arXiv subjects

Reinaldo Santos-Lima

Publications and source records attributed to Reinaldo Santos-Lima.

10 recordsLinked to original sources

arXiv2025

Spectral Learning of Magnetized Plasma Dynamics: A Neural Operator Application

Fourier neural operators (FNOs) provide a mesh-independent way to learn solution operators for partial differential equations, yet their efficacy for magnetized turbulence is largely unexplored. Here we train an FNO surrogate for the 2-D Orszag-Tang vortex, a canonical non-ideal magnetohydrodynamic (MHD) benchmark, across an ensemble of viscosities and magnetic diffusivities. On unseen parameter settings the model achieves a mean-squared error of $\approx 6 \times 10^{-3}$ in velocity and $\approx 10^{-3}$ in magnetic field, reproduces energy spectra and dissipation rates within $96\%$ accuracy, and retains temporal coherence over long timescales. Spectral analysis shows accurate recovery of large- and intermediate-scale structures, with degradation at the smallest resolved scales due to Fourier-mode truncation. Relative to a UNet baseline the FNO cuts error by $97\%$, and compared with a high-order finite-volume solver it delivers a $25\times$ inference speed-up, offering a practical path to rapid parameter sweeps in MHD simulations.

astro-ph.HE↗

arXiv2023

Tomographic Imaging of the Sagittarius Spiral Arm's Magnetic Field Structure

The Galactic global magnetic field is thought to play a vital role in shaping Galactic structures such as spiral arms and giant molecular clouds. However, our knowledge of magnetic field structures in the Galactic plane at different distances is limited, as measurements used to map the magnetic field are the integrated effect along the line of sight. In this study, we present the first-ever tomographic imaging of magnetic field structures in a Galactic spiral arm. Using optical stellar polarimetry over a $17' \times 10'$ field of view, we probe the Sagittarius spiral arm. Combining these data with stellar distances from the $Gaia$ mission, we can isolate the contributions of five individual clouds along the line of sight by analyzing the polarimetry data as a function of distance. The observed clouds include a foreground cloud ($d < 200$ pc) and four clouds in the Sagittarius arm at 1.23 kpc, 1.47 kpc, 1.63 kpc, and 2.23 kpc. The column densities of these clouds range from 0.5 to $2.8 \times 10^{21}~\mathrm{cm}^{-2}$. The magnetic fields associated with each cloud show smooth spatial distributions within their observed regions on scales smaller than 10 pc and display distinct orientations. The position angles projected on the plane-of-sky, measured from the Galactic north to east, for the clouds in increasing order of distance are $135^\circ$, $46^\circ$, $58^\circ$, $150^\circ$, and $40^\circ$, with uncertainties of a few degrees. Notably, these position angles deviate significantly from the direction parallel to the Galactic plane.

astro-ph.GA↗

arXiv2021

Magnetic Field Orientation in Self-Gravitating Turbulent Molecular Clouds

Stars form inside molecular cloud filaments from the competition of gravitational forces with turbulence and magnetic fields. The exact orientation of these filaments with the magnetic fields depends on the strength of these fields, the gravitational potential, and the line-of-sight (LOS) relative to the mean field. To disentangle these effects we employ three-dimensional magnetohydrodynamical numerical simulations that explore a wide range of initial turbulent and magnetic states, i.e., sub-Alfvénic to super-Alfvénic turbulence, with and without gravity. We use histogram of relative orientation (HRO) and the associated projected Rayleigh statistics (PRS) to study the orientation of density and, in order to compare with observations, the integrated density relative to the magnetic field. We find that in sub-Alfvénic systems the initial coherence of the magnetic is maintained inside the cloud and filaments form perpendicular to the field. This trend is not observed in super-Alfvénic models, where the lines are dragged by gravity and turbulence and filaments are mainly aligned to the field. The PRS analysis of integrated maps shows that LOS effects are important only for sub-Alfvénic clouds. When the LOS is perpendicular to the initial field orientation most of the filaments are perpendicular to the projected magnetic field. The inclusion of gravity increases the number of dense structures perpendicular to the magnetic field, reflected as lower values of the PRS for denser regions, regardless of whether the model is sub- or super-Alfvénic. The comparison of our results with observed molecular clouds reveal that most are compatible with sub-Alfvénic models.

astro-ph.GA↗

arXiv2021

Identification of plasma modes in Galactic turbulence with synchrotron polarization

Magneto-hydrodynamic (MHD) turbulence is ubiquitous and a fundamental ingredient underlying many astrophysical phenomena. The multiphase nature of interstellar medium and diversity of driving mechanisms give rise to spatial variation of turbulence properties, particularly their plasma properties. There has been no observational diagnosis of the plasma modes so far beyond the solar system. Here we report the identification of different plasma modes in various Galactic environments, including active star forming zones and supernova remnants, based on our synchrotron polarization analysis. The observed high degree of consistency between the $γ-$ray excess in Cygnus cocoon and the location of magnetosonic modes provides strong observational evidence for the long-advocated theory that magnetosonic modes dominate the cosmic ray (CR) scattering and acceleration. Our results open up a new avenue for the study of interstellar turbulence and demonstrate the indispensability to account for their plasma property in all the relevant processes including CR transport and star formation.

physics.plasm-ph↗

arXiv2018

Kinetic-MHD simulations of gyroresonance instability driven by CR pressure anisotropy

The transport of cosmic rays (CRs) is crucial for the understanding of almost all high-energy phenomena. Both pre-existing large-scale magnetohydrodynamic (MHD) turbulence and locally generated turbulence through plasma instabilities are important for the CR propagation in astrophysical media. The potential role of the resonant instability triggered by CR pressure anisotropy to regulate the parallel spatial diffusion of low-energy CRs ($\lesssim 100$ GeV) in the interstellar and intracluster medium of galaxies (ISM and ICM) has been showed in previous theoretical works. This work aims to study the gyroresonance instability via direct numerical simulations, in order to access quantitatively the wave-particle scattering rates. For this we employ a 1D PIC-MHD code to follow the growth and saturation of the gyroresonance instability. We extract from the simulations the pitch-angle diffusion coefficient $D_{μμ}$ produced by the instability during the linear and saturation phases, and a very good agreement (within a factor of 3) is found with the values predicted by the quasilinear theory (QLT). Our results support the applicability of the QLT for modeling the scattering of low-energy CRs by the gyroresonance instability in the complex interplay between this instability and the large-scale MHD turbulence.

astro-ph.HE↗

arXiv2016

Magnetic fields in early protostellar disk formation

We consider formation of accretion disks from a realistically turbulent molecular gas using 3D MHD simulations. In particular, we analyze the effect of the fast turbulent reconnection described by the Lazarian & Vishniac (1999) model for the removal of magnetic flux from a disk. With our numerical simulations we demonstrate how the fast reconnection enables protostellar disk formation resolving the so-called "magnetic braking catastrophe". In particular, we provide a detailed study of the dynamics of a 0.5 M$_\odot$ protostar and the formation of its disk for up to several thousands years. We measure the evolution of the mass, angular momentum, magnetic field, and turbulence around the star. We consider effects of two processes that strongly affect the magnetic transfer of angular momentum, both of which are based on turbulent reconnection: the first, "reconnection diffusion", removes the magnetic flux from the disk, the other involves the change of the magnetic field's topology, but does not change the absolute value of the magnetic flux through the disk. We demonstrate that for the first mechanism, turbulence causes a magnetic flux transport outward from the inner disk to the ambient medium, thus decreasing the coupling of the disk to the ambient material. A similar effect is achieved through the change of the magnetic field's topology from a split monopole configuration to a dipole configuration. We explore how both mechanisms prevent the catastrophic loss of disk angular momentum and compare both above turbulent reconnection mechanisms with alternative mechanisms from the literature.

astro-ph.SR↗

arXiv2016

Turbulence-induced magnetic fields in shock precursors

Galactic cosmic rays are believed to be mostly accelerated at supernova shocks. However, the interstellar magnetic field is too weak to efficiently accelerate galactic cosmic rays up to the highest energies, i.e. $10^{15}$ eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic-ray nonresonant streaming instability has been claimed to be responsible for the amplification of precursor magnetic fields. However, an alternative mechanism has been proposed in which the cosmic-ray pressure gradient forms the shock precursor and drives turbulence, amplifying the magnetic field via the small-scale dynamo. A key ingredient for the mechanism to operate are the inhomogeneities present in the interstellar medium (ISM). These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different ISM conditions through 3D MHD numerical simulations.

astro-ph.HE↗

arXiv2014

Runaway stars as cosmic ray injectors inside molecular clouds

Giant molecular clouds (GMCs) are a new population of gamma-ray sources, being the target of cosmic rays (CRs) -- locally accelerated or not --. These clouds host very young stellar clusters where massive star formation takes place. Eventually, some of the stars are ejected from the clusters, becoming runaway stars. These stars move supersonically through the cloud and develop bowshocks where particles can be accelerated up to relativistic energies. As a result, the bowshocks present non-thermal emission, and inject relativistic protons in the cloud. These protons diffuse in the GMC interacting with the matter. We present a model for the non-thermal radiation generated by protons and secondary pairs accelerated in the bowshocks of massive runaways stars within young GMCs. We solve the transport equation for primary protons and secondary pairs as the stars move through the cloud. We present non-thermal emissivity maps in radio and in gamma rays as a function of time. We obtain X-ray luminosities of the order $\sim$ ${ 10^{32}}$ erg~s$^{-1}$ and gamma-ray luminosities $\sim$ $10^{34}$ erg~s$^{-1}$. We conclude that, under some assumptions, relativistic protons from massive runaway stars interacting with matter in GMCs give rise to extended non-thermal sources.

astro-ph.HE↗

arXiv2011

Magnetic Flux Transport by turbulent reconnection in astrophysical flows

The role of MHD turbulence in astrophysical environments is still highly debated. An important question that permeates this debate is the transport of magnetic flux. This is particularly important, for instance, in the context of star formation. When clouds collapse gravitationally to form stars, there must be some magnetic flux transport. otherwise the new born stars would have magnetic fields several orders of magnitude larger than the observed ones. Also, the magnetic flux that is dragged in the late stages of the formation of a star can remove all the rotational support from the accretion disk that grows around the protostar. The efficiency of the mechanism which is often invoked to allow the transport of magnetic fields in the different stages of star formation, namely, the ambipolar diffusion, has been lately put in check. We here discuss an alternative mechanism for magnetic flux transport which is based on turbulent fast magnetic reconnection. We review recent results obtained from 3D MHD numerical simulations that indicate that this mechanism is very efficient for decoupling and transport magnetic flux from the inner denser regions to the outskirts of collapsing clouds in the different stages of star formation. We also discuss this mechanism in the context of dynamo processes and speculate that it can play a role both in the solar dynamo and in accretion disk dynamo processes.

astro-ph.SR↗

arXiv2011

Particle Acceleration by Magnetic Reconnection in AGNs and in the IGM

There is no single mechanism by which fast particles are accelerated in astrophysical environments, and it is now recognized that the data require a rich variety of different mechanisms operating under different conditions. The mechanisms discussed in the literature include varying magnetic fields in compact sources, stochastic processes in turbulent environments, and acceleration behind shocks. An alternative, much less explored mechanism so far, involves particle acceleration within magnetic reconnection sites. In this work, we explore this mechanism in the AGN framework and show that particles are efficiently accelerated through a first-order Fermi process and have an exponential growth of energy. We also address briefly the propagation of cosmic rays (CRs) in the intergalactic medium (IGM). Since the latter is a collisionless environment, kinetic effects must be considered which will affect the turbulent magnetic field distribution and therefore, the CR propagation.

astro-ph.HE↗