D. Sokoloff - Academia.edu (original) (raw)
Papers by D. Sokoloff
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their impor... more Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their importance for star formation, they govern the transport of cosmic rays, relevant to the launch and regulation of galactic outflows and winds, which in turn are pivotal in shaping the structure of halo magnetic fields. Mapping the small-scale structure of interstellar magnetic fields in many nearby galaxies is crucial to understand the interaction between gas and magnetic fields, in particular how gas flows are affected. Elucidation of the magnetic role in, e.g., triggering star formation, forming and stabilising spiral arms, driving outflows, gas heating by reconnection and magnetising the intergalactic medium has the potential to revolutionise our physical picture of the ISM and galaxy evolution in general. Radio polarisation observations in the very nearest galaxies at high frequencies (≥ 3 GHz) and with high spatial resolution (≤ 5) hold the key here. The galaxy survey with SKA1 that we propose will also be a major step to understand the galactic dynamo, which is important for models of galaxy evolution and for astrophysical magnetohydrodynamics in general. Field amplification by turbulent gas motions, which is crucial for efficient dynamo action, has been investigated so far only in simulations, while compelling evidence of turbulent fields from observations is still lacking.
Lecture Notes in Physics
ABSTRACT
The relation between magnetic and material arms in models for spiral galaxies
Physical Review E, 1998
A shell model of magnetohydrodynamic turbulence, which allows one to conserve all the integrals o... more A shell model of magnetohydrodynamic turbulence, which allows one to conserve all the integrals of motion in both two and three dimensions, is proposed and studied. We demonstrate that this model reproduces basic facts known in the small-scale turbulent dynamo theory. In particular, we consider a process of redistribution of magnetic helicity generated by the mean-field dynamo, described in the model as magnetic forcing, into a small-scale magnetic field. We argue that the resulting equilibrium magnetic field spectrum strongly depends on the level of magnetic helicity and cross helicity, introduced by the large scales. The spectra with spectral index ''Ϫ5/3'' dominate if the cross helicity vanishes. If the level of cross helicity is high ͑correlated velocity and magnetic field͒ the spectra depend on the magnetic helicity: the strong magnetic helicity suppresses any cascade providing steep spectra, while the vanishing helicity of turbulent magnetic fields results in the occurrence of Kraichnan-Iroshnikov spectral index ''Ϫ3/2.'' ͓S1063-651X͑98͒04103-8͔
We study the generation and maintenance of large-scale magnetic elds in barred galaxies. We take ... more We study the generation and maintenance of large-scale magnetic elds in barred galaxies. We take a velocity eld (with strong noncircular components) from a published gas dynamical simulation of Athanassoula (1992), and use this as input to a galactic dynamo calculation. Our work is largely motivated by recent high quality VLA radio observations of the barred galaxy NGC 1097, and we compare our results in detail with the regular magnetic elds deduced from these observations. We are able to reproduce most of the conspicuous large-scale features of the observed regular eld, including the eld structure in the central regions, by using a simple mean- eld dynamo model in which the intensity of interstellar turbulence (more precisely, the turbulent diusivity) is enhanced by a factor of 2{6 in the dust lanes and near the circumnuclear ring. We argue that magnetic elds can be dynamically important, and therefore should be included in models of gas flow in barred galaxies.
Multiscale magnetic fields in spiral galaxies: evolution
We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand M... more We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand Minima can be associated with random fluctuations of the parameters governing the solar dynamo. We consider fluctuations of the alpha coefficient in the conventional Parker migratory dynamo, and also in slightly more sophisticated dynamo models, and demonstrate that they can mimic the gross features of the phenomenon of the occurrence of Grand Minima over suitable parameter ranges. The temporal distribution of these Grand Minima appears chaotic, with a more or less exponential waiting time distribution, typical of Poisson processes. In contrast, however, the available reconstruction of Grand Minima statistics based on cosmogenic isotope data demonstrates substantial deviations from this exponential law. We were unable to reproduce the non-Poissonic tail of the waiting time distribution either in the framework of a simple alpha-quenched Parker model or in its straightforward generalization...
Geophysical & Astrophysical Fluid Dynamics, 2004
... J. , 543: 1027–1043. [CrossRef], [Web of Science ®] View all references), to illustrate the s... more ... J. , 543: 1027–1043. [CrossRef], [Web of Science ®] View all references), to illustrate the scale of the relevant literature. A more systematic presentation of this literature can be found eg in a recent review by Ossendrijver (200321. Ossendrijver, M. 2003. “The stellar dynamo”. ...
Astronomische Nachrichten, 2006
A fraction of solar active regions are observed to have current helicity of a sign that contradic... more A fraction of solar active regions are observed to have current helicity of a sign that contradicts the polarity law for magnetic helicity; this law corresponds to the well-known Hale polarity law for sunspots. A significant excess of active regions with the "wrong" sign of helicity is seen to occur just at the beginning of the cycle. We compare these observations with predictions from a dynamo model based on principles of helicity conservation, discussed by Zhang et al. (2006). This model seems capable of explaining only a fraction of the regions with the wrong sign of the helicity. We attribute the remaining excess to additional current helicity production from the twisting of rising magnetic flux tubes, as suggested by Choudhuri et al. (2004). We estimate the relative contributions of this effect and that connected with the model based on magnetic helicity conservation.
Proceedings of the International Astronomical Union
We present results of convective turbulent dynamo simulations including a coronal layer in a sphe... more We present results of convective turbulent dynamo simulations including a coronal layer in a spherical wedge. We find an equatorward migration of the radial and azimuthal fields similar to the behavior of sunspots during the solar cycle. The migration of the field coexist with a spoke-like differential rotation and anti-solar (clockwise) meridional circulation. Even though the migration extends over the whole convection zone, the mechanism causing this is not yet fully understood.
Monthly Notices of the Royal Astronomical Society, 2015
In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, w... more In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, we suggest an observational test for small-scale dynamo action based on statistics of anti-Hale sunspot groups. As we have shown, according to theoretical expectations the smallscale dynamo action has to provide a population of sunspot groups which do not follow the Hale polarity law, and the density of such groups on the time-latitude diagram is expected to be independent on the phase of the solar cycle. Correspondingly, a percentage of the anti-Hale groups is expected to reach its maximum values during solar minima. For several solar cycles, we considered statistics of anti-Hale groups obtained by several scientific teams, including ours, to find that the percentage of anti-Hale groups becomes indeed maximal during a solar minimum. Our interpretation is that this fact may be explained by the small-scale dynamo action inside the solar convective zone.
We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand M... more We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand Minima can be associated with random fluctuations of the parameters governing the solar dynamo. We consider fluctuations of the alpha-coefficient in the conventional Parker migratory dynamo, and also in slightly more sophisticated dynamo models, and demonstrate that they can mimic the gross features of the phenomenon of the occurrence of Grand Minima over a suitable parameter range. The temporal distribution of these Grand Minima appears chaotic, with a more or less exponential waiting time distribution, typical of Poisson processes. In contrast however, the available reconstruction of Grand Minima statistics based on cosmogenic isotope data demonstrates substantial deviations from this exponential law. We were unable to reproduce the non-Poissonic tail of the waiting time distribution either in the framework of a simple alpha-quenched Parker model, or in its straightforward generalizatio...
We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with ra... more We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of t...
Astronomische Nachrichten
Magnetic fields in galaxies exist on various spatial scales. Large-scale magnetic fields are thou... more Magnetic fields in galaxies exist on various spatial scales. Large-scale magnetic fields are thought to be generated by the − Ω dynamo. Small-scale galactic magnetic fields (1 kpc and below) can be generated by tangling the large-scale field. The analysis of field structures with the help of polarized radio continuum emission is hampered by the effect of Faraday dispersion (due to fluctuations in magnetic field and/or thermal electron density), which shifts the signals from large to small scales. At long observation wavelengths, large-scale magnetic fields may become invisible, as in the case of the spectro-polarimetric data cube of the spiral galaxy NGC 6946 observed with the Westerbork Synthesis Radio Telescope in the wavelength range 17-23 cm. The application of rotation measure (RM) synthesis alone does not overcome this problem. We propose to decompose the Faraday data cube into data cubes at different spatial scales by a wavelet transform. Signatures of the "magnetic arms" observed in NGC 6946 at shorter wavelengths then become visible. Our method allows the use of small-scale structures as tracers of large-scale magnetic field patterns in data cubes at long wavelengths, as provided by new-generation radio telescopes.
Astronomy & Astrophysics
Context. The magnetic field configurations in several nearby spiral galaxies contain magnetic arm... more Context. The magnetic field configurations in several nearby spiral galaxies contain magnetic arms that are sometimes located between the material arms. The nearby barred galaxy M83 provides an outstanding example of a spiral pattern seen in tracers of gas and magnetic field. Aims. We analyse the spatial distribution of magnetic fields in M83 and their relation to the material spiral arms. Methods. Isotropic and anisotropic wavelet transforms are used to decompose the images of M83 in various tracers to quantify structures in a range of scales from 0.2 to 10 kpc. We used radio polarization observations at λ6.2 cm and λ13 cm obtained with the VLA, Effelsberg and ATCA telescopes and APEX sub-mm observations at 870 μm, which are first published here, together with maps of the emission of warm dust, ionized gas, molecular gas, and atomic gas. Results. The spatial power spectra are similar for the tracers of dust, gas, and total magnetic field, while the spectra of the ordered magnetic field are significantly different. As a consequence, the wavelet cross-correlation between all material tracers and total magnetic field is high, while the structures of the ordered magnetic field are poorly correlated with those of other tracers. The magnetic field configuration in M83 contains pronounced magnetic arms. Some of them are displaced from the corresponding material arms, while others overlap with the material arms. The pitch angles of the magnetic and material spiral structures are generally similar. The magnetic field vectors at λ6.2 cm are aligned with the outer material arms, while significant deviations occur in the inner arms and, in particular, in the bar region, possibly due to non-axisymmetric gas flows. Outside the bar region, the typical pitch angles of the material and magnetic spiral arms are very close to each other at about 10 •. The typical pitch angle of the magnetic field vectors is about 20 • larger than that of the material spiral arms. Conclusions. One of the main magnetic arms in M83 is displaced from the gaseous arms similarly to the galaxy NGC 6946, while the other main arm overlaps a gaseous arm, similar to what is observed in M51. We propose that a regular spiral magnetic field generated by a mean-field dynamo is compressed in material arms and partly aligned with them. The interaction of galactic dynamo action with a transient spiral pattern is a promising mechanism for producing such complicated spiral patterns as in M83.
Astronomy & Astrophysics
Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission... more Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission of the interacting galaxies when compared with the typical emission from similar non-interacting galaxies. This increase is believed to be associated with an increase in the star formation rate and the turbulent magnetic fields resulting from the encounter, while the regular magnetic field is usually believed to decrease as a result of the encounter. Aims. We attempt to verify these expectations. Methods. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field resulting from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase in the star formation rate caused by the encounter. Results. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction. This leads to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. Conclusions. The encounter causes enhanced total and polarized emission without increase in the regular magnetic field strength. The increase in synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase in the star formation rate, depending on the choice of parameters. The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr). However, a long-lasting field reversal of the regular magnetic field may result.
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their impor... more Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their importance for star formation, they govern the transport of cosmic rays, relevant to the launch and regulation of galactic outflows and winds, which in turn are pivotal in shaping the structure of halo magnetic fields. Mapping the small-scale structure of interstellar magnetic fields in many nearby galaxies is crucial to understand the interaction between gas and magnetic fields, in particular how gas flows are affected. Elucidation of the magnetic role in, e.g., triggering star formation, forming and stabilising spiral arms, driving outflows, gas heating by reconnection and magnetising the intergalactic medium has the potential to revolutionise our physical picture of the ISM and galaxy evolution in general. Radio polarisation observations in the very nearest galaxies at high frequencies (≥ 3 GHz) and with high spatial resolution (≤ 5) hold the key here. The galaxy survey with SKA1 that we propose will also be a major step to understand the galactic dynamo, which is important for models of galaxy evolution and for astrophysical magnetohydrodynamics in general. Field amplification by turbulent gas motions, which is crucial for efficient dynamo action, has been investigated so far only in simulations, while compelling evidence of turbulent fields from observations is still lacking.
Lecture Notes in Physics
ABSTRACT
The relation between magnetic and material arms in models for spiral galaxies
Physical Review E, 1998
A shell model of magnetohydrodynamic turbulence, which allows one to conserve all the integrals o... more A shell model of magnetohydrodynamic turbulence, which allows one to conserve all the integrals of motion in both two and three dimensions, is proposed and studied. We demonstrate that this model reproduces basic facts known in the small-scale turbulent dynamo theory. In particular, we consider a process of redistribution of magnetic helicity generated by the mean-field dynamo, described in the model as magnetic forcing, into a small-scale magnetic field. We argue that the resulting equilibrium magnetic field spectrum strongly depends on the level of magnetic helicity and cross helicity, introduced by the large scales. The spectra with spectral index ''Ϫ5/3'' dominate if the cross helicity vanishes. If the level of cross helicity is high ͑correlated velocity and magnetic field͒ the spectra depend on the magnetic helicity: the strong magnetic helicity suppresses any cascade providing steep spectra, while the vanishing helicity of turbulent magnetic fields results in the occurrence of Kraichnan-Iroshnikov spectral index ''Ϫ3/2.'' ͓S1063-651X͑98͒04103-8͔
We study the generation and maintenance of large-scale magnetic elds in barred galaxies. We take ... more We study the generation and maintenance of large-scale magnetic elds in barred galaxies. We take a velocity eld (with strong noncircular components) from a published gas dynamical simulation of Athanassoula (1992), and use this as input to a galactic dynamo calculation. Our work is largely motivated by recent high quality VLA radio observations of the barred galaxy NGC 1097, and we compare our results in detail with the regular magnetic elds deduced from these observations. We are able to reproduce most of the conspicuous large-scale features of the observed regular eld, including the eld structure in the central regions, by using a simple mean- eld dynamo model in which the intensity of interstellar turbulence (more precisely, the turbulent diusivity) is enhanced by a factor of 2{6 in the dust lanes and near the circumnuclear ring. We argue that magnetic elds can be dynamically important, and therefore should be included in models of gas flow in barred galaxies.
Multiscale magnetic fields in spiral galaxies: evolution
We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand M... more We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand Minima can be associated with random fluctuations of the parameters governing the solar dynamo. We consider fluctuations of the alpha coefficient in the conventional Parker migratory dynamo, and also in slightly more sophisticated dynamo models, and demonstrate that they can mimic the gross features of the phenomenon of the occurrence of Grand Minima over suitable parameter ranges. The temporal distribution of these Grand Minima appears chaotic, with a more or less exponential waiting time distribution, typical of Poisson processes. In contrast, however, the available reconstruction of Grand Minima statistics based on cosmogenic isotope data demonstrates substantial deviations from this exponential law. We were unable to reproduce the non-Poissonic tail of the waiting time distribution either in the framework of a simple alpha-quenched Parker model or in its straightforward generalization...
Geophysical & Astrophysical Fluid Dynamics, 2004
... J. , 543: 1027–1043. [CrossRef], [Web of Science ®] View all references), to illustrate the s... more ... J. , 543: 1027–1043. [CrossRef], [Web of Science ®] View all references), to illustrate the scale of the relevant literature. A more systematic presentation of this literature can be found eg in a recent review by Ossendrijver (200321. Ossendrijver, M. 2003. “The stellar dynamo”. ...
Astronomische Nachrichten, 2006
A fraction of solar active regions are observed to have current helicity of a sign that contradic... more A fraction of solar active regions are observed to have current helicity of a sign that contradicts the polarity law for magnetic helicity; this law corresponds to the well-known Hale polarity law for sunspots. A significant excess of active regions with the "wrong" sign of helicity is seen to occur just at the beginning of the cycle. We compare these observations with predictions from a dynamo model based on principles of helicity conservation, discussed by Zhang et al. (2006). This model seems capable of explaining only a fraction of the regions with the wrong sign of the helicity. We attribute the remaining excess to additional current helicity production from the twisting of rising magnetic flux tubes, as suggested by Choudhuri et al. (2004). We estimate the relative contributions of this effect and that connected with the model based on magnetic helicity conservation.
Proceedings of the International Astronomical Union
We present results of convective turbulent dynamo simulations including a coronal layer in a sphe... more We present results of convective turbulent dynamo simulations including a coronal layer in a spherical wedge. We find an equatorward migration of the radial and azimuthal fields similar to the behavior of sunspots during the solar cycle. The migration of the field coexist with a spoke-like differential rotation and anti-solar (clockwise) meridional circulation. Even though the migration extends over the whole convection zone, the mechanism causing this is not yet fully understood.
Monthly Notices of the Royal Astronomical Society, 2015
In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, w... more In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, we suggest an observational test for small-scale dynamo action based on statistics of anti-Hale sunspot groups. As we have shown, according to theoretical expectations the smallscale dynamo action has to provide a population of sunspot groups which do not follow the Hale polarity law, and the density of such groups on the time-latitude diagram is expected to be independent on the phase of the solar cycle. Correspondingly, a percentage of the anti-Hale groups is expected to reach its maximum values during solar minima. For several solar cycles, we considered statistics of anti-Hale groups obtained by several scientific teams, including ours, to find that the percentage of anti-Hale groups becomes indeed maximal during a solar minimum. Our interpretation is that this fact may be explained by the small-scale dynamo action inside the solar convective zone.
We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand M... more We consider to what extent the long-term dynamics of cyclic solar activity in the form of Grand Minima can be associated with random fluctuations of the parameters governing the solar dynamo. We consider fluctuations of the alpha-coefficient in the conventional Parker migratory dynamo, and also in slightly more sophisticated dynamo models, and demonstrate that they can mimic the gross features of the phenomenon of the occurrence of Grand Minima over a suitable parameter range. The temporal distribution of these Grand Minima appears chaotic, with a more or less exponential waiting time distribution, typical of Poisson processes. In contrast however, the available reconstruction of Grand Minima statistics based on cosmogenic isotope data demonstrates substantial deviations from this exponential law. We were unable to reproduce the non-Poissonic tail of the waiting time distribution either in the framework of a simple alpha-quenched Parker model, or in its straightforward generalizatio...
We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with ra... more We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of t...
Astronomische Nachrichten
Magnetic fields in galaxies exist on various spatial scales. Large-scale magnetic fields are thou... more Magnetic fields in galaxies exist on various spatial scales. Large-scale magnetic fields are thought to be generated by the − Ω dynamo. Small-scale galactic magnetic fields (1 kpc and below) can be generated by tangling the large-scale field. The analysis of field structures with the help of polarized radio continuum emission is hampered by the effect of Faraday dispersion (due to fluctuations in magnetic field and/or thermal electron density), which shifts the signals from large to small scales. At long observation wavelengths, large-scale magnetic fields may become invisible, as in the case of the spectro-polarimetric data cube of the spiral galaxy NGC 6946 observed with the Westerbork Synthesis Radio Telescope in the wavelength range 17-23 cm. The application of rotation measure (RM) synthesis alone does not overcome this problem. We propose to decompose the Faraday data cube into data cubes at different spatial scales by a wavelet transform. Signatures of the "magnetic arms" observed in NGC 6946 at shorter wavelengths then become visible. Our method allows the use of small-scale structures as tracers of large-scale magnetic field patterns in data cubes at long wavelengths, as provided by new-generation radio telescopes.
Astronomy & Astrophysics
Context. The magnetic field configurations in several nearby spiral galaxies contain magnetic arm... more Context. The magnetic field configurations in several nearby spiral galaxies contain magnetic arms that are sometimes located between the material arms. The nearby barred galaxy M83 provides an outstanding example of a spiral pattern seen in tracers of gas and magnetic field. Aims. We analyse the spatial distribution of magnetic fields in M83 and their relation to the material spiral arms. Methods. Isotropic and anisotropic wavelet transforms are used to decompose the images of M83 in various tracers to quantify structures in a range of scales from 0.2 to 10 kpc. We used radio polarization observations at λ6.2 cm and λ13 cm obtained with the VLA, Effelsberg and ATCA telescopes and APEX sub-mm observations at 870 μm, which are first published here, together with maps of the emission of warm dust, ionized gas, molecular gas, and atomic gas. Results. The spatial power spectra are similar for the tracers of dust, gas, and total magnetic field, while the spectra of the ordered magnetic field are significantly different. As a consequence, the wavelet cross-correlation between all material tracers and total magnetic field is high, while the structures of the ordered magnetic field are poorly correlated with those of other tracers. The magnetic field configuration in M83 contains pronounced magnetic arms. Some of them are displaced from the corresponding material arms, while others overlap with the material arms. The pitch angles of the magnetic and material spiral structures are generally similar. The magnetic field vectors at λ6.2 cm are aligned with the outer material arms, while significant deviations occur in the inner arms and, in particular, in the bar region, possibly due to non-axisymmetric gas flows. Outside the bar region, the typical pitch angles of the material and magnetic spiral arms are very close to each other at about 10 •. The typical pitch angle of the magnetic field vectors is about 20 • larger than that of the material spiral arms. Conclusions. One of the main magnetic arms in M83 is displaced from the gaseous arms similarly to the galaxy NGC 6946, while the other main arm overlaps a gaseous arm, similar to what is observed in M51. We propose that a regular spiral magnetic field generated by a mean-field dynamo is compressed in material arms and partly aligned with them. The interaction of galactic dynamo action with a transient spiral pattern is a promising mechanism for producing such complicated spiral patterns as in M83.
Astronomy & Astrophysics
Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission... more Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission of the interacting galaxies when compared with the typical emission from similar non-interacting galaxies. This increase is believed to be associated with an increase in the star formation rate and the turbulent magnetic fields resulting from the encounter, while the regular magnetic field is usually believed to decrease as a result of the encounter. Aims. We attempt to verify these expectations. Methods. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field resulting from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase in the star formation rate caused by the encounter. Results. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction. This leads to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. Conclusions. The encounter causes enhanced total and polarized emission without increase in the regular magnetic field strength. The increase in synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase in the star formation rate, depending on the choice of parameters. The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr). However, a long-lasting field reversal of the regular magnetic field may result.