arXiv subjects

Vladimir Zhdankin

Publications and source records attributed to Vladimir Zhdankin.

At least 19 recordsLinked to original sources

arXiv2026

A Linear Instability and Damping in the Acoustic Dispersion Relation of Fluids Subject to Inverse Compton Drag

Radiation drag from inverse-Compton scattering of an external radiation field changes the acoustic dispersion relation of a relativistic fluid. We derive the linear dispersion relation and show that, depending on the electron distribution and fluid sound speed, radiation drag either damps both modes or damps one while driving the other unstable. The unstable growth is robust at short wavelengths but slow and overdamped at long wavelengths, where the linear analysis is limited by the background acceleration. The resulting growth and damping rates are of order the bulk radiative acceleration rate, so whenever radiation drag is important for the background flow, its effects on fluid perturbations are also important. A purely damped region exists for a wide range of parameters. Radiation drag also makes acoustic waves dispersive: The phase speed deviates from the ordinary sound speed at longer wavelengths, and approaches the sound speed at short wavelengths. We validate the derived rates using special-relativistic hydrodynamic simulations. We apply the derived relation to astrophysical jets such as gamma-ray bursts, blazars in the broad-line region of the host active galactic nucleus, and quasar jets traveling through the CMB and galactic radiation field. We find that the derived instability, damping and dispersion may be relevant across a range of relativistic outflow conditions.

astro-ph.HE↗

arXiv2026

Nonthermal Particle Acceleration by Magnetic Pumping in Pulsating Plasmas

We present a new "pulsating box" setup to investigate particle acceleration in high-beta plasmas undergoing compression-expansion cycles. Our fully kinetic simulations show that particles are efficiently accelerated by magnetic pumping, producing nonthermal energy distributions with power-law tails. Numerical results are in excellent agreement with a generalized maximum entropy model that we derive, linking the power-law index of distributions to the injected energy. Our results are relevant for understanding the origin of high-energy particles in space and astrophysical plasmas.

astro-ph.HE↗

arXiv2026

Fast Magnetosonic Turbulence in Two-Dimensional Relativistic Plasmas

We present fully kinetic simulations of driven 2D turbulence in a relativistic plasma, designed for the first time to induce a fast magnetosonic cascade. As the driving strength increases, turbulence transitions from a weak wave-dominated regime to strong shock-driven dynamics. Using spatiotemporal Fourier analysis, we identify fast modes, finding that the weak turbulence regime exhibits spectral properties that are in excellent agreement with theoretical expectations. Our results are relevant for the modeling of turbulence in high-energy astrophysical plasmas.

physics.plasm-ph↗

arXiv2025

Radiative relativistic turbulence as an in situ pair-plasma source in blazar jets

As powerful gamma-ray engines, blazars -- relativistic plasma jets launched toward Earth from active galactic nuclei -- are manifestly high-energy particle accelerators. Yet, exactly how these jets accelerate particles as well as what they are made of both remain largely mysterious. In this work, we argue that these issues may be linked through the gamma-ray emission for which blazars are renowned. Namely, high-energy photons produced at sites of intense particle acceleration could be absorbed by soft radiation within the jet, enriching it with electron-positron pairs. We explore this possibility in the specific context of particle acceleration by magnetized radiative relativistic turbulence. Using a combination of theory, particle-in-cell simulations, and Fokker-Planck modeling, we identify and characterize a novel pair-production-mediated equilibration mechanism in such turbulence. Initially, turbulent energy injection outpaces radiative cooling, leading to runaway particle acceleration and gamma-ray radiation. Then, gamma-ray absorption begets copious newborn pairs, slowing subsequent particle acceleration. This eventually brings particle acceleration into balance with radiative cooling and shuts down pair production: a pair-enriched final equilibrium. We estimate that this process could significantly load jets of flat-spectrum radio quasars with fresh pairs. These results represent an important connection between particle acceleration and plasma composition in blazar jets.

astro-ph.HE↗

arXiv2025

First-principles measurement of ion and electron energization in collisionless accretion flow

We present the largest 3D Particle-in-Cell shearing-box simulations of turbulence driven by the magnetorotational instability, for the first time employing the realistic proton-to-electron mass ratio. We investigate the energy partition between relativistically hot electrons and subrelativistic ions in turbulent accretion flows, a regime relevant to collisionless, radiatively inefficient accretion flows of supermassive black holes, such as those targeted by the Event Horizon Telescope. We provide a simple empirical formula to describe the measured heating ratio between ions and electrons, which can be used for more accurate global modeling of accretion flows with standard fluid approaches such as general-relativistic magnetohydrodynamics.

astro-ph.HE↗

arXiv2025

Energy Diffusion and Advection Coefficients in Kinetic Simulations of Relativistic Plasma Turbulence

Turbulent, relativistic nonthermal plasmas are ubiquitous in high-energy astrophysical systems, as inferred from broadband nonthermal emission spectra. The underlying turbulent nonthermal particle acceleration (NTPA) processes have traditionally been modelled with a Fokker-Planck (FP) diffusion-advection equation for the particle energy distribution. We test FP-type NTPA theories by performing and analysing particle-in-cell (PIC) simulations of turbulence in collisionless relativistic pair plasma. By tracking large numbers of particles in simulations with different initial magnetisation and system size, we first test and confirm the applicability of the FP framework. We then measure the FP energy diffusion ($D$) and advection ($A$) coefficients as functions of particle energy $γm c^2$, and compare their dependence to theoretical predictions. At high energies, we robustly find $D \sim γ^2$ for all cases. Hence, we fit $D = D_0 γ^2$ and find a scaling consistent with $D_0 \sim σ^{3/2}$ at low instantaneous magnetisation $σ(t)$, flattening to $D_0 \sim σ$ at higher $σ\sim 1$. We also find that the power-law index $α(t)$ of the particle energy distribution converges exponentially in time. We build and test an analytic model connecting the FP coefficients and $α(t)$, predicting $A(γ) \sim γ\log γ$. We confirm this functional form in our measurements of $A(γ,t)$, which allows us to predict $α(t)$ through the model relations. Our results suggest that the basic second-order Fermi acceleration model, which predicts $D_0 \sim σ$, may not be a complete description of NTPA in turbulent plasmas. These findings encourage further application of tracked particles and FP coefficients as a diagnostic in kinetic simulations of various astrophysically relevant plasma processes like collisionless shocks and magnetic reconnection.

astro-ph.HE↗

arXiv2024

Scale Statistics of Current Sheets in Relativistic Collisionless Plasma Turbulence

We analyze distributions of the spatial scales of coherent intermittent structures -- current sheets -- obtained from fully kinetic, two-dimensional simulations of relativistic plasma turbulence using unsupervised machine-learning data dissection. We find that the distribution functions of sheet length $\ell$ (longest scale of the analyzed structure in the direction perpendicular to the dominant guide field) and curvature $r_c$ (radius of a circle fitted to the structures) can be well-approximated by power-law distributions, indicating self-similarity of the structures. The distribution for the sheet width $w$ (shortest scale of the structure) peaks at the kinetic scales and decays exponentially at larger values. The data shows little or no correlation between $w$ and $\ell$, as expected from theoretical considerations. The typical $r_c$ depends linearly on $\ell$, which indicates that the sheets all have a similar curvature relative to their sizes. We find a weak correlation between $r_c$ and $w$. Our results can be used to inform realistic magnetohydrodynamic sub-grid models for plasma turbulence in high-energy astrophysics.

astro-ph.HE↗

arXiv2024

Collisionless Magnetorotational Turbulence in Pair Plasmas: Steady-state Dynamics, Particle Acceleration, and Radiative Cooling

We present 3D fully kinetic shearing-box simulations of pair-plasma magnetorotational turbulence with unprecedented macro-to-microscopic scale separation. While retrieving the expected fluid behavior of the plasma at large scales, we observe a steepening of turbulent spectra at kinetic scales and substantial angular-momentum transport linked with kinetic processes. For the first time, we provide a definitive demonstration of nonthermal particle acceleration in kinetic magnetorotational turbulence agnostically of initial conditions, by means of a novel strategy exploiting synchrotron cooling.

astro-ph.HE↗

arXiv2023

Particle acceleration by magnetic Rayleigh-Taylor instability: mechanism for flares in black-hole accretion flows

We study the magnetic Rayleigh-Taylor instability in relativistic collisionless plasma, as an astrophysical process for nonthermal particle acceleration. We consider dense plasma on top of a highly magnetized cavity with sheared magnetic field. Using particle-in-cell simulations, we show that small plumes grow and merge progressively to form a large-scale plume, which broadens to drive rapid magnetic reconnection in the cavity. We find that this leads to efficient particle acceleration capable of explaining flares from the inner accretion flow onto the black hole Sgr A*.

astro-ph.HE↗

arXiv2023

Dimensional measures of generalized entropy

Entropy is useful in statistical problems as a measure of irreversibility, randomness, mixing, dispersion, and number of microstates. However, there remains ambiguity over the precise mathematical formulation of entropy, generalized beyond the additive definition pioneered by Boltzmann, Gibbs, and Shannon (applicable to thermodynamic equilibria). For generalized entropies to be applied rigorously to nonequilibrium statistical mechanics, we suggest that there is a need for a physically interpretable (dimensional) framework that can be connected to dynamical processes operating in phase space. In this work, we introduce dimensional measures of entropy that admit arbitrary invertible weight functions (subject to curvature and convergence requirements). These "dimensional entropies" have physical dimensions of phase-space volume and represent the extent of level sets of the distribution function. Dimensional entropies with power-law weight functions (related to Rényi and Tsallis entropies) are particularly robust, as they do not require any internal dimensional parameters due to their scale invariance. We also point out the existence of composite entropy measures that can be constructed from functionals of dimensional entropies. We calculate the response of the dimensional entropies to perturbations, showing that for a structured distribution, perturbations have the largest impact on entropies weighted at a similar phase-space scale. This elucidates the link between dynamics (perturbations) and statistics (entropies). Finally, we derive corresponding generalized maximum-entropy distributions. Dimensional entropies may be useful as a diagnostic (for irreversibility) and for theoretical modeling (if the underlying irreversible processes in phase space are understood) in chaotic and complex systems, such as collisionless systems of particles with long-range interactions.

cond-mat.stat-mech↗

arXiv2023

Magnetogenesis in a collisionless plasma: from Weibel instability to turbulent dynamo

We report on a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of ``seed'' magnetic fields, the formation of turbulence, and the inductive amplification of fields by the fluctuation dynamo. Driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary ``seed'' magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until they reach energy equipartition with the turbulent flow. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.

astro-ph.HE↗

arXiv2022

Synchrotron Firehose Instability

We demonstrate using linear theory and particle-in-cell (PIC) simulations that a synchrotron-cooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into small-amplitude, kinetic-scale magnetic-field fluctuations, which pitch-angle scatter the particles and bring the plasma to a near-thermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron-positron and electron-ion plasmas. As a byproduct of the growing electron-firehose magnetic field fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ion-cyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black-hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient bursts of radiation.

astro-ph.HE↗

arXiv2022

Generalized entropy production in collisionless plasma flows and turbulence

Collisionless plasmas exhibit nonthermal particle distributions after being energized; as a consequence, they enter a state of low Boltzmann-Gibbs (BG) entropy relative to the thermal state. The Vlasov equations predict that in a collisionless plasma with closed boundaries, BG entropy is formally conserved, along with an infinite set of Casimir invariants; this provides a seemingly strong constraint that may explain how plasmas maintain low entropy. Nevertheless, it is commonly believed that entropy production is enabled by phase mixing or nonlinear entropy cascades. The question of whether such anomalous entropy production occurs, and of how to characterize it quantitatively, is a fundamental problem in plasma physics. We construct a new theoretical framework for characterizing entropy production (in a generalized sense) based on ideally conserved dimensional quantities derived from the Casimir invariants; these are referred to as ``Casimir momenta'' and they generalize BG entropy. The growth of Casimir momenta relative to the average particle momentum indicates entropy production. We apply this framework to quantify entropy production in particle-in-cell simulations of laminar flows and turbulent flows driven in relativistic plasma, where efficient nonthermal particle acceleration is enabled. We demonstrate that a large amount of anomalous entropy is produced by turbulence despite nonthermal features. These results have implications for reduced modeling of nonthermal particle acceleration and for diagnosing irreversible dissipation in collisionless plasmas such as the solar wind and Earth's magnetosphere. Dimensional representations of generalized entropy analogous to Casimir momenta may be useful for other problems in statistical physics.

astro-ph.HE↗

arXiv2022

Nonthermal particle acceleration from maximum entropy in collisionless plasmas

Dissipative processes cause collisionless plasmas in many systems to develop nonthermal particle distributions with broad power-law tails. The prevalence of power-law energy distributions in space/astrophysical observations and kinetic simulations of systems with a variety of acceleration and trapping (or escape) mechanisms poses a deep mystery. We consider the possibility that such distributions can be modeled from maximum-entropy principles, when accounting for generalizations beyond the Boltzmann-Gibbs entropy. Using a dimensional representation of entropy (related to the Renyi and Tsallis entropies), we derive generalized maximum-entropy distributions with a power-law tail determined by the characteristic energy scale at which irreversible dissipation occurs. By assuming that particles are typically energized by an amount comparable to the free energy (per particle) before equilibrating, we derive a formula for the power-law index as a function of plasma parameters for magnetic dissipation in systems with sufficiently complex topologies. The model reproduces several results from kinetic simulations of relativistic turbulence and magnetic reconnection.

astro-ph.HE↗

arXiv2022

Fully kinetic shearing-box simulations of magnetorotational turbulence in 2D and 3D. I. Pair plasmas

The magnetorotational instability (MRI) is a fundamental mechanism determining the macroscopic dynamics of astrophysical accretion disks. In collisionless accretion flows around supermassive black holes, MRI-driven plasma turbulence cascading to microscopic (i.e. kinetic) scales can result in enhanced angular-momentum transport and redistribution, nonthermal particle acceleration, and a two-temperature state where electrons and ions are heated unequally. However, this microscopic physics cannot be captured with standard magnetohydrodynamic (MHD) approaches typically employed to study the MRI. In this work, we explore the nonlinear development of MRI turbulence in a pair plasma, employing fully kinetic Particle-in-Cell (PIC) simulations in two and three dimensions. First, we thoroughly study the axisymmetric MRI with 2D simulations, explaining how and why the 2D geometry produces results that differ substantially from MHD expectations. We then perform the largest (to date) 3D simulations, for which we employ a novel shearing-box approach, demonstrating that 3D PIC models can reproduce the mesoscale (i.e. MHD) MRI dynamics in sufficiently large runs. With our fully kinetic simulations, we are able to describe the nonthermal particle acceleration and angular-momentum transport driven by the collisionless MRI. Since these microscopic processes ultimately lead to the emission of potentially measurable radiation in accreting plasmas, our work is of prime importance to understand current and future observations from first principles, beyond the limitations imposed by fluid (MHD) models. While in this first study we focus on pair plasmas for simplicity, our results represent an essential step toward designing more realistic electron-ion simulations, on which we will focus in future work.

astro-ph.HE↗

arXiv2021

Kinetic Simulations of Imbalanced Turbulence in a Relativistic Plasma: Net Flow and Particle Acceleration

Turbulent high-energy astrophysical systems often feature asymmetric energy injection: for instance, Alfven waves propagating from an accretion disk into its corona. Such systems are "imbalanced": the energy fluxes parallel and anti-parallel to the large-scale magnetic field are unequal. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In the present study, we investigate externally-driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using three-dimensional particle-in-cell (PIC) simulations. We find that the injected electromagnetic momentum efficiently converts into plasma momentum, resulting in net motion along the background magnetic field with speeds up to a significant fraction of lightspeed. This discovery has important implications for the launching of accretion disk winds. We also find that although particle acceleration in imbalanced turbulence operates on a slower timescale than in balanced turbulence, it ultimately produces a power-law energy distribution similar to balanced turbulence. Our results have ramifications for black hole accretion disk coronae, winds, and jets.

astro-ph.HE↗

arXiv2021

Particle energization in relativistic plasma turbulence: solenoidal versus compressive driving

Many high-energy astrophysical systems contain magnetized collisionless plasmas with relativistic particles, in which turbulence can be driven by an arbitrary mixture of solenoidal and compressive motions. For example, turbulence in hot accretion flows may be driven solenoidally by the magnetorotational instability or compressively by spiral shock waves. It is important to understand the role of the driving mechanism on kinetic turbulence and the associated particle energization. In this work, we compare particle-in-cell simulations of solenoidally driven turbulence with similar simulations of compressively driven turbulence. We focus on plasma that has an initial beta of unity, relativistically hot electrons, and varying ion temperature. Apart from strong large-scale density fluctuations in the compressive case, the turbulence statistics are similar for both drives, and the bulk plasma is described reasonably well by an isothermal equation of state. We find that nonthermal particle acceleration is more efficient when turbulence is driven compressively. In the case of relativistically hot ions, both driving mechanisms ultimately lead to similar power-law particle energy distributions, but over a different duration. In the case of non-relativistic ions, there is significant nonthermal particle acceleration only for compressive driving. Additionally, we find that the electron-to-ion heating ratio is less than unity for both drives, but takes a smaller value for compressive driving. We demonstrate that this additional ion energization is associated with the collisionless damping of large-scale compressive modes via perpendicular electric fields.

astro-ph.HE↗

arXiv2021

Spontaneous magnetization of collisionless plasma through the action of a shear flow

We study in a fully kinetic framework the generation of seed magnetic fields through the Weibel instability driven in an initially unmagnetized plasma by a large-scale shear force. We develop an analytical model that describes the development of thermal pressure anisotropy via phase mixing, the ensuing exponential growth of magnetic fields in the linear Weibel stage, and its saturation when the seed magnetic fields become strong enough to instigate gyromotion of particles and thereby inhibit their free-streaming. The predicted scaling dependencies of the saturated seed fields on key parameters (e.g., ratio of system scale to electron skin depth, the forcing amplitude) are confirmed by 3D and 2D particle-in-cell simulations using an electron-positron plasma. This work demonstrates the spontaneous magnetization of a collisionless plasma through large-scale motions as simple as a shear flow, and therefore has important implications for magnetogenesis in dilute astrophysical systems.

physics.plasm-ph↗