Magnetic helicity tensor for an anisotropic turbulence

The evolution of the magnetic helicity tensor for a nonzero mean magnetic field and for large magnetic Reynolds numbers in an anisotropic turbulence is studied. It is shown that the isotropic and anisotropic parts of the magnetic helicity tensor have different characteristic times of evolution. The time of variation of the isotropic part of the magnetic helicity tensor is much larger than the correlation time of the turbulent velocity field. The anisotropic part of the magnetic helicity tensor changes for the correlation time of the turbulent velocity field. The mean turbulent flux of the magnetic helicity is calculated as well. It is shown that even a small anisotropy of turbulence strongly modifies the flux of the magnetic helicity. It is demonstrated that the tensor of the magnetic part of the alpha effect for weakly inhomogeneous turbulence is determined only by the isotropic part of the magnetic helicity tensor.     

Energy transfers in dynamos with small magnetic Prandtl numbers

We perform numerical simulation of dynamo with magnetic Prandtl number Pm = 0.2 on 1024³ grid, and compute the energy fluxes and the shell-to-shell energy transfers. These computations indicate that the magnetic energy growth takes place mainly due to the energy transfers from large-scale velocity field to large-scale magnetic field and that the magnetic energy flux is forward. The steady-state magnetic energy is much smaller than the kinetic energy, rather than equipartition; this is because the magnetic Reynolds number is near the dynamo transition regime. We also contrast our results with those for dynamo with Pm = 20 and decaying dynamo.     

对颗粒物质运动的一致性进行控制的随机力场

在颗粒流的研究中引入了一个正态分布的随机力场, 并通过计算机模拟研究了该力场对均匀颗粒流的影响. 结果发现: 随机力场基本上不改变均匀颗粒流的平均密度和速度, 对颗粒流密度的涨落也影响很小. 随机力场对均匀颗粒流的影响主要体现在它可提高速度的涨落, 它与颗粒体系的耗散性质相抗衡, 使颗粒流维持一定的波动能量. 研究结果还显示: 通过随机力场所获得的波动能量并没有均匀分布到各个自由度上, 由于颗粒体系的耗散性质颗粒体系难以达到能均分状态. 关键词： 颗粒物质 随机力 分子动力学模拟     

Brownian motion in gravitationally interacting systems

We derive a model that describes the dynamics of a Brownian particle, such as a massive black hole, in a stellar system dominated by gravitational forces, and examine whether it achieves a state of equipartition of kinetic energy with the stars. This problem has been considered before only for stellar systems with an isothermal Maxwellian distribution of velocities; here we study other examples and confirm our calculations with N-body simulations. We show that in certain cases the black hole's steady state kinetic energy can be very far from equipartition.     

Numerical solutions of the isotropic 3-wave kinetic equation

We show that the isotropic 3-wave kinetic equation is equivalent to the mean-field rate equations for an aggregation–fragmentation problem with an unusual fragmentation mechanism. This analogy is used to write the theory of 3-wave turbulence almost entirely in terms of a single scaling parameter. A new numerical method for solving the kinetic equation over a large range of frequencies is developed by extending Lee’s method for solving aggregation equations. The new algorithm is validated against some analytical calculations of the Kolmogorov–Zakharov (KZ) constant for some families of model interaction coefficients. The algorithm is then applied to study some wave turbulence problems in which the finiteness of the dissipation scale is an essential feature. Firstly, it is shown that for finite capacity cascades, the dissipation of energy becomes independent of the cut-off frequency as this cut-off is taken to infinity. This is an explicit indication of the presence of a dissipative anomaly. Secondly, a preliminary numerical study is presented of the so-called bottleneck effect in a wave turbulence context. It is found that the structure of the bottleneck depends non-trivially on the interaction coefficient. Finally, some results are presented on the complementary phenomenon of thermalisation in closed wave systems which demonstrates explicitly for the first time the existence of so-called mixed solutions of the kinetic equation which exhibit aspects of both KZ and equilibrium equipartition spectra.     

Formation of Suprathermal Particle Fluxes in a Strongly Turbulent Plasma

We consider the problem on the formation of suprathermal particle fluxes by electrostatic structures in strongly turbulent cosmic plasmas. It is shown that regions with a strong plasma turbulence can be large accelerators of charged particles. We give solutions of the stationary kinetic equation in a turbulent layer for different acceleration regimes and estimate the efficiency of diffusion over the longitudinal and transverse velocities of particles with respect to the magnetic field. The transverse diffusion in velocity space is more efficient for ions and leads to strong isotropization of ion fluxes. Electrons move almost along the magnetic field. We reveal the conditions under which the regular force in a nonuniform magnetic field influences the stochastic-acceleration process. The average energy of axial motion of the particles and the particle fluxes at large distances from the injection region are estimated. Ions and electrons can be accelerated up to comparable energies. We analyze the characteristic features of the motion of the relativistic-particle beams. It is shown that strong plasma turbulence can form particle beams with specific energies. The proposed mechanism is useful for explanation of the properties of energetic particles in cosmic plasmas with magnetic-field-aligned currents, e.g., in high-latitude regions of planetary magnetospheres, force-free configurations of the solar corona, and the solar wind.     

Magnetic-field generation in Kolmogorov turbulence

We analyze the initial, kinematic stage of magnetic field evolution in an isotropic and homogeneous turbulent conducting fluid with a rough velocity field, v(l) approximately l(alpha), alpha<1. This regime is relevant to the problem of magnetic field generation in fluids with small magnetic Prandtl number, i.e., with Ohmic resistivity much larger than viscosity. We propose that the smaller the roughness exponent alpha, the larger the magnetic Reynolds number that is needed to excite magnetic fluctuations. This implies that numerical or experimental investigations of magnetohydrodynamic turbulence with small Prandtl numbers need to achieve extremely high resolution in order to describe magnetic phenomena adequately.     

Energy spectra stemming from interactions of Alfvén waves and turbulent eddies

We present a numerical analysis of an incompressible decaying magnetohydrodynamic turbulence run on a grid of 1536{3} points. The Taylor Reynolds number at the maximum of dissipation is approximately 1100, and the initial condition is a superposition of large-scale Arn'old-Beltrami-Childress flows and random noise at small scales, with no uniform magnetic field. The initial kinetic and magnetic energies are equal, with negligible correlation. The resulting energy spectrum is a combination of two components, each moderately resolved. Isotropy obtains in the large scales, with a spectral law compatible with the Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear interactions due to Alfvén waves; scaling of structure functions confirms the non-Kolmogorovian nature of the flow in this range. At small scales, weak turbulence emerges with a k{perpendicular}{-2} spectrum, the perpendicular direction referring to the local quasiuniform magnetic field.     

Time scale to ergodicity in the Fermi-Pasta-Ulam system

We study the approach to near-equipartition in the N-dimensional Fermi-Pasta-Ulam Hamiltonian with quartic (hard spring) nonlinearity. We investigate numerically the time evolution of orbits with initial energy in some few low-frequency linear modes. Our results indicate a transition where, above a critical energy which is independent of N, one can reach equipartition if one waits for a time proportional to N(2). Below this critical energy the time to equipartition is exponentially long. We develop a theory to determine the time evolution and the excitation of the nonlinear modes based on a resonant normal form treatment of the resonances among the oscillators. Our theory predicts the critical energy for equipartition, the time scale to equipartition, and the form of the nonlinear modes below equipartition, in qualitative agreement with the numerical results. (c) 1995 American Institute of Physics.     

Photoconductivity of a two-dimensional electron system with spin-orbit coupling in AC magnetic field

The response of an electron system to a dc probe field is analyzed in the case when an initial deviation of conduction-electron spin degrees of freedom from equilibrium in a microwave magnetic field induces combined resonance transitions in the electron system. It is shown that a perturbation of spin degrees of freedom is converted into kinetic energy and modifies transport coefficients, leading to oscillations of diagonal components of the conductivity tensor.     

Turbulent particle transport in magnetized plasmas

Particle transport in magnetized plasmas is investigated with a fluid model of drift wave turbulence. An analytical calculation shows that magnetic field curvature and thermodiffusion drive an anomalous pinch. The curvature driven pinch velocity is consistent with the prediction of turbulence equipartition theory. The thermodiffusion flux is found to be directed inward for a small ratio of electron to ion pressure gradient, and it reverses its sign when increasing this ratio. Numerical simulations confirm that a turbulent particle pinch exists. It is mainly driven by curvature for equal ion and electron heat sources. The sign and relative weights of the curvature and thermodiffusion pinches are consistent with the analytical calculation.     

Dynamos at extreme magnetic Prandtl numbers: insights from shell models

We present a magnetohydrodynamic (MHD) shell model suitable for computation of various energy fluxes of MHD turbulence for very small and very large magnetic Prandtl numbers Pm; such computations are inaccessible to direct numerical simulations. For small Pm, we observe that both kinetic and magnetic energy spectra scale as k^?5/3 in the inertial range, but the dissipative magnetic energy scales as k^?11/3exp?(? k/k_η). Here the kinetic energy at large length scale feeds the large-scale magnetic field that cascades to small-scale magnetic field, which gets dissipated by Joule heating. The large-Pm dynamo has a similar behaviour except that the dissipative kinetic energy scales as k^?13/3. For this case, the large-scale velocity field transfers energy to the large-scale magnetic field, which gets transferred to small-scale velocity and magnetic fields; the energy of the small-scale magnetic field also gets transferred to the small-scale velocity field, and the energy thus accumulated is dissipated by the viscous force.     

The Hall effect and oscillating decay of a magnetic field

The decay of a strong magnetic field in a conducting matter is considered. It is shown that nondissipative Hall currents can considerably change the behavior of the field when it decays. The nonlinear character of the Hall effect leads to the generation of fields of high multipolarity even for most simple initial magnetic configurations. In particular, the evolution of an initially dipole configuration may give rise not only to quadrupole or higher poloidal harmonics but also to a toroidal field that is other than zero only inside the conductor. The nonlinear Hall currents relate different harmonics to each other and, in a sufficiently strong field, may provide efficient energy exchange between them. Due to this redistribution of the magnetic energy, the evolution of different harmonics has an oscillating character. The oscillation period is determined by the characteristic time of Hall drift and may be fairly short in strong magnetic fields.     

Evolution of spin dynamics in electron-doped cuprate superconductors

Within the kinetic energy driven superconducting mechanism, the evolution of the magnetic excitations of the electron-doped cuprates in the superconducting state is studied. It is shown that there is a broad commensurate low energy magnetic scattering peak, while the magnetic resonance energy is located among this broad commensurate low energy scattering range. This broad commensurate low energy magnetic scattering disperses outward into a continuous ring-like incommensurate magnetic scattering at high energy.     

能量输运对柱状等离子体开关磁场穿透过程的影响

 对于高密度、导通时间为μs级的柱状等离子体开关，利用磁流体动力学理论(MHD)，对其导通阶段的磁场穿透过程进行了模拟，得到了磁场分布随时间的变化；研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明：对于高密度等离子体开关，磁场以远大于磁扩散速率的速度穿透到等离子体中；在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下，激波区域的等离子体温度显著升高，这进一步加速了磁场穿透；当考虑能量输运方程时，开关导通时间为0.87 μs，比等温模型的结果0.92 μs短，与实验结果0.87 μs相一致。     

微波磁场和斜入射对介质表面次级电子倍增的影响

分别研究了微波磁场和斜入射微波电场对介质表面次级电子倍增的影响.利用particle-in-cell/Monte Carlo方法,获得了微波磁场和斜入射微波电场条件下电子数量、介质表面直流场、电子平均能量和介质表面吸收功率的时间变化图像.模拟结果表明,斜入射和微波磁场虽然会显著影响电子的平均能量,但对电子数量和介质表面吸收功率的影响并不大,因此不会对微波介质表面击穿产生太大作用.     

Qualitative Estimation of Kinetic Energy Components of Longitudinal Continuous Motion of Magnetized Relativistic Electrons within the Landau Level

It is shown in the framework of the relativistic quantum mechanics that direct subtraction of the interaction energy of an electron with a magnetic field from the kinetic energy or its addition to the latter in the strength function contradicts the well-known fact that the magnetic field does no positive work on a charged particle.     

Landau Diamagnetism,Pauli Paramagnetism,and Determination of Longitudinal and Transverse Kinetic Energy Components of a Degenerate Nonrelativistic Electron Gas

The Fermi energy, density of average kinetic energy, and average density of kinetic energy of the transverse finite motion of an electron gas of a specified concentration are calculated taking into account Landau diamagnetism and Pauli paramagnetism. The kinetic energy of a longitudinal continuous electron motion along the direction of the external magnetic field H is estimated. It is shown that the kinetic energy of the longitudinal continuous motion vanishes with increase in the external magnetic field strength in the quantum limit where the maximum Landau quantum number N _m = 0. For N _m > 0, the longitudinal kinetic energy component of a degenerate electron gas somewhat increases with magnetic field strength. The cause of the erroneous result is discussed.     

Orthogonal interaction of bernstein mode with ion-acoustic wave through plasma maser effect

Interaction of Bernstein mode wave with ion-acoustic turbulence is treated as the plasma maser effect. Ion-acoustic turbulence is considered as resonant mode, propagating orthogonal to the test Bernstein mode. The external magnetic field, in the direction of ion-acoustic turbulence, is playing key role in transferring energy in upconversion process from resonant low frequency wave to the high frequency wave through modulated electric field. It is shown that the direct coupling term and the associated reverse absorbtion effect do not contribute individually to the growth or damping to the Bernstein mode. Only the polarization coupling term is found to be effective in the energy upconversion process as the external magnetic field is providing momentum in the direction of propagation of Bernstein mode. The polarization coupling term is identified to be the dominant upconversion factor in the plasma maser effect.     

可压缩湍流研究进展

高马赫数可压缩湍流的运动是一个多尺度多过程的物理现象。采用了多过程分解的方法,将可压缩湍流分解为剪切和胀压过程,分析其统计行为和动力学行为。发展了一种新的紧致差分和WENO格式相结合的混合型数值格式,准确模拟了可压缩湍流场;研究了其多尺度多过程行为和对粒子的输运影响;研究了激波结构对湍流场的影响;在高雷诺数可压缩湍流中,证明存在惯性区,其中流运动和压力做功引起的动能流通量都是常数;证明可压缩湍流中存在从大尺度到小尺度的动能级串过程;证明动能流通量的剪切部分和胀压部分在惯性区都为常数;分析亚格子应力项和亚格子质量流动项对动能级串的影响。