arXiv subjects

Pablo Dmitruk

Publications and source records attributed to Pablo Dmitruk.

At least 19 recordsLinked to original sources

arXiv2026

Study of the 1/f spectrum using virtual spacecraft sampling in MHD turbulence

We investigate the appearance of a low-frequency 1/f magnetic spectrum in three-dimensional incompressible magnetohydrodynamic turbulence using direct numerical simulations and virtual spacecraft sampling. Our goal is to determine how the measured temporal spectra depend on the mean magnetic guide field, the probe velocity relative to the Alfv\'en speed, and the sampling angle with respect to the guide field. We find that the clearest 1/f ranges are obtained for stronger guide fields and are favored by faster sampler trajectories oriented more nearly perpendicular to the mean magnetic field. To characterize this behavior, we introduce a quantitative score that measures the quality and spectral coverage of the detected 1/f interval. We further show that, as the probe speed increases, the measured temporal spectra become progressively more consistent with a direct mapping between spatial and temporal fluctuations, particularly for nearly perpendicular sampling in the strong guide field case. These results indicate that the presence and clarity of a temporal 1/f range depend not only on the underlying turbulent dynamics, but also on the geometry and speed of the sampling process, with implications for the interpretation of low-frequency in situ measurements in the solar wind.

astro-ph.SR↗

arXiv2025

Turbulence in the terrestrial magnetosheath: space-time correlation using the Magnetospheric Multiscale mission

Spatiotemporal correlation of magnetic field fluctuations is investigated using the Magnetospheric Multiscale mission in the terrestrial magnetosheath. The first observation of the turbulence propagator in space emerges through analysis of more than a thousand intervals. Results show clear features of spatial and spectral anisotropy, leading to a distinct behavior of relaxation times in the directions parallel and perpendicular to the mean magnetic field. Full space-time investigation of the Taylor hypothesis reveals a scale-dependent anisotropy of magnetosheath fluctuations that can be compared to the effect of flow propagation on spacecraft frame time decorrelation rates as well as with Eulerian estimates. The turbulence propagator reveals that the amplitudes of the perpendicular modes decorrelate according to sweeping or Alfv\'enic propagation mechanisms. The decorrelation time of parallel modes instead does not depend on the parallel wavenumber, which could be due to resonant interactions. Through direct observation, this study provides unprecedented insight into the space-time structure of turbulent space plasmas, while giving critical constraints for theoretical and numerical models.

physics.space-ph↗

arXiv2025

Waviness and self-sustained turbulence in plane Couette-Poiseuille flow

Direct numerical simulations of a Couette Poiseuille flow were performed near the transition to turbulence to investigate the nonlinear relationship between streak waviness and rolls. This relationship is a key step in Waleffe's model for a self sustaining process (SSP). Simulations were conducted for Reynolds numbers ranging from 500 to 940, and a range of initial perturbation amplitudes was used. In these simulations, the streaks, rolls, and streak waviness initially grow. The optimal time for this growth closely matches the linear transient growth period for small perturbations, but is much shorter when the initial perturbations are large and highly nonlinear. For higher Reynolds numbers and large initial perturbations, the velocity field reaches a turbulent steady state, while in the remaining cases the flow relaminarizes. The main result is that the waviness of the streaks is a quadratic function of the rolls, provided that the roll amplitude is sufficiently large.

physics.flu-dyn↗

arXiv2025

An Intermittent Model for the $1/f$ Spectrum in the Pristine Solar Wind

We present a statistical, observational study of the $1/f$ range of solar wind turbulence, where $f$ denotes frequency, using in situ data from the Parker Solar Probe (PSP). We compute the energy cascade rate using the third order law of incompressible magnetohydrodynamic (MHD) turbulence, incorporating expansion terms to account for solar wind dynamics. Our results reveal a $1/\tau$ dependence of the energy cascade rate, where $\tau$ is the temporal lag, within the $1/f$ range, in contrast to the constant cascade rate in the inertial range. To explain this behavior, we propose a new intermittent model predicting a $1/\ell$ scaling of the cascade rate, where $\ell$ represents the spatial lag. The analysis of the probability density function (PDF) of magnetic field increments confirms the intermittent nature of the parallel fluctuation component, whereas the perpendicular fluctuations are found to be quasi Gaussian. These findings provide new insights into energy transfer processes in the $1/f$ range of solar wind turbulence, with potential applications in planetary magnetosheaths.

astro-ph.SR↗

arXiv2025

The Energy Cascade Rate in Supersonic Magnetohydrodynamic Turbulence

Three-dimensional direct numerical simulations (DNS) are implemented to investigate the energy cascade rate in compressible isothermal magnetohydrodynamic (MHD) turbulence. Utilizing an exact law derived from the K\'arm\'an-Howarth equation, we examine the contributions of flux and non-flux terms to the cascade rate across a broad range of sonic and Alfv\'enic Mach numbers, from subsonic to supersonic regimes and varying mean magnetic fields. Cascade rates are computed using on-grid 3-D decomposition and two plasma increment approaches: signed and absolute values. Anisotropy induced by strong magnetic fields is analyzed through angular-dependent scaling of the cascade terms. Moreover, the increment calculation method significantly influences the relative contributions of flux and non-flux terms, with absolute methods tending to overestimate the latter. These findings extend current studies of compressible turbulence and offer critical insights into energy transfer mechanisms relevant to many astrophysical phenomena.

physics.plasm-ph↗

arXiv2024

Direct comparison of the energization of self-consistent charged particles vs test particles in a turbulent plasma

The test particle approach is a widely used method for studying the dynamics of charged particles in complex electromagnetic fields and has been successful in explaining particle energization in turbulent plasmas. However, this approach is fundamentally not self-consistent, as test particles do not generate their own electromagnetic fields and therefore do not interact with their surroundings realistically. In this work, we compare the energization of a population of test protons in a magnetofluid to that of a plasma composed of self-consistent particles. We use a compressible Hall magnetohydrodynamic (CHMHD) model for the test particle case and a hybrid particle-in-cell (HPIC) approach for the self-consistent case, conducting both 2D and 3D simulations. We calculate the rate of energization and conversion to thermal energy in both models, finding a higher temperature for the test particle case. Additionally, we examine the distribution of suprathermal particles and find that, in the test particle scenario, these particles eventually occupy the entire domain, while in the self-consistent case, suprathermal particles are confined to specific regions. We conclude that while test particles capture some qualitative features of their self-consistent counterparts, they miss finer phenomena and tend to overestimate energization.

physics.plasm-ph↗

arXiv2022

A Statistical Study of the Compressible Energy Cascade Rate in Solar Wind Turbulence: Parker Solar Probe Observations

We investigated incompressible and compressible magnetohydrodynamic (MHD) energy cascade rates in the solar wind at different heliocentric distances. We used in situ magnetic field and plasma observations provided by the Parker Solar Probe (PSP) mission and exact relations in fully developed turbulence. To estimate the compressible cascade rate, we applied two recent exact relations for compressible isothermal and polytropic MHD turbulence, respectively. Our observational results show a clear increase of the absolute value of the compressible and incompressible cascade rates as we get closer to the Sun. Moreover, we obtained an increase in both isothermal and polytropic cascade rates with respect to the incompressible case as compressibility increases in the plasma. Further discussion about the relation between the compressibility and the heliocentric distance is carried out. Furthermore, we compared both exact relations as compressibility increases in the solar wind and although we note a slightly trend to observe larger cascades using a polytropic closure, we obtained essentially the same cascade rate in the range of compressibility observed. Finally, we investigated the signed incompressible and compressible energy cascade rates and its connection with the real cascade rate.

astro-ph.SR↗

arXiv2022

Effect of electromagnetic boundary conditions on the onset of small-scale dynamos driven by convection

We present a high-order numerical study of the dependence of the dynamo onset on different electromagnetic boundary conditions, in convecting Boussinesq flows forced by a temperature gradient. Perfectly conducting boundaries, vacuum, and mixed electromagnetic boundary conditions are considered, using a method that treats fields and boundary conditions with close to spectral accuracy. Having one or two conducting boundaries greatly facilitates dynamo action. For the mixed case it is shown that the critical magnetic Reynolds number becomes independent of the Rayleigh number, Ra, for sufficiently large Ra.

physics.flu-dyn↗

arXiv2022

Behavior of hydrodynamic and magnetohydrodynamic turbulence in a rotating sphere with precession and dynamo action

The effect of precession in a rotating sphere filled with fluid was studied with direct numerical simulations, both in the incompressible hydrodynamics (HD) and magnetohydrodynamics (MHD) scenarios. In both cases the asymptotic state and its dependence with both rotating and precession frequency was analyzed. For the MHD case no self-sustaining dynamos were found for the prograde precession case, whereas on the other hand a critical retrograde precession frequency was found above which dynamo action is self-sustained. It was also found that these correspond to small-scale dynamos with a developed turbulent regime. Furthermore, it is observed the presence of reversals of the magnetic dipole moment with greater waiting times between reversals for smaller precession frequencies.

physics.flu-dyn↗

arXiv2022

Test particle energization of heavy ions in magnetohydrodynamic turbulence

In the present work, we study the energization and displacement of heavy ions through the use of test particles interacting with the electromagnetic fields of magnetohydrodynamic (MHD) turbulence. These fields are obtained from pseudospectral direct numerical solutions (DNSs) of the compressible three-dimensional MHD equations with a strong background magnetic field. We find particle energization to be predominantly perpendicular as the ions become heavier (lower charge-to-mass ratio) and that high displacement is detrimental for perpendicular energization. On the other hand, perpendicular displacement is unaffected by the charge-to-mass ratio, which we explain with a simple guide center model. Using Voronoi tessellation along with this model, we analyze preferential concentration and find that particles behave as tracers in the perpendicular plane, clustering in regions with $\nabla_\perp\cdot\mathbf{u}_\perp < 0$. These regions also have $(\nabla\times\mathbf{E})_z < 0$, which is optimal for perpendicular energization, thus providing a mechanism to understand precedent results.

astro-ph.SR↗

arXiv2022

Vector potential-based MHD solver for non-periodic flows using Fourier continuation expansions

A high-order method to evolve in time electromagnetic and velocity fields in conducting fluids with non-periodic boundaries is presented. The method has a small overhead compared with fast FFT-based pseudospectral methods in periodic domains. It uses the magnetic vector potential formulation for accurately enforcing the null divergence of the magnetic field, and allowing for different boundary conditions including perfectly conducting walls or vacuum surroundings, two cases relevant for many astrophysical, geophysical, and industrial flows. A spectral Fourier continuation method is used to accurately represent all fields and their spatial derivatives, allowing also for efficient solution of Poisson equations with different boundaries. A study of conducting flows at different Reynolds and Hartmann numbers, and with different boundary conditions, is presented to study convergence of the method and the accuracy of the solenoidal and boundary conditions.

physics.comp-ph↗

arXiv2021

Spatio temporal analysis of waves in compressively driven magnetohydrodynamics turbulence

Using direct numerical simulations (DNSs), the interaction between linear waves and turbulence under the compressible magnetohydrodynamic (CMHD) approach was studied. A set of DNSs in three dimensions for a spatial resolution of $128^3$ and $256^3$ were performed. A parametric study was carried out varying the sonic Mach number, the mean magnetic field and the compressibility amplitude of the forcing. Spatio-temporal spectra of the magnetic energy were built and analyzed, allowing for direct identification of all wave modes in a CMHD turbulent system and quantification of the amount of energy in each mode as a function of the wave number. Thus, linear waves were detected, that is Alfvén waves and fast and slow magnetosonic waves. Furthermore, different responses of the plasma were found according to whether the Mach number or the mean magnetic field was varied. On the other hand, making use of spatio-temporal spectra and two different integration methods, we accurately quantified the amount of energy present in each of the normal modes. Finally, although the presence of linear waves was observed, in all the cases studied the system was mainly dominated by the non-linear dynamics of the plasma.

physics.plasm-ph↗

arXiv2020

Fourier continuation method for incompressible fluids with boundaries

We present a Fourier Continuation-based parallel pseudospectral method for incompressible fluids in cuboid non-periodic domains. The method produces dispersionless and dissipationless derivatives with fast spectral convergence inside the domain, and with very high order convergence at the boundaries. Incompressibility is imposed by solving a Poisson equation for the pressure. Being Fourier-based, the method allows for fast computation of spectral transforms. It is compatible with uniform grids (although refined or nested meshes can also be implemented), which in turn allows for explicit time integration at sufficiently high Reynolds numbers. Using a new parallel code named SPECTER we illustrate the method with two problems: channel flow, and plane Rayleigh-Bénard convection under the Boussinesq approximation. In both cases the method yields results compatible with previous studies using other high-order numerical methods, with mild requirements on the time step for stability.

physics.comp-ph↗

arXiv2019

Magnetic structure, dipole reversals, and 1/f noise in resistive MHD spherical dynamos

A parametric study of the magnetic dipole behavior in resistive incompressible MHD inside a rotating sphere is performed, using direct numerical simulations and considering Reynolds and Ekman numbers as controlling parameters. The tendency is to obtain geodynamo-like magnetic dipole reversal regimes for sufficiently small Ekman and large Reynolds numbers. The typical dipole latitude obtained in the reversal regime is around 40 degrees (with respect to the rotation axis of the sphere). A statistical analysis of waiting times between dipole reversals is also performed, obtaining a non-Poissonian distribution of waiting times, indicating long-term memory effects. We also report the presence of a $1/f$ frequency power spectrum in the magnetic dipole time-series, which also shows a tendency to grow toward lower frequencies as the Ekman number is decreased.

physics.flu-dyn↗

arXiv2018

Interplay between Alfvén and magnetosonic waves in compressible magnetohydrodynamics turbulence

Using spatio-temporal spectra we show direct evidence of excitation of magnetosonic and Alfvén waves in three-dimensional compressible magnetohydrodynamic turbulence at small Mach numbers. For the plasma pressure dominated regime, or high $β$ regime (with $β$ the ratio between fluid and magnetic pressure), and for the magnetic pressure dominated regime, or low $β$ regime, we study magnetic field fluctuations parallel and perpendicular to a guide magnetic field $\textbf{B}_0$. In the low $β$ case we find excitation of compressible and incompressible fluctuations, with a transfer of energy towards Alfvénic modes and to a lesser extent towards magnetosonic modes. In particular, we find signatures of the presence of fast magnetosonic waves in a scenario compatible with that of weak turbulence. In the high $β$ case, fast and slow magnetosonic waves are present, with no clear trace of Alfvén waves, and a significant part of the energy is carried by two-dimensional turbulent eddies.

physics.plasm-ph↗

arXiv2018

Energy cascade rate in isothermal compressible magnetohydrodynamic turbulence

Three-dimensional direct numerical simulations are used to study the energy cascade rate in isothermal compressible magnetohydrodynamic turbulence. Our analysis is guided by a two-point exact law derived recently for this problem in which flux, source, hybrid, and mixed terms are present. The relative importance of each term is studied for different initial subsonic Mach numbers $M_S$ and different magnetic guide fields ${\bf B}_0$. The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field ${\bf B}_0$, unlike the other terms whose modulus increase significantly with $M_S$ and ${\bf B}_0$. In particular, for strong ${\bf B}_0$ the source and hybrid terms are dominant at small scales with almost the same amplitude but with a different sign. A statistical analysis made with an isotropic decomposition based on the SO(3) rotation group is shown to generate spurious results in presence of ${\bf B}_0$, when compared with an axisymmetric decomposition better suited to the geometry of the problem. Our numerical results are compared with previous analyses made with in-situ measurements in the solar wind and the terrestrial magnetosheath.

physics.plasm-ph↗

arXiv2017

Charged particle diffusion in isotropic random magnetic fields

The investigation of the diffusive transport of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider diffusion of charged particles in fully three-dimensional isotropic turbulent magnetic fields with no mean field, which may be pertinent to many astrophysical situations. We identify different ranges of particle energy depending upon the ratio of the Larmor radius of the charged particle to the characteristic outer length scale of the turbulence. Two different theoretical models are proposed to calculate the diffusion coefficient, each applicable to a distinct range of particle energies. The theoretical results are compared with those from computer simulations, showing good agreement.

physics.space-ph↗

arXiv2016

von Kármán-Howarth equation for three-dimensional two-fluid plasmas

We derive the von Kármán-Howarth equation for a full three dimensional incompressible two-fluid plasma. In the long-time limit and for very large Reynolds numbers we obtain the equivalent of the hydrodynamic "four-fifth" law. This exact law predicts the scaling of the third-order two-point correlation functions, and puts a strong constraint on the plasma turbulent dynamics. Finally, we derive a simple expression for the 4/5 law in terms of third-order structure functions, which is appropriate for comparison with in-situ measurements in the solar wind at different spatial ranges.

physics.plasm-ph↗