Driven dust-charge fluctuation and chaotic ion dynamics in the plasma sheath and pre-sheath regions

Possible existence of chaotic oscillations in ion dynamics in the sheath and pre-sheath regions of a dusty plasma, induced by externally driven dust-charge fluctuation, is presented in this work. In a complex plasma, dust charge fluctuation occurs continuously with time due to the variation of electron and ions current flowing into the dust particles. In most of the works related to dust-charge fluctuation, theoretically it is assumed that the average dust-charge fluctuation follows the the plasma perturbation, while in reality, the dust-charge fluctuation is a semi-random phenomena, fluctuating about some average value. The very cause of dust-charge fluctuation in a dusty plasma also points to the fact that these fluctuations can be driven externally by changing electron and ion currents to the dust particles. With the help of a \emph{hybrid}-Particle in Cell-Monte Carlo (\emph{h}-PIC-MCC) code in this work, we use the plasma sheath as a candidate for driving the dust-charge fluctuation by periodically exposing the sheath-side wall to UV radiation, causing photoemission of electrons, which in turn drive the dust-charge fluctuation. We show that this \emph{driven} dust-charge fluctuation can induce a chaotic response in the ion dynamics in the sheath and the pre-sheath regions.Comment: 16 pages, 11 figure

Simulating Rayleigh-Taylor induced magnetohydrodynamic turbulence in prominences

Solar prominences represent large-scale condensations suspended against gravity within the solar atmosphere. The Rayleigh-Taylor (RT) instability is proposed to be one of the important fundamental processes leading to the generation of dynamics at many spatial and temporal scales within these long-lived, cool, and dense structures amongst the solar corona. We run 2.5D ideal magnetohydrodynamic (MHD) simulations with the open-source MPI-AMRVAC code far into the nonlinear evolution of an RT instability perturbed at the prominence-corona interface. Our simulation achieves a resolution down to $\sim 23$ km on a 2D $(x,y)$ domain of size 30 Mm $\times$ 30 Mm. We follow the instability transitioning from a multi-mode linear perturbation to its nonlinear, fully turbulent state. Over the succeeding $\sim 25$ minute period, we perform a statistical analysis of the prominence at a cadence of $\sim 0.858$ s. We find the dominant guiding $B_z$ component induces coherent structure formation predominantly in the vertical velocity $V_y$ component, consistent with observations, demonstrating an anisotropic turbulence state within our prominence. We find power-law scalings in the inertial range for the velocity, magnetic, and temperature fields. The presence of intermittency is evident from the probability density functions of the field fluctuations, which depart from Gaussianity as we consider smaller and smaller scales. In exact agreement, the higher-order structure functions quantify the multifractality, in addition to different scale characteristics and behavior between the longitudinal and transverse directions. Thus, the statistics remain consistent with the conclusions from previous observational studies, enabling us to directly relate the RT instability to the turbulent characteristics found within quiescent prominence.Comment: 21 pages, 17 figures, Accepted for publication in Astronomy and Astrophysic

Swift generator for three-dimensional magnetohydrodynamic turbulence

International audienceMagnetohydrodynamic turbulence is central to laboratory and astrophysical plasmas, and is invoked for interpreting many observed scalings. Verifying predicted scaling law behavior requires extreme-resolution direct numerical simulations (DNS), with needed computing resources excluding systematic parameter surveys. We here present an analytic generator of realistically looking turbulent magnetic fields, that computes threedimensional (3D) O(1000 3) solenoidal vector fields in minutes to hours on desktop computers. Our model is inspired by recent developments in 3D incompressible fluid turbulence theory, where a Gaussian white noise vector subjected to a nonlinear transformation results in an intermittent, multifractal random field. Our B × C model has only few parameters that have clear geometric interpretations. We directly compare a (costly) DNS with a swiftly B × C-generated realization, in terms of its (1) characteristic sheetlike structures of current density, (2) volume-filling aspects across current intensity, (3) power-spectral behavior, (4) probability distribution functions of increments for magnetic field and current density, structure functions, and spectra of exponents, and (5) partial variance of increments. The model even allows to mimic time-evolving magnetic and current density distributions and can be used for synthetic observations on 3D turbulent data cubes