How Alfvén waves induce compressive flows in the neighborhood of a 2.5D magnetic null-point.

The aim of the present study is to provide insight on the induced compressive perturbations together with the modifications of the environmental parameters in the course of Alfvén wave interaction with a solar magnetic null-point. The shock-capturing Godunov-type code PLUTO is used to solve the set of ideal magnetohydrodynamic equations. The nonlinear effects connected with an initial Alfvén pulse nearing a magnetic null point induces fast and slow magnetoacoustic waves with anti phase conduct. The induced current density and flows are independent of the local plasma-[Formula: see text] at the reconnection site. The induced inflows and outflows highly depend on the polarization. The inflows have a stronger effect compared to the outflows in both the x and y directions showing its peak in the x-direction. The dominant wave that couples to flows is the fast wave due to the in-phase harmony between perturbations of the compressive parameters and the fast wave. The induced current density possesses a steady orientation at the reconnection site which governs the diffusion or propagation of the waves. Induced perturbations by the nonlinear forces together with their back reaction on the Alfvén wave have a significant role in the current density excitation being responsible for the creation of inflows and outflows that are possible candidates for the creation of solar jets which has a significant contribution towards coronal seismology.

Particle-in-cell Simulations of the Whistler Heat-flux Instability in Solar Wind Conditions.

In collision-poor plasmas from space, e.g., solar wind or stellar outflows, the heat flux carried by the strahl or beaming electrons is expected to be regulated by the self-generated instabilities. Recently, simultaneous field and particle observations have indeed revealed enhanced whistler-like fluctuations in the presence of counter-beaming populations of electrons, connecting these fluctuations to the whistler heat-flux instability (WHFI). This instability is predicted only for limited conditions of electron beam-plasmas, and has not yet been captured in numerical simulations. In this Letter we report the first simulations of WHFI in particle-in-cell setups, realistic for the solar wind conditions, and without temperature gradients or anisotropies to trigger the instability in the initiation phase. The velocity distributions have a complex reaction to the enhanced whistler fluctuations conditioning the instability saturation by a decrease of the relative drifts combined with induced (effective) temperature anisotropies (heating the core electrons and pitch-angle and energy scattering the strahl). These results are in good agreement with a recent quasilinear approach, and support therefore a largely accepted belief that WHFI saturates at moderate amplitudes. In the anti-sunward direction the strahl becomes skewed with a pitch-angle distribution decreasing in width as electron energy increases, which seems to be characteristic of self-generated whistlers and not to small-scale turbulence.

Nonlinear three-wave interaction in pair plasmas.

It is shown that nonlinear three-wave interaction, described by vector-product-type nonlinearities, in pair plasmas implies much more restrictive conditions for a double energy transfer, as compared to electron-ion plasmas.

Kinetic instability of the dust acoustic mode in inhomogeneous, partially magnetized plasma with both positively and negatively charged grains.

A purely kinetic instability of the dust acoustic mode in inhomogeneous plasmas is discussed. In the presence of a magnetic field, electrons and ions may be magnetized while at the same time dust grains may remain unmagnetized. Although the dynamics of the light species is strongly affected by the magnetic field, the dust acoustic mode may still propagate in practically any direction. The inhomogeneity implies a source of free energy for an instability that develops through the diamagnetic drift effects of the magnetized species. It is shown that this may be a powerful mechanism for the excitation of dust acoustic waves. The analysis presented in the work is also directly applicable to plasmas containing both positive and negative ions and electrons, provided that at least one of the two ion species is unmagnetized.

Ion temperature gradient instability in a dusty plasma.

An analysis of the temperature-gradient-driven ( eta(i) ) instability of drift waves in dusty plasma is presented. Various limits that allow for the coupling of the drift wave with the dynamics of dust grains are discussed. In particular, the cases of tiny (magnetized) and relatively heavy (unmagnetized) grains are studied. It is shown that in both limits the behavior of the eta(i) mode is considerably affected by the dust dynamics. The growth rate turns out to be higher in the presence of dust, and the instability threshold is lower, resulting in a more unstable plasma.

Interchange mode in the presence of dust.

The linear and nonlinear development of an electrostatic interchange mode which involves a magnetized nonuniform electron-ion fluid in the presence of nonuniform static charged dust grains is investigated. The charge on grains is taken as spatially dependent, and the consequences of that condition are investigated. It is shown that standardly accepted stabilization of the interchange mode in the presence of negatively charged grains can be violated due to the spatial dependence of the charge on grains. Also, the ion drift, which is caused by the action of a gravity term perpendicular to the magnetic field lines, is taken as nonuniform as a result of the magnetic field nonuniformity, and it is shown that due to such a nonuniformity the instability condition can be significantly modified. In the nonlinear regime several types of coherent stationary vortex structures are found: namely, dipolar and tripolar vortices and vortex chains. The dipolar vortex is found to propagate in the direction of the ion drift, while the tripole and vortex chains are carried by the drift flow. The spatial dependence of these structures is determined by parameters describing the nonuniformity of the equilibrium plasma.

Analytical description of a neutral-induced tripole vortex in a plasma.

An analytical description of a stationary triple vortex, observed in a cylindrical plasma, is presented. The concentration of neutrals, which is rather high in the experiment, turns out to be of crucial importance due to a spatially dependent distribution. In the radial direction the neutral concentration is paraboliclike, yielding an effective radial force directed towards the axis of the system. This neutral force causes the rotation of the plasma in the direction which is opposite to the E-->xB--> drift. The stationary triple vortex develops for a starting Gaussian-density distribution and a rigid-body rotation of the plasma column.

Three-wave interaction in a self-gravitating fluid.

Nonlinear three-wave interaction is investigated in rotating self-gravitating astrophysical fluids. Both direct and inverse cascades are found. The latter should be of importance for the formation of structures in rotating astrophysical objects like protogalaxies and galaxies. Linear gravitational instability is shown to be a process that develops on much longer time scales, compared to the nonlinear wave interaction, and the nonlinear precipitation of energy from the linearly unstable, slowly contracting mode towards smaller spatial and time scales is shown to be possible.

Effects of dust charge fluctuations on current-driven dust-ion-acoustic waves.

The effects of fluctuating grain charge on current-driven dust-ion-acoustic modes are investigated in the presence of various physical parameters which are varied in order to describe the behavior of the mode in detail. Variation of the charge clearly influences the ion-acoustic mode by changing of its frequency and growth rate. We report here the existence of a dust charge fluctuation mode in the system, and it is shown that the mode is unstable for a negative charge on the dust per unit volume not exceeding some critical value.

Intermediate shocks in three-dimensional magnetohydrodynamic bow-shock flows with multiple interacting shock fronts

Simulation results of three-dimensional (3D) stationary magnetohydrodynamic (MHD) bow-shock flows around perfectly conducting spheres are presented. For strong upstream magnetic field a new complex bow-shock flow topology arises consisting of two consecutive interacting shock fronts. It is shown that the leading shock front contains a segment of intermediate 1-3 shock type. This is the first confirmation in 3D that intermediate shocks, which were believed to be unphysical for a long time, can be formed and can persist for small-dissipation MHD in a realistic flow configuration.