499 research outputs found
Plasma turbulence at ion scales: a comparison between PIC and Eulerian hybrid-kinetic approaches
Kinetic-range turbulence in magnetized plasmas and, in particular, in the
context of solar-wind turbulence has been extensively investigated over the
past decades via numerical simulations. Among others, one of the widely adopted
reduced plasma model is the so-called hybrid-kinetic model, where the ions are
fully kinetic and the electrons are treated as a neutralizing (inertial or
massless) fluid. Within the same model, different numerical methods and/or
approaches to turbulence development have been employed. In the present work,
we present a comparison between two-dimensional hybrid-kinetic simulations of
plasma turbulence obtained with two complementary approaches spanning about two
decades in wavenumber - from MHD inertial range to scales well below the ion
gyroradius - with a state-of-the-art accuracy. One approach employs hybrid
particle-in-cell (HPIC) simulations of freely-decaying Alfv\'enic turbulence,
whereas the other consists of Eulerian hybrid Vlasov-Maxwell (HVM) simulations
of turbulence continuously driven with partially-compressible large-scale
fluctuations. Despite the completely different initialization and
injection/drive at large scales, the same properties of turbulent fluctuations
at are observed. The system indeed self-consistently
"reprocesses" the turbulent fluctuations while they are cascading towards
smaller and smaller scales, in a way which actually depends on the plasma beta
parameter. Small-scale turbulence has been found to be mainly populated by
kinetic Alfv\'en wave (KAW) fluctuations for , whereas KAW
fluctuations are only sub-dominant for low-.Comment: 18 pages, 4 figures, accepted for publication in J. Plasma Phys.
(Collection: "The Vlasov equation: from space to laboratory plasma physics"
Timing mirror structures observed by Cluster with a magnetosheath flow model
The evolution of structures associated with mirror modes during their flow in
the Earth's magnetosheath is studied. The fact that the related magnetic
fluctuations can take distinct shapes, from deep holes to high peaks, has
been assessed in previous works on the observational, modeling and numerical
points of view. In this paper we present an analytical model for the flow
lines and velocity magnitude inside the magnetosheath. This model is used to
interpret almost 10 years of Cluster observations of mirror structures: by
back tracking each isolated observation to the shock, the "age", or flow
time, of these structures is determined together with the geometry of the
shock. Using this flow time the evolutionary path of the structures may be
studied with respect to different quantities: the distance to mirror
threshold, the amplitude of mirror fluctuations and the skewness of the
magnetic amplitude distribution as a marker of the shape of the structures.
These behaviours are confronted to numerical simulations which confirm the
dynamical perspective gained from the association of the statistical analysis
and the analytical model: magnetic peaks are mostly formed just behind the
shock and are quickly overwhelmed by magnetic holes as the plasma conditions
get more mirror stable. The amplitude of the fluctuations are found to
saturate before the skewness vanishes, i.e. when both structures
quantitatively balance each other, which typically occurs after a flow time
of 100â200 s in the Earth's magnetosheath. Comparison with other astrophysical
contexts is discussed
Fast Acceleration of Transrelativistic Electrons in Astrophysical Turbulence
Highly energetic, relativistic electrons are commonly present in many
astrophysical systems, from solar flares to the intra-cluster medium, as
indicated by observed electromagnetic radiation. However, open questions remain
about the mechanisms responsible for their acceleration, and possible
re-acceleration. Ubiquitous plasma turbulence is one of the possible universal
mechanisms. We study the energization of transrelativistic electrons in
turbulence using hybrid particle-in-cell, which provide a realistic model of
Alfv\'{e}nic turbulence from MHD to sub-ion scales, and test particle
simulations for electrons. We find that, depending on the electron initial
energy and turbulence strength, electrons may undergo a fast and efficient
phase of energization due to the magnetic curvature drift during the time they
are trapped in dynamic magnetic structures. In addition, electrons are
accelerated stochastically which is a slower process that yields lower maximum
energies. The combined effect of these two processes determines the overall
electron acceleration. With appropriate turbulence parameters, we find that
superthermal electrons can be accelerated up to relativistic energies. For
example, with heliospheric parameters and a relatively high turbulence level,
rapid energization to MeV energies is possible.Comment: Accepted for publication in The Astrophysical Journa
The role of parametric instabilities in turbulence generation and proton heating: Hybrid simulations of parallel propagating Alfv\'en waves
Large amplitude Alfv\'en waves tend to be unstable to parametric
instabilities which result in a decay process of the initial wave into
different daughter waves depending upon the amplitude of the fluctuations and
the plasma beta. The propagation angle with respect to the mean magnetic field
of the daughter waves plays an important role in determining the type of decay.
In this paper, we revisit this problem by means of multi-dimensional hybrid
simulations. In particular, we study the decay and the subsequent nonlinear
evolution of large-amplitude Alfv\'en waves by investigating the saturation
mechanism of the instability and its final nonlinear state reached for
different wave amplitudes and plasma beta conditions. As opposed to
one-dimensional simulations where the Decay instability is suppressed for
increasing plasma beta values, we find that the decay process in
multi-dimensions persists at large values of the plasma beta via the
filamentation/magnetosonic decay instabilities. In general, the decay process
acts as a trigger both to develop a perpendicular turbulent cascade and to
enhance mean field-aligned wave-particle interactions. We find indeed that the
saturated state is characterized by a turbulent plasma displaying a
field-aligned beam at the Alfv\'en speed and increased temperatures that we
ascribe to the Landau resonance and pitch angle scattering in phase space
Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: from fluid to kinetic modeling
The nonlinear evolution of collisionless plasmas is typically a multi-scale
process where the energy is injected at large, fluid scales and dissipated at
small, kinetic scales. Accurately modelling the global evolution requires to
take into account the main micro-scale physical processes of interest. This is
why comparison of different plasma models is today an imperative task aiming at
understanding cross-scale processes in plasmas. We report here the first
comparative study of the evolution of a magnetized shear flow, through a
variety of different plasma models by using magnetohydrodynamic, Hall-MHD,
two-fluid, hybrid kinetic and full kinetic codes. Kinetic relaxation effects
are discussed to emphasize the need for kinetic equilibriums to study the
dynamics of collisionless plasmas in non trivial configurations. Discrepancies
between models are studied both in the linear and in the nonlinear regime of
the magnetized Kelvin-Helmholtz instability, to highlight the effects of small
scale processes on the nonlinear evolution of collisionless plasmas. We
illustrate how the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field direction
during the linear evolution when kinetic effects are taken into account. Even
if we found that small scale processes differ between the different models, we
show that the feedback from small, kinetic scales to large, fluid scales is
negligable in the nonlinear regime. This study show that the kinetic modeling
validates the use of a fluid approach at large scales, which encourages the
development and use of fluid codes to study the nonlinear evolution of
magnetized fluid flows, even in the colisionless regime
The oblique firehose instability in a bi-kappa magnetized plasma
In this work, we derive a dispersion equation that describes the excitation
of the oblique (or Alfv\'en) firehose instability in a plasma that contains
both electron and ion species modelled by bi-kappa velocity distribution
functions. The equation is obtained with the assumptions of low-frequency waves
and moderate to large values of the parallel (respective to the ambient
magnetic field) plasma beta parameter, but it is valid for any direction of
propagation and for any value of the particle gyroradius (or Larmor radius).
Considering values for the physical parameters typical to those found in the
solar wind, some solutions of the dispersion equation, corresponding to the
unstable mode, are presented. In order to implement the dispersion solver,
several new mathematical properties of the special functions occurring in a
kappa plasma are derived and included. The results presented here suggest that
the superthermal characteristic of the distribution functions leads to
reductions to both the maximum growth rate of the instability and of the
spectral range of its occurrence
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