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Thierry Passot

Publications and source records attributed to Thierry Passot.

At least 19 recordsLinked to original sources
arXiv2026

Sub-sonic compressible magnetohydrodynamic turbulence I. Alfv\'enic and fast-magnetosonic injection, amplitude dependence, and compressibility effects

We investigate how sub-sonic compressible magnetohydrodynamic (MHD) turbulence properties that are relevant for cosmic-ray (CR) transport in the Galaxy are affected by the nature and amplitude of initial fluctuations, and by the plasma compressibility $\beta$. We perform 3D simulations of decaying compressible ideal-MHD turbulence at $1024^3$ resolution with the PLUTO code. The level of density fluctuations in fully developed turbulence is insensitive to whether this state is reached starting from Alfv\'enic or fast-magnetosonic perturbations. Fast-magnetosonic injection is characterized by an early phase of rapid shock dissipation, followed by a turbulence-dominated decay with a rate comparable to that of the Alfv\'enic case. The contribution of fast-magnetosonic fluctuations in fully developed turbulence remains relevant only when the initial injection consists exclusively of fast modes. Large-amplitude turbulence ($\delta B/B_0>1$) is characterized by a nearly isotropic Kolmogorov or Iroshnikov-Kraichnan spectrum for Alfv\'enic or fast-magnetosonic injection, respectively. At low amplitudes ($\delta B/B_0\ll1$), both initial Alfv\'enic and mixed-wave perturbations lead to strongly anisotropic turbulence with spectra $\propto k_\perp^{-5/3}$ and $\propto k_z^{-2}$ (becoming steeper at $\beta\gg1$), whereas fast-magnetosonic perturbations produce a turbulent state populated by shocks with a nearly isotropic $k^{-2}$ spectrum. Magnetic-field curvature and mirror structures are strongly sensitive to fluctuation amplitude and plasma $\beta$. The predicted -2.5 power-law scaling emerges only in the large-amplitude regime at high $\beta$. This work highlights that features of sub-sonic compressible MHD turbulence that may affect CR transport are sensitive to large-scale conditions and to the plasma $\beta$. Their effect on CR diffusion and field-line random walk is the object of Paper II.

physics.plasm-ph↗
arXiv2024

Impact of pressure anisotropy on the cascade rate of Hall-MHD turbulence with biadiabatic ions

The impact of ion pressure anisotropy on the energy cascade rate of Hall-MHD turbulence with biadiabatic ions and isothermal electrons is evaluated in three-dimensional direct numerical simulations, using the exact (or third-order) law derived in \citet{simon_exact_2022}. It is shown that pressure anisotropy can enhance or reduce the cascade rate, depending on the scales, in comparison with the prediction of the exact law with isotropic pressure, by an amount that correlates well with pressure anisotropy $a_p=\frac{p_\perp}{p_\parallel}\neq1$ that develops in simulations initialized with an isotropic pressure (${a_p}_0=1$). A simulation with initial pressure anisotropy, ${a_p}_0=4$, confirms this trend, exhibiting a stronger impact on the cascade rate, both in the inertial range and at larger scales, close to the forcing scales. Furthermore, a Fourier-based numerical method, to compute exact laws in numerical simulations in the full $(\ell_\perp,\ell_\parallel)$ increment plane, is presented.

physics.plasm-ph↗
arXiv2022

Turbulent regimes in collisions of 3D Alfvén-wave packets

Using 3D gyrofluid simulations, we revisit the problem of Alfven-wave (AW) collisions as building blocks of the Alfvenic cascade and their interplay with magnetic reconnection at magnetohydrodynamic (MHD) scales. Depending on the large-scale nonlinearity parameter $χ_0$ (the ratio between AW linear propagation time and nonlinear turnover time), different regimes are observed. For strong nonlinearities ($χ_0\sim1$), turbulence is consistent with a dynamically aligned, critically balanced cascade--fluctuations exhibit a scale-dependent alignment $\sinθ_k\propto k_\perp^{-1/4}$, a $k_\perp^{-3/2}$ spectrum and $k_\|\propto k_\perp^{1/2}$ spectral anisotropy. At weaker nonlinearities (small $χ_0$), a spectral break marking the transition between a large-scale weak regime and a small-scale $k_\perp^{-11/5}$ tearing-mediated range emerges, implying that dynamic alignment occurs also for weak nonlinearities. At $χ_0<1$ the alignment angle $θ_{k_\perp}$ shows a stronger scale dependence than in the $χ_0\sim1$ regime, i.e. $\sinθ_k\propto k_\perp^{-1/2}$ at $χ_0\sim0.5$, and $\sinθ_k\propto k_\perp^{-1}$ at $χ_0\sim0.1$. Dynamic alignment in the weak regime also modifies the large-scale spectrum, scaling roughly as $k_\perp^{-3/2}$ for $χ_0\sim0.5$ and as $k_\perp^{-1}$ for $χ_0\sim0.1$. A phenomenological theory of dynamically aligned turbulence at weak nonlinearities that can explain these spectra and the transition to the tearing-mediated regime is provided; at small $χ_0$, the strong scale dependence of the alignment angle combines with the increased lifetime of turbulent eddies to allow tearing to onset and mediate the cascade at scales that can be larger than those predicted for a critically balanced cascade by several orders of magnitude. Such a transition to tearing-mediated turbulence may even supplant the usual weak-to-strong transition.

astro-ph.SR↗
arXiv2021

Bridging hybrid- and full-kinetic models with Landau-fluid electrons: I. 2D magnetic reconnection

Magnetic reconnection (MR) plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in-situ measurements from space missions allow to study naturally occurring MR processes in great detail. Alongside direct measurements, numerical simulations play a key role in investigating the fundamental physics underlying MR. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with their computational cost, is crucial to efficiently address the problem. We consider a new plasma model that includes a refined electron response within the hybrid-kinetic framework (kinetic ions, fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D MR, with particular focus on its robustness during the non-linear stage, is evaluated. We perform 2D simulations of MR with moderate guide field by means of three different plasma models: a hybrid-Vlasov-Maxwell model with isotropic, isothermal electrons, a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and a full-kinetic one. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of MR, as well as several aspects of the associated electron micro-physics and its feedback onto proton dynamics. This includes the global evolution of MR and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species pressure anisotropy. In particular, anisotropy driven instabilities (such as firehose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons anisotropy during the non-linear stage of MR. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability.

physics.plasm-ph↗
arXiv2021

Inverse cascade and magnetic vortices in kinetic Alfvén-wave turbulence

A Hamiltonian two-field gyrofluid model for kinetic Alfvén waves (KAWs) in a magnetized electron-proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be nonlocal and the ratio $χ_f$ of the wave period to the characteristic nonlinear time at the driving scale affect some of its properties. For example, at small values of $χ_f$, parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $χ_f$, leading to an inverse cascade. For $β_e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $β_e$, the ratio of the KAW to the Alfvén frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form.

physics.plasm-ph↗
arXiv2019

Modeling imbalanced collisionless Alfvén wave turbulence with nonlinear diffusion equations

A pair of nonlinear diffusion equations in Fourier space} is used to study the dynamics of strong Alfvén-wave turbulence, from MHD to electron scales. Special attention is paid to the regime of imbalance between the energies of counter-propagating waves commonly observed in the solar wind (SW), especially in regions relatively close to the Sun. In the collisionless regime where dispersive effects arise at scales comparable to or larger than those where dissipation becomes effective, the imbalance produced by a given injection rate of generalized cross-helicity (GCH), which is an invariant, is much larger than in the corresponding collisional regime described by the usual (or reduced) magnetohydrodynamics. The combined effect of high imbalance and ion Landau damping induces a steep energy spectrum for the transverse magnetic field at sub-ion scales. This spectrum is consistent with observations in highly Alfvenic regions of the SW, such as trailing edges, but does not take the form of a transition range continued at smaller scales by a shallower spectrum. This suggests that the observed spectra displaying such a transition result from the superposition of contributions originating from various streams with different degrees of imbalance. Furthermore, when imbalanced energy injection is supplemented at small scales in an already fully developed turbulence, for example under the effect of magnetic reconnection, a significant enhancement of the imbalance at all scales is observed.

physics.plasm-ph↗
arXiv2018

Fluid and gyrofluid modeling of low-$β_e$ plasmas: phenomenology of kinetic Alfvén wave turbulence

Reduced fluid models including electron inertia and ion finite Larmor radius corrections are derived asymptotically, both from fluid basic equations and from a gyrofluid model. They apply to collisionless plasmas with small ion-to-electron equilibrium temperature ratio and low $β_e$, where $β_e$ indicates the ratio between the equilibrium electron pressure and the magnetic pressure exerted by a strong, constant and uniform magnetic guide field. The consistency between the fluid and gyrofluid approaches is ensured when choosing ion closure relations prescribed by the underlying ordering. A two-field reduction of the gyrofluid model valid for arbitrary equilibrium temperature ratio is also introduced, and is shown to have a noncanonical Hamiltonian structure. This model provides a convenient framework for studying kinetic Alfvén wave turbulence, from MHD to sub-$d_e$ scales (where $d_e$ holds for the electron skin depth). Magnetic energy spectra are phenomenologically determined within energy and generalized helicity cascades in the perpendicular spectral plane. Arguments based on absolute statistical equilibria are used to predict the direction of the transfers, pointing out that, within the sub-ion range associated with a $k_\perp^{-7/3}$ transverse magnetic spectrum, the generalized helicity could display an inverse cascade if injected at small scales, for example by reconnection processes.

physics.plasm-ph↗
arXiv2009

Non-modal approach to linear theory: marginal stability and the dissipation of turbulent fluctuations

The non-modal approach for a linearized system differs from a normal mode analysis by following the temporal evolution of some perturbed equilibria, and therefore includes transient effects. We employ a non-modal approach for studying the stability of a bi-Maxwellian magnetized plasma using the Landau fluid model, which we briefly describe. We show that bi-Maxwellian stable equilibria can support transient growth of some physical quantities, and we study how these transients behave when an equilibrium approaches its marginally stable condition. This is relevant to anisotropic plasma, that are often observed in the solar wind with a temperature anisotropy close to values that can trigger a kinetic instability. The results obtained with a non-modal approach are relevant to a re-examination of the concept of linear marginal stability. Moreover, we discuss the topic of the dissipation of turbulent fluctuations, suggesting that the non-modal approach should be included in future studies.

physics.plasm-ph↗
arXiv2009

Transient growth in stable collisionless plasma

The first kinetic study of transient growth for a collisionless homogeneous Maxwellian plasma in a uniform magnetic field is presented. A system which is linearly stable may display transient growth if the linear operator describing its evolution is non-normal, so that its eigenvectors are non-orthogonal. In order to include plasma kinetic effects a Landau fluid model is employed. The linear operator of the model is shown to be non-normal and the results suggest that the nonnormality of a collisionless plasma is intrinsically related to its kinetic nature, with the transient growth being more accentuated for smaller scales and higher plasma beta. The results based on linear spectral theory have been confirmed with nonlinear simulations.

physics.plasm-ph↗
arXiv2006

Molecular cloud evolution. I. Molecular cloud and thin CNM sheet formation

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution 1D simulations predict the thermodynamic conditions in the cold layer. After $\sim 1$ Myr of evolution, the layer has column density $\sim 2.5 \times 10^{19} \psc$, thickness $\sim 0.03$ pc, temperature $\sim 25$ K and pressure $\sim 6650$ K $\pcc$. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the 1D simulations, the inflows into the sheets produce line profiles with a central line of width $\sim 0.5 \kms$ and broad wings of width $\sim 1 \kms$. 3D numerical simulations show that the cold layer develops turbulent motions and increases its thickness, until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from $\sim 7.5$ Myr for inflow Mach number $\Mr = 2.4$ to $> 80$ Myr for $\Mr = 1.03$. These numbers should be considered upper limits. The highest-density turbulent gas (HDG, $n > 100 \pcc$) is always overpressured with respect to the mean WNM pressure by factors 1.5--4, even though we do not include self-gravity. The intermediate-density gas (IDG, $10 < n [{\rm cm}^ {-3}] < 100$) has a significant pressure scatter that increases with $\Mr$, so that at $\Mr = 2.4$, a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves, and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

astro-ph↗
arXiv2003

Twofold effect of Alfven waves on the transverse gravitational instability

This paper is devoted to the study of the gravitational instability of a medium permeated by a uniform magnetic field along which a circularly polarized Alfven wave propagates. We concentrate on the case of perturbations purely transverse to the ambient field by means of direct numerical simulations of the MHD equations and of a linear stability analysis performed on a moderate amplitude asymptotic model. The Alfven wave provides an extra stabilizing pressure when the scale of perturbations is sufficiently large or small compared with the Jeans length $L_J$. However, there is a band of scales around $L_J$ for which the Alfven wave is found to have a destabilizing effect. In particular, when the medium is stable in absence of waves, the gravitational instability can develop when the wave amplitude lies in an appropriate range. This effect appears to be a consequence of the coupling between Alfven and magnetosonic waves. The prediction based on a WKB approach that the Alfven wave pressure tensor is isotropic and thus opposes gravity in all directions is only recovered for large amplitude waves for which the coupling between the different MHD modes is negligible.

astro-ph↗
arXiv2002

Magnetic Pressure-Density Correlation in Compressible MHD Turbulence

We discuss magnetic pressure and density fluctuations in strongly turbulent isothermal MHD flows in "1+2/3" dimensions. We first consider "simple" nonlinear MHD waves, which allow us show that the slow and fast modes have different asymptotic dependences of the magnetic pressure B^2 vs. rho. For the slow mode, B^2 ~= c_1-c_2 rho, while for the fast mode, B^2 ~= rho^2. We also perform a perturbative analysis to investigate Alfven wave pressure, recovering previous results that B^2 ~= rho^gamma_e, with gamma_e ~= 2, 3/2 and 1/2 at respectively large, moderate and low M_a. This variety of scalings implies that a single polytropic description of magnetic pressure is not possible in general, since the relation between B^2 and rho depends on which mode dominates the density fluctuation production, which in turn depends on the angle between the magnetic field and the direction of wave propagation, and on the Alfvenic Mach number M_a. Typically, at small M_a, the slow mode dominates, and B is ANTIcorrelated with rho. At large M_a, both modes contribute to density fluctuation production, and the magnetic pressure decorrelates from density, exhibiting a large scatter, which however decreases towards higher densities. In this case, the unsystematic behavior of the magnetic pressure causes the density PDF to generally maintain the lognormal shape corresponding to non-magnetic isothermal turbulence, except when the slow mode dominates, in which case the PDF develops an excess at low densities. Our results are consistent with the low values and apparent lack of correlation between the magnetic field strength and density in surveys of the lower-density molecular gas, and also with the recorrelation apparently seen at higher densities, if M_a is relatively large there.

astro-ph↗
arXiv2002

Thermal Instability and Magnetic Pressure in the Turbulent ISM

We review recent results on the nonlinear development of thermal instability in the context of the turbulent atomic interstellar medium (ISM). First, we pre- sent a brief summary of the linear theory, remarking that, in the atomic ISM, the wave mode is stable at small scales. Next, we revisit the growth of isolated entropy perturbations in initially unstable gas, as a function of the ratio $η$ of the cooling to the dynamical crossing times. Third, we consider the evolution of {\it velocity} perturbations. These correspond to the wave mode, and are stable at moderate amplitudes and small scales, as confirmed numerically. Fourth, we consider the behavior of magnetic pressure in turbulent regimes. We propose that recent findings of a poor B-rho correlation at low rho are due to the different B-rho scalings for the slow and fast modes of non- linear MHD waves. This implies that, in fully turbulent regimes, the magnetic field may not be a very efficient source of pressure, and that polytropic de- scriptions of magnetic pressure are probably not adequate. Finally, we discuss simulations of the ISM (and resolution issues) concerned with the possibility of significant amounts of gas being in the ``lukewarm'' temperature range be- tween the warm and cold stable phases. The mass fraction in this range in- creases, and the phase segregation decreases, as smaller scales are considered. We attribute this to the enhanced stability of moderate, adiabatic-like veloc- ity fluctuations with $η\gg 1$, to the recycling of gas from the dense to the diffuse phase by stellar energy injection, and to the magnetic field not being strongly turbulent there, possibly providing additional stability. Final- ly, we suggest that the lukewarm gas can be observationally distinguished through simultaneous determination of two of its thermodynamic variables.

astro-ph↗
arXiv2000

The Density Probability Distribution Function in Turbulent, Isothermal, Magnetized Flows in Slab Geometry

We investigate the behavior of the magnetic pressure, $b^2$, in fully turbulent MHD flows in ``1+2/3'' dimensions by means of its effect on the probability density function (PDF) of the density field. We start by reviewing our previous results for general polytropic flows, according to which the value of the polytropic exponent $γ$ determines the functional shape of the PDF. A lognormal density PDF appears in the isothermal ($γ=1$) case, but a power-law tail at either large or small densities appears for large Mach numbers when $γ>1$ and $γ< 1$, respectively. In the isothermal magnetic case, the relevant parameter is the field fluctuation amplitude, $\dbb$. A lognormal PDF still appears for small field fluctuations (generally the case for {\it large mean fields}), but a significant low-density excess appears at large fluctuation amplitudes ({\it weak mean fields}), similar to the behavior at $γ> 1$ of polytropic flows. We interpret these results in terms of simple nonlinear MHD waves, for which the magnetic pressure behaves linearly with the density in the case of the slow mode, and quadratically in the case of the fast wave. Finally, we discuss some implications of these results, in particular the fact that the effect of the magnetic field in modifying the PDF is strongest when the mean field is weak.

astro-ph↗
arXiv2000

Phase Dynamics of Nearly Stationary Patterns in Activator-Inhibitor Systems

The slow dynamics of nearly stationary patterns in a FitzHugh-Nagumo model are studied using a phase dynamics approach. A Cross-Newell phase equation describing slow and weak modulations of periodic stationary solutions is derived. The derivation applies to the bistable, excitable, and the Turing unstable regimes. In the bistable case stability thresholds are obtained for the Eckhaus and the zigzag instabilities and for the transition to traveling waves. Neutral stability curves demonstrate the destabilization of stationary planar patterns at low wavenumbers to zigzag and traveling modes. Numerical solutions of the model system support the theoretical findings.

nlin.PS↗
arXiv1999

On the Effects of Projection on Morphology

We study the effects of projection of three-dimensional (3D) data onto the plane of the sky by means of numerical simulations of turbulence in the interstellar medium including the magnetic field, parameterized cooling and diffuse and stellar heating, self-gravity and rotation. We compare the physical-space density and velocity distributions with their representation in position-position-velocity (PPV) space (``channel maps''), noting that the latter can be interpreted in two ways: either as maps of the column density's spatial distribution (at a given line-of-sight (LOS) velocity), or as maps of the spatial distribution of a given value of the LOS velocity (weighted by density). This ambivalence appears related to the fact that the spatial and PPV representations of the data give significantly different views. First, the morphology in the channel maps more closely resembles that of the spatial distribution of the LOS velocity component than that of the density field, as measured by pixel-to-pixel correlations between images. Second, the channel maps contain more small-scale structure than 3D slices of the density and velocity fields, a fact evident both in subjective appearance and in the power spectra of the images. This effect may be due to a pseudo-random sampling (along the LOS) of the gas contributing to the structure in a channel map: the positions sampled along the LOS (chosen by their LOS velocity) may vary significantly from one position in the channel map to the next.

astro-ph↗
arXiv1998

Turbulence as an Organizing Agent in the ISM

We discuss HD and MHD compressible turbulence as a cloud-forming and cloud-structuring mechanism in the ISM. Results from a numerical model of the turbulent ISM at large scales suggest that the phase-like appearance of the medium, the typical values of the densities and magnetic field strengths in the intercloud medium, as well as the velocity dispersion-size scaling relation in clouds may be understood as consequences of the interstellar turbulence. However, the density-size relation appears to only hold for the densest clouds, suggesting that low-column density clouds, which are hardest to observe, are turbulent transients. We then explore some properties of highly compressible polytropic turbulence, in one and several dimensions, applicable to molecular cloud scales. At low values of the polytropic index $γ$, turbulence may induce the gravitational collapse of otherwise linearly stable clouds, except if they are magnetically subcritical. The nature of the density fluctuations in the high Mach-number limit depends on $γ$, and in no case resembles that resulting from Burgers turbulence. In the isothermal ($γ=1$) case, the dispersion of $\ln (ρ)$ scales like the turbulent Mach number. The latter case is singular with a lognormal density pdf, while power-law tails develop at high (resp. low) densities for $γ<1$ (resp. $γ>1$).

astro-ph↗