Elisabete M. de Gouveia Dal Pino

Magnetic Fields in Diffuse Media

Future Observations of Cosmic Magnetic Fields with LOFAR, SKA and its precursors.- Magnetic fields with the Atacama Large Millimeter/submillimeter Array.- Synchrotron radiation and Faraday rotation.- Interstellar Grain Alignment - Observational Status.- Magnetic Field Measurement with Ground State Alignment.- Kinetic Turbulence.- Interstellar Polarization and Magnetic Turbulence.- MHD Turbulence and its Implications.- The intermittency of ISM turbulence : what do the observations tell us?.- Ambipolar Diffusion.- Magnetic Reconnection in Astrophysical Environments.- Interstellar MHD Turbulence and Star Formation.- Observations of Magnetic Fields in Molecular Clouds-Testing Star Formation Theory.- Magnetic Fields in Star Formation.- Particle Acceleration by Magnetic Reconnection.- Cosmic Ray transport in turbulent magnetic field.- Magnetic Fields in the Milky Way.- Magnetic fields in galaxies.- Simulations of galactic dynamos.- Cosmic rays in galaxy clusters and their interaction with magnetic fields.- Turbulence in the intracluster medium.

Multiple outflow episodes from protostars: Three-dimensional models of intermittent jets

Short abstract: We present fully 3-D simulations of supersonic, radiatively cooling intermittent jets with intermediate and long variability periods (that is, periods of the order of or longer than, the dynamical time scale of the jet). Variations of intermediate period elucidate the formation and evolution of chains of internal regularly spaced radiative shocks, which in this work are identified with the observed emission knots of protostellar jets. Variations of long period elucidate the formation of multiple bow shock structures separated by long trails of diffuse gas, which resemble those observed in systems like HH111 and HH46/47. The time variability of the outflow is probably associated with observed irruptive events in the accretion process around the protostars. In our simulations, the outflow variations are produced by periodically turning on the outflow with a highly supersonic velocity and periodically turning off it to a low velocity regime. In the case of velocity variations of intermediate period, we find, as in previous work, that the shock structures form a train of regularly spaced emitting features which move away from the source with a velocity close to that of the outflow, have high radial motions, and produce low intensity spectra, as required by the observations. As they propagate downstream, the shocks widen and dissipate due to the expulsion of material sideways to the cocoon by the high pressure gradients of the postshock gas. This fading explains the most frequent occurence of knots closer to the driving source. In the case of the long period velocity variability, our simulations have produced a pair of bow-shock-like structures separated by a trail almost

Particle acceleration in turbulence and weakly stochastic reconnection.

In this Letter we analyze the energy distribution evolution of test particles injected in three dimensional (3D) magnetohydrodynamic (MHD) simulations of different magnetic reconnection configurations. When considering a single Sweet-Parker topology, the particles accelerate predominantly through a first-order Fermi process, as predicted in and demonstrated numerically in . When turbulence is included within the current sheet, the acceleration rate is highly enhanced, because reconnection becomes fast and independent of resistivity and allows the formation of a thick volume filled with multiple simultaneously reconnecting magnetic fluxes. Charged particles trapped within this volume suffer several head-on scatterings with the contracting magnetic fluctuations, which significantly increase the acceleration rate and results in a first-order Fermi process. For comparison, we also tested acceleration in MHD turbulence, where particles suffer collisions with approaching and receding magnetic irregularities, resulting in a reduced acceleration rate. We argue that the dominant acceleration mechanism approaches a second order Fermi process in this case.

Deflection of Ultra-High-Energy Cosmic Rays by the Galactic Magnetic Field: From the Sources to the Detector

We report the results of three-dimensional simulations of the trajectories of ultra-high-energy (UHE) protons and Fe nuclei (with energies E = 4 × 1019 and 2.5 × 1020 eV), propagating through the Galactic magnetic field (GMF) from the sources to the detector. A uniform distribution of antiparticles is backtracked from the detector, at the Earth, to the halo of the Galaxy. We assume an axisymmetric, large-scale spiral magnetic field that permeates both the disk and the halo. A normal field component to the Galactic plane (Bz) is also included in part of the simulations. We find that the presence of a large-scale GMF does not generally affect the arrival directions of the protons, although the inclusion of a Bz component may cause significant deflection of the lower energy protons (E = 4 × 1019 eV). Error boxes larger than or equal to ~5° are most expected in this case. On the other hand, in the case of heavy nuclei, the arrival direction of the particles is strongly dependent on the coordinates of the particle source. The deflection may be high enough (>20°) as to make any identification of the sources extremely difficult unless the real magnetic field configuration is determined accurately. Moreover, not every incoming particle direction is allowed between a given source and the detector. This generates sky patches that are virtually unobservable from the Earth. In the particular case of the UHE events of Yakutsk, Flys Eye, and Akeno, they come from locations for which the deflection caused by the assumed magnetic field is not significant.

Three-dimensional Magnetohydrodynamic Simulations of Radiatively Cooling, Pulsed Jets

We here investigate, by means of fully three-dimensional smoothed particle magnetohydrodynamic numerical simulations, the effects of different initial magnetic field configurations on the evolution of overdense, radiatively cooling, pulsed jets using the following different initial magnetic field topologies: (1) longitudinal, (2) helical geometry permeating both the jet and the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as different values of the magnetic field strength (? 0.1-? or B 260-0 ?G). The presence of a helical or toroidal field tends to affect the global characteristics of the fluid more than a longitudinal field. However, the relative differences that have been previously detected in two-dimensional simulations involving distinct magnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head, inhibiting its fragmentation in the early evolution of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex morphology at the jet head (as required by the observations of Herbig-Haro [HH] jets) even in the MHD jet models with helical or toroidal configurations. The detailed structure and emission properties of the internal working surfaces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of ?) improves the jet collimation and amplifies the density (by factors up to 1.4 and 4) and the H? intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and ? 1-0.1 relative to a nonmagnetic jet. As a consequence, the corresponding I[S II]/IH? ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to nonmagnetic calculations by about the same amounts, although the intensity estimates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical magnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional jet simulations with initial toroidal magnetic fields is found in the three-dimensional flows or even in the ? 0.1 case. The implications of our results for HH jets are briefly discussed.

Astrophysical jets and outflows

Abstract Highly collimated supersonic jets and less collimated outflows are observed to emerge from a wide variety of astrophysical objects. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (like galactic black holes or microquasars, and X-ray binary stars), and in the nuclei of active galaxies (AGNs). Despite their different physical scales (in size, velocity, and amount of energy transported), they have strong morphological similarities. What physics do they share? These systems are either hydrodynamic or magnetohydrodynamic (MHD) in nature and are, as such, governed by non-linear equations. While theoretical models helped us to understand the basic physics of these objects, numerical simulations have been allowing us to go beyond the one-dimensional, steady-state approach extracting vital information. In this lecture, the formation, structure, and evolution of the jets are reviewed with the help of observational information, MHD and purely hydrodynamical modeling, and numerical simulations. Possible applications of the models particularly to YSOs and AGN jets are addressed.

Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets

We investigate the effect of magnetic fields on the propagation dynamics and the morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional smoothed particle magnetohydrodynamic simulations. Evaluated for a set of parameters that are mainly suitable for protostellar jets (with density ratios between those of the jet and the ambient medium η≈3-10, and ambient Mach number Ma≈24), these simulations are also compared with baseline nonmagnetic and adiabatic calculations. Two initial magnetic field topologies (in approximate equipartition with the gas, β=pth/pB1) are considered: (1) a helical field and (2) a longitudinal field, both of which permeate both the jet and the ambient medium. We find that, after amplification by compression and reorientation in nonparallel shocks at the working surface, the magnetic field that is carried backward with the shocked gas into the cocoon improves the jet collimation relative to the purely hydrodynamic (HD) systems, but this effect is larger in the presence of the helical field. In both magnetic configurations, low-amplitude, approximately equally spaced (λ≈2-4Rj) internal shocks (which are absent in the HD systems) are produced by magnetohydrodynamic (MHD) Kelvin-Helmholtz reflection pinch modes. The longitudinal field geometry also excites nonaxisymmetric helical modes that cause some beam wiggling. The strength and amount of these modes are, however, reduced (by about 2 times) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio, η, between the jet and the ambient medium also reduces, in general, the number of the internal shocks. As a consequence, the weakness of the induced internal shocks makes it doubtful that the magnetic pinches could by themselves produce the bright knots observed in the overdense, radiatively cooling protostellar jets. Magnetic fields may leave also important signatures on the head morphologies of the radiative cooling jets. The amplification of the nonparallel components of the magnetic fields, particularly in the helical field geometry, reduces the postshock compressibility and increases the postshock cooling length. This may lead to stabilization of the cold shell of shocked material that develops at the head against both the Rayleigh-Taylor and global thermal instabilities. As a consequence, the clumps that develop by fragmentation of the shell in the HD jets tend to be depleted in the helical field geometry. The jet immersed in the longitudinal field, on the other hand, still retains the clumps, although they have their densities decreased relative to the HD counterparts. As stressed in our previous work, since the fragmented shell structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of protostellar jets, this result suggests that, as long as (equipartition) magnetic fields are present, they should probably be predominantly longitudinal at the heads of these jets.

Magnetic Field Effects on the Head Structure of Protostellar Jets

We present the results of three-dimensional smooth particle magnetohydrodynamics numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Together with a baseline nonmagnetic calculation, two initial magnetic configurations (in approximate equipartition with the gas) are considered: (1) a helical field and (2) a longitudinal field, both of which permeate both the jet and the ambient medium. We find that magnetic fields have important effects on the dynamics and structure of radiative cooling jets, especially at the head. The presence of a helical field suppresses the formation of the clumpy structure that is found to develop at the head of purely hydrodynamical jets by fragmentation of the cold shell of shocked material. On the other hand, a cooling jet embedded in a longitudinal magnetic field retains clumpy morphology at its head. This fragmented structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of protostellar jets. This suggests that a strong (equipartition) helical magnetic field configuration is ruled out at the jet head. Therefore, if strong magnetic fields are present, they are probably predominantly longitudinal in those regions. In both magnetic configurations, we find that the confining pressure of the cocoon is able to excite short-wavelength MHD Kelvin-Helmholtz pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it is likely that they could only play a secondary role in the formation of the bright knots observed in protostellar jets.

THE ROLE OF DIFFUSIVITY QUENCHING IN FLUX-TRANSPORT DYNAMO MODELS

In the nonlinear phase of a dynamo process, the back-reaction of the magnetic field upon the turbulent motion results in a decrease of the turbulence level and therefore in a suppression of both the magnetic field amplification (the α-quenching effect) and the turbulent magnetic diffusivity (the η-quenching effect). While the former has been widely explored, the effects of η-quenching in the magnetic field evolution have rarely been considered. In this work, we investigate the role of the suppression of diffusivity in a flux-transport solar dynamo model that also includes a nonlinear α-quenching term. Our results indicate that, although for α-quenching the dependence of the magnetic field amplification with the quenching factor is nearly linear, the magnetic field response to η-quenching is nonlinear and spatially nonuniform. We have found that the magnetic field can be locally amplified in this case, forming long-lived structures whose maximum amplitude can be up to ~2.5 times larger at the tachocline and up to ~2 times larger at the center of the convection zone than in models without quenching. However, this amplification leads to unobservable effects and to a worse distribution of the magnetic field in the butterfly diagram. Since the dynamo cycle period increases when the efficiency of the quenching increases, we have also explored whether the η-quenching can cause a diffusion-dominated model to drift into an advection-dominated regime. We have found that models undergoing a large suppression in η produce a strong segregation of magnetic fields that may lead to unsteady dynamo-oscillations. On the other hand, an initially diffusion-dominated model undergoing a small suppression in η remains in the diffusion-dominated regime.

SPATIAL GROWTH OF CURRENT-DRIVEN INSTABILITY IN RELATIVISTIC ROTATING JETS AND THE SEARCH FOR MAGNETIC RECONNECTION

Using the three-dimensional relativistic magnetohydrodynamic code RAISHIN, we investigated the influence of radial density profile on the spatial development of the current-driven kink instability along magnetized rotating, relativistic jets. For the purpose of our study, we used a non-periodic computational box, the jet flow is initially established across the computational grid, and a precessional perturbation at the inlet triggers the growth of the kink instability. We studied light as well as heavy jets with respect to the environment depending on the density profile. Different angular velocity amplitudes have been also tested. The results show the propagation of a helically kinked structure along the jet and relatively stable configuration for the lighter jets. The jets appear to be collimated by the magnetic field and the flow is accelerated due to conversion of electromagnetic into kinetic energy. We also identify regions of high current density in filamentary current sheets, indicative of magnetic reconnection, which are associated to the kink unstable regions and correlated to the decrease of the sigma parameter of the flow. We discuss the implications of our findings for Poynting-flux dominated jets in connection with magnetic reconnection process. We find that fast magnetic reconnection may be driven by the kink-instability turbulence and govern the transformation of magnetic into kinetic energy thus providing an efficient way to power and accelerate particles in AGN and gamma-ray-burst relativistic jets.