Magnetic Fields in Disks and Halos of Spiral Galaxies

Spiral galaxies host interstellar magnetic fields of 4-15 μG total strength. A significant fraction of the field lines shows large-scale structures. At face-on or moderately inclined view, the field lines run generally parallel to the spiral arms, either with uniform direction with respect to azimuthal angle (axisymmetric spiral, ASS), with one reversal along azimuthal angle (bisymmetric spiral, BSS), or with spiral orientation without dominating direction.At edge-on view, the field is concentrated in a thin disk, often surrounded by a thick radio disk with field lines mostly parallel to the plane, similar to the quadrupole-type dynamo field. Radio polarization data from NGC891 indicate that the thermal gas seen in Hα is responsible for Faraday depolarization. The required scaleheight of the field of ~4 kpc is comparable to the value expected in case of energy equipartition between magnetic fields and cosmic rays. The interacting edge-on galaxy NGC 4631 shows a much larger radio halo with field lines perpendicular to the disk, possibly driven by a strong galactic wind or the result of a dipole-type halo field.Field lines bending out of the plane are also visible in face-on galaxies as regions with high rotation measures and low star-formation activity. The resemblance to the phenomenon of the solar corona suggests to call them “galactic coronal holes”.

Download Full-text

Dynamo in Astrophysics

Symposium - International Astronomical Union ◽

10.1017/s0074180900189636 ◽

1990 ◽

Vol 140 ◽

pp. 83-89

Author(s):

A.A. Ruzmaikin

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Turbulent Flow ◽

Large Scale ◽

Spiral Galaxies ◽

Clusters Of Galaxies ◽

Magnetic Structures ◽

Random Magnetic Field ◽

Astrophysical Objects

The fast dynamo acting in a turbulent flow explains the origin of magnetic fields in astrophysical objects. Stellar cycles and large-scale magnetic fields in spiral galaxies reflect the behaviour of a mean magnetic field. Intermittent magnetic structures in clusters of galaxies are associated with random magnetic field.

Download Full-text

Probing interstellar magnetic fields with Supernova remnants

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030087 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 75-80 ◽

Cited By ~ 2

Author(s):

Roland Kothes ◽

Jo-Anne Brown

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Supernova Remnants ◽

Large Scale ◽

Galactic Plane ◽

Field Configuration ◽

Large Scale Magnetic Field ◽

Rotation Measure ◽

Field Lines ◽

The Magnetic Field

AbstractAs Supernova remnants expand, their shock waves are freezing in and compressing the magnetic field lines they encounter; consequently we can use Supernova remnants as magnifying glasses for their ambient magnetic fields. We will describe a simple model to determine emission, polarization, and rotation measure characteristics of adiabatically expanding Supernova remnants and how we can exploit this model to gain information about the large scale magnetic field in our Galaxy. We will give two examples: The SNR DA530, which is located high above the Galactic plane, reveals information about the magnetic field in the halo of our Galaxy. The SNR G182.4+4.3 is located close to the anti-centre of our Galaxy and reveals the most probable direction where the large-scale magnetic field is perpendicular to the line of sight. This may help to decide on the large-scale magnetic field configuration of our Galaxy. But more observations of SNRs are needed.

Download Full-text

Galactic Dynamos Without Sharp Boundaries: Non-Axisymmetric Fields in Axisymmetric Disks?

Symposium - International Astronomical Union ◽

10.1017/s0074180900189697 ◽

1990 ◽

Vol 140 ◽

pp. 115-116

Author(s):

R. Meinel ◽

D. Elstner ◽

G. Rüdiger ◽

F. Krause

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Spiral Galaxies ◽

Galactic Magnetic Fields ◽

Axisymmetric Structure ◽

Galactic Dynamos

Radio polarization observations of spiral galaxies suggest the existence of large-scale galactic magnetic fields which are of either axisymmetric -spiral (ASS) or bisymmetric-spiral (BSS), i.e. non-axisymmetric, structure (cf. Beck, 1939). Clear evidence for a BSS field was indicated for M31 by M. Krause et al. (1989).

Download Full-text

Examining the optical intensity and magnetic field expansion factor in the open magnetic field regions associated with coronal holes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320000393 ◽

2019 ◽

Vol 15 (S354) ◽

pp. 228-231

Author(s):

Chia-Hsien Lin ◽

Guan-Han Huang ◽

Lou-Chuang Lee

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Coronal Holes ◽

Closed Field ◽

Logarithmic Scale ◽

Expansion Factor ◽

Astrophysical Journal ◽

Field Lines ◽

The Relationship ◽

Open Magnetic Field

AbstractCoronal holes can be identified as the darkest regions in EUV or soft X-ray images with predominantly unipolar magnetic fields (LIRs) or as the regions with open magnetic fields (OMF). Our study reveals that only 12% of OMF regions are coincident with LIRs. The aim of this study is to investigate the conditions that affect the EUV intensity of OMF regions. Our results indicate that the EUV intensity and the magnetic field expansion factor of the OMF regions are weakly positively correlated when plotted in logarithmic scale, and that the bright OMF regions are likely to locate inside or next to the regions with closed field lines. We empirically determined a linear relationship between the expansion factor and the EUV intensity. The relationship is demonstrated to improve the consistency from 12% to 23%. The results have been published in Astrophysical Journal (Huang et al. 2019).

Download Full-text

Observations of Galactic Magnetic Fields

Highlights of Astronomy ◽

10.1017/s153929960001474x ◽

2002 ◽

Vol 12 ◽

pp. 712-715 ◽

Cited By ~ 1

Author(s):

Rainer Beck

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Density Wave ◽

Upstream Region ◽

Galactic Magnetic Fields ◽

Barred Galaxies ◽

Angular Momentum Transport ◽

Central Regions ◽

Spiral Structures ◽

Field Lines

AbstractMagnetic fields are anchored in gas clouds. Field lines are tangled in spiral arms, but highly regularbetweenthe arms. The similarity of pitch angles between gaseous and magnetic arms suggests a coupling between the density wave and the magnetic wave. Observations of large-scale patterns in Faraday rotation favour a dynamo origin of the regular fields. Fields in barred galaxies do not reveal the strong shearing shocks observed in the cold gas, but swing smoothly from the upstream region into the bar. Magnetic fields are important for the dynamics of gas clouds, for the formation of spiral structures, bars and halos, and for mass and angular momentum transport in central regions.

Download Full-text

Magnetic fields in our Milky Way Galaxy and nearby galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313002561 ◽

2012 ◽

Vol 8 (S294) ◽

pp. 213-224 ◽

Cited By ~ 4

Author(s):

JinLin Han

Keyword(s):

Star Formation ◽

Magnetic Fields ◽

Large Scale ◽

Milky Way ◽

Spiral Galaxies ◽

Large Scale Structure ◽

Scale Structure ◽

Measure Data ◽

Submillimeter Wavelength ◽

Nearby Galaxies

AbstractMagnetic fields in our Galaxy and nearby galaxies have been revealed by starlight polarization, polarized emission from dust grains and clouds at millimeter and submillimeter wavelength, the Zeeman effect of spectral lines or maser lines from clouds or clumps, diffuse radio synchrotron emission from relativistic electrons in interstellar magnetic fields, and the Faraday rotation of background radio sources as well as pulsars for our Milky Way. It is easy to get a global structure for magnetic fields in nearby galaxies, while we have observed many details of magnetic fields in our Milky Way, especially by using pulsar rotation measure data. In general, magnetic fields in spiral galaxies probably have a large-scale structure. The fields follow the spiral arms with or without the field direction reversals. In the halo of spiral galaxies magnetic fields exist and probably also have a large-scale structure as toroidal and poloidal fields, but seem to be slightly weaker than those in the disk. In the central region of some galaxies, poloidal fields have been detected as vertical components. Magnetic field directions in galaxies seem to have been preserved during cloud formation and star formation, from large-scale diffuse interstellar medium to molecular clouds and then to the cloud cores in star formation regions or clumps for the maser spots. Magnetic fields in galaxies are passive to dynamics.

Download Full-text

Coronal Structure and Solar Wind

Symposium - International Astronomical Union ◽

10.1017/s0074180900067383 ◽

1980 ◽

Vol 91 ◽

pp. 73-78

Author(s):

J. N. Tandon

Keyword(s):

Solar Wind ◽

Large Scale ◽

Coronal Holes ◽

Solar Magnetic Fields ◽

Coronal Structure ◽

Active Centers ◽

Hydromagnetic Wave ◽

Field Lines ◽

Open Magnetic Field ◽

Coronal Structures

Recent observations of large scale coronal structures and solar wind have been studied. The intercorrelation of the two have been qualitatively explained through the focussing of solar-ion streams taking account of the local and general solar magnetic fields. This explains the association of coronal holes with weak, diverging open magnetic field lines and envisages the transfer of hydromagnetic wave energy from nearby active centers to account for the enhanced outflow of solar wind associated with coronal holes.

Download Full-text

Formation of Coronal Holes and the Global Magnetic Field Distribution

International Astronomical Union Colloquium ◽

10.1017/s025292110002501x ◽

1994 ◽

Vol 144 ◽

pp. 65-67 ◽

Cited By ~ 1

Author(s):

V. Bumba ◽

M. Klvaňa ◽

V. Rušin ◽

M. Rybanský ◽

G. T. Buyukliev

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Coronal Holes ◽

Activity Cycle ◽

Close Relation ◽

Solar Activity Cycle ◽

Cycle Maximum ◽

Global Magnetic Field ◽

Mutual Relations ◽

The Individual

The photoelectric magnetograph of the Ondřejov Observatory was reconstructed in 1990 (Klvaňa and Bumba, 1994; Klvaňaet al, 1994). During 1991 and 1992, several hundred sets of measurements were obtained, mostly in line Fel 5253.47 Å. It has been found that some of the measurements are distributed very favorably around coronal holes, sometimes covering smaller parts and in a few cases even larger parts of their areas.Both 1991 and 1992 were exceptional as regards their relation to the phase of the ending solar activity cycle (No 22): while the period of the secondary cycle maximum (mainly the southern solar hemisphere) took place in 1991, the year 1992 coincided with the initial stage of its declining branch. Since the formation of coronal holes is in close relation to the dynamics of the global distribution of solar magnetic fields, we thought that before starting to investigate the detailed connections of the individual coronal holes with particular local magnetic fields, it might be interesting to study their mutual relations also on a large scale.

Download Full-text

Examination of the EUV Intensity in the Open Magnetic Field Regions Associated with Coronal Holes

10.5194/egusphere-egu2020-1501 ◽

2020 ◽

Author(s):

Guan-Han Huang ◽

Chia-Hsien Lin ◽

Lou Chuang Lee

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Order Approximation ◽

Coronal Holes ◽

Closed Field ◽

Spatial And Temporal Distributions ◽

Field Lines ◽

First Order Approximation ◽

Log Normal ◽

Open Magnetic Field

Coronal holes can be identified as the regions with magnetic field lines extending far away from the Sun, or the darkest regions in EUV/X-ray images with predominantly unipolar magnetic fields. A comparison between the locations of our determined regions with open magnetic field lines (OMF) and regions with low EUV intensity (LIR) reveals that only 12% of the OMF regions coincide with the LIRs. The aim of this study is to investigate the conditions leading to the different brightnesses of OMF regions, and to provide a means to predict whether an OMF region would be bright or dark. Examining the statistical distribution profiles of the magnetic field expansion factor (fs) and Atmospheric Imaging Assembly 193 &#197; intensity (I193) reveals that both profiles are approximately log-normal. The analysis of the spatial and temporal distributions of fs and I193 indicates that the bright OMF regions often are inside or next to regions with closed field lines, including quiet-Sun regions and regions with strong magnetic fields. Examining the relationship between I193 and fs reveals a weak positive correlation between log I193 and log fs , with a correlation coefficient &#8776; 0.39. As a first-order approximation, the positive relationship is determined to be log I193 = 0.62 log fs + 1.51 based on the principle of the whitening/dewhitening transformation. This linear relationship is demonstrated to increase the consistency between the OMF regions and LIRs from 12% to 23%.

Download Full-text

3D numerical simulations of magnetic field evolution in barred galaxies and in spiral galaxies under influence of tidal forces

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311007320 ◽

2010 ◽

Vol 6 (S274) ◽

pp. 381-384

Author(s):

Katarzyna Otmianowska-Mazur ◽

Katarzyna Kulpa-Dybeł ◽

Barbara Kulesza-Żydzik ◽

Hubert Siejkowski ◽

Grzegorz Kowal

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Spiral Galaxies ◽

Cosmic Ray ◽

Mhd Equations ◽

Back Reaction ◽

Barred Galaxies ◽

Tidal Forces ◽

Field Evolution

AbstractWe present the results of the three-dimensional, fully non-linear MHD simulations of the large-scale magnetic field evolution in a barred galaxy with the back reaction of magnetic field to gas. We also include the process of the cosmic-ray driven dynamo. In addition, we check what physical processes are responsible for the magnetic field evolution in the tidally influenced spiral galaxies. We solve the MHD equations for the gas and magnetic field in a spiral galaxy with gravitationally prescribed bulge, disk and halo which travels along common orbit with the second body. In order to compare our modeling results with the observations we also construct the maps of high-frequency (Faraday rotation-free) polarized radio emission from the simulated magnetic fields. The model accounts for the effects of projection and limited resolution.We found that the obtained magnetic field configurations are highly similar to the observed maps of the polarized intensity of barred galaxies, because the modeled vectors form coherent structures along the bar and spiral arms. We also found a physical explanation of the problem of inconsistency between the velocity and magnetic fields character present in this type of galaxies. Due to the dynamical influence of the bar, the gas forms spiral waves which go radially outward. Each spiral arm forms the magnetic arm which stays much longer in the disk than the gaseous spiral structure. The modeled total energy of magnetic field and magnetic flux grows exponentially due to the action of the cosmic-ray driven dynamo. We also obtained the polarization maps of tidally influenced spiral galaxies which are similar to observations.

Download Full-text