Z箍缩等离子体磁重联现象

分析了磁重联对晕等离子体加速和能量平衡过程的影响。分析表明晕等离子体向轴心的加速过程分为两个阶段:第一阶段晕等离子体在磁压或热压(依赖于丝数)作用下向轴心运动;第二阶段晕等离子体由于磁重联过程被加速到阿尔芬速度,并到达轴心形成先驱等离子体。估算表明重联层的厚度与离子的惯性长度具有相同的数量级,电流片内电子运动和离子的运动是解耦合的。在内爆滞止阶段电荷分离产生强的径向电场,这个电场将磁能转化为等离子体轴向(z方向)动能,内爆动能和轴向动能共同转化为X射线辐射能,以此解释了X射线能量大于内爆动能这一观测结果。分析了磁重联电磁脉冲的性质,对于1 MA驱动条件,电磁脉冲的总能量可达kJ量级。     

Z箍缩等离子体磁重联现象

分析了磁重联对晕等离子体加速和能量平衡过程的影响。分析表明晕等离子体向轴心的加速过程分为两个阶段:第一阶段晕等离子体在磁压或热压(依赖于丝数)作用下向轴心运动;第二阶段晕等离子体由于磁重联过程被加速到阿尔芬速度,并到达轴心形成先驱等离子体。估算表明重联层的厚度与离子的惯性长度具有相同的数量级,电流片内电子运动和离子的运动是解耦合的。在内爆滞止阶段电荷分离产生强的径向电场,这个电场将磁能转化为等离子体轴向(z方向)动能,内爆动能和轴向动能共同转化为X射线辐射能,以此解释了X射线能量大于内爆动能这一观测结果。分析了磁重联电磁脉冲的性质,对于1 MA驱动条件,电磁脉冲的总能量可达kJ量级。     

Three-dimensional dynamics of collisionless magnetic reconnection in large-scale pair plasmas

Using the largest three-dimensional particle-in-cell simulations to date, collisionless magnetic reconnection in large-scale electron-positron plasmas without a guide field is shown to involve complex interaction of tearing and kink modes. The reconnection onset is patchy and occurs at multiple sites which self-organize to form a single, large diffusion region. The diffusion region tends to elongate in the outflow direction and become unstable to secondary kinking and formation of "plasmoid-rope" structures with finite extent in the current direction. The secondary kink folds the reconnection current layer, while plasmoid ropes at times follow the folding of the current layer. The interplay between these secondary instabilities plays a key role in controlling the time-dependent reconnection rate in large-scale systems.     

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.     

Temperature gradients in fast collisionless magnetic reconnection

Temperature gradients are shown to deform and shift the magnetic islands that grow during fast collisionless reconnection when electron inertia decouples the plasma motion from the magnetic field. A kinetic electron model describes the collisionless processes during the reconnection of field lines originating in regions with different temperatures. Using a novel model of the reconnecting instability as a surface mode, the kinetic effects are treated analytically in the linear and nonlinear stages of the instability of a current-carrying low-beta plasma slab in a strong magnetic guide field.     

Chaotic collisionless reconnection in Jupiter's nightside magnetosphere

Summary Recently it has been shown that an X-type magnetic neutral line may form in Jupiter's nightside magnetosphere as the result of the current flowing in the plasma disc. Still, a collisionless reconnection mechanism is required to tear up the magnetic field. Recently, Büchner and Zelenyi showed that the chaotization of the electron motion can lead to fast collisionless reconnection in the Earth's magnetotail. In their theory, enhanced pitch angle diffusion is obtained when the curvature parameter κ_e decreases to κ_e ≃ 1.6. We apply Büchner and Zelenyi's theory to Jupiter's magnetosphere. The curvature parameter is obtained from the measured plasma parameters and from a self-consistent equilibrium model of the magnetic field, and the results for κ_e show that the larger the assumed plasma disc, the larger is the range where κ_e is less than or equal to the stochasticity threshold 1.6. This indicates that chaotic magnetic-field line reconnection may occur in Jupiter's nightside magnetosphere around 60R _J from the planet, and is consistent with thein situ magnetic-field observations, that show the signature of magnetic islands and of tearing-mode instability in the relevant region. We speculate that these processes, differently from the Earth's case, occur in a semi-steady way. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Three-species collisionless reconnection: effect of O+ on magnetotail reconnection

The nature of collisionless reconnection in a three-species plasma composed of a heavy species, protons, and electrons is examined. In addition to the usual two length scales present in two-species reconnection, there are two additional larger length scales in the system: one associated with a "heavy whistler" which produces a large scale quadrupolar out-of-plane magnetic field, and one associated with the "heavy Alfvén" wave which can slow the outflow speed and thus the reconnection rate. The consequences for reconnection with O+ present in the magnetotail are discussed.     

Resistive wall mode in collisionless quasistationary plasmas

The stability analysis of the n=1 resistive wall mode is carried out for a simplified model of collisionless tokamak plasma. It is found that the trapped particle compressibility and the resonance between the mode and the precession drift frequency lead to a significant improvement of the beta stability limits. It is shown that, within the frame of the simplified model, the resistive wall mode can be fully suppressed and the plasma can be stable up to the wall beta limits for a slow plasma rotation.     

Low-frequency relativistic electromagnetic solitons in collisionless plasmas

A relativistic electromagnetic soliton solution in the model of a one-dimensional, unbounded, cold, collisionless plasma is obtained without using the envelope approximation. The breaking of solitons with over-critical amplitudes is observed. The stability of undercritical solitons and the breaking of overcritical solitons are demonstrated by a particle-in-cell computer simulation. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 33–38 (10 July 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Kinetic models of shocks in collisionless plasmas

Summary The role played by kinetic effects in some shock waves is described with particular attention being given to perpendicular shocks which are, at present, better understood than any other type. At low values of Mach number and β, the way in which instabilities produce an enhanced resistivity is described, while at higher Mach numbers the nonfluid behaviour of the ions with resultant ion heating is emphasized.

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Riassunto Si descrive il ruolo svolto da effetti cinetici in alcune onde d'urto con particolare attenzione per gli uti perpendicolari che sono, attualmente, meglio compresi di ogni altro tipo. Si descrive il modo in cui instabilit⦏ producono una miglior resistività per bassi valori del numero di Mach e di β, mentre per numeri di Mach più alti si mette in evidenza il comportamento non fluido degli ioni con risultante riscaldamento ionico.

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Резюме Описывается роль, которую играют кинетические эффекты в некоторых ударных волнах, причем, особое внимание уделяется перпендикулярным ударным волнам, которые в настоящее время исследованы лучше, чем другие типы ударных волн. При малых значениях числа Маха и β описывается способ, в котором неустойчивости приводят к увеличенному сопротивлению, тогда как при больших значения числа Маха отмечается поведение ионов, не свойственное жидкости, с результирующим нагревом ионов.

     

New measure of the dissipation region in collisionless magnetic reconnection

A new measure to identify a small-scale dissipation region in collisionless magnetic reconnection is proposed. The energy transfer from the electromagnetic field to plasmas in the electron's rest frame is formulated as a Lorentz-invariant scalar quantity. The measure is tested by two-dimensional particle-in-cell simulations in typical configurations: symmetric and asymmetric reconnection, with and without the guide field. The innermost region surrounding the reconnection site is accurately located in all cases. We further discuss implications for nonideal MHD dissipation.     

Nonlinear dispersion of stationary waves in collisionless plasmas

A nonlinear dispersion of a general stationary wave in collisionless plasma is obtained in a nondifferential form expressed in terms of a single-particle oscillation-center Hamiltonian. For electrostatic oscillations in nonmagnetized plasma, considered as a paradigmatic example, the linear dielectric function is generalized, and the trapped particle contribution to the wave frequency shift Δω is found analytically as a function of the wave amplitude a. Smooth distributions yield Δω ～ a(1/2), as usual. However, beamlike distributions of trapped electrons result in different power laws, or even a logarithmic nonlinearity, which are derived as asymptotic limits of the same dispersion relation.     

Very high Mach-number electrostatic shocks in collisionless plasmas

The kinetic theory of collisionless electrostatic shocks resulting from the collision of plasma slabs with different temperatures and densities is presented. The theoretical results are confirmed by self-consistent particle-in-cell simulations, revealing the formation and stable propagation of electrostatic shocks with very high Mach numbers (M>10), well above the predictions of the classical theories for electrostatic shocks.     

Dynamics of quadruple laser beams in collisionless plasmas

Dynamics of a quadruple laser beam propagating through collisionless plasma has been investigated. The dielectric function of the plasma has been obtained for ponderomotive nonlinearity. Moment theory has been used to find the solution of nonlinear Schrodinger wave equation for the field of laser beam. Particular emphases is put on the variations of both the beam width and longitudinal phase delay with distance of propagation through the plasma. Self-trapping of the laser beam has been also investigated.