778 research outputs found
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
Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability
In collisionless and weakly collisional plasmas, such as hot accretion flows
onto compact objects, the magnetorotational instability (MRI) can differ
significantly from the standard (collisional) MRI. In particular, pressure
anisotropy with respect to the local magnetic-field direction can both change
the linear MRI dispersion relation and cause nonlinear modifications to the
mode structure and growth rate, even when the field and flow perturbations are
small. This work studies these pressure-anisotropy-induced nonlinearities in
the weakly nonlinear, high-ion-beta regime, before the MRI saturates into
strong turbulence. Our goal is to better understand how the saturation of the
MRI in a low collisionality plasma might differ from that in the collisional
regime. We focus on two key effects: (i) the direct impact of self-induced
pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii)
the influence of pressure anisotropy on the "parasitic instabilities" that are
suspected to cause the mode to break up into turbulence. Our main conclusions
are: (i) The mirror instability regulates the pressure anisotropy in such a way
that the linear MRI in a collisionless plasma is an approximate nonlinear
solution once the mode amplitude becomes larger than the background field (just
as in MHD). This implies that differences between the collisionless and
collisional MRI become unimportant at large amplitudes. (ii) The break up of
large amplitude MRI modes into turbulence via parasitic instabilities is
similar in collisionless and collisional plasmas. Together, these conclusions
suggest that the route to magnetorotational turbulence in a collisionless
plasma may well be similar to that in a collisional plasma, as suggested by
recent kinetic simulations. As a supplement to these findings, we offer
guidance for the design of future kinetic simulations of magnetorotational
turbulence.Comment: Submitted to Journal of Plasma Physic
Transition from collisionless to collisional MRI
Recent calculations by Quataert et al. (2002) found that the growth rates of
the magnetorotational instability (MRI) in a collisionless plasma can differ
significantly from those calculated using MHD. This can be important in hot
accretion flows around compact objects. In this paper we study the transition
from the collisionless kinetic regime to the collisional MHD regime, mapping
out the dependence of the MRI growth rate on collisionality. A kinetic closure
scheme for a magnetized plasma is used that includes the effect of collisions
via a BGK operator. The transition to MHD occurs as the mean free path becomes
short compared to the parallel wavelength 2\pi/k_{\Par}. In the weak magnetic
field regime where the Alfv\'en and MRI frequencies are small compared
to the sound wave frequency k_{\Par} c_0, the dynamics are still effectively
collisionless even if , so long as the collision frequency \nu
\ll k_{\Par} c_{0}; for an accretion flow this requires \nu \lsim \Omega
\sqrt{\beta}. The low collisionality regime not only modifies the MRI growth
rate, but also introduces collisionless Landau or Barnes damping of long
wavelength modes, which may be important for the nonlinear saturation of the
MRI.Comment: 20 pages, 4 figures, submitted to ApJ with a clearer derivation of
anisotropic pressure closure from drift kinetic equatio
Electron-scale reduced fluid models with gyroviscous effects
Reduced fluid models for collisionless plasmas including electron inertia and
finite Larmor radius corrections are derived for scales ranging from the ion to
the electron gyroradii. Based either on pressure balance or on the
incompressibility of the electron fluid, they respectively capture kinetic
Alfv\'en waves (KAWs) or whistler waves (WWs), and can provide suitable tools
for reconnection and turbulence studies. Both isothermal regimes and Landau
fluid closures permitting anisotropic pressure fluctuations are considered. For
small values of the electron beta parameter , a perturbative
computation of the gyroviscous force valid at scales comparable to the electron
inertial length is performed at order , which requires second-order
contributions in a scale expansion. Comparisons with kinetic theory are
performed in the linear regime. The spectrum of transverse magnetic
fluctuations for strong and weak turbulence energy cascades is also
phenomenologically predicted for both types of waves. In the case of moderate
ion to electron temperature ratio, a new regime of KAW turbulence at scales
smaller than the electron inertial length is obtained, where the magnetic
energy spectrum decays like , thus faster than the
spectrum of WW turbulence.Comment: 29 pages, 4 figure
Linear theory of nonlocal transport in a magnetized plasma
A system of nonlocal electron-transport equations for small perturbations in
a magnetized plasma is derived using the systematic closure procedure of V. Yu.
Bychenkov et al., Phys. Rev. Lett. 75, 4405 (1995). Solution to the linearized
kinetic equation with a Landau collision operator is obtained in the diffusive
approximation. The Fourier components of the longitudinal, oblique, and
transversal electron fluxes are found in an explicit form for quasistatic
conditions in terms of the generalized forces: the gradients of density and
temperature, and the electric field. The full set of nonlocal transport
coefficients is given and discussed. Nonlocality of transport enhances electron
fluxes across magnetic field above the values given by strongly collisional
local theory. Dispersion and damping of magnetohydrodynamic waves in weakly
collisional plasmas is discussed. Nonlocal transport theory is applied to the
problem of temperature relaxation across the magnetic field in a laser hot
spot.Comment: 27 pages, 13 figure
Pressure anisotropy and small spatial scales induced by velocity shear
Non-Maxwellian metaequilibria can exist in low-collisionality plasmas as
evidenced by satellite and laboratory measurements. By including the full
pressure tensor dynamics in a fluid plasma model, we show that a sheared
velocity field can provide an effective mechanism that makes an initial
isotropic state anisotropic and agyrotropic. We discuss how the propagation of
magneto-elastic waves can affect the pressure tensor anisotropization and its
spatial filamentation which are due to the action of both the magnetic field
and flow strain tensor. We support this analysis by a numerical integration of
the nonlinear equations describing the pressure tensor evolution.Comment: 5 pages, 3 Figure
Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection
Non-Maxwellian electron velocity space distribution functions (EVDF) are
useful signatures of plasma conditions and non-local consequences of
collisionless magnetic reconnection. In the past, EVDFs were obtained mainly
for antiparallel reconnection and under the influence of weak guide-fields in
the direction perpendicular to the reconnection plane. EVDFs are, however, not
well known, yet, for oblique (or component-) reconnection in dependence on
stronger guide-magnetic fields and for the exhaust (outflow) region of
reconnection away from the diffusion region. In view of the multi-spacecraft
Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of
collisionless magnetic reconnection in dependence on the guide-field strength
from small () to very strong () guide-fields, taking
into account the feedback of the self-generated turbulence. For this sake, we
carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM
code. We obtained anisotropic EVDFs and electron beams propagating along the
separatrices as well as in the exhaust region of reconnection. The beams are
anisotropic with a higher temperature in the direction perpendicular rather
than parallel to the local magnetic field. The beams propagate in the direction
opposite to the background electrons and cause instabilities. We also obtained
the guide-field dependence of the relative electron-beam drift speed, threshold
and properties of the resulting streaming instabilities including the strongly
non-linear saturation of the self-generated plasma turbulence. This turbulence
and its non-linear feedback cause non-adiabatic parallel electron acceleration
and EVDFs well beyond the limits of the quasi-linear approximation, producing
phase space holes and an isotropizing pitch-angle scattering.Comment: 21 pages, 8 figures. Revised to match with the version published in
Physics of Plasmas. An abridged version of the abstract is shown her
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