Ritoku Horiuchi

Particle simulation study of collisionless driven reconnection in a sheared magnetic field

Nonlinear development of collisionless driven reconnection and the consequent energy conversion process between the field and particles in a sheared magnetic field are investigated by means of a two-and-one-half-dimensional particle simulation. Magnetic reconnection takes place in two steps irrespective of a longitudinal magnetic field, but the growth rate of the reconnection field varies in proportion to the E×B drift velocity at an input boundary. It is clearly observed that the triggering mechanism of collisionless driven reconnection for the fast growing phase changes from an electron meandering dominance in a weak longitudinal field to an electron inertia dominance in a strong field. The electron acceleration and heating take place in the reconnection area under the influence of reconnection electric field, while the electron energy is converted to the ion energy through the action of an electrostatic (ambipolar) field excited by magnetic compression in the downstream. It is also found that, in the p...

Three-dimensional particle simulation of plasma instabilities and collisionless reconnection in a current sheet

Generation of anomalous resistivity and dynamical development of collisionless reconnection in the vicinity of a magnetically neutral sheet are investigated by means of a three-dimensional particle simulation. For no external driving source, two different types of plasma instabilities are excited in the current layer. The lower hybrid drift instability (LHDI) is observed to grow in the periphery of current layer in an early period, while a drift kink instability (DKI) is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI. A reconnection electric field grows at the neutral sheet in accordance with the excitation of the DKI. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. It is also found that the anisotropic ion distribution is formed thro...

Particle simulation study of driven magnetic reconnection in a collisionless plasma

Driven magnetic reconnection in a collisionless plasma, ‘‘collisionless driven reconnection,’’ is investigated by means of two‐and‐one‐half‐dimensional particle simulation. Magnetic reconnection develops in two steps, i.e., slow reconnection, which takes place in the early stage of the compression when the current layer is compressed as thin as the orbit amplitude of an ion meandering motion (ion current layer), and subsequent fast reconnection, which takes place in the late stage when the electron current is concentrated into the narrow region with a spatial scale comparable to the orbit amplitude of an electron meandering motion (electron current layer). The global dynamic evolution of magnetic reconnection is controlled by the physics of the ion current layer. The maximum reconnection rate is roughly in proportion to the driving electric field. It is also found that both ion heating and electron heating take place in accordance with the formation of two current layers and the ion temperature becomes tw...

Electromagnetic instability and anomalous resistivity in a magnetic neutral sheet

An electromagnetic instability in a magnetic neutral sheet is investigated by means of a two‐and‐one‐half dimensional (21/2‐D) semi‐implicit particle simulation code. Electromagnetic waves are excited slowly in a field null region after saturation of the lower hybrid drift waves excited in an early time on both sides of the neutral sheet. This instability is found to be a new instability, independent of the lower hybrid drift instability. Examination of its characteristic properties indicates that the new instability is highly related to the meandering motions of ions in the neutral sheet. The growth of the instability gives rise to anomalous resistivity in the neutral sheet current.

Magnetohydrodynamic Vlasov simulation of the toroidal Alfvén eigenmode

A new simulation method has been developed to investigate the excitation and saturation processes of toroidal Alfven eigenmodes (TAE modes). The background plasma is described by a magnetohydrodynamic (MHD) fluid model, while the kinetic evolution of energetic alpha particles is followed by the drift kinetic equation. The magnetic fluctuation of n=2 mode develops and saturates at the level of 1.8×10−3 of the equilibrium field when the initial beta of alpha particles is 2% at the magnetic axis. After saturation, the TAE mode amplitude shows an oscillatory behavior with a frequency corresponding to the bounce frequency of the alpha particles trapped by the TAE mode. The decrease of the power transfer rate from the alpha particles to the TAE mode, which is due to the trapped particle effect of a finite‐amplitude wave, causes the saturation. From the linear growth rate the saturation level can be estimated.

Collisionless driven reconnection in an open system

Particle simulation studies of collisionless driven reconnection in an open system are presented. Collisionless reconnection evolves in two steps in accordance with the formation of two current layers, i.e., an ion current layer in the early ion phase and an electron current layer in the late electron phase. After the electron current layer is formed inside the ion current layer, the system relaxes gradually to a steady state when convergent plasma flow is driven by an external electric field with a narrow input window. On the other hand, when the convergent plasma flow is driven from the wide input window, magnetic reconnection takes place in an intermittent manner, due to the frequent formation of magnetic islands in the vicinity of neutral sheet.

Full magnetohydrodynamic simulation of the tilting instability in a field‐reversed configuration

Nonlinear evolution of the tilting instability in a field‐reversed configuration (FRC) is investigated by means of a three‐dimensional full magnetohydrodynamic simulation. Three types of plasma model are considered, i.e., case (a) where the plasma is confined by a uniform external field, case (b) where the plasma is confined by a mirror external field, and case (c) where the plasma rotates around the major axis. For the prolate FRC the internal tilt mode cannot stay at a low amplitude but keeps growing irrespective of whether the mirror field exists or not. When the initial configuration is substantially deformed, the instability triggers an external mode at the plasma–vacuum boundary because of the pressure imbalance. The outgoing flow driven by the external mode tears the plasma into two pieces, and the torn pieces move away toward the axial edges. The growth time is nearly equal to the transit time for the Alfven wave to propagate over the plasma length. When the plasma is driven to spin with the Mach ...

Molecular dynamics simulation of amphiphilic molecules in solution : Micelle formation and dynamic coexistence

The micelle formation and the dynamic coexistence in amphiphilic solution are investigated by molecular dynamics simulation of coarse-grained rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that three kinds of isolated micelles (disk, cylindrical, and spherical micelles) are observed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disk into a cylinder, and then into a sphere as the hydrophilic interaction increases whereas it is not so sensitive to the variation of the hydrophobic interaction. This fact indicates that the hydrophilic interaction plays an important role in determining the micellar shape in the range of the interaction parameters used. It is also found that in a certain interaction parameter range, two kinds of micellar shapes coexist dynamically. From the detailed analyses of the dynamic coexistence, it is ascertained that the dynamic coexistence of a cylindrical micelle and a spherical micelle accompanies the coalescence and fragmentation of micelles while that of a disk micelle and a cylindrical micelle does not, but exhibits the continuous change between them.

Long Time Scale Evolution of Collisionless Driven Reconnection in a Two-Dimensional Open System

Long time scale evolution of collisionless driven reconnection in an open system is investigated by means of two-dimensional full particle simulation based on an open boundary model. Collisionless reconnection is externally driven by the plasma inflow, which is mainly controlled by two key parameters of an external driving electric field, i.e., the strength E0 and the early nonuniformity scale xd. The strength E0 controls reconnection rate, while the scale xd controls the current layer shape and thus the magnetic-field configuration. It is found that the dynamical behavior of collisionless reconnection is sensitive to xd and less to E0 in our simulation parameter range. In the small xd case, the system evolves toward a steady state in which the reconnection rate is balanced with the external driving field E0. As xd increases, the reconnection evolution exhibits an intermittent phenomenon because of the frequent excitation of magnetic islands near the original X point.

Self‐organization and energy relaxation in a three‐dimensional magnetohydrodynamic plasma

A three‐dimensional self‐organization process in an incompressible dissipative plasma is investigated in detail by means of a magnetohydrodynamic simulation. In an initial phase the total magnetic energy rapidly decreases via magnetic reconnection driven by a global ideal unstable mode, while the total magnetic helicity decreases very slowly throughout the simulation run. This selective dissipation is found to be caused by the excitation of nonhelical, small spatial scale structures of the magnetic field (‘‘magnetic bubble’’) in the vicinity of the reconnection points. Magnetic reconnection takes place at minimum points of the magnetic helicity density where the magnetic bubbles are predominantly created. The disappearance of the bubbles leads to a rapid dissipation of the total magnetic energy. While the magnetic helicity is globally conserved during this process, its spectral density is transferred to the low wavenumber region (‘‘inverse cascade’’). After the rapid dissipation of the magnetic energy, th...