. Under certain conditions the bursty type reconnection is expected to be important for the onset of magnetospheric substorms and their energization.
The authors present a family of nonlinear solutions for magnetic field annihilation in two dimensions. These solutions include fully the effects of viscosity and resistivity and are a generalization of the Sonnerup and Priest model, where an irrotational stagnation point flow carries straight field lines toward a long, thin current sheet. Here, they allow for the vorticity in the inflow. When this is low, there is a unique solution for the flow and magnetic field. The current sheet adjusts its dimensions to accommodate different inflows. It is widest for a negative imposed vorticity and increases in width as the resistivity of viscosity is increased. When the imposed vorticity is large and negative, however, the solutions become nonunique, the flow pattern becomes cellular, and current sheets develop at the cell boundaries. These results, then, show that it is possible to have many more different types of inflow matched to full solutions for the current sheet than have been considered hitherto.
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