scholarly journals The supernova-regulated ISM – VI. Magnetic effects on the structure of the interstellar medium

2019 ◽  
Vol 488 (4) ◽  
pp. 5065-5074 ◽  
Author(s):  
C C Evirgen ◽  
F A Gent ◽  
A Shukurov ◽  
A Fletcher ◽  
P J Bushby
Keyword(s):  
Magnetic Field ◽  
Interstellar Medium ◽  
Vertical Distribution ◽  
Large Scale ◽  
Dynamo Action ◽  
Hot Gas ◽  
Downward Pressure ◽  
Large Scale Magnetic Field ◽  
The Vertical Distribution ◽  
The Magnetic Field

ABSTRACT We explore the effect of magnetic fields on the vertical distribution and multiphase structure of the supernova-driven interstellar medium in simulations that admit dynamo action. As the magnetic field is amplified to become dynamically significant, gas becomes cooler and its distribution in the disc becomes more homogeneous. We attribute this to magnetic quenching of vertical velocity, which leads to a decrease in the cooling length of hot gas. A non-monotonic vertical distribution of the large-scale magnetic field strength, with the maximum at |z| ≈ 300 pc causes a downward pressure gradient below the maximum which acts against outflow driven by SN explosions, while it provides pressure support above the maximum.

scholarly journals Probing interstellar magnetic fields with Supernova remnants

2008 ◽  
Vol 4 (S259) ◽  
pp. 75-80 ◽  
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.

scholarly journals The Toroidal Magnetic Field inside the Sun

1991 ◽  
Vol 130 ◽  
pp. 187-189
Author(s):  
V.N. Krivodubskij ◽  
A.E. Dudorov ◽  
A.A. Ruzmaikin ◽  
T.V. Ruzmaikina
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Convection Zone ◽  
The Sun ◽  
Poloidal Magnetic Field ◽  
Radial Gradient ◽  
The Core ◽  
Large Scale Magnetic Field ◽  
Magnetic Buoyancy ◽  
The Magnetic Field

Analysis of the fine structure of the solar oscillations has enabled us to determine the internal rotation of the Sun and to estimate the magnitude of the large-scale magnetic field inside the Sun. According to the data of Duvall et al. (1984), the core of the Sun rotates about twice as fast as the solar surface. Recently Dziembowski et al. (1989) have showed that there is a sharp radial gradient in the Sun’s rotation at the base of the convection zone, near the boundary with the radiative interior. It seems to us that the sharp radial gradients of the angular velocity near the core of the Sun and at the base of the convection zone, acting on the relict poloidal magnetic field Br, must excite an intense toroidal field Bф, that can compensate for the loss of the magnetic field due to magnetic buoyancy.

scholarly journals Large-scale dynamos in rapidly rotating plane layer convection

2018 ◽  
Vol 612 ◽  
pp. A97 ◽  
Author(s):  
P. J. Bushby ◽  
P. J. Käpylä ◽  
Y. Masada ◽  
A. Brandenburg ◽  
B. Favier ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Plane Layer ◽  
Cycle Period ◽  
Dynamo Action ◽  
Horizontal Magnetic Field ◽  
Vortex Instability ◽  
Large Scale Vortex ◽  
The Mean ◽  
The Magnetic Field

Context.Convectively driven flows play a crucial role in the dynamo processes that are responsible for producing magnetic activity in stars and planets. It is still not fully understood why many astrophysical magnetic fields have a significant large-scale component.Aims.Our aim is to investigate the dynamo properties of compressible convection in a rapidly rotating Cartesian domain, focusing upon a parameter regime in which the underlying hydrodynamic flow is known to be unstable to a large-scale vortex instability.Methods.The governing equations of three-dimensional non-linear magnetohydrodynamics (MHD) are solved numerically. Different numerical schemes are compared and we propose a possible benchmark case for other similar codes.Results.In keeping with previous related studies, we find that convection in this parameter regime can drive a large-scale dynamo. The components of the mean horizontal magnetic field oscillate, leading to a continuous overall rotation of the mean field. Whilst the large-scale vortex instability dominates the early evolution of the system, the large-scale vortex is suppressed by the magnetic field and makes a negligible contribution to the mean electromotive force that is responsible for driving the large-scale dynamo. The cycle period of the dynamo is comparable to the ohmic decay time, with longer cycles for dynamos in convective systems that are closer to onset. In these particular simulations, large-scale dynamo action is found only when vertical magnetic field boundary conditions are adopted at the upper and lower boundaries. Strongly modulated large-scale dynamos are found at higher Rayleigh numbers, with periods of reduced activity (grand minima-like events) occurring during transient phases in which the large-scale vortex temporarily re-establishes itself, before being suppressed again by the magnetic field.

Dynamo action in rotating convection

2009 ◽  
Vol 5 (H15) ◽  
pp. 347-347
Author(s):  
Gustavo Guerrero ◽  
Elisabete M. de Gouveia Dal Pino
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Hydrostatic Equilibrium ◽  
Computational Domain ◽  
Dynamo Action ◽  
Rotating Convection ◽  
Helical Turbulence ◽  
Large Scale Magnetic Field ◽  
Unstable Layer ◽  
Scale Shear

AbstractWe present MHD numerical simulations of a rotating turbulent convection system in a 3D domain (we have used the finite volume, Goudunov type MHD code PLUTO (Mignone et al. 2007)). Rotating convection is the natural scenario for the study of the dynamo action which is able to generate a large scale magnetic field, like the observed in the sun. Though we have neglected in the present approach the Ω effect, due to a large scale shear, our model is appropriate to test the controversial existence of the so called α effect that arises from helical turbulence (e.g. Cattaneo & Hughes 2006, Käpylä et al. 2009). We start with a two-layer piece-wise polytropic region in hydrostatic equilibrium (e.g. Ziegler 2002), considering one stable overshoot layer at the bottom and a convectively unstable layer at the top of the computational domain. We have allowed this hydrodynamic system to evolve up to the steady state, i.e., after about 10 turnover times (τ). Then, we introduced a seed magnetic field and let the system evolve for more ~40 τ. Our preliminary results are summarized below in Figure 2.

scholarly journals Recurrent flux emergence from dynamo-generated fields

2010 ◽  
Vol 6 (S271) ◽  
pp. 407-408
Author(s):  
Jörn Warnecke ◽  
Axel Brandenburg
Keyword(s):  
Magnetic Field ◽  
Time Series ◽  
Large Scale ◽  
Field Structure ◽  
Flux Emergence ◽  
Magnetic Field Structure ◽  
Forcing Function ◽  
Large Scale Magnetic Field ◽  
The Magnetic Field

Abstractwe investigate the emergence of a large-scale magnetic field. This field is dynamo-generated by turbulence driven with a helical forcing function. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Time series of the magnetic field structure show recurrent plasmoid ejections.

The nonlinear properties of a large-scale dynamo driven by helical forcing

2002 ◽  
Vol 456 ◽  
pp. 219-237 ◽  
Author(s):  
FAUSTO CATTANEO ◽  
DAVID W. HUGHES ◽  
JEAN-CLAUDE THELEN
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Initial Conditions ◽  
Spatial Scales ◽  
Mean Field ◽  
Forced Flow ◽  
Small Scale ◽  
Dynamo Action ◽  
Initial State ◽  
The Magnetic Field

By considering an idealized model of helically forced flow in an extended domain that allows scale separation, we have investigated the interaction between dynamo action on different spatial scales. The evolution of the magnetic field is studied numerically, from an initial state of weak magnetization, through the kinematic and into the dynamic regime. We show how the choice of initial conditions is a crucial factor in determining the structure of the magnetic field at subsequent times. For a simulation with initial conditions chosen to favour the growth of the small-scale field, the evolution of the large-scale magnetic field can be described in terms of the α-effect of mean field magnetohydrodynamics. We have investigated this feature further by a series of related numerical simulations in smaller domains. Of particular significance is that the results are consistent with the existence of a nonlinearly driven α-effect that becomes saturated at very small amplitudes of the mean magnetic field.

scholarly journals The magnetic field structure in NGC 253 in presence of a galactic wind

2008 ◽  
Vol 4 (S259) ◽  
pp. 509-514 ◽  
Author(s):  
Volker Heesen ◽  
M. Krause ◽  
R. Beck ◽  
R.-J. Dettmar
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Cosmic Ray ◽  
Field Structure ◽  
Radio Continuum ◽  
Magnetic Field Structure ◽  
Galactic Wind ◽  
Radio Halo ◽  
Large Scale Magnetic Field ◽  
The Magnetic Field

AbstractWe present radio continuum polarimetry observations of the nearby edge-on galaxy NGC 253 which possesses a very bright radio halo. Using the vertical synchrotron emission profiles and the lifetimes of cosmic-ray electrons, we determined the cosmic-ray bulk speed as 300±30 km s−1, indicating the presence of a galactic wind in this galaxy. The large-scale magnetic field was decomposed into a toroidal axisymmetric component in the disk and a poloidal component in the halo. The poloidal component shows a prominent X-shaped magnetic field structure centered on the nucleus, similar to the magnetic field observed in other edge-on galaxies. Faraday rotation measures indicate that the poloidal field has an odd parity (antisymmetric). NGC 253 offers the possibility to compare the magnetic field structure with models of galactic dynamos and/or galactic wind flows.

scholarly journals Multi-Frequency Radio Observations of Spiral Galaxies and Their Interpretation

1990 ◽  
Vol 140 ◽  
pp. 187-196 ◽  
Author(s):  
M. Krause
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Spiral Galaxies ◽  
Radio Observations ◽  
Dynamo Theory ◽  
Polarization Data ◽  
Open Questions ◽  
Large Scale Magnetic Field ◽  
Future Work ◽  
The Magnetic Field

After a brief historical summary of radio observations of spiral galaxies I review the methods of analyzing radio polarization data in view of the magnetic field. Special attention is drawn to the Faraday rotation and depolarization effects and to the identification of the large-scale magnetic field structure. The present observational results and open questions are discussed in terms of the predictions of the dynamo theory and prospects on future work are given.

The Large-Scale Magnetic Field and Sunspot Cycles

2000 ◽  
Vol 179 ◽  
pp. 161-162
Author(s):  
V. I. Makarov ◽  
A. G. Tlatov
Keyword(s):  
Magnetic Field ◽  
Spherical Harmonics ◽  
Legendre Polynomials ◽  
Large Scale ◽  
Magnetic Moments ◽  
Radial Component ◽  
Wolf Number ◽  
Spherical Functions ◽  
Large Scale Magnetic Field ◽  
The Magnetic Field

Extended abstractWe report on the correlation between the large scale magnetic field and sunspot cycles during the last 80 years that was found by Makarovet al. (1999) and Makarov & Tlatov (2000) in H-αspherical harmonics of the large scale magnetic field for 1915–1999. The sum of intensities of the low modes 1 = 1 and 3, A(t), was used for comparison with the Wolf number, W(t). It was shown that the large scale magnetic field cycles, A(t), precede the sunspot cycles, W(t), by 5.5 years.Let us consider the behaviour in time of the harmonics with low numbers 1 = 1 and 1 = 3. The radial component B(r) of the magnetic field may be expanded in terms of the spherical harmonicswhereθandϕare the latitude and longitude,are Legendre polynomials andandare coefficients of expansion on the spherical functions.The magnetic moments of a dipole (1 = 1) and an octopole (1 = 3) are determined by the following equations:Let us enter the parameter describing their intensity,

scholarly journals ACCRETION DISKS WITH A LARGE SCALE MAGNETIC FIELD AROUND BLACK HOLES

2013 ◽  
Vol 53 (A) ◽  
pp. 677-682
Author(s):  
Gennady Bisnovatyi-Kogan ◽  
Alexandr S. Klepnev ◽  
Richard V.E. Lovelace
Keyword(s):  
Magnetic Field ◽  
Black Holes ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Large Scale ◽  
Outward Diffusion ◽  
Large Scale Magnetic Field ◽  
Jet Collimation ◽  
The Magnetic Field ◽  
Inner Parts

We consider accretion disks around black holes at high luminosity, and the problem of the formation of a large-scale magnetic field in such disks, taking into account the non-uniform vertical structure of the disk. The structure of advective accretion disks is investigated, and conditions for the formation of optically thin regions in central parts of the accretion disk are found. The high electrical conductivity of the outer layers of the disk prevents outward diffusion of the magnetic field. This implies a stationary state with a strong magnetic field in the inner parts of the accretion disk close to the black hole, and zero radial velocity at the surface of the disk. The problem of jet collimation by magneto-torsion oscillations is investigated.

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