arXiv subjects

Mario Riquelme

Publications and source records attributed to Mario Riquelme.

13 recordsLinked to original sources

arXiv2026

Transport of electrons in tangled magnetic fields

Cosmic magnetic fields are typically inhomogeneous and often highly tangled due to large-scale plasma flows, turbulence, and instabilities. If the variations in the magnetic field occur on scales that are large compared to the gyro-radius of the plasma electrons, the electrons are primarily confined to gyro-centre trajectories along the field lines. Therefore, in-situ electron measurements help us map out the connectivity of the magnetic field in space plasmas. Gyro-centre drifts, wave-particle interactions, trapping, and cross-field diffusion are processes related to field inhomogeneities and fluctuations; they have the potential to modify or even disrupt the transport of electrons along field lines. We introduce the basic principles of electron transport in tangled magnetic fields and review the creation of tangled fields through turbulence and instabilities as well as the modulation of parallel electron transport through kinetic instabilities. We then describe trapping and de-trapping effects in inhomogeneous magnetic fields, as well as electron diffusion and energisation across the magnetic field. The transport of electrons in tangled fields results from a complex interplay of plasma processes that occur on a broad range of scales. A combination of in-situ plasma measurements, remote-sensing plasma observations, and plasma theory and simulations is required to resolve this contemporary challenge to the fields of heliophysics and astrophysics.

physics.space-ph↗

arXiv2025

Double Power-law Electron Spectra in Solar Flares Due to Temperature Anisotropy Instabilities

Despite extensive observational and theoretical efforts, the physical processes responsible for shaping the diversity of accelerated electron spectra observed in solar flares remain poorly understood. We use 2D particle-in-cell (PIC) simulations of magnetized plasmas subject to continuous shear-driven magnetic amplification to investigate whether electron temperature anisotropy instabilities in above-the-loop-top (ALT) regions can account for this diversity. We explore how the resulting spectra depend on key plasma parameters: the initial electron temperature $T_e$ and the initial ratio of electron cyclotron to plasma frequencies, $f_e = ω_{ce}/ω_{pe}$. In our simulations, the adiabatic evolution of the plasma generates electron temperature anisotropy with the electron temperature perpendicular to the magnetic field being larger than the parallel temperature. This eventually drives electromagnetic instabilities capable of scattering and accelerating electrons. The simulations consistently produce nonthermal tails in the electron spectra whose hardness increases with the initial value of $f_e$, while depending only weakly on $T_e$. For runs in which $f_e \lesssim 1.2$, the spectra exhibit double power-law shapes with downward (knee-like) breaks, and the electron scattering is dominated by OQES modes. In runs with $f_e\gtrsim 1.5$, PEMZ modes dominate and produce harder double power-law spectra with upward (elbow-like) breaks. Cases that include the $f_e\sim 1.2-1.5$ transition yield nearly single power-laws that end with bump-like breaks. Our results support the role of temperature anisotropy instabilities in accelerating electrons in ALT regions, offering a promising framework to help explain the wide range of nonthermal electron spectra reported in solar flare observations.

astro-ph.SR↗

arXiv2024

HESS J1745$-$290 spectrum explained by a transition in the diffusion regime of PeV cosmic rays in the Sgr A* accretion flow

Context. The diffuse TeV gamma-ray emission detected in the inner $\sim$ 100 pc of the Galactic Center suggests the existence of a central cosmic-ray accelerator reaching $\sim$ PeV energies. It is interesting to associate this so-called PeVatron with the point source HESS J1745$-$290, whose position is consistent with that of the central supermassive black hole, Sgr A*. However, the point source shows a spectral break at a few TeV, which is not shown by the diffuse emission, challenging this association. Aims. We seek to build an emission model for the point source consistent with both emissions being produced by the same population of relativistic protons, continuously injected with a power-law spectrum up to $\sim$ PeV energies, near Sgr A*. Methods. In our model, we assume that the point source is produced by hadronic collisions between the cosmic rays and the gas in the accretion flow of Sgr A*. The cosmic-ray density is calculated taking into consideration cosmic-ray transport due to diffusion and advection, while the properties of the gas are obtained from previous numerical simulations of the accretion flow. Results. Our model succeeds in explaining both the point source and the diffuse emission with the same cosmic rays injected in the vicinity of Sgr A*, as long as the coherence length of the magnetic turbulence in the accretion flow is $l_c\sim(1-3)\times 10^{14}\,\mathrm{cm}$. The spectral break of the point source appears naturally due to an energy-dependent transition in the way the cosmic rays diffuse within the inner $\sim 0.1$ pc of the accretion flow (where most of the emission is produced) Conclusions. Our model supports the idea that Sgr A* can be a PeVatron, whose accelerated cosmic rays give rise to both the point source and the diffuse emission. Future TeV telescopes, like CTAO, will be able to test this model.

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

Particle-in-cell Simulations of the Magnetorotational Instability in Stratified Shearing Boxes

The magnetorotational instability (MRI) plays a crucial role in regulating the accretion efficiency in astrophysical accretion disks. In low-luminosity disks around black holes, such as Sgr A* and M87, Coulomb collisions are infrequent, making the MRI physics effectively collisionless. The collisionless MRI gives rise to kinetic plasma effects that can potentially affect its dynamic and thermodynamic properties. We present 2D and 3D particle-in-cell (PIC) plasma simulations of the collisionless MRI in stratified disks using shearing boxes with net vertical field. We use pair plasmas, with initial $β=100$ and concentrate on sub-relativistic plasma temperatures ($k_BT \lesssim mc^2$). Our 2D and 3D runs show disk expansion, particle and magnetic field outflows, and a dynamo-like process. They also produce magnetic pressure dominated disks with (Maxwell stress dominated) viscosity parameter $α\sim 0.5-1$. By the end of the simulations, the dynamo-like magnetic field tends to dominate the magnetic energy and the viscosity in the disks. Our 2D and 3D runs produce fairly similar results, and are also consistent with previous 3D MHD simulations. Our simulations also show nonthermal particle acceleration, approximately characterized by power-law tails with temperature dependent spectral indices $-p$. For temperatures $k_BT \sim 0.05-0.3\, mc^2$, we find $p\approx 2.2-1.9$. The maximum accelerated particle energy depends on the scale separation between MHD and Larmor-scale plasma phenomena in a way consistent with previous PIC results of magnetic reconnection-driven acceleration. Our study constitutes a first step towards modeling from first principles potentially observable stratified MRI effects in low-luminosity accretion disks around black holes.

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↗

arXiv2021

Stochastic Electron Acceleration by Temperature Anisotropy Instabilities Under Solar Flare Plasma Conditions

Using 2D particle-in-cell (PIC) plasma simulations we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top (ALT) sources in solar flares. We focus on the long-term effect of $T_{e,\perp} > T_{e,\parallel}$ instabilities by driving the anisotropy growth during the entire simulation time, through imposing a shearing or a compressing plasma velocity ($T_{e,\perp}$ and $T_{e,\parallel}$ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes $T_{e,\perp}/T_{e,\parallel}$ grow due to electron magnetic moment conservation, and amplifies the ratio $ω_{ce}/ω_{pe}$ from $\sim 0.53$ to $\sim 2$ ($ω_{ce}$ and $ω_{pe}$ are the electron cyclotron and plasma frequencies, respectively). In the regime $ω_{ce}/ω_{pe}\lesssim 1.2-1.7$ the instability is dominated by oblique, quasi-electrostatic (OQES) modes, and the acceleration is inefficient. When $ω_{ce}/ω_{pe}$ has grown to $ω_{ce}/ω_{pe}\gtrsim 1.2-1.7$, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z (PEMZ) modes. After $ω_{ce}/ω_{pe}$ reaches $\sim 2$, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power-law of index $α_s \sim$ 2.9 plus a high-energy bump reaching $\sim 300$ keV. In the compressing runs, $α_s \sim$ 3.7 with a spectral break above $\sim 500$ keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting a critical role played by the type of anisotropy driving, $ω_{ce}/ω_{pe}$ and the electron temperature in the efficiency of the acceleration.

astro-ph.SR↗

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↗

arXiv2018

PIC Simulations of Velocity-Space Instabilities in a Decreasing Magnetic Field: Viscosity and Thermal Conduction

We use particle-in-cell (PIC) simulations of a collisionless, electron-ion plasma with a decreasing background magnetic field, $B$, to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If $B$ decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with $p_{||,j} > p_{\perp,j}$ ($p_{||,j}$ and $p_{\perp,j}$ represent the pressure of species $j$ ($=i$ or $e$) parallel and perpendicular to the magnetic field). Linear theory indicates that, for sufficiently large anisotropies, different velocity-space instabilities can be triggered. These instabilities, which grow on scales comparable to the electron and ion Larmor radii, in principle have the ability to pitch-angle scatter the particles, limiting the growth of the anisotropies. Our PIC simulations focus on the nonlinear, saturated regime of the instabilities. This is done through the permanent decrease of the magnetic field by an imposed shear in the plasma. Our results show that, in the regime $2 \lesssim β_j \lesssim 20$ ($β_j \equiv 8πp_j/B^2$), the saturated ion and electron pressure anisotropies are controlled by the combined effect of the oblique ion firehose (OIF) and the fast magnetosonic/whistler (FM/W) instabilities. These instabilities grow preferentially on the ion Larmor radius scale, and make the ion and electron pressure anisotropies nearly equal: $Δp_e/p_{||,e} \approx Δp_i/p_{||,i}$ (where $Δp_j=p_{\perp,j} - p_{||,j}$). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons along the mean magnetic field, which we find strongly depends on whether $B$ decreases or increases. Our results can be applied in studies of low collisionality plasmas such as the solar wind, the intracluster medium, and some accretion disks around black holes.

astro-ph.HE↗

arXiv2017

Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

We use 2D particle-in-cell (PIC) simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, \textbf{B}. In this setup, an electron pressure anisotropy with $p_{\perp,e} > p_{||,e}$ naturally arises due to the adiabatic invariance of the electron magnetic moment ($p_{||,e}$ and $p_{\perp,e}$ are the pressures parallel and perpendicular to \textbf{B}). If the anisotropy is large enough, the whistler instability arises, efficiently scattering the electrons and limiting $Δp_e$ ($\equiv p_{\perp,e}-p_{||,e}$). In this context, $Δp_e$ taps into the plasma velocity shear, producing electron heating by the so called anisotropic viscosity. In our simulations, we permanently drive the growth of $|\textbf{B}|$ by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially $β_e\sim 1$, gradually decreasing its efficiency for larger values of $β_e$ ($\equiv 8πp_e/|\textbf{B}|^2$). If initially $β_e \sim 1$, the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with spectral index $\sim 3.7$. In these cases, the nonthermal tail accounts for $\sim 5\%$ of the electrons, and for $\sim 15\%$ of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.

astro-ph.HE↗

arXiv2016

PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction

In low-collisionality plasmas, velocity-space instabilities are a key mechanism providing an effective collisionality for the plasma. We use particle-in-cell (PIC) simulations to study the interplay between electron and ion-scale velocity-space instabilities and their effect on electron pressure anisotropy, viscous heating, and thermal conduction. The adiabatic invariance of the magnetic moment in low-collisionality plasmas leads to pressure anisotropy, $p_{\perp,j} > p_{||,j}$, if the magnetic field $\vec{B}$ is amplified ($p_{\perp,j}$ and $p_{||,j}$ denote the pressure of species $j$ [electron, ion] perpendicular and parallel to $\vec{B}$). If the resulting anisotropy is large enough, it can in turn trigger small-scale plasma instabilities. Our PIC simulations explore the nonlinear regime of the mirror, ion-cyclotron, and electron whistler instabilities, through continuous amplification of the magnetic field $|\vec{B}|$ by an imposed shear in the plasma. In the regime $1 \lesssim β_j \lesssim 20$ ($β_j \equiv 8πp_j/|\vec{B}|^2$), the saturated electron pressure anisotropy, $Δp_e/p_{||,e}$, is determined mainly by the (electron-lengthscale) whistler marginal stability condition, with a modest factor of $\sim 1.5-2$ decrease due to the trapping of electrons by the mirrors. We explicitly calculate the mean free path of the electrons and ions along the mean magnetic field and provide a simple physical prescription for the mean free path and thermal conductivity in low-collisionality $β_j \gtrsim 1$ plasmas. Our results imply that velocity-space instabilities likely decrease the thermal conductivity of plasma in the outer parts of massive, hot, galaxy clusters. We also discuss the implications of our results for electron heating and thermal conduction in low-collisionality accretion flows onto black holes, including Sgr A* in the Galactic Center.

physics.plasm-ph↗

arXiv2015

PIC Simulations of Continuously Driven Mirror and Ion Cyclotron Instabilities in High Beta Astrophysical and Heliospheric Plasmas

We use particle-in-cell (PIC) simulations to study the nonlinear evolution of ion velocity space instabilities in an idealized problem in which a background velocity shear continuously amplifies the magnetic field. We simulate the astrophysically relevant regime where the shear timescale is long compared to the ion cyclotron period, and the plasma beta is ~ 1-100. The background field amplification in our calculation is meant to mimic processes such as turbulent fluctuations or MHD-scale instabilities. The field amplification continuously drives a pressure anisotropy with the perpendicular pressure larger than the parallel pressure, and the plasma becomes unstable to the mirror and ion cyclotron instabilities. In all cases, the nonlinear state is dominated by the mirror instability, not the ion cyclotron instability, and the plasma pressure anisotropy saturates near the threshold for the linear mirror instability. The magnetic field fluctuations initially undergo exponential growth but saturate in a secular phase in which the fluctuations grow on the same timescale as the background magnetic field (with δB ~ 0.3 in the secular phase). At early times, the ion magnetic moment is well-conserved but once the fluctuation amplitudes exceed δB ~ 0.1 , the magnetic moment is no longer conserved but instead changes on a timescale comparable to that of the mean magnetic field. We discuss the implications of our results for low-collisionality astrophysical plasmas, including the near-Earth solar wind and low-luminosity accretion disks around black holes.

astro-ph.HE↗

arXiv2005

Neutrino emission rates in highly magnetized neutron stars revisited

Magnetars are a subclass of neutron stars whose intense soft-gamma-ray bursts and quiescent X-ray emission are believed to be powered by the decay of a strong internal magnetic field. We reanalyze neutrino emission in such stars in the plausibly relevant regime in which the Landau band spacing of both protons and electrons is much larger than kT (where k is the Boltzmann constant and T is the temperature), but still much smaller than the Fermi energies. Focusing on the direct Urca process, we find that the emissivity oscillates as a function of density or magnetic field, peaking when the Fermi level of the protons or electrons lies about 3kT above the bottom of any of their Landau bands. The oscillation amplitude is comparable to the average emissivity when the Landau band spacing mentioned above is roughly the geometric mean of kT and the Fermi energy (excluding mass), i. e., at fields much weaker than required to confine all particles to the lowest Landau band. Since the density and magnetic field strength vary continuously inside the neutron star, there will be alternating surfaces of high and low emissivity. Globally, these oscillations tend to average out, making it unclear whether there will be any observable effects.

astro-ph↗