211 research outputs found
Kinetic cascade in solar-wind turbulence: 3D3V hybrid-kinetic simulations with electron inertia
Understanding the nature of the turbulent fluctuations below the ion
gyroradius in solar-wind turbulence is a great challenge. Recent studies have
been mostly in favor of kinetic Alfv\'en wave (KAW) type of fluctuations, but
other kinds of fluctuations with characteristics typical of magnetosonic,
whistler and ion Bernstein modes, could also play a role depending on the
plasma parameters. Here we investigate the properties of the sub-proton-scale
cascade with high-resolution hybrid-kinetic simulations of freely-decaying
turbulence in 3D3V phase space, including electron inertia effects. Two proton
plasma beta are explored: the "intermediate" and "low"
regimes, both typically observed in solar wind and corona. The
magnetic energy spectum exhibits and power laws
at , while they are slightly steeper at . Nevertheless,
both regimes develop a spectral anisotropy consistent with at , and pronounced small-scale intermittency.
In this context, we find that the kinetic-scale cascade is dominated by
KAW-like fluctuations at , whereas the low- case presents a
more complex scenario suggesting the simultaneous presence of different types
of fluctuations. In both regimes, however, a non-negligible role of ion
Bernstein type of fluctuations at the smallest scales seems to emerge.Comment: 6 pages, 5 figures, final version published in The Astrophysical
Journal Letters: Cerri, Servidio & Califano, ApJL 846, L18 (2017
The third-order law for increments in magnetohydrodynamic turbulence with constant shear
We extend the theory for third-order structure functions in homogeneous
incompressible magnetohydrodynamic (MHD) turbulence to the case in which a
constant velocity shear is present. A generalization is found of the usual
relation [Politano and Pouquet, Phys. Rev. E, 57 21 (1998)] between third-order
structure functions and the dissipation rate in steady inertial range
turbulence, in which the shear plays a crucial role. In particular, the
presence of shear leads to a third-order law which is not simply proportional
to the relative separation. Possible implications for laboratory and space
plasmas are discussed
Generation of non-Gaussian statistics and coherent structures in ideal magnetohydrodynamics
Spectral method simulations of ideal magnetohydrodynamics are used to investigate production of coherent small scale structures, a feature of fluid models that is usually associated with inertial range signatures of nonuniform dissipation, and the associated emergence of non-Gaussian statistics. The near-identical growth of non-Gaussianity in ideal and nonideal cases suggests that generation of coherent structures and breaking of self-similarity are essentially ideal processes. This has important implications for understanding the origin of intermittency in turbulence
Magnetic field reversals and long-time memory in conducting flows
Employing a simple ideal magnetohydrodynamic model in spherical geometry,we
show that the presence of either rotation or finite magnetic helicity is
sufficient to induce dynamical reversals of the magnetic dipole moment. The
statistical character of the model is similar to that of terrestrial magnetic
field reversals, with the similarity being stronger when rotation is
present.The connection between long time correlations, noise, and
statistics of reversals is supported, consistent with earlier suggestions.Comment: accepted in Physical Review
Thermodynamic ground states of platinum metal nitrides
The thermodynamic stabilities of various phases of the nitrides of the
platinum metal elements are systematically studied using density functional
theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal
structures, in which the metal ions occupy simple tetragonal lattice sites,
have lower formation enthalpies at ambient conditions than any previously
proposed structures. The region of stability with respect to those structures
extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal
structures at this pressure are thermodynamically stable also with respect to
phase separation. The fact that the local density and generalized gradient
approximations predict different values of the absolute formation enthalpies as
well different relative stabilities between simple tetragonal and the pyrite or
marcasite structures are further discussed.Comment: 5 pages, 4 figure
Phase space dynamics of unmagnetized plasmas: collisionless and collisional regimes
Eulerian electrostatic kinetic simulations of unmagnetized plasmas (kinetic
electrons and motionless protons) with high-frequency equilibrium perturbations
have been employed to investigate the phase space energy transfer across
spatial and velocity scales, associated with the resonant interaction of
electrons with the self-induced electric field. Numerical runs cover a wide
range of collisionless and weakly collisional plasma regimes. An analysis
technique based on the Fourier-Hermite transform of the particle distribution
function allows to point out how kinetic processes trigger the phase space
energy cascade, which is instead inhibited at finer scales when collisions are
turned on. Numerical results are presented and discussed for the cases of
linear wave Landau damping, nonlinear electron trapping, bump-on-tail and
two-stream instabilities. A more realistic situation of turbulent Langmuir
fluctuations is also discussed in detail. Fourier-Hermite transform shows an
energy spread, highly conditioned by collisions, which involves velocity scales
more quickly than the spatial scales, even when nonlinear effects are dominant.
This results in anisotropic spectra whose slopes are compatible with
theoretical expectations. Finally, an exact conservation law has been derived,
which describes the time evolution of the free energy of the system, taking
into account the collisional dissipation.Comment: 15 pages, 7 figure
The Mediational Role of Coping Strategies in the Relationship Between Self-Esteem and Risk of Internet Addiction
The aim of the present study is to explore, through a mediation model, the relationship among self-esteem, coping strategies, and the risk of Internet addiction in a sample of 300 Italian university students. We submitted the data to a descriptive, mediational comparison between variables (t-test), and correlational statistical analyses. The results confirmed the effect of self-esteem on the risk of Internet addiction. However, we found that the introduction of coping strategies as a mediator gives rise to partial mediation. A low level of self-esteem is a predictor of avoidance-oriented coping that, in turn, affects the risk of Internet addiction
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