Magnetic Reconnection In Collisionless Plasma Research Articles

Magnetic reconnection during sawteeth crashes

Sawteeth oscillations are periodic relaxations of the core plasma density and temperature in tokamaks. The rise of the temperature due to external heating is terminated by the crash phase, which involves magnetic reconnection. This is the case of fast magnetic reconnection in collisionless plasmas (Lundquist number S≥108) with a strong guide field. (The toroidal magnetic field in a tokamak is a few orders larger compared to the reconnected helical field.) Experimental measurements show non-linear behavior before and during the crash phase. Simplified single-fluid models are not able to explain the reconnection dynamics during the crash, and two-fluid effects have to be considered. In this case, numerical simulations give good agreement with the observations for the crash duration. At the same time, the present simulations explain experimentally observed phenomena only partially, and several questions remain an area of active research: evolution during the crash, the onset of the fast phase, the existence of the post-cursors, the degree of stochasticity, and others. This overview paper summarizes the current understanding of the crash process, highlights remaining problems, and shows connections to magnetic reconnection research in other plasmas.

How anomalous resistivity accelerates magnetic reconnection

Whether turbulence induced anomalous resistivity (AR) can facilitate a fast magnetic reconnection in collisionless plasma is a subject of active debate for decades. Recent space observations suggest that the reconnection rate can be higher than the Hall-reconnection rate and turbulent dissipation is required. In this paper, using particle-in-cell simulations, we present a case study of how AR produced by Buneman instability accelerates magnetic reconnection. We first show that the AR/drag produced by Buneman instability in a thin electron current layer (1) can dissipate magnetic energy stored in the current layer through dissipation of the kinetic energy of electron beams; (2) the inhomogeneous drag caused by wave couplings spontaneously breaks the magnetic field lines and causes impulsive fast non-Hall magnetic reconnection on electron-scales with a mean rate reaching of 0.6 VA. We then show that a Buneman instability driven by intense electron beams around the x-point in a 3D magnetic reconnection significantly enhances the dissipation of the magnetic energy. Electron-scale magnetic reconnections driven by the inhomogeneous drag around the x-line enhance the reconnection electric field and the in-plane perpendicular magnetic field. About 40% of the released magnetic energy is converted into electron thermal energy by AR while 50% is converted into kinetic energy of the electron beams through the acceleration by the reconnection electric field. The enhanced magnetic energy dissipation is balanced by a net Poynting flux in-flow. About 10% of the released magnetic energy is brought out by an enhanced Poynting flux out-flow. These results suggest that AR with sufficient intensity and electron-scale inhomogeneity can significantly accelerate magnetic reconnection.

Properties of the first-order Fermi acceleration in fast magnetic reconnection driven by turbulence in collisional magnetohydrodynamical flows

Fast magnetic reconnection may occur in different astrophysical sources, producing flare-like emission and particle acceleration. Currently, this process is being studied as an efficient mechanism to accelerate particles via a first-order Fermi process. In this work we analyse the acceleration rate and the energy distribution of test particles injected in three-dimensional magnetohydrodynamical (MHD) domains with large-scale current sheets where reconnection is made fast by the presence of turbulence. We study the dependence of the particle acceleration time with the relevant parameters of the embedded turbulence, i.e., the Alfv\'en speed $V_{\rm A}$, the injection power $P_{\rm inj}$ and scale $k_{\rm inj}$ ($k_{\rm inj} = 1/l_{\rm inj}$). We find that the acceleration time follows a power-law dependence with the particle kinetic energy: $t_{acc} \propto E^{\alpha}$, with $0.2 < \alpha < 0.6$ for a vast range of values of $c / V_{\rm A} \sim 20 - 1000$. The acceleration time decreases with the Alfv\'en speed (and therefore with the reconnection velocity) as expected, having an approximate dependence $t_{acc} \propto (V_{\rm A} / c)^{-\kappa}$, with $\kappa \sim 2.1- 2.4$ for particles reaching kinetic energies between $1 - 100 \, m_p c^2$, respectively. Furthermore, we find that the acceleration time is only weakly dependent on the $P_{\rm inj}$ and $l_{\rm inj}$ parameters of the turbulence. The particle spectrum develops a high-energy tail which can be fitted by a hard power-law already in the early times of the acceleration, in consistency with the results of kinetic studies of particle acceleration by magnetic reconnection in collisionless plasmas.

Decoupling of Electron and Ion Dynamics in Driven Magnetic Reconnection in Collisionless Plasmas

Decoupling of Electron and Ion Dynamics in Driven Magnetic Reconnection in Collisionless Plasmas

Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case

The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field (E→es) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→es around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field (E→||) from E→ind to accelerate particles along the magnetic field. We explain the electron and ion dynamics and their velocity distributions and flow structures during the time-dependent driven reconnection as they move from the upstream to the downstream. In particular, we address the following key physics issues: (1) the decoupling of electron and ion dynamics due to meandering orbits around the field reversal region and the generation of a pair of converging bipolar electrostatic electric field (E→es) around the reconnection region; (2) the slowdown of electron and ion inflow velocities due to acceleration/deceleration of electrons and ions by E→es as they move across the neutral sheet; (3) how the reconnection current layer is enhanced and how the orbit meandering particles are accelerated inside the reconnection region by E→ind; (4) why the electron outflow velocity from the reconnection region reaches super-Alfvenic speed and the ion outflow velocity reaches Alfvenic speed; (5) how the quadrupole magnetic field is produced and how E→|| is produced; (6) how electrons and ions are accelerated by E→|| around the separatrix region; (7) why electrons have a flat-top parallel velocity distribution in the upstream just outside the reconnection region as observed in the magnetotail; (8) how electron and ion dynamics decouple and how the monopolar electrostatic electric field is produced around the separatrix region; (9) how ions gain energy as they move across the separatrix region into the downstream and how the ion velocity distribution is thermalized in the far downstream; and (10) how electrons move across the separatrix region and in the downstream and how the electron velocity distribution is thermalized in the far downstream. Finally, the main energy source for driving magnetic reconnection and particle acceleration/heating is the inductive electric field, which accelerates both electrons and ions around the reconnection current layer and separatrix regions.

Publisher's Note: “Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case” [Phys. Plasmas 22, 101205 (2015)

Publisher's Note: “Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case” [Phys. Plasmas <b>22</b>, 101205 (2015)

The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These “wavefronts” maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, “stir” the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence, and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta.

Dissipation mechanism in 3D collisionless magnetic reconnection

Dissipation processes responsible for fast magnetic reconnection in collisionless plasma are investigated using 3D electromagnetic particle-in-cell simulations. The present study compares two simulation runs; 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 the drift kink mode evolves along the current density and deforms the current sheet structure drastically. Although fast reconnection is achieved in both the cases, it is found that 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 discussed that in 3D reconnection the thickness of the electron current sheet becomes larger than the local electron inertia length.

Nonlinear variational method for predicting fast collisionless magnetic reconnection

A mechanism for fast magnetic reconnection in collisionless plasma is studied for understanding sawtooth collapse in tokamak discharges using a two-fluid model for cold ions and electrons. Explosive growth of the tearing mode enabled by electron inertia is analytically estimated using an energy principle with a nonlinear displacement map. Decrease in the potential energy in the nonlinear regime (where the island width exceeds the electron skin depth) is found to be steeper than in the linear regime, resulting in accelerated reconnection. Release of potential energy by such a fluid displacement leads to unsteady and strong convective flow, which is not damped by the small dissipation effects in high-temperature tokamak plasmas. Direct numerical simulation in slab geometry substantiates the theoretical prediction of the nonlinear growth.

Solar activity studies: From a magnetohydrodynamics description to a plasma perspective

Solar activity, in which there is an explosive release of magnetic energy in the solar atmosphere, is one of the most spectacular manifestations of space plasma activity. Non-potential, free magnetic energy is accumulated by the interaction between plasma and magnetic fields in the photosphere and sub-photospheric layer. In the photosphere and lower solar atmosphere, plasma is only partially ionized and there is three-species (or three-fluid) plasma. Cowling conductivity should therefore be considered in explaining solar observations. On the other hand, the explosive release of magnetic energy is believed to be caused by magnetic reconnection in the corona, where plasma is fully ionized and essentially collisionless. However, for collisionless plasma, we seem to have no precise quantitative description or formulation of its conductivity. To physically understand magnetic reconnection in collisionless plasma is an important task in making further progress in solar activity studies. This article discusses the plasma perspective of solar activity studies. The discussion begins with solar observations, and then, an analysis of the plasma problems that we face and need to better understand.

Dissipation mechanism in 3D magnetic reconnection

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)).

Role of electron physics in the development of turbulent magnetic reconnection in collisionless plasmas

Magnetic reconnection is important to the dynamics of many astrophysical and fusion plasmas but our understanding of it is incomplete. Petaflop-scale simulations of the evolution of turbulent magnetic reconnection in a three-dimensional plasma indicate that it proceeds in a way that is dramatically different from classical theory.

Reconnection onset in the magnetotail: Particle simulations with open boundary conditions

The mechanism of the onset of magnetic reconnection in collisionless plasmas in the tails of planetary magnetospheres and similar processes in the solar corona is one of the most fundamental and yet not fully solved problems of space plasma physics. Modeling the onset with particle codes requires either extremely large simulation boxes or open boundary conditions. In this Letter we report on simulations of reconnection in the magnetotail that incorporate open boundaries. In a simulation setup with the initial geometry similar to that of the GEM Reconnection Challenge bursts of spontaneous reconnection are detected in outflow regions that mimic magnetotails. They strongly resemble the ion tearing instability predicted by Schindler [1974] as a mechanism of magnetospheric substorms. Quenching the onset by replacing open boundary conditions for particles with their reintroduction reveals the key role of passing particles in the tearing destabilization.

A study of two-dimensional particle simulation of magnetic reconnection

Using two-dimensional full particle simulation, the process of magnetic reconnection in collisionless plasma is investigated and the velocity distributions of ions and electrons in various districts are obtained. As shown by the results of calculation, the Hall current produced by the different characters of electrons and ions in the diffusion region gives rise to a quadrupolar distribution of the y-component of the magnetic field, B y . The velocity distributions of ions and electrons deviate from the initial Maxwell distributions, and exhibit nonlocal multiple distributions. At the same time, the electric field generated by the magnetic reconnection leads to the acceleration and heating of the electrons in the vicinity of the X-point. Hence a high-energy tail is formed in the energy spectral distribution of electrons.

Single-particle dynamics in collisionless magnetic reconnection.

The role of single-particle dynamics in driven magnetic reconnection in collisionless plasmas is investigated experimentally and analytically. The trapping of particle orbits in the magnetic cusp is observed to allow fast reconnection in the absence of a macroscopic current layer, at a rate identical to that of vacuum. The development of an electrostatic potential structure around the magnetic X line during reconnection is predicted theoretically and observed experimentally.

Structure of the dissipation region during collisionless magnetic reconnection

Collisionless magnetic reconnection is studied using a 2 1/2‐dimensional hybrid code including Hall dynamics and electron inertia. The simulations reveal that the dissipation region develops a two‐scale structure: an inner electron region and an outer ion region. Close to the X line is a region with a scale of c/ωpe, the electron collisionless skin depth, where the electron flows completely dominate those of the ions and the frozen‐in magnetic flux constraint is broken. Outside of this region and encompassing the rest of the dissipation region, which scales like c/ωpi, the ion inertial length, is the Hall region where the electrons are frozen‐in to the magnetic field but the ions are not, allowing the two species to flow at different velocities. The decoupling of electron and ion motion in the dissipation region has important implications for the rate of magnetic reconnection in collisionless plasma: the ions are not constrained to flow through the very narrow region where the frozen‐in constraint is broken so that ion flux into the dissipation region is large. For the simulations which have been completed to date, the resulting rate of reconnection is a substantial fraction of the Alfvén velocity and is controlled by the ions, not the electrons. The dynamics of the ions is found to be inherently nonfluid‐like, with multiple ion beams present both at the X line and at the downstream boundary between the inflow and outflow plasma. The reconnection rate is only slightly affected by the temperature of the inflowing ions and in particular the structure of the dissipation region is controlled by the ion inertial length c/ωpi and not the ion Larmor radius based on the incoming ion temperature.

Structure of the dissipation region during magnetic reconnection in collisionless plasma

The results of an analytic and numerical investigation of the structure of the X line during steady state magnetic reconnection in collisionless plasma are presented. The structure of the X line essentially depends on a single dimensionless parameter F, which is a measure of the influx of plasma into the reconnection region. For small F the self‐consistent plasma current driven at the X line is small, and the magnetic fields are nearly unchanged from the initial vacuum state. With increasing F the current driven at the X line becomes large, and the dissipation region collapses in the direction of the inflow and elongates along the outflow. For sufficiently large F the velocity of the plasma ejected from the X line exceeds the local Alfvén velocity. In this regime a fast mode shock forms at the outflow end of the dissipation region which slows the high‐velocity outflow plasma to the subsonic flow characteristic of the broader outflow region. Finally, at a critical plasma flux Fc the dissipation region collapses to zero thickness; no steady solutions are found for F &gt; Fc. By matching the energy dissipated at the neutral line with the change in global magnetic energy, a self‐consistent equation for F is derived which indicates that F always adjusts so that F ⪝ Fc. Predictions of reconnection rates and associated parameters for the geomagnetic tail are presented which are in reasonable agreement with observations.

Magnetic reconnection in collisionless plasmas: Prescribed fields

The structure of the dissipation region during magnetic reconnection in collisionless plasma has been investigated. We consider a prescribed two‐dimensional magnetic x line configuration with an imposed reconnection electric field Ey. Particles are injected onto a computational grid, their orbits are integrated, and the moments of the distribution function are stored. The structure of the dissipation region depends on only two variables: a normalized reconnection electric field Ê and the opening angle θ of the separatrices of the magnetic field. An important conclusion of the work is that there is no linear relationship between the current sheet velocity νy and the electric field. For a small normalized electric field the maximum νy is localized away from the x line, is diamagnetic in origin (produced by local pressure gradients), and is independent of Ê as Ê → 0. For a large normalized electric field the effective life‐time of particles in the reconnection region scales as Ê−1/3 and νy, ∝ Ê2/3. In this limit, particles are ejected from the reconnection region as high‐velocity, gyrophase‐bunched beams. These beams produce an irregular filamentary current distribution in the outflow region. The beams which are ejected along the center of the outflow region are eventually trapped by the magnetic field while the beams ejected just downstream from the separatrix continue to move with high velocity out of the computational region. Analytic expressions for the current sheet velocity (νy), inflow and outflow velocities, and the scale size of the dissipation region are derived. Over the entire range of Ê, significant temperature anisotropies are produced, with T⊥ &lt; T∥ in the inflow region and T⊥ ≫ T∥ in the outflow region. Implications of our results for understanding magnetic reconnection in magnetospheric plasma and the source of energetic particles measured in the plasma sheet boundary layer and in explaining recent observations of fine scale magnetic turbulence in the central plasma sheet are discussed.

New regimes of magnetic reconnection in collisionless plasmas.

An implicit particle-in-cell code has been used to study collisionless magnetic reconnection of a current sheet. The reconnection process is found to occur in three distinct stages: (1) early spontaneous reconnection fed by the free energy of an initial anisotropy in the electron component; (2) coalescence of the resulting small-scale filaments of electron current, accompanied by electron jetting; and (3) oscillatory flow of electrons through the magnetic $X$ point, superposed on continuing nonlinear growth of ionmediated reconneciton. The time evolution of stage 3 is strongly dependent on $\frac{{M}_{i}}{{m}_{e}}$.

Magnetic reconnection in collisionless plasmas

A class of processes involving magnetic field reconnection, in collisionless plasmas and magnetic configurations where the field undergoes a finite change of direction, is investigated. Reconnecting modes that rely on the effects of electron Landau resonance and density gradient for their excitation are found to require the analytical or numerical treatment of four consecutive asymptotic regions. The influence of finite electron temperature gradient in the region where the effects of electron Landau resonance prevail, and the convection of energy toward the region where ion Landau resonance is dominant tend to dampen these modes. Conversely, significant distortions of the ion distribution can follow their excitation. The relevance of the obtained results to experimental observations on laboratory plasmas and in space physics is discussed. Different processes are involved with magnetic reconnection in magnetic configurations where the field does not have appreciable shear but has a neutral surface on which it vanishes.