Computer studies on the three-dimensional spontaneous fast reconnection model as a nonlinear instability

The spontaneous evolution of fast reconnection is studied in three dimensions by extending (in the z direction) the previous two-dimensional model that considered only the x-y plane [M. Ugai, Phys. Fluids B 4, 2953 (1992)]. It is demonstrated that the reconnection development strongly depends on three-dimensional effects; only when the central current sheet is sufficiently long in the z direction, say more than a few times larger than the current sheet width, the fast reconnection mechanism fully develops by the self-consistent coupling between the global reconnection flow and the current-driven anomalous resistivity. In this case, the reconnection flow can grow so powerfully as to enhance the current density (the current-driven resistivity) locally near an X line; otherwise, such a vital reconnection flow cannot be caused. The resulting quasisteady fast reconnection mechanism is significantly confined in the z direction, where a strong (Alfvénic) plasma jet results from standing switch-off shocks; accordingly, a large-scale plasmoid is formed and propagates in the middle of the system. It is concluded that the well-known two-dimensional spontaneous fast reconnection model can reasonably be extended to three dimensions.

Computer simulations study the temporal dynamics of magnetic reconnection in a current sheet system under a continuous local plasma injection imposed on the inflow boundary. The basic reconnection process is strongly influenced by the resistivity model. For a uniform resistivity model, the field lines become flattened with time so that the resulting quasisteady configuration involves a long diffusion region and the reconnection rate strongly depends on the resistivity, in good agreement with the Sweet–Parker mechanism. For an anomalous resistivity model, magnetic reconnection drastically occurs, leading to development of a large-scale plasmoid. Once an effective resistivity is triggered, a quasisteady fast reconnection mechanism spontaneously develops even in the absence of the specified boundary condition, in good agreement with the spontaneous fast reconnection model. When the imposed plasma injection is larger than the one inherent to the spontaneous fast reconnection mechanism, the fast reconnection mechanism can no longer proceed steadily and multiple X points and plasmoids result. It is concluded that it is not the boundary condition that drives the fast reconnection mechanism; the boundary condition may simply initiate the onset of magnetic reconnection (or effective resistivity) but has no essential effect on the fast reconnection development.

The self-consistent behavior of an isolated current-sheet system enclosed by free boundaries is numerically studied in the framework of magnetohydrodynamics. Initiated by a disturbance, the middle current sheet drastically collapses into a thin layer in a collisionless plasma. It is argued that an anomalous resistance should limit the extreme current concentration. When the current sheet, initially of width 2L, becomes thin, an anomalous resistivity η is introduced that increases with the relative electron–ion drift velocity ‖VD‖ when ‖VD‖ exceeds a threshold value VC. It is found that, when the reconnection flow has just been coupled to the anomalous resistivity, the reconnection growth rate [normalized by (L/VA)−1 ] becomes extremely large and scales as (∂η/∂‖VD‖)0.6V2.2C (normalization is based on the initial configuration); accordingly, the fast reconnection mechanism is set up on a short time scale and leads to the rapid collapse of the system at large. The initial growth of the fast reconnection is shown to be in good agreement with the underlying physical mechanism of the modified tearing instability.

Dissipation processes responsible for fast magnetic reconnection in collisionless plasmas are investigated using 3D electromagnetic particle-in-cell simulations. The present study revisits the two simulation runs performed in the previous study (Fujimoto, Phys. Plasmas 16, 042103 (2009)); one with small system size in the current density direction, and the other with larger system size. In the case with small system size, the reconnection processes are almost the same as those in 2D reconnection, while in the other case a kink mode evolves along the current density and deforms the current sheet structure drastically. Although fast reconnection is achieved in both the cases, the dissipation mechanism is very different between them. In the case without kink mode, the electrons transit the electron diffusion region without thermalization, so that the magnetic dissipation is supported by the inertia resistivity alone. On the other hand, in the kinked current sheet, the electrons are not only accelerated in bulk, but they are also partly scattered and thermalized by the kink mode, which results in the anomalous resistivity in addition to the inertia resistivity. It is demonstrated that in 3D reconnection the thickness of the electron current sheet becomes larger than the local electron inertia length, consistent with the theoretical prediction in Fujimoto and Sydora (Phys. Plasmas 16, 112309 (2009)).

Fast magnetic reconnection mechanism triggered by current sheet ejection is studied in plasma merging laboratory experiment with a strong guide field. The current sheet is hardly compressed in the presence of the guide field and slow steady reconnection evolves in the early phase. It then transits to impulsively fast magnetic reconnection when the current sheet is ejected from the X point region. Behavior of the current sheet is investigated in detail by comparison with the fast reconnection case provided by anomalous resistivity.

The spontaneous fast reconnection model is applied to the earthward fast flow events observed in the near‐Earth plasma sheet. Here, the earthward fast flow events include both of bursty bulk flow events and flow burst events. In order to apply it directly to actual observations, virtual probes are located in the plasma sheet region in the three‐dimensional simulation domain so that we can directly observe the temporal variations of plasma quantities in accordance with the growth and proceeding of the fast reconnection. In this model, magnetic reconnection drastically evolves and Alfvénic fast plasma jet flows in the very restricted narrow channel, and a large‐scale plasmoid is formed ahead of the fast plasma jet. The results of virtual observation of these evolutions are found to be in good agreement with actual satellite observations. At the same time, in the lobe region, travelling compression regions (TCRs) are observed in connection with the fast flow events. The temporal profiles of magnetic fields detected by the virtual probes are also in good agreement with actual satellite observations. It is concluded that the earthward fast flow events and earthward TCR events result from the fast reconnection mechanism.

It is well known that magnetic pulsations of long periods impulsively occur in accordance with the sudden onset of geomagnetic substorms and drastic enhancement of electrojets in the ionosphere. On the basis of the spontaneous fast reconnection model, the present paper examines the physical mechanism by which both magnetic pulsations and strong electrojets are impulsively driven by the fast (Alfvenic) reconnection jet. When a large-scale plasmoid [or traveling compression region (TCR)], directly caused by the fast reconnection jet, collides with the magnetic loop footpoint, strong electrojets are impulsively driven in a finite extent in the loop footpoint in accordance with the evolution of the current wedge and the generator current circuit. Simultaneously, magnetohydrodynamic (Alfven) waves, accompanied by the TCR, are reflected from the electrojet layer, leading to impulsive magnetic pulsations ahead of the loop footpoint because of the interaction (or resonance) between the reflected waves and the waves traveling toward the footpoint. The pulsations propagate outward in all directions from the source region of the wave reflection, and the pulsation periods are typically estimated to be of several tens of seconds.

In the last two decades, a wide variety of plasmoids events have been observed, ranging from space and astrophysical phenomenon to magnetically confined laboratory plasmas, in which there are a lot of evidence of observational plasmoid-like features supported by direct large-scaled computer simulations. A super-Alfvénic instability, named plasmoid instability, occurs in an extended current sheet, when the Lundquist number exceeds a critical value. The large-aspect-ratio current sheet is fragmented by generating, growing, coalescing and ejecting of plasmoids so that this phenomenon has been proposed as a possible mechanism for fast reconnection scenario. This super-Alfvénic plasmoid instability has been usedin the significant new development of reconnection theory, and thus can provide alternative and more convincing mechanism for fast reconnection. In this work, a “driving” kind of shear flow in the out-of-plane direction is imposed on a two-dimensional, three-component magnetohydrodynamic model with a current sheet system to study the dynamic process of the plasmoids in a current sheet system. The effect of the width and strength of the driving flow on the reconnection rate of plasmoids are numerically analyzed in detail. It is found that the plasmoids are easily formed in the case of strong and wide out-of-plane driving flow. The reconnection rate and the number of the plasmoids increase with the driving flow width and/or driving flow strength increasing. In the presence of guiding field, it is found that the symmetry of the plasmoids is broken in the reconnection plane. In addition, for the fixed guiding field, the growth rate of plasmoids increases much faster when the strength of driving flow increases.

In this paper we study resistive processes in the pre—are phase of eruptive —ares by means of the 2.5- dimensional MHD numerical simulation. According to many detailed observations of solar —ares, their evolution is characterized by several phases, each of which has a distinct nature. In the —rst phase, some kinds of radiation begin to be enhanced gradually, which implies the occurrence of the pre—are heating. Then, at a certain time, that gradual energy-release phase is replaced by the violent energy-release phase in which a huge amount of energy is released in various forms. So far, the nature of this violent energy- release phase has been well studied by using a —are model based on the fast magnetic reconnection, although those problems of the pre—are heating and the transition from the gradual energy-release phase to the violent one have not been sufficiently discussed yet. In this paper, in order to tackle these prob- lems, we start with a 2.5-dimensional force-free current sheet under a uniformly distributed resistivity, which is subject to a very small random velocity perturbation. At —rst the evolution enters on the linear stage of tearing instability and later a sufficient amount of thermal energy is produced in the nonlinear stage, which is considered to have a relation with the pre—are heating. In this nonlinear stage, the com- ponent of magnetic —elds perpendicular to the sheet (perpendicular magnetic —elds) —ows away from X- points formed in the sheet and eventually the current sheet collapses at these points. This collapse strongly reduces the thickness of the sheet if the magnetic Reynolds number is quite large and the plasma beta is quite low. Since the formation of thin current sheet leads to the occurrence of locally enhanced resistivity (anomalous resistivity), the transition from the gradual energy-release phase under a uniformly distributed resistivity to the violent one under a locally enhanced anomalous resistivity can be accomplished, which causes the fast magnetic reconnection responsible for various explosive phenomena in the Sun. Subject headings: MHDSun: coronaSun: —aresSun: magnetic —elds

Standard magnetohydrodynamic (MHD) theory predicts reconnection rate that is far too slow to account for a wide variety of reconnection events observed in space and laboratory plasmas. Therefore, it was commonly accepted that some non-MHD (kinetic) effects play a crucial role in fast reconnection. A recently renewed interest in simple MHD models is associated with the so-called plasmoid instability of reconnecting current sheets. Although it is now evident that this effect can significantly enhance the rate of reconnection, many details of the underlying multiple-plasmoid process still remain controversial. Here, we report results of a high-resolution computer simulation which demonstrate that fast albeit intermittent magnetic reconnection is sustained by numerous small-scale Petschek-type shocks spontaneously formed in the current sheet due to its plasmoid instability.

Magnetic reconnection is widely believed to be involved in dynamical phenomena such as solar flares or magnetospheric substorms. The Sweet–Parker model of magnetic reconnection in a Y-type geometry predicts a characteristic time scale proportional to S1/2 (where S is the Lundquist number), which is too slow to account for the observed time scales. The Petschek model, in contrast, predicts a time scale proportional to ln S in an X-point geometry. Numerical magnetohydrodynamic (MHD) simulations in the high-S regime generally validate the Sweet–Parker model, unless the resistivity is enhanced in the diffusion region to large values (such that typically S<103). It is demonstrated in this paper that nonlinear reconnection dynamics in a Harris sheet driven by inward boundary flows occurs on a nonlinear time scale that is proportional to S1/5 and thus has a weaker dependence on resistivity than the Sweet–Parker time scale. The current sheet amplitude at the separatrix (spanning Y points) grows algebraically in the linear regime but is suddenly enhanced after it makes a transition to the nonlinear regime. An analytical calculation is given for both the linear and the nonlinear regimes, and supported by two-dimensional resistive MHD simulations. The features of sudden current sheet enhancement and fast reconnection, controlled by boundary flows, are relevant to the phenomena of substorm onset or the impulsive phase of flares.

Fast magnetic reconnection plays a fundamental role in driving explosive\ndynamics and heating in the solar chromosphere. The reconnection time scale of\ntraditional models is shortened at the onset of the coalescence instability,\nwhich forms a turbulent reconnecting current sheet through plasmoid\ninteraction. In this work we aim to investigate the role of partial ionisation\non the development of fast reconnection through the study of the coalescence\ninstability of plasmoids. Unlike the processes occurring in fully ionised\ncoronal plasmas, relatively little is known about how fast reconnection\ndevelops in partially ionised plasmas of the chromosphere. We present 2.5D\nnumerical simulations of coalescing plasmoids in a single fluid\nmagnetohydrodynamic (MHD) model, and a two-fluid model of a partially ionised\nplasma (PIP). We find that in the PIP model, which has the same total density\nas the MHD model but an initial plasma density two orders of magnitude smaller,\nplasmoid coalescence is faster than the MHD case, following the faster thinning\nof the current sheet and secondary plasmoid dynamics. Secondary plasmoids form\nin the PIP model where the effective Lundquist number $S = 7.8 \\cdot 10^3$, but\nare absent from the MHD case where $S = 9.7 \\cdot 10^3$: these are responsible\nfor a more violent reconnection. Secondary plasmoids also form in linearly\nstable conditions as a consequence of the non-linear dynamics of the neutrals\nin the inflow. In the light of these results we can affirm that two-fluid\neffects play a major role on the processes occurring in the solar chromosphere.\n

view Abstract Citations (113) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Turbulent Reconnection Strauss, H. R. Abstract The reconnection rate in the presence of tearing mode turbulence is calculated. Tearing modes can produce turbulence in three-dimensional sheared magnetic fields. They produce a turbulent anomalous electron viscosity or hyperresistivity. Using quasi-linear and direct interaction approximations, the scaling of the hyperresistivity with magnetic perturbation strength and other parameters is estimated. This yields a reconnection rate, or inflow Mach number M, scaling as delta B, the ratio of rms magnetic fluctuations to mean magnetic field. This can greatly exceed the Sweet-Parker rate. The rate of heating of the current sheet is also large and can be many orders of magnitude larger than in a Sweet-Parker current sheet. Hyperresistivity produced by tearing mode turbulence may explain both fast reconnection and the heating of the solar corona. Publication: The Astrophysical Journal Pub Date: March 1988 DOI: 10.1086/166104 Bibcode: 1988ApJ...326..412S Keywords: Magnetic Field Reconnection; Solar Corona; Tearing Modes (Plasmas); Current Sheets; Plasma Turbulence; Resistance Heating; Solar Physics; HYDROMAGNETICS; SUN: CORONA; TURBULENCE full text sources ADS |

Aims.We report the observation by Parker Solar Probe (PSP) of magnetic structures in the solar wind that present a strong peak in their magnetic field magnitude with an embedded central current sheet. Similar structures have been observed, either at the Earth’s magnetopause and called interlinked flux tubes, or in the solar wind and called interplanetary field enhancements.Methods.In this work, we first investigate two striking events in detail; one occurred in the regular slow solar wind on November 2, 2018 and the other was observed during a heliospheric current sheet crossing on November 13, 2018. They both show the presence of a central current sheet with a visible ion jet and general characteristics consistent with the occurrence of magnetic reconnection. We then performed a survey of PSP data from encounters 1 to 4 and find 18 additional events presenting an increase in the magnetic field magnitude of over 30% and a central current sheet. We performed a statistical study on the 20 “magnetic increases with central current sheet” (MICCS), with 13 observed in the regular slow solar wind with a constant polarity (i.e., identical strahl direction), and 7 which were specifically observed near a heliospheric current sheet crossing.Results.We analyze and discuss the general properties of the structures, including the duration, location, amplitude, and magnetic topology, as well as the characteristics of their central current sheet. We find that the latter has a preferential orientation in the TN plane of the RTN frame. We also find no significant change in the dust impact rate in the vicinity of the MICCS under study, leading us to conclude that dust probably plays no role in the MICCS formation and evolution. Our findings are overall consistent with a double flux tube-configuration that would result from initially distinct flux tubes which interact during solar wind propagation.

We calculate the anomalous resistivity (AR) due to electrostatic waves, including possibly the lower hybrid wave and electron beam mode, around the secondary islands in the reconnection region observed by the Cluster spacecraft. Our main findings are: AR is important on the reconnection separatrix layer but heavily suppressed at the central current sheet where Bx ∼ 0. Moreover, there is a highly asymmetric pattern of AR across the island along the outflow direction, with much larger AR on one side of island than on the other side. Our results may be helpful in understanding the role of AR in reconnection.