Asymmetric Reconnection Research Articles

The magnetic field reconnection site and dissipation region

Magnetic field reconnection is said to involve an ion diffusion region surrounding an electron diffusion region. Because of uncertainties in the meanings of these terms and on the physical parameters that characterize them, this paper defines the reconnection site as a region having an electron scale size and containing magnetic fields from four topologies, and the dissipation region, as having an ion scale size and surrounding the reconnection site. Two-dimensional, asymmetric, open, particle-in-cell simulations, with and without guide fields, examine these regions. It is found that significant values of (E+UI×B) and/or (E+Ue×B) are not confined to either the reconnection site or the dissipation region. The reconnection site is uninteresting because, for asymmetric reconnection, it does not contain processes that serve to locate it, such as electron acceleration, parallel electric fields, super-Alfvenic electron flow, maximum electron beta, electron nongyrotopy, or demagnetized thermal electrons. However, the surrounding dissipation region exhibits these features.

Regions associated with electron physics in asymmetric magnetic field reconnection

Spatial relationships between regions containing signatures of electron physics in asymmetric reconnection with a guide field are examined using simulations and space observations that are in excellent agreement. These electron physics regions do not completely overlap, are not confined to sizes ∼electron skin depth, and do not surround the X‐line. The electron ideal Ohm's law, E + Ue × B = 0, is violated over many ion inertia lengths in the outflow direction. Thus, pressure terms and, to a lesser extent, inertia terms in the Generalized Ohm's Law are important in producing electron physics on ion scales. Parallel electric fields that are necessary but not sufficient for reconnection are found in the simulation and in space on ion scales in the outflow direction. They account for ∼10% of j · E, averaged over the current sheet. The electron exhaust jet exists over a shorter length for asymmetric reconnection than it does for symmetric reconnection.

Comment on “Scaling of asymmetric magnetic reconnection: General theory and collisional simulations” [Phys. Plasmas 14, 102114 (2007)

Recently, Cassak and Shay [Phys. Plasmas 14, 102114 (2007)] proposed a new scaling for asymmetric reconnection referred to as Sweet–Parker-type scaling. We set into question the derivation of those scaling laws.

Two‐spacecraft observations of an interplanetary slow shock

Slow‐mode shocks in space have been identified via one spacecraft observation by many authors since 1970. However, Feng et al. suggested that the analysis using single‐spacecraft observation based only on the Rankine‐Hugoniot relations could misinterpret a tangential discontinuity as a slow shock. In this paper, we identify a slow‐mode shock using a model fitting based on both the Rankine‐Hugoniot relations and intraspacecraft timing. As a result, we found a slow‐shock solution that, including the local parameters and propagation time, agrees well with the observations. The slow shock may be associated with an asymmetric reconnection exhaust at a heliospheric current sheet.

Magnetic Reconnection by a Self-RetreatingXLine

Particle-in-cell simulations of collisionless magnetic reconnection are performed to study asymmetric reconnection in which an outflow is blocked by a hard wall while leaving sufficiently large room for the outflow of the opposite direction. This condition leads to a slow, roughly constant motion of the diffusion region away from the wall, the so-called "X-line retreat." The typical retreat speed is approximately 0.1 times the Alfvén speed. At the diffusion region, ion flow pattern shows strong asymmetry and the ion stagnation point and the X line are not collocated. A surprise, however, is that the reconnection rate remains the same unaffected by the retreat motion.

Scaling of asymmetric Hall magnetic reconnection

The scaling of the reconnection rate and ion and electron outflow speeds with upstream magnetic field strengths and plasma mass densities during asymmetric collisionless (Hall) reconnection without a guide field is studied using two‐dimensional two‐fluid simulations. The results agree with a recent theory by Cassak and Shay (2007). It is found that the normalized reconnection rate is on the order of 0.1 and is independent of the asymmetry in field or density. Signatures of asymmetric Hall reconnection and applications to the magnetopause are briefly discussed.

Observations and simulations of asymmetric magnetic field reconnection

Comparisons of asymmetric magnetic field reconnection at the subsolar magnetopause are made between satellite data and computer simulations for the ratio of the antiparallel components of the magnetic fields at the boundaries being about three and the density ratio being 10 to 30. The bipolar electric field and quadrupolar magnetic field of symmetric reconnection are not found in either the simulations or the experimental data because the Hall term in the generalized Ohm's Law (the leading term on its right side on the ion scale) is a factor up to 100 smaller on the magnetosheath side of the current layer than on the magnetospheric side. Average plasma densities and fields at six THEMIS magnetopause crossings agree with open simulations provided that the experimental data is in the joint variance, not minimum variance, frame. This is because the magnetic field component normal to the assumed planar current sheet is not constant for asymmetric reconnection while it is forced to be approximately constant by the mathematics of minimum variance analyses. A north‐south asymmetry of the fields and flows in asymmetric reconnection is found in both the simulations and the space measurements. The simulations and the satellite data both have regions the order of the electron skin depth in size that contain significant electric fields. In such structures, parallel electric fields exist in the simulation throughout the current layer. Such parallel fields are similar to those reported earlier from Polar satellite data.

Scaling of asymmetric magnetic reconnection: General theory and collisional simulations

A Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed. Scaling laws for the reconnection rate, outflow speed, the density of the outflow, and the structure of the dissipation region are derived from first principles. These results are independent of the dissipation mechanism. It is shown that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated, leading to a bulk flow of plasma across the X-line. The scaling laws are verified using two-dimensional resistive magnetohydrodynamics numerical simulations for the special case of asymmetric magnetic fields with symmetric density. Observational signatures and applications to reconnection in the magnetosphere are discussed.

Origin of the interhemispheric potential mismatch of merging cells for interplanetary magnetic field BY‐dominated periods

When the dawn‐to‐dusk component of the interplanetary magnetic field (IMF BY) is dominant, ionospheric convection exhibits a basic two‐cell pattern with significant dawn‐dusk and interhemispheric asymmetries. For IMF BY > 0 the duskside merging cell potential in the Northern Hemisphere is much higher than that in the Southern Hemisphere, and the dawnside merging cell potential in the Southern Hemisphere is much higher than that in the Northern Hemisphere. The situation is reversed for IMF BY < 0. This interhemispheric potential mismatch originates from reconnection of overdraped lobe field lines and closed flankside field lines. This type of north‐south asymmetric reconnection does not affect the merging cell potentials in the same hemisphere as the reconnection point, whereas in the opposite hemisphere, it diminishes the potential of the dawnside (or duskside) Dungey‐type merging cell. Thus the total dawnside (or duskside) merging cell potential in one hemisphere is smaller than that in the other hemisphere by the reconnection voltage associated with the asymmetric reconnection.

Wind observations of asymmetric magnetic reconnection in the distant magnetotail

On February 23, 2001, during southward and strongly dawnward IMF, Wind traversed the distant magnetotail near XGSM = −90 RE. Wind encountered the mantle in the southern hemisphere and a nearly empty lobe in the northern hemisphere. In the current layer (plasma sheet), high‐speed tailward plasma jets with plasma density intermediate between the mantle and the lobe were detected. The speed of these flows were 96–99% of the Alfvén speed measured in the deHoffmann‐Teller frame. We interpret these observations as evidence for asymmetric reconnection involving a rotational discontinuity in the distant tail when the density in the two inflow regions are vastly different. This is in contrast to typical symmetric magnetotail reconnection involving two lobes of equal density where the flow acceleration across the slow shock is usually sub‐Alfvénic and the outflow density is enhanced relative to the inflow density. Hall effects are also observed in this event.

The dynamics of plasmoid in asymmetric spontaneous fast reconnection

The spontaneous fast reconnection evolution is studied in asymmetric magnetic field configuration. In particular, it is investigated how shear flow influences in magnetosheath region to the propagation of plasmoid results from magnetic reconnection using two-dimensional magnetohydrodynamic simulations. According to the fast reconnection development, the resulting large-scale plasmoids swell and propagate. Once the plasmoid fully develops, the propagation speed becomes almost constant in both the symmetric and asymmetric magnetic field configuration. An asymmetric plasmoid swells predominantly in the region of a weaker magnetic field and propagates along the field lines. The associated shock structure standing at the plasma boundary is the ordinary slow shock irrespective of the intensity of shear flow. However, velocity of plasmoid is proportional to shear flow velocity.

Magnetohydrodynamic study of adiabatic supersonic and subsonic expansion accelerations in spontaneous fast magnetic reconnection

The thermodynamic supersonic expansion acceleration mechanism associated with the spontaneous fast magnetic reconnection is studied by two-dimensional magnetohydrodynamic (MHD) simulations and the Rankine–Hugoniot analysis. The reconnection outflow jet can steadily exceed the Alfvén velocity measured in the upstream magnetic field region. Such a high speed jet cannot be explained by the Petschek model. According to previous studies, when supersonic (superfast) plasma jets generated by a pair of slow shocks expand in the direction normal to the jet, the jets can be further accelerated beyond the Alfvén velocity by the adiabatic supersonic expansion process. The expansion process is caused by the swelling of the plasmoid (magnetic loop). In this paper, it is theoretically shown that the sound Mach number of the reconnection jet generated by slow shocks is determined by the plasma density and beta value in the upstream magnetic field region, in which asymmetric reconnection models are also studied. Then, the theoretical prediction of the Mach number is related to the onset of the supersonic expansion acceleration process in MHD simulations. In addition, it is shown that, also when the reconnection jet is subsonic, the jet is further accelerated by the adiabatic subsonic expansion mechanism.

Basic Physical Mechanism of Fast Reconnection Evolution in Space Plasmas

Large dissipative events, such as solar flares and geomagnetic substorms, may result from sudden onset of fast (explosive) magnetic reconnection. Hence, it is a long-standing problem to find the physical mechanism that makes magnetic reconnection explosive; in particular, how can the fast magnetic reconnection explosively evolve in space plasmas? In this respect, we have proposed the spontaneous fast reconnection model as a nonlinear instability that grows by the positive feedback between plasma microphysics (anomalous resistivity) and macrophysics (global reconnection flow). On the basis of MHD simulations, we demonstrate for a variety of physical situations that the fast reconnection mechanism involving slow shocks in fact evolves explosively as a nonlinear instability and is sustained quasi-steadily on the nonlinear saturation phase. Also, distinct plasma processes, such as large-scale plasmoid propagation, magnetic loop development and loop-top heating, and asymmetric fast reconnection evolution, directly result from the spontaneous fast reconnection model. Obviously, MHD simulations are very useful in understanding the basic physics of explosive fast reconnection evolution in space plasmas. However, they cannot treat the details of microphysics near an X neutral point, which should be precisely studied in the coming 21st century.

Computer simulations of asymmetric spontaneous fast reconnection

The spontaneous fast reconnection model is extended to the general asymmetric situations, where magnetic reconnection may take place between the field lines that are anchored in different magnetic dipole sources. It is demonstrated for a wide variety of parameters that the asymmetric fast reconnection mechanism can evolve as a nonlinear instability, so that an asymmetric plasmoid swells predominantly in the region of a weaker magnetic field and propagates along the field lines. In the central diffusion region, the secondary tearing is likely to take place and significant erosion of the stronger magnetic field occurs; accordingly, the X neutral point moves with time, where the (current-driven) anomalous resistivity is found to be always locally enhanced, allowing the fast reconnection mechanism to be sustained quasisteadily and extended outwards further. The associated shock structure standing at the boundary of the stronger magnetic field is identified with the ordinary slow shock, in general, combined with an intermediate wave in the presence of sheared field. On the other hand, the shock standing at the boundary of the weaker magnetic field has an intermediate shock-like structure near the diffusion region, which propagates along the shock layer to finally become the ordinary combination of a slow shock and a finite-amplitude rotational intermediate wave at the plasmoid boundary.

A 2D numerical study of explosive events in the solar atmosphere

Two-dimensional (2D) compressible magnetohydrodynamic simulations are performed to explore the idea that the asymmetric reconnection between newly emerging intranetwork magnetic field flux and pre-existing network flux causes the explosive events in the solar atmosphere. The dependence of the reconnection rate as a function of time on the density and temperature of the emerging flux are investigated. For a Lundquist number of Lu= 5000 we find that the tearing mode instability can lead to the formation and growth of small magnetic islands. Depending on the temperature and density ratio of the emerging plasma, the magnetic island can be lifted upward and convected out of the top boundary, or is suppressed downward and convected out of the top boundary, or is suppressed downward nad submerged below the bottom boundary. The motions of the magnetic islands with different direction are accompanied respectively with upward or downward high velocity flow which might be associated with the red- and blue-shifted components detected in the explosive events.

Field line stochasticity during the m=1 reconnection

A full three-dimensional nonlinear magnetohydrodynamic (MHD) simulation using a high number of modes supports a model of the m=1 asymmetric resistive reconnection in tokamaks. The current sheet associated with the internal kink mode exhibits a toroidal modulation mainly due to a finite toroidal pressure effect that leads to an asymmetric m=1 magnetic island. The main effect of the modulation is to drive the growth of (m±1,m) magnetic islands in the plasma on a time scale imposed by the m=1 reconnection process. As the reconnection proceeds, different magnetic island chains with m of order 10 overlap, leading to an annular stochastic region around the m=1 separatrix. The diffusion in this stochastic layer depends exponentially on the current state of the reconnection, and can reach a value high enough to expel the electronic temperature in less than about 100 μsec if the modulation is greater than about 1.0, that is, for low-shear and/or high-pressure gradient equilibrium values on the q=1 surface.

Asymmetric reconnection and stochasticity in tokamaks

A model of the asymmetric internal reconnection occurring in toroidal geometry when pressure effects are taken into account is proposed. This model predicts a toroidal modulation of the current sheet, which is strong and mainly pressure driven for a rather low shear value on the q=1 surface. This modulation gives magnetic field line stochasticity in the vicinity of the m=1 separatrix, increasing during the reconnection process and reaching a value that can explain a complete expulsion of the electronic energy before the completion of the reconnection if the central value of the safety factor is somewhat below 0.8. This model is compared with a full toroidal MHD simulation including toroidal harmonics up to n=11

Effects of an inhomogeneous impurity distribution in a field-reversed theta pinch

A 140-kJ theta-pinch device originally constructed for spectroscopic investigations of highly ionized atoms is described. Atoms of solid elements are introduced into the initial plasma by laser-driven ablation from solid targets. The stability of the field-reversed plasma configuration is investigated with and without impurities. An inhomogeneous impurity distribution leads to the ejection of the plasma as a whole. This phenomenon is interpreted as the result of asymmetric field line reconnection at the ends.