OA@INAFhttps://openaccess.inaf.itThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 04 Jun 2023 10:53:34 GMT2023-06-04T10:53:34Z501713D Relativistic MHD numerical simulations of X-shaped radio sourceshttp://hdl.handle.net/20.500.12386/27144Title: 3D Relativistic MHD numerical simulations of X-shaped radio sources
Authors: Rossi, P.; Bodo, G.; Capetti, A.; Massaglia S.
Abstract: A significant fraction of extended radio sources presents a peculiar X-shaped radio morphology: in addition to the classical double lobed structure, radio emission is also observed along a second axis of symmetry in the form of diffuse wings or tails. In a previous investigation we showed the existence of a connection between the radio morphology and the properties of the host galaxies. Motivated by this connection we performed two-dimensional numerical simulations showing that X-shaped radio sources may naturally form as a jet propagates along the major axis a highly elliptical density distribution, because of the fast expansion of the cocoon along the minor axis of the distribution.
Aims: We intend to extend our analysis by performing three-dimensional numerical simulations and investigating the role of different parameters in determining the formation of the X-shaped morphology.
Methods: The problem is addressed by numerical means, carrying out three-dimensional relativistic magnetohydrodynamic simulations of bidirectional jets propagating in a triaxial density distribution.
Results: We show that only jets with power ≲ 1044 erg s-1 can give origin to an X-shaped morphology and that a misalignment of 30° between the jet axis and the major axis of the density distribution is still favourable to the formation of this kind of morphology. In addition we compute synthetic radio emission maps and polarization maps.
Conclusions: In our scenario for the formation of X-shaped radio sources only low power FRII can give origin to such kind of morphology. Our synthetic emission maps show that the different observed morphologies of X-shaped sources can be the result of similar structures viewed under different perspectives.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/20.500.12386/271442017-01-01T00:00:00ZA Constrained Transport Method for the Solution of the Resistive Relativistic MHD Equationshttp://hdl.handle.net/20.500.12386/29339Title: A Constrained Transport Method for the Solution of the Resistive Relativistic MHD Equations
Authors: Mignone, A.; Mattia, G.; BODO, Gianluigi; Del Zanna, L.
Abstract: We describe a novel Godunov-type numerical method for solving the equations
of resistive relativistic magnetohydrodynamics. In the proposed approach, the
spatial components of both magnetic and electric fields are located at zone
interfaces and are evolved using the constrained transport formalism. Direct
application of Stokes' theorem to Faraday's and Ampere's laws ensures that the
resulting discretization is divergence-free for the magnetic field and
charge-conserving for the electric field. Hydrodynamic variables retain,
instead, the usual zone-centred representation commonly adopted in
finite-volume schemes. Temporal discretization is based on Runge-Kutta
implicit-explicit (IMEX) schemes in order to resolve the temporal scale
disparity introduced by the stiff source term in Ampere's law. The implicit
step is accomplished by means of an improved and more efficient Newton-Broyden
multidimensional root-finding algorithm. The explicit step relies on a
multidimensional Riemann solver to compute the line-averaged electric and
magnetic fields at zone edges and it employs a one-dimensional Riemann solver
at zone interfaces to update zone-centred hydrodynamic quantities. For the
latter, we introduce a five-wave solver based on the frozen limit of the
relaxation system whereby the solution to the Riemann problem can be decomposed
into an outer Maxwell solver and an inner hydrodynamic solver. A number of
numerical benchmarks demonstrate that our method is superior in stability and
robustness to the more popular charge-conserving divergence cleaning approach
where both primary electric and magnetic fields are zone-centered. In addition,
the employment of a less diffusive Riemann solver noticeably improves the
accuracy of the computations.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/20.500.12386/293392019-01-01T00:00:00ZA fluid-particle hybrid framework for the PLUTO code: applications to non-thermal emission in jetshttp://hdl.handle.net/20.500.12386/25153Title: A fluid-particle hybrid framework for the PLUTO code: applications to non-thermal emission in jets
Authors: Vaidya, B.; Mignone, A.; BODO, Gianluigi; MASSAGLIA, SILVANO
Abstract: We present an implementation of a fully parallel hybrid framework for the evolution of Lagrangian particles coupled to a MHD fluid for the PLUTO code. For the applications of interest, particles represent ensembles of electrons whose spectral energy distribution is governed by a kinetic transport equation that takes into account different physical processes such as diffusive shock acceleration, synchrotron emission and adiabatic expansion. An application to model non-thermal emission from extragalactic jets shows the effectiveness and strength of the approach in describing not only the dynamics but also the radiation properties of jets and, in general, of high-energy astrophysical plasma environments.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/20.500.12386/251532016-01-01T00:00:00ZA Particle Module for the PLUTO Code. I. An Implementation of the MHD-PIC Equationshttp://hdl.handle.net/20.500.12386/28223Title: A Particle Module for the PLUTO Code. I. An Implementation of the MHD-PIC Equations
Authors: Mignone, A.; BODO, Gianluigi; Vaidya, B.; Mattia, G.
Abstract: We describe an implementation of a particle physics module available for the PLUTO code appropriate for the dynamical evolution of a plasma consisting of a thermal fluid and a nonthermal component represented by relativistic charged particles or cosmic rays (CRs). While the fluid is approached using standard numerical schemes for magnetohydrodynamics, CR particles are treated kinetically using conventional Particle-In-Cell (PIC) techniques. The module can be used either to describe test-particle motion in the fluid electromagnetic field or to solve the fully coupled magnetohydrodynamics (MHD)-PIC system of equations with particle backreaction on the fluid as originally introduced by Bai et al. Particle backreaction on the fluid is included in the form of momentum-energy feedback and by introducing the CR-induced Hall term in Ohm’s law. The hybrid MHD-PIC module can be employed to study CR kinetic effects on scales larger than the (ion) skin depth provided that the Larmor gyration scale is properly resolved. When applicable, this formulation avoids resolving microscopic scales, offering substantial computational savings with respect to PIC simulations. We present a fully conservative formulation that is second-order accurate in time and space, and extends to either the Runge-Kutta (RK) or the corner transport upwind time-stepping schemes (for the fluid), while a standard Boris integrator is employed for the particles. For highly energetic relativistic CRs and in order to overcome the time-step restriction, a novel subcycling strategy that retains second-order accuracy in time is presented. Numerical benchmarks and applications including Bell instability, diffusive shock acceleration, and test-particle acceleration in reconnecting layers are discussed.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/20.500.12386/282232018-01-01T00:00:00ZA Particle Module for the PLUTO Code. II. Hybrid Framework for Modeling Nonthermal Emission from Relativistic Magnetized Flowshttp://hdl.handle.net/20.500.12386/28222Title: A Particle Module for the PLUTO Code. II. Hybrid Framework for Modeling Nonthermal Emission from Relativistic Magnetized Flows
Authors: Vaidya, Bhargav; Mignone, Andrea; BODO, Gianluigi; ROSSI, Paola; MASSAGLIA, SILVANO
Abstract: We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic magnetohydrodynamic (MHD) flow. Our method makes use of Lagrangian particles moving through an Eulerian grid where the (relativistic) MHD equations are solved concurrently. Lagrangian particles follow fluid streamlines and represent ensembles of (real) relativistic particles with a finite energy distribution. The spectral distribution of each particle is updated in time by solving the relativistic cosmic ray transport equation based on local fluid conditions. This enables us to account for a number of physical processes, such as adiabatic expansion, synchrotron and inverse Compton emission. An accurate semi-analytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In the presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration is presented. Our approach relies on a refined shock-detection algorithm and updates the particle energy distribution based on the shock compression ratio, magnetic field orientation, and amount of (parameterized) turbulence. The evolved distribution from each Lagrangian particle is further used to produce observational signatures like emission maps and polarization signals, accounting for proper relativistic corrections. We further demonstrate the validity of this hybrid framework using standard numerical benchmarks and evaluate the applicability of such a tool to study high-energy emission from extragalactic jets.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/20.500.12386/282222018-01-01T00:00:00ZAstrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementationhttp://hdl.handle.net/20.500.12386/23591Title: Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementation
Authors: Vaidya, B.; Mignone, A.; BODO, Gianluigi; Massaglia, S.
Abstract: Context. An equation of state (EoS) is a relation between thermodynamic state variables and it is essential for closing the set of equations describing a fluid system. Although an ideal EoS with a constant adiabatic index Γ is the preferred choice owing to its simplistic implementation, many astrophysical fluid simulations may benefit from a more sophisticated treatment that can account for diverse chemical processes. Aims: In the present work we first review the basic thermodynamic principles of a gas mixture in terms of its thermal and caloric EoS by including effects like ionization, dissociation, and temperature dependent degrees of freedom such as molecular vibrations and rotations. The formulation is revisited in the context of plasmas that are either in equilibrium conditions (local thermodynamic- or collisional excitation-equilibria) or described by non-equilibrium chemistry coupled to optically thin radiative cooling. We then present a numerical implementation of thermally ideal gases obeying a more general caloric EoS with non-constant adiabatic index in Godunov-type numerical schemes. Methods: We discuss the necessary modifications to the Riemann solver and to the conversion between total energy and pressure (or vice versa) routinely invoked in Godunov-type schemes. We then present two different approaches for computing the EoS. The first employs root-finder methods and it is best suited for EoS in analytical form. The second is based on lookup tables and interpolation and results in a more computationally efficient approach, although care must be taken to ensure thermodynamic consistency. Results: A number of selected benchmarks demonstrate that the employment of a non-ideal EoS can lead to important differences in the solution when the temperature range is 500-10<SUP>4</SUP> K where dissociation and ionization occur. The implementation of selected EoS introduces additional computational costs although the employment of lookup table methods (when possible) can significantly reduce the overhead by a factor of ~ 3-4.
Acknowledgments: We would like to thank the referee, Prof. A. Raga, for his constructive suggestions on the paper. B.V. is grateful to the funding support from the University of Torino under the contract: “Progetto di Ateneo-Compagnia di SanPaolo”.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/20.500.12386/235912015-01-01T00:00:00ZFully Convective Magneto-rotational Turbulence in Large Aspect-ratio Shearing Boxeshttp://hdl.handle.net/20.500.12386/23477Title: Fully Convective Magneto-rotational Turbulence in Large Aspect-ratio Shearing Boxes
Authors: BODO, Gianluigi; Cattaneo, F.; Mignone, A.; ROSSI, Paola
Abstract: We present a numerical study of turbulence and dynamo action in stratified shearing boxes with both finite and zero net magnetic flux. We assume that the fluid obeys the perfect gas law and has finite thermal diffusivity. The latter is chosen to be small enough so that vigorous convective states develop. The properties of these convective solutions are analyzed as the aspect ratio of the computational domain is varied and as the value of the mean field is increased. For the cases with zero net flux, we find that a well-defined converged state is obtained for large enough aspect ratios. In the converged state, the dynamo can be extremely efficient and can generate substantial toroidal flux. We identify solutions in which the toroidal field is mostly symmetric about the mid-plane and solutions in which it is mostly anti-symmetric. The symmetric solutions are found to be more efficient at transporting angular momentum and can give rise to a luminosity that is up to an order of magnitude larger than the corresponding value for the anti-symmetric states. In the cases with a finite net flux, the system appears to spend most of the time in the symmetric states.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/20.500.12386/234772015-01-01T00:00:00ZGlobal Properties of Fully Convective Accretion Disks from Local Simulationshttp://hdl.handle.net/20.500.12386/23476Title: Global Properties of Fully Convective Accretion Disks from Local Simulations
Authors: BODO, Gianluigi; Cattaneo, F.; Mignone, A.; Ponzo, F.; ROSSI, Paola
Abstract: We present an approach to deriving global properties of accretion disks from the knowledge of local solutions derived from numerical simulations based on the shearing box approximation. The approach consists of a two-step procedure. First, a local solution valid for all values of the disk height is constructed by piecing together an interior solution obtained numerically with an analytical exterior radiative solution. The matching is obtained by assuming hydrostatic balance and radiative equilibrium. Although in principle the procedure can be carried out in general, it simplifies considerably when the interior solution is fully convective. In these cases, the construction is analogous to the derivation of the Hayashi tracks for protostars. The second step consists of piecing together the local solutions at different radii to obtain a global solution. Here we use the symmetry of the solutions with respect to the defining dimensionless numbers—in a way similar to the use of homology relations in stellar structure theory—to obtain the scaling properties of the various disk quantities with radius.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/20.500.12386/234762015-01-01T00:00:00ZLinear stability analysis of magnetized jets: the rotating casehttp://hdl.handle.net/20.500.12386/25143Title: Linear stability analysis of magnetized jets: the rotating case
Authors: BODO, Gianluigi; Mamatsashvili, G.; ROSSI, Paola; Mignone, A.
Abstract: We perform a linear stability analysis of magnetized rotating cylindrical jet flows in the approximation of zero thermal pressure. We focus our analysis on the effect of rotation on the current driven mode and on the unstable modes introduced by rotation. We find that rotation has a stabilizing effect on the current driven mode only for rotation velocities of the order of the Alfvén velocity. Rotation introduces also a new unstable centrifugal buoyancy mode and the `cold' magnetorotational instability. The first mode is analogous to the Parker instability with the centrifugal force playing the role of effective gravity. The magnetorotational instability can be present, but only in a very limited region of the parameter space and is never dominant. The current driven mode is characterized by large wavelengths and is dominant at small values of the rotational velocity, while the buoyancy mode becomes dominant as rotation is increased and is characterized by small wavelengths.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/20.500.12386/251432016-01-01T00:00:00ZLinear stability analysis of magnetized relativistic rotating jetshttp://hdl.handle.net/20.500.12386/29342Title: Linear stability analysis of magnetized relativistic rotating jets
Authors: BODO, Gianluigi; George Mamatsashvili; ROSSI, Paola; Andrea Mignone
Abstract: We carry out a linear stability analysis of a magnetized relativistic
rotating cylindrical jet flow using the approximation of zero thermal pressure.
We identify several modes of instability in the jet: Kelvin-Helmholtz, current
driven and two kinds of centrifugal-buoyancy modes -- toroidal and poloidal.
The Kelvin-Helmholtz mode is found at low magnetization and its growth rate
depends very weakly on the pitch parameter of the background magnetic field and
on rotation. The current driven mode is found at high magnetization, the values
of its growth rate and the wavenumber, corresponding to the maximum growth,
increase as we decrease the pitch parameter of the background magnetic field.
This mode is stabilized by rotation, especially, at high magnetization. The
centrifugal-buoyancy modes, arising due to rotation, tend also to be more
stable when magnetization is increased. Overall, relativistic jet flows appear
to be more stable with respect to their non-relativistic counterpart.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/20.500.12386/293422019-01-01T00:00:00ZLinear Wave Propagation for Resistive Relativistic Magnetohydrodynamicshttp://hdl.handle.net/20.500.12386/28270Title: Linear Wave Propagation for Resistive Relativistic Magnetohydrodynamics
Authors: Mignone, A.; Mattia, G.; BODO, Gianluigi
Abstract: We present a linear mode analysis of the relativistic MHD equations in the
presence of finite electrical conductivity. Starting from the fully
relativistic covariant formulation, we derive the dispersion relation in the
limit of small linear perturbations. It is found that the system supports ten
wave modes which can be easily identified in the limits of small or large
conductivities. In the resistive limit, matter and electromagnetic fields
decouple and solution modes approach pairs of light and acoustic waves as well
as a number of purely damped (non-propagating) modes. In the opposite (ideal)
limit, the frozen-in condition applies and the modes of propagation coincide
with a pair of fast magnetosonic, a pair of slow and Alfv\'en modes, as
expected. In addition, the contact mode is always present and it is unaffected
by the conductivity. For finite values of the conductivity, the dispersion
relation gives rise to either pairs of opposite complex conjugate roots or
purely imaginary (damped) modes. In all cases, the system is dissipative and
also dispersive as the phase velocity depends nonlineary on the wavenumber.
Occasionally, the group velocity may exceed the speed of light although this
does not lead to superluminal signal propagation.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/20.500.12386/282702018-01-01T00:00:00ZMagnetic Helicities and Dynamo Action in Magneto-rotational Turbulencehttp://hdl.handle.net/20.500.12386/26844Title: Magnetic Helicities and Dynamo Action in Magneto-rotational Turbulence
Authors: BODO, Gianluigi; Cattaneo, F.; Mignone, A.; ROSSI, Paola
Abstract: We examine the relationship between magnetic flux generation and magnetic
helicity in a simple model of dynamo action. We consider dynamo action driven
by Magneto-Rotational Turbulence (MRT) within the shearing-box approxima-
tion. We consider magnetically open boundary conditions that allow a flux of
helicity in or out of the computational domain. We circumvent the problem of
the lack of gauge invariance in open domains by choosing a particular gauge –
the winding gauge – that provides a natural interpretation in terms of average
winding number of pairwise field lines. We use this gauge precisely to define and
measure the helicity and helicity flux for several realizations of dynamo action.
We find that the system as a whole does not break reflectional symmetry and
the total helicity remains small even in cases when substantial magnetic flux is
generated. We find no particular connection between the generation of magnetic
flux – large-scale dynamo action – and the helicity or the helicity flux though
the volume. We suggest that this result may be due to the essentially nonlinear
nature of the dynamo processes in MRT.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/20.500.12386/268442017-01-01T00:00:00ZMaking Faranoff-Riley I radio sources II. The effects of jet magnetizationhttp://hdl.handle.net/20.500.12386/29347Title: Making Faranoff-Riley I radio sources II. The effects of jet magnetization
Authors: Massaglia, S.; BODO, Gianluigi; ROSSI, Paola; Capetti, S.; Mignone, A.
Abstract: Radio sources of low power are the most common in the universe. Their jets
typically move at nonrelativistic velocity and show plume-like morphologies
that in many instances appear distorted and bent. We investigate the role of
magnetic field on the propagation and evolution of low-power jets and the
connection between the field intensity and the resulting morphology. The
problem is addressed by means of three-dimensional magnetohydrodynamic (MHD)
simulations. We consider supersonic jets that propagate in a stratified medium.
The ambient temperature increases with distance from the jet origin maintaining
constant pressure. Jets with low magnetization show an enhanced collimation at
small distances with respect to hydrodynamic (HD) cases studied in a previous
paper. These jets eventually evolve in a way similar to the HD cases. Jets with
higher magnetization are affected by strong nonaxisymmetric modes that lead to
the sudden jet energy release. From there on, distorted plumes of jet material
move at subsonic velocities. This transition is associated with the formation
of structures reminiscent of the `warm spots' observed in wide-angle-tail (WAT)
sources.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/20.500.12386/293472019-01-01T00:00:00ZMaking Faranoff-Riley I radio sources. I. Numerical hydrodynamic 3D simulations of low-power jetshttp://hdl.handle.net/20.500.12386/25152Title: Making Faranoff-Riley I radio sources. I. Numerical hydrodynamic 3D simulations of low-power jets
Authors: MASSAGLIA, SILVANO; BODO, Gianluigi; ROSSI, Paola; CAPETTI, Alessandro; Mignone, A.
Abstract: Context. Extragalactic radio sources have been classified into two classes, Fanaroff-Riley I and II, which differ in morphology and radio power. Strongly emitting sources belong to the edge-brightened FR II class, and weakly emitting sources to the edge-darkened FR I class. The origin of this dichotomy is not yet fully understood. Numerical simulations are successful in generating FR II morphologies, but they fail to reproduce the diffuse structure of FR Is. <BR /> Aims: By means of hydro-dynamical 3D simulations of supersonic jets, we investigate how the displayed morphologies depend on the jet parameters. Bow shocks and Mach disks at the jet head, which are probably responsible for the hot spots in the FR II sources, disappear for a jet kinetic power ℒ<SUB>kin</SUB> ≲ 10<SUP>43</SUP> erg s<SUP>-1</SUP>. This threshold compares favorably with the luminosity at which the FR I/FR II transition is observed. <BR /> Methods: The problem is addressed by numerical means carrying out 3D HD simulations of supersonic jets that propagate in a non-homogeneous medium with the ambient temperature that increases with distance from the jet origin, which maintains constant pressure. <BR /> Results: The jet energy in the lower power sources, instead of being deposited at the terminal shock, is gradually dissipated by the turbulence. The jets spread out while propagating, and they smoothly decelerate while mixing with the ambient medium and produce the plumes characteristic of FR I objects. <BR /> Conclusions: Three-dimensionality is an essential ingredient to explore the FR I evolution becausethe properties of turbulence in two and three dimensions are very different, since there is no energy cascade to small scales in two dimensions, and two-dimensional simulations with the same parameters lead to FRII-like behavior.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/20.500.12386/251522016-01-01T00:00:00ZMHD simulations of three-dimensional resistive reconnection in a cylindrical plasma columnhttp://hdl.handle.net/20.500.12386/25151Title: MHD simulations of three-dimensional resistive reconnection in a cylindrical plasma column
Authors: Striani, E.; Mignone, A.; Vaidya, B.; BODO, Gianluigi; Ferrari, Attilio
Abstract: Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as S<SUP>1/2</SUP> (where S is the Lundquist number) and yields time-scales several order of magnitude larger than the observed ones. Earlier two-dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of S (`fast reconnection' regime) due to the presence of the secondary tearing instability that takes place for S ≳ 1 × 10<SUP>4</SUP>. We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at S ≃ 1 × 10<SUP>3</SUP>.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/20.500.12386/251512016-01-01T00:00:00ZNonlinear Transverse Cascade and Sustenance of MRI Turbulence in Keplerian Disks with an Azimuthal Magnetic Fieldhttp://hdl.handle.net/20.500.12386/27115Title: Nonlinear Transverse Cascade and Sustenance of MRI Turbulence in Keplerian Disks with an Azimuthal Magnetic Field
Authors: Gogichaishvili, D.; Mamatsashvili, G.; Horton, W.; Chagelishvili, G.; BODO, Gianluigi
Abstract: We investigate magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI) in Keplerian
disks with a nonzero net azimuthal magnetic field using shearing box simulations. As distinct from most previous
studies, we analyze turbulence dynamics in Fourier (k-) space to understand its sustenance. The linear growth of MRI
with azimuthal field has a transient character and is anisotropic in Fourier space, leading to anisotropy of nonlinear
processes in Fourier space. As a result, the main nonlinear process appears to be a new type of angular redistribution of
modes in Fourier space – the nonlinear transverse cascade – rather than usual direct/inverse cascade. We demonstrate
that the turbulence is sustained by interplay of the linear transient growth of MRI (which is the only energy supply for
the turbulence) and the transverse cascade. These two processes operate at large length scales, comparable to box size
and the corresponding small wavenumber area, called vital area in Fourier space is crucial for the sustenance, while
outside the vital area direct cascade dominates. The interplay of the linear and nonlinear processes in Fourier space is
generally too intertwined for a vivid schematization. Nevertheless, we reveal the basic subcycle of the sustenance that
clearly shows synergy of these processes in the self-organization of the magnetized flow system. This synergy is quite
robust and persists for the considered different aspect ratios of the simulation boxes. The spectral characteristics of the
dynamical processes in these boxes are qualitatively similar, indicating the universality of the sustenance mechanism
of the MRI-turbulence.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/20.500.12386/271152017-01-01T00:00:00ZRecollimation shocks and radiative losses in extragalactic relativistic jetshttp://hdl.handle.net/20.500.12386/28224Title: Recollimation shocks and radiative losses in extragalactic relativistic jets
Authors: BODO, Gianluigi; TAVECCHIO, Fabrizio
Abstract: We present the results of state-of-the-art simulations of recollimation shocks induced by the interaction of a relativistic jet with an external medium, including the effect of radiative losses of the shocked gas. Our simulations confirm that - as suggested by earlier semi-analytical models - the post-shock pressure loss induced by radiative losses may lead to a stationary equilibrium state characterized by a very strong focusing of the flow, with the formation of quite narrow nozzles, with cross-sectional radii as small as 10<SUP>-3</SUP> times the length scale of the jet. We also study the time-dependent evolution of the jet structure induced by a density perturbation injected at the flow base. The set-up and the results of the simulations are particularly relevant for the interpretation of the observed rapid variability of the γ-ray emission associated to flat spectrum radio quasars. In particular, the combined effects of jet focusing and Doppler beaming on the observed radiation make it possible to explain the sub-hour flaring events such as that observed in the flat specrum radio quasar PKS 1222+216 by MAGIC.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/20.500.12386/282242018-01-01T00:00:00Z