arXiv subjects

Francisco Ley

Publications and source records attributed to Francisco Ley.

7 recordsLinked to original sources

arXiv2026

On the double-adiabatic equations in the relativistic regime

We revisit the double adiabatic evolution equations and extend them to the relativistic and ultrarelativistic regimes. We analytically solve the relativistic, time-dependent drift kinetic equation for a homogeneous, magnetized, collisionless plasma and obtain a solution explicitly dependent on the magnetic field and density variations. In the case of an initial relativistic Maxwellian distribution, a natural extension to an anisotropic Maxwell-Jüttner is obtained. We calculate the moments of this time-dependent solution and obtain analytical expressions for the evolution of the perpendicular and parallel pressures in the ultrarelativistic case. We numerically solve the moment equations in the relativistic case and obtain general expressions for the double-adiabatic equations in this regime. We confirm our results using fully kinetic particle-in-cell simulations of shearing and compressing boxes. Our findings can be readily applied to relativistic species including cosmic-rays and electron-positron pairs, present in astrophysical plasmas like pulsar wind nebulae, astrophysical jets, black hole accretion flows, and Van Allen radiation belts.

physics.plasm-ph↗

arXiv2026

Reduced Effective Viscosity from Anisotropic Transport and Plasma Instabilities in the Sloshing Cores of Galaxy Clusters

The $\sim μ$G magnetic field in the intracluster medium (ICM) introduces a pressure anisotropy with respect to the magnetic field's direction that manifests as an anisotropic viscous stress. Plasma instabilities arising from the pressure anisotropy crossing certain thresholds force it to marginally stable values, reducing viscous transport. Additionally, the feedback of this anisotropic pressure on the velocity field has been predicted to lead to a form of self-organization that also can reduce viscous dissipation without affecting the collisionality. In this work, we present high-resolution Braginskii-MHD simulations of a galaxy cluster core with sloshing gas motions and turbulence, including the effects of anisotropic viscous stress and different simple prescriptions for limiting the pressure anisotropy due to plasma instabilities. Braginskii viscosity has an expected, though modest, effect on suppressing Kelvin-Helmholtz instabilities at sloshing cold front surfaces, dependent on how the pressure anisotropy is limited. Due to the sloshing motions, the magnetic field's strength can become high enough in places that the pressure anisotropy need not be limited. Nevertheless, the combined effect of the limiters and the turbulent structure of the magnetic field in all simulations is that the effective viscosity is much lower than the isotropic Spitzer value in a significant fraction of the core region. However, we find that this reduced viscosity is capable of steepening the velocity-amplitude spectrum and transferring a small fraction of the turbulent kinetic energy into heat. Finally, we present evidence for magneto-immutable dynamics in our simulations.

astro-ph.GA↗

arXiv2026

The Role of Whistler and Ion Cyclotron Waves in Particle Escape from Mirror Modes in the Intracluster Medium

Electron and ion-cyclotron waves are well known to exist in solar system plasmas but their existence and importance in galaxy clusters is an open question. Guided by numerical simulations, (Ley et al. (2024)) argued that whistlers (electron-cyclotron) and ion-cyclotron (IC) waves are generated by trapped particles in mirror modes in the nonlinear stages of the mirror instability under ICM conditions. Building on this work, we construct a novel particle propagation simulation of the ICM plasma based on the static electromagnetic field configuration from the fully kinetic particle-in-cell (PIC) simulation of the nonlinear mirror instability by (Ley et al. (2024)). We study how the trapping rate of particles is related to the secondary waves driven by mirror modes. We observe that secondary whistlers and IC waves enhance trapped particle escape from mirror modes. We measure the particle-wave scattering rate by whistlers and IC waves, demonstrate that the scattering rates and wave amplitudes follow the proportionality relation expected from quasilinear theory, and show the existence of a significant correlation between scattering rates and the excitation of secondary instabilities.

astro-ph.HE↗

arXiv2023

Secondary Whistler and Ion-cyclotron Instabilities driven by Mirror Modes in Galaxy Clusters

Electron cyclotron waves (whistlers), are commonly observed in plasmas near Earth and the solar wind. In the presence of nonlinear mirror modes, bursts of whistlers, usually called lion roars, have been observed within low magnetic field regions associated to these modes. In the intracluster medium (ICM) of galaxy clusters, the excitation of the mirror instability is expected, but it is not yet clear whether electron and ion cyclotron waves can also be present under conditions where gas pressure dominates over magnetic pressure (high $β$). In this work, we perform fully kinetic particle-in-cell (PIC) simulations of a plasma subject to a continuous amplification of the mean magnetic field $\textbf{B}(t)$ to study the nonlinear stages of the mirror instability and the ensuing excitation of whistler and ion cyclotron (IC) waves under ICM conditions. Once mirror modes reach nonlinear amplitudes, both whistler and IC waves start to emerge simultaneously, with sub-dominant amplitudes, propagating in low-$\textbf{B}$ regions, and quasi-parallel to $\textbf{B}(t)$. We show that the underlying source of excitation is the pressure anisotropy of electrons and ions trapped in mirror modes with loss-cone type distributions. We also observe that IC waves play an essential role in regulating the ion pressure anisotropy at nonlinear stages. We argue that whistler and IC waves are a concomitant feature at late stages of the mirror instability even at high-$β$, and therefore expected to be present in astrophysical environments like the ICM. We discuss the implications of our results for collisionless heating and dissipation of turbulence in the ICM.

astro-ph.HE↗

arXiv2023

Electron Re-acceleration via Ion Cyclotron Waves in the Intracluster Medium

In galaxy clusters, the intracluster medium (ICM) is expected to host a diffuse, long-lived, and invisible population of "fossil" cosmic-ray electrons (CRe) with 1-100 MeV energies. These CRe, if re-accelerated by 100x in energy, can contribute synchrotron luminosity to cluster radio halos, relics, and phoenices. Re-acceleration may be aided by CRe scattering upon the ion-Larmor-scale waves that spawn when ICM is compressed, dilated, or sheared. We study CRe scattering and energy gain due to ion cyclotron (IC) waves generated by continuously-driven compression in 1D fully kinetic particle-in-cell simulations. We find that pitch-angle scattering of CRe by IC waves induces energy gain via magnetic pumping. In an optimal range of IC-resonant momenta, CRe may gain up to ~10-30% of their initial energy in one compress/dilate cycle with magnetic field amplification ~3-6x, assuming adiabatic decompression without further scattering and averaging over initial pitch angle.

astro-ph.HE↗

arXiv2022

A Heating Mechanism via Magnetic Pumping in the Intracluster Medium

Turbulence driven by AGN activity, cluster mergers and galaxy motion constitutes an attractive energy source for heating the intracluster medium (ICM). How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization (precluding viscous heating by Coulomb processes). Kunz et al. 2011 proposed a viable heating mechanism based on the anisotropy of the plasma pressure (gyroviscous heating) under ICM conditions. The present paper builds upon that work and shows that particles can be gyroviscously heated by large-scale turbulent fluctuations via magnetic pumping. We study how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated and demonstrate that the particle distribution function acquires a high energy tail. For this, we perform particle-in-cell simulations where we periodically vary the mean magnetic field $\textbf{B}(t)$. When $\textbf{B}(t)$ grows (dwindles), a pressure anisotropy $P_{\perp}>P_{\parallel}$ ($P_{\perp}< P_{\parallel}$) builds up ($P_{\perp}$ and $P_{\parallel}$ are, respectively, the pressures perpendicular and parallel to $\textbf{B}(t)$). These pressure anisotropies excite mirror ($P_{\perp}>P_{\parallel}$) and oblique firehose ($P_{\parallel}>P_{\perp}$) instabilities, which trap and scatter the particles, limiting the anisotropy and providing a channel to heat the plasma. The efficiency of this mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear stage. We provide a simplified analytical heating model that captures the phenomenology involved. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM.

astro-ph.HE↗

arXiv2019

Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field

Using 1D and 2D particle-in-cell (PIC) simulations of a plasma with a growing magnetic field $\vec{B}$, we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As $\vec{B}$ grows, an ion pressure anisotropy $p_{\perp,i} > p_{||,i}$ arises, due to the adiabatic invariance of the ion magnetic moment ($p_{||,i}$ and $p_{\perp,i}$ are the ion pressures parallel and perpendicular to $\vec{B}$). When initially $β_i = 0.5$ ($β_i \equiv 8πp_i/|\vec{B}|^2$, where $p_i$ is the ion isotropic pressure), the pressure anisotropy is limited mainly by inelastic pitch-angle scattering provided by the IC instability, which in turn produces a non-thermal tail in the ion energy spectrum. After $\vec{B}$ is amplified by a factor $\sim 2.7$, this tail can be approximated as a power-law of index $\sim 3.4$ plus two non-thermal bumps, and accounts for $2-3\%$ of the ions and $\sim 18\%$ of their kinetic energy. On the contrary, when initially $β_i =2$, the ion scattering is dominated by the mirror instability and the acceleration is suppressed. This implies that efficient ion acceleration requires that initially $β_i \lesssim 1$. Although we focus on cases where $\vec{B}$ is amplified by plasma shear, we check that the acceleration occurs similarly if $\vec{B}$ grows due to plasma compression. Our results are valid in a sub-relativistic regime where the ion thermal energy is $\sim 10\%$ of the ion rest mass energy. This acceleration process can thus be relevant in the inner region of low-luminosity accretion flows around black holes.

astro-ph.HE↗