31 research outputs found
Cosmic ray driven dynamo in galactic disks. A parameter study
We present a parameter study of the magnetohydrodynamical dynamo driven by
cosmic rays in the interstellar medium (ISM) focusing on the efficiency of
magnetic field amplification and the issue of energy equipartition between
magnetic, kinetic and cosmic ray (CR) energies. We perform numerical CR-MHD
simulations of the ISM using the extended version of ZEUS-3D code in the
shearing box approximation and taking into account the presence of Ohmic
resistivity, tidal forces and vertical disk gravity. CRs are supplied in
randomly distributed supernova (SN) remnants and are described by the
diffusion-advection equation, which incorporates an anisotropic diffusion
tensor. The azimuthal magnetic flux and total magnetic energy are amplified
depending on a particular choice of model parameters. We find that the most
favorable conditions for magnetic field amplification correspond to magnetic
diffusivity of the order of 3\times 10^{25} \cm^2\s^{-1}, SN rates close to
those observed in the Milky Way, periodic SN activity corresponding to spiral
arms, and highly anisotropic and field-aligned CR diffusion. The rate of
magnetic field amplification is relatively insensitive to the magnitude of SN
rates in a rage of spanning 10% up to 100% of realistic values. The timescale
of magnetic field amplification in the most favorable conditions is 150 Myr, at
galactocentric radius equal to 5 kpc. The final magnetic field energies
fluctuate near equipartition with the gas kinetic energy. In all models CR
energy exceeds the equipartition values by a least an order of magnitude, in
contrary to the expected equipartition. We suggest that the excess of cosmic
rays can be attributed to the fact that the shearing-box does not permit cosmic
rays to leave the system along the horizontal magnetic field.Comment: 12 papges, 11 figures, accepted for publication in Astronomy and
Astrophysic
Reconnection Studies Under Different Types of Turbulence Driving
We study a model of fast magnetic reconnection in the presence of weak
turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional
direct numerical simulations. The model has been already successfully tested in
Kowal et al. (2009) confirming the dependencies of the reconnection speed
on the turbulence injection power and the injection scale
expressed by a constraint
and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in
order to drive turbulence, we injected velocity fluctuations in Fourier space
with frequencies concentrated around , as described in
Alvelius (1999). In this paper we extend our previous studies by comparing fast
magnetic reconnection under different mechanisms of turbulence injection by
introducing a new way of turbulence driving. The new method injects velocity or
magnetic eddies with a specified amplitude and scale in random locations
directly in real space. We provide exact relations between the eddy parameters
and turbulent power and injection scale. We performed simulations with new
forcing in order to study turbulent power and injection scale dependencies. The
results show no discrepancy between models with two different methods of
turbulence driving exposing the same scalings in both cases. This is in
agreement with the Lazarian and Vishniac (1999) predictions. In addition, we
performed a series of models with varying viscosity . Although Lazarian
and Vishniac (1999) do not provide any prediction for this dependence, we
report a weak relation between the reconnection speed with viscosity,
.Comment: 19 pages, 9 figures. arXiv admin note: text overlap with
arXiv:0903.205
Cosmic-ray driven dynamo in the medium of irregular galaxy
We investigate the cosmic ray driven dynamo in the interstellar medium of
irregular galaxy. The observations (Chyzy et al. 2000, 2003) show that the
magnetic field in irregular galaxies is present and its value reaches the same
level as in spiral galaxies. However the conditions in the medium of irregular
galaxy are very unfavorable for amplification the magnetic field due to slow
rotation and low shearing rate.
In this work we present numerical model of the interstellar medium in
irregular galaxies. The model includes magnetohydrodynamical dynamo driven by
cosmic rays in the interstellar medium provided by random supernova explosions.
We describe models characterized by different shear and rotation. We find that
even slow galactic rotation with low shearing rate gives amplification of the
magnetic field. Simulations have shown that high amount of the magnetic energy
flow out off the simulation region becoming an efficient source of
intergalactic magnetic fields.Comment: 2 pages, 2 figures, To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
3D MHD simulations of magnetic field evolution and radio polarization of barred galaxies
Aims. We study numerically the large-scale gas and magnetic field
evolution of barred galaxies in the gravitational potential of a disk, bulge, halo, and
bar. We solve non-linear MHD equations including the back-reaction of the magnetic field
to the gas. We do not take into account any dynamo process.
Methods. We apply the numerical MHD code to calculate the model of the
galaxy in three dimensions. We construct realistic maps of high-frequency (Faraday
rotation free) polarized radio emission on the basis of the simulated magnetic fields. The
polarization model includes the effects of projection and limited resolution.
Results. The main result is that our modeled polarization maps resemble
the radio polarization structures observed in barred galaxies. The modeled polarization
B-vectors distribution along the bar and between spiral
arms resembles the observed topology of the magnetic field in barred galaxies. Our
calculations for several different rotational velocities and sound speeds give the same
result we got in our previous earlier published model. The reason of this behaviour is the
dynamical evolution of the bar that causes gas to form spiral waves going radially
outward. A gaseous spiral arms in turn generates magnetic ones, which live much longer in
the inter-arm disk space than the gaseous pattern
3D model of magnetic fields evolution in dwarf irregular galaxies
Radio observations show that magnetic fields are present in dwarf irregular
galaxies (dIrr) and its strength is comparable to that found in spiral
galaxies. Slow rotation, weak shear and shallow gravitational potential are the
main features of a typical dIrr galaxy. These conditions of the interstellar
medium in a dIrr galaxy seem to unfavourable for amplification of the magnetic
field through the dynamo process. Cosmic-ray driven dynamo is one of the
galactic dynamo model, which has been successfully tested in case of the spiral
galaxies. We investigate this dynamo model in the ISM of a dIrr galaxy. We
study its efficiency under the influence of slow rotation, weak shear and
shallow gravitational potential. Additionally, the exploding supernovae are
parametrised by the frequency of star formation and its modulation, to
reproduce bursts and quiescent phases. We found that even slow galactic
rotation with a low shearing rate amplifies the magnetic field, and that rapid
rotation with a low value of the shear enhances the efficiency of the dynamo.
Our simulations have shown that a high amount of magnetic energy leaves the
simulation box becoming an efficient source of intergalactic magnetic fields.Comment: 4 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
The effect of supernova rate on the magnetic field evolution in barred galaxies
Context. For the first time, our magnetohydrodynamical numerical calculations provide results for a three-dimensional model of barred galaxies involving a cosmic-ray driven dynamo process that depends on star formation rates. Furthermore, we argue that the cosmic-ray driven dynamo can account for a number of magnetic features in barred galaxies, such as magnetic arms observed along the gaseous arms, magnetic arms in the inter-arm regions, polarized emission that is at the strongest in the central part of the galaxy, where the bar is situated, polarized emission that forms ridges coinciding with the dust lanes along the leading edges of the bar, as well as their very strong total radio intensity. Aims. Our numerical model probes what kind of physical processes could be responsible for the magnetic field topology observed in barred galaxies (modes, etc.). We compare our modelled results directly with observations, constructing models of high-frequency (Faraday rotation-free) polarized radio emission maps out of the simulated magnetic field and cosmic ray pattern in our modeled galaxy. We also take the effects of projection into account as well as the limited resolution. Methods. We applied global 3D numerical calculations of a cosmic-ray driven dynamo in barred galaxies with different physical input parameters such as the supernova (SN) rate. Results. Our simulation results lead to the modelled magnetic field structure similar to the one observed on the radio maps of barred galaxies. Moreover, they cast new light on a number of properties in barred and spiral galaxies, such as fast exponential growth of the total magnetic energy to the present values. The quadrupole modes of magnetic field are often identified in barred galaxies, but the dipole modes (e.g., in NGC 4631) are found very seldom. In our simulations the quadrupole configuration dominates and the dipole configuration only appears once in the case of model S100, apparently as a consequence of the choice of the random number seed. Synthetic radio maps of our models display X-type structure similar to what is observed in real galaxies. Conclusions. We conclude that a cosmic-ray driven dynamo process in barred galaxies can amplify magnetic fields efficiently. The fastest rate of magnetic field increase is 195 yr for a SN frequency of 1/50 yr-1.The obtained strength of magnetic field corresponds to the observational values (a few in spiral arms). The polarization and rotation measure maps also agree with observations. We found the effect of shifting magnetic arms in 4 models (out of the sample of 5)
Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear
We analyze the electromotive force (EMF) terms and basic assumptions of the
linear and nonlinear dynamo theories in our three-dimensional (3D) numerical
model of the Parker instability with cosmic rays and shear in a galactic disk.
We also apply the well known prescriptions of the EMF obtained by the nonlinear
dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the
EMF reconstructed from their prescriptions corresponds to the EMF obtained
directly from our numerical models. We show that our modeled EMF is fully
nonlinear and it is not possible to apply any of the considered nonlinear
dynamo approximations due to the fact that the conditions for the scale
separation are not fulfilled.Comment: 15 pages, 12 figure
Cosmic ray driven dynamo in barred and ringed galaxies
We study the global evolution of the magnetic field and interstellar medium (ISM) of the barred and ringed galaxies in the presence of non-axisymmetric components of the potential, i.e. the bar and/or the oval perturbations. The magnetohydrodynamical dynamo is driven by cosmic rays (CR), which are continuously supplied to the disk by supernova (SN) remnants. Additionally, weak, dipolar and randomly oriented magnetic field is injected to the galactic disk during SN explosions. To compare our results directly with the observed properties of galaxies we construct realistic maps of high-frequency polarized radio emission. The main result is that CR driven dynamo can amplify weak magnetic fields up to few G within few Gyr in barred and ringed galaxies. What is more, the modelled magnetic field configuration resembles maps of the polarized intensity observed in barred and ringed galaxies
Can the cosmic-ray driven dynamo model explain the observations of the polarized emission of edge-on galaxies ?
In the present paper we construct maps of polarized synchrotron radio
emission of a whole galaxy, based on local models of the cosmic ray (CR) driven
dynamo. We perform numerical simulations of the dynamo in local Cartesian
domains, with shear-periodic boundary conditions, placed at the different
galactocentric radii. Those local solutions are concatenated together to
construct the synchrotron images of the whole galaxy. The main aim of the paper
is to compare the model results with the observed radio continuum emission from
nearly edge-on spiral galaxy. On the basis of the modeled evolution of the
magnetic field structure, the polarization maps can be calculated at different
time-steps and at any orientation of the modeled galaxy. For the first time a
self-consistent cosmic-ray electron distribution is used to integrate
synchrotron emissivity along the line of sight. Finally, our maps are convolved
with the given radiotelescope beam. We show that it is possible to reconstruct
the extended magnetic halo structures of the edge-on galaxies (so called
X-shaped structures).Comment: 7 pages, 7 figures. ApJ, accepte