An Island Coalescence Scenario for Near-Earth Current Disruption in the Magnetotail

A current disruption and dipolarization scenario associated with island coalescences in the near-Earth region is proposed. The thin and elongated current-sheet built up during the growth phase is unstable due to a tearing mode instability that leads to formation of multiple magnetic islands （or magnetic flux ropes in the three dimensional case） in the near-Earth region. The growth rate of the tearing mode shoual be different in different locations because the rate is in general determined by the external driving force and the local plasma sheet properties. When the rate of the magnetic reconnection in the mid-tail region around 20RE is much larger than that in other locations, the strong bulk earthward flows resulting from the fast reconnection in the mid-tail drive the earthward convection and the coalescence of the magnetic islands. Consequently, the cross-tail current in the near-Earth region is suddenly disrupted and the geometry of the magnetic field changes from tall-like to dipolar-like in the ideal time scale. This proposed scenario is tested by Hall MHD simulation and is compared with the observations.     

Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

We demonstrate the electrical beat note analysis and radio frequency(RF) injection locking of a continuous wave(cw) terahertz quantum cascade laser(QCL) emitting around 3 THz(~100 μm). In free running the beat note frequency of the QCL shows a shift of ~180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II,and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.     

Whistler-Mode Waves Growth by a Generalized Relativistic Kappa-Type Distribution

The instability of field-aligned Whistler-mode waves in space plasmas is studied by using a recently developed generalized relativistic kappa-type （KT） distribution. Numerical calculations are performed for a direct comparison between the new KT distribution and the current kappa distribution. We show that the wave growth for the KT distribution tends to occur in the lower wave frequency （e.g.,ω 〈~0.1Ω） due to a larger fractional number of the resonant electrons ηrel （which controls the wave growth）, while primarily locating in the higher wave frequency for the kappa distribution. Moreover, the relativistic anisotropy Are1 by the KT distribution is found to be smaller than that by the kappa distribution, leading to a smaller peak of wave growth. The results present a further understanding of plasma wave instability particularly in those plasmas where relativistic electrons are present.     

Connection of Screw Instability with Electric Current in an Accretion Disc around a Black Hole

The screw instability of the magnetic field is discussed based on its poloidal configuration generated by a single toroidal electric current flowing in the equatorial plane of a Kerr black hole （BH）. The rotation of the BH relative to the disc induces an electromotive force, which in turn results in a poloidal electric current. By using Ampere＇s law, we calculate the toroidal component of the magnetic field and derive a criterion for the screw instability of the magnetic field connecting the rotating BH with its surrounding disc. It is determined that the screw instability is related to two parameters： the radius of the disc and the BH spin. The occurrence of screw instability is depicted in a parameter space. In addition, we discuss the effect of the screw instability on magnetic extraction of energy from the rotating BH.     

Toroidicity and shape dependence of peeling mode growth rates in axisymmetric toroidal plasmas

The growth rate of the peeling mode instability with large toroidal mode number is calculated for general axisymmetric toroidal plasmas, including tokamaks and the spherical torus(ST) equilibia by using formalism presented by Connor et al.Analytic equilibia with non-zero edge current density and quasi-uniform current profiles are assumed. It is found that in sharp D-shape tokamak plasma, the derivative of the safety factor with respect to the poloidal flux becomes very large,making the perturbed poloidal motion very large, in turn making a significant reduction of the growth rate of the peeling mode, similar to the X-point effect in diverted plasma. The large aspect ratio effect is also studied, which reduces the growth rate further.     

Analytical Interaction of the Acoustic Wave and Turbulent Flame

A modified resonance model of a weakly turbulent flame in a high-frequency acoustic wave is derived analytically. Under the mechanism of Darrieus-Landau instability, the amplitude of flame wrinkles, which is as functions of the expansion coefficient and the perturbation wave number, increases greatly independent of the stationary＇ turbulence. The high perturbation wave number makes the resonance easier to be triggered but weakened with respect to the extra acoustic wave. In a closed burning chamber with the acoustic wave induced by the flame itself, the high perturbation wave number is to restrain the resonance for a realistic flame.     

Dynamic Processes of Cross-Tail Current in the Near-Earth Magnetotail

Current dynamic processes in realistic magnetotail geometry simulations under various driven conditions and Hall effects. are studied by Hall magnetohydrodynamic （MHD） Associated with the external driving force, a thin current sheet with a broad extent is built up in the near-Earth magnetotail. The time evolution for the formation of the current sheet comprises two phases： slow growth and a fast impulsive phase before the near-Earth disruption of the current sheet resulting from the fast magnetic reconnection. The simulation results indicate that as the external driving force increases, the site and the tailward speed of the near-Earth current disruption region are closer to the Earth and faster, respectively. Whether the near-Earth disruption of the current sheet takes place or not is mainly controlled by Hall effects. It is found that there is no sudden disruption of the current sheet in the near-Earth region if the ion inertial length is below di= 0.04.     

Coupling between velocity and interface perturbations in cylindrical Rayleigh–Taylor instability

Rayleigh–Taylor instability(RTI) in cylindrical geometry initiated by velocity and interface perturbations is investigated analytically through a third-order weakly nonlinear(WN) model. When the initial velocity perturbation is comparable to the interface perturbation, the coupling between them plays a significant role. The difference between the RTI growth initiated only by a velocity perturbation and that only by an interface perturbation in the WN stage is negligibly small. The effects of the mode number on the first three harmonics are discussed respectively. The low-mode number perturbation leads to large amplitudes of RTI growth. The Atwood number and initial perturbation dependencies of the nonlinear saturation amplitude of the fundamental mode are analyzed clearly. When the mode number of the perturbation is large enough,the WN results in planar geometry are recovered.     

Suppression of Rayleigh-Taylor instability by gyroviscosity and sheared axial flow in imploding plasma pinches

The synergistic stabilizing effect of gyroviscosity and sheared axial flow on the Rayleigh-Taylor instability in Z-pinch implosions is studied by means of the incompressible viscid magneto-hydrodynamic equations.The gyroviscosity(or finite Larmor radius) effects are introduced in the momentum equation through an anisotropic ion stress tensor.Dispersion relation with the effect of a density discontinuity is derived.The results indicate that the short-wavelength modes of the Rayleigh-Taylor instability are easily stabilized by the gyroviscosity effects.The long wavelength modes are stabilized by the sufficient sheared axial flow.However,the synergistic effects of the finite Larmor radius and sheared axial flow can heavily mitigate the Rayleigh-Taylor instability.This synergistic effect can compress the Rayleigh-Taylor instability to a narrow wave number region.Even with a sufficient gyroviscosity and large enough flow velocity,the synergistic effect can completely suppressed the Rayleigh-Taylor instability in whole wave number region.     

Two-Dimensional Wave Motion on the Charged Surface of a Viscous Liquid

The wave motion on the charged surface of a viscous Newtonian liquid is solved as an initial-value problem. Both the leaky dielectric and perfect dielectric cases are considered. The amplitude of wave is assumed to be small. The electric field induced by surface charge is shown to have a generally destabilizing effect on surface wave. The neutral stability curve is drawn in the （G, N~） plane （G： the gravitational bond number; N~： the electrical Bond number）. The Ohnesorge number, Taylor-Melcher number and permittivity ratio have little influence on the neutral stability curve. It is testified that the classical normal mode method cannot predict wave behaviour at small times.     

Numerical analysis of the optimized performance of the electron cyclotron wave system in a HL-2M tokamak

The capabilities of current drive, neoclassical tearing mode(NTM) stabilization, and sawtooth control are analyzed for the electron–cyclotron wave(ECW) system in a HL-2M tokamak. Better performance of the upper launcher is demonstrated in comparison with that of a dropped upper launcher, in terms of J_(EC)/J_(bs) for NTM stabilization and I_(ECCD)/(?ρtor)~2 for sawtooth control. 1-MW ECW power is enough for the 3/2 NTM stabilization, and 1.8-MW ECW power is required to suppress 2/1 NTM in a single null divertor equilibrium with 1.2-MA toroidal current with the upper launcher. Optimization simulation of electron–cyclotron current drive(ECCD) is carried out for three mirrors in an equatorial port, indicating that the middle mirror has a good performance compared with the top and bottom mirrors. The results for balanced co- and counter-ECCD in an equatorial port are also presented.     

Acoustic Rotation Modes in Complex Plasmas

Acoustic rotation modes in complex plasmas are investigated in a cylindrical system with an axial symmetry. The linear mode solution is derived. The mode in an infinite area is reduced to a classical dust acoustic wave in the region away from the centre. When the dusty plasma is confined in a finite region, the breathing and rotating-void behaviour are observed. Vivid structures of different mode number solutions are illustrated.     

Three-Dimensional Waves in Tilt Thermal Boundary Layers

We numerically and theoretically study the stabilities of tilt thermal boundary layers immersed in stratified air. An interesting phenomenon is revealed： the stationary longitudinal-roll mode becomes unstable to some oscillating state even when the Grashof number is smaller than its corresponding critical value. By stability analysis, this phenomenon is explained in terms of a new three-dimensional wave mode. The effect of the tilt angle on the stability of the boundary flows is investigated. Since the new three-dimensional wave is found to be the most unstable mode when the title angle is between 30° and 64°, it is expected to play an important role in the transition to turbulence.     

Cluster Observation of Eelectrostatic Solitary Waves around Magnetic Null Point in Thin Current Sheet

Electrostatic solitary waves （ESWs） are observed in the vicinity of the magnetic null of the widely studied magnetic reconnection taking place at the near-earth tail when current sheet becomes dramatic thinning during substorm time on 1 October 2001. We use the Imada method for the 2-D reconnection model and study the characteristics of ESWs near the X-line region and the magnetic null points. The result shows that the amplitude of the observed ESWs in the vicinity of X-line region ranges from 0.1mV/m to 5mV/m, and the amplitude is larger near the magnetic null points. The generation mechanism and the role of ESWs associated with magnetic reconnection are also discussed.     

Nonlinear saturation amplitude of cylindrical Rayleigh Taylor instability

The nonlinear saturation amplitude （NSA） of the fundamental mode in the classical Rayleigh-Taylor instability with a cylindrical geometry for an arbitrary Atwood number is analytically investigated by considering the nonlinear corrections up to the third order. The analytic results indicate that the effects of the initial radius of the interface （r0） and the Atwood number （A） play an important role in the NSA of the fundamental mode. The NSA of the fundamental mode first increases gently and then decreases quickly with increasing A. For a given A, the smaller the ro/λ（λ is the perturbation wavelength）, the larger the NSA of the fundamental mode. When ro/λ is large enough （r0 〉〉 λ）, the NSA of the fundamental mode is reduced to the prediction in the previous literatures within the framework of the third-order perturbation theory.     

Simulation on effect of poloidal power spectrum on lower hybrid wave propagation

The coupling of lower hybrid wave to the plasma is a crucial issue for efficient current drive in tokamaks. This paper establishes a new coupling model which assumes the antenna to be a curved face and the plasma to be a cylinder. Power spectrum considering the coupling between wave-guides in both poloidal and toroidal direction is simply estimated and discussed. The effect of the poloidal wave vector on wave propagation, power deposition and driven current is also investigated with the help of lower hybrid current drive code. Results show that the poloidal wave vector affects the ray tracing, and also has effect on power deposition and driven current. The effect of the poloidal wave vector on power deposition and driven current profile depends on plasma parameters. Preliminary studies suggest that it seems possible to control the current profile by adjusting the poloidal phase difference between the waveguide in poloidal direction.     

Instability and Death of Spiral Wave in a Two-Dimensional Array of Hindmarsh-Rose Neurons

Spiral wave could be observed in the excitable media, the neurons are often excitable within appropriate parameters. The appearance and formation of spiral wave in the cardiac tissue is linked to monomorphic ventricular tachycardia that can denervate into polymorphic tachycardia and ventricular fibrillation. The neuronal system often consists of a large number of neurons with complex connections. In this paper, we theoretically study the transition from spiral wave to spiral turbulence and homogeneous state （death of spiral wave） in two-dimensional array of the Hindmarsh-Rose neuron with completely nearest-neighbor connections. In our numerical studies, a stable rotating spiral wave is developed and selected as the initial state, then the bifurcation parameters are changed to different values to observe the transition from spiral wave to homogeneous state, breakup of spiral wave and weak change of spiral wave, respectively. A statistical factor of synchronization is defined with the mean field theory to analyze the transition from spiral wave to other spatial states, and the snapshots of the membrane potentials of all neurons and time series of mean membrane potentials of all neurons are also plotted to discuss the change of spiral wave. It is found that the sharp changing points in the curve for factor of synchronization vs. bifurcation parameter indicate sudden transition from spiral wave to other states. And the results are independent of the number of neurons we used.     

Numerical Simulation on a Possible Formation Mechanism of Interplanetary Magnetic Cloud Boundaries

The formation med2aniRm of the interplanetary magnetic cloud (MC) boundaries is numerically investigated by simulating the interactions between an MC of some initial momentum and a local interplanetary current sheet.The compressible 2.51:) MHD equations are solved. R~sults show that the magnetic reconnection process is a possible formation mechanism when an MC interacts with a surrounding current sheet. A number of interesting features are found. For instance, the front boundary of the MCs is a magnetic reconnection boundary that could be caused by a driven reconnection ahead of the cloud, and the tail boundary might be caused by the driving of the entrained flow as a result of the Bernoulli principle. Analysis of the magnetic field and plasma data demonstrates that at these two boundaries appear large value of the plasma parameter β, clear increase of plasma temperature and density, distinct decrease of magnetic magnitude, and a transition of maguetic field direction of about 180 degrees. The outcome of the present simulation agrees qualitatively with the observational results on MC boundary inferred from IMP-8, etc.     

Development and transition of spiral wave in the coupled Hindmarsh--Rose neurons in two-dimensional space

The dynamics and the transition of spiral waves in the coupled Hindmarsh--Rose (H--R) neurons in two-dimensional space are investigated in the paper. It is found that the spiral wave can be induced and developed in the coupled HR neurons in two-dimensional space, with appropriate initial values and a parameter region given. However, the spiral wave could encounter instability when the intensity of the external current reaches a threshold value of 1.945. The transition of spiral wave is found to be affected by coupling intensity D and bifurcation parameter r. The spiral wave becomes sparse as the coupling intensity increases, while the spiral wave is eliminated and the whole neuronal system becomes homogeneous as the bifurcation parameter increases to a certain threshold value. Then the coupling action of the four sub-adjacent neurons, which is described by coupling coefficient D’, is also considered, and it is found that the spiral wave begins to breakup due to the introduced coupling action from the sub-adjacent neurons (or sites) and together with the coupling action of the nearest-neighbour neurons, which is described by the coupling intensity D.     

Numerical Simulation on a Possible Formation Mechanism of Interplanetary Magnetic Cloud Boundaries

The formation mechanism of the interplanetary magnetic cloud (MC) boundaries is numerically investigatedby simulating the interactions between an MC of some initial momentum and a local interplanetary current sheet.The compressible 2.5D MHD equations are solved. Results show that the magnetic reconnection process is a possibleformation mechanism when an MC interacts with a surrounding current sheet. A number of interesting features arefound. For instance, the front boundary of the MCs is a magnetic reconnection boundary that could be caused by adriven reconnection ahead of the cloud, and the tail boundary might be caused by the driving of the entrained flowas a result of the Bernoulli principle. Analysis of the magnetic field and plasma data demonstrates that at these twoboundaries appear large value of the plasma parameterβ, clear increase of plasma temperature and density, distinctdecrease of magnetic magnitude, and a transition of magnetic field direction of about 180 degrees. The outcome of thepresent simulation agrees qualitatively with the observational results on MC boundary inferred from IMP-8, etc.