scholarly journals Magnetic Activity Following Re-Accretion on to Galaxies

2004 ◽  
Vol 217 ◽  
pp. 174-176
Author(s):  
H. Nishikori ◽  
M. Machida ◽  
R. Matsumoto
Keyword(s):  
Magnetic Fields ◽  
Galactic Center ◽  
Plasma Heating ◽  
Three Dimensional ◽  
Magnetic Activity ◽  
Initial Condition ◽  
Disk Drives ◽  
Magnetorotational Instability ◽  
Mhd Simulations ◽  
Dark Matters

We carried out global three-dimensional magnetohydrody-namical (MHD) simulations of galactic gaseous disks re-accreting intergalactic plasma. As the initial condition, we assume that a rotating slender torus is formed at 10kpc from the galactic center. We assume a gravitational potential generated by bulge stars, disk stars and dark matters. Numerical results indicate that magnetorotational instability (MRI) growing in the torus amplifies magnetic fields and generates turbulence. The Maxwell stress enhanced by turbulent magnetic fields drives mass accretion of the disk gas. The amplification of magnetic fields in the accreting gas disk drives magnetic activities such as flares and plasma heating due to magnetic reconnection. The magnetic activity is maintained for time scales longer than the accretion time scale, typically 5Gyr.

scholarly journals Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

Solar Physics ◽  
2021 ◽  
Vol 296 (8) ◽  
Author(s):  
J. Threlfall ◽  
J. Reid ◽  
A. W. Hood
Keyword(s):  
Magnetic Fields ◽  
Three Dimensional ◽  
Coronal Loops ◽  
Flux Tubes ◽  
Single Thread ◽  
Mhd Instabilities ◽  
Mhd Instability ◽  
Mhd Simulations ◽  
Ideal Mhd ◽  
Model Geometry

AbstractMagnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of single- and multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release.

scholarly journals Three-Dimensional MHD Simulations of a Subcluster Plasma Moving in Turbulent ICM

2006 ◽  
Vol 2 (S235) ◽  
pp. 189-189
Author(s):  
N. Asai ◽  
N. Fukuda ◽  
R. Matsumoto
Keyword(s):  
Heat Conduction ◽  
Magnetic Fields ◽  
Three Dimensional ◽  
Cold Fronts ◽  
Mhd Simulations ◽  
Anisotropic Heat Conduction ◽  
Magnetohydrodynamic Simulations

AbstractWe carried out 3D magnetohydrodynamic simulations of a subcluster moving in turbulent ICM by including anisotropic heat conduction. Since magnetic fields stretched along the subcluster surface suppress the heat conduction across the front, cold fronts are formed and sustained.

scholarly journals Feedback Regulated Turbulence, Magnetic Fields, and Star Formation Rates in Galactic Disks

2015 ◽  
Vol 11 (S315) ◽  
pp. 38-41
Author(s):  
Chang-Goo Kim ◽  
Eve C. Ostriker
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Formation Rate ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Quasi Equilibrium ◽  
Galactic Disks ◽  
Heating Rates ◽  
Mhd Simulations ◽  
Vertical Support

AbstractWe use three-dimensional magnetohydrodynamic (MHD) simulations to investigate the quasi-equilibrium states of galactic disks regulated by star formation feedback. We incorporate effects from massive-star feedback via time-varying heating rates and supernova (SN) explosions. We find that the disks in our simulations rapidly approach a quasi-steady state that satisfies vertical dynamical equilibrium. The star formation rate (SFR) surface density self-adjusts to provide the total momentum flux (pressure) in the vertical direction that matches the weight of the gas. We quantify feedback efficiency by measuring feedback yields, ηc≡ Pc/ΣSFR (in suitable units), for each pressure component. The turbulent and thermal feedback yields are the same for HD and MHD simulations, ηth ~ 1 and ηturb ~ 4, consistent with the theoretical expectations. In MHD simulations, turbulent magnetic fields are rapidly generated by turbulence, and saturate at a level corresponding to ηmag,t ~ 1. The presence of magnetic fields enhances the total feedback yield and therefore reduces the SFR, since the same vertical support can be supplied at a smaller SFR. We suggest further numerical calibrations and observational tests in terms of the feedback yields.

scholarly journals 8.10. Three-dimensional global MHD simulations of jet formation in active galactic nuclei

1998 ◽  
Vol 184 ◽  
pp. 363-364
Author(s):  
R. Matsumoto ◽  
K. Shibata
Keyword(s):  
Active Galactic Nuclei ◽  
Accretion Disks ◽  
Three Dimensional ◽  
Galactic Nuclei ◽  
Equatorial Region ◽  
Astrophysical Jets ◽  
Magnetorotational Instability ◽  
Jet Formation ◽  
Mhd Simulations ◽  
Avalanche Flow

Magnetically driven jets from accretion disks are considered to be the most promising models of astrophysical jets. Uchida & Shibata (1985) and Shibata & Uchida (1986) first carried out two-dimensional nonlinear MHD simulations of jet formation from a magnetized disk. Matsumoto et al. (1996) applied the Uchida-Shibata model to a gas torus in active galactic nuclei and showed that the surface layer of the torus accretes faster than the equatorial region like an avalanche because magnetic braking most effectively extracts angular momentum from that layer. A magnetized torus subjects to global non-axisymmetric instabilities (Curry & Pudritz 1996) and local magnetorotational instability (Balbus & Hawley 1991). We carried out three-dimensional global MHD simulations to show the non-axisymmetric effects on the torus, avalanche flow and jet formation.

scholarly journals Three-Dimensional Global MHD Simulations of Magnetised Accretion Disks and Jets

1997 ◽  
Vol 163 ◽  
pp. 443-447 ◽  
Author(s):  
R. Matsumoto ◽  
K. Shibata
Keyword(s):  
Surface Layer ◽  
Magnetic Fields ◽  
Accretion Disks ◽  
Large Scale ◽  
Three Dimensional ◽  
Maximum Speed ◽  
Jet Formation ◽  
Mhd Simulations ◽  
Avalanche Flow ◽  
Magnetic Braking

AbstractWe carried out three-dimensional global MHD simulations of jet formation from an accretion disk threaded by large-scale magnetic fields. Numerical results show that bipolar jets with maximum speed υjet ~ υKepler are created. The surface layer of the disk accretes faster than the equatorial part because magnetic braking most effectively affects that layer. Accretion proceeds along spiral channels which correspond to the surface avalanche flow appearing in previous axisymmetric simulations. Spirally shaped low β (= Pgas/Pmag < 1) regions appear in the innermost part of accretion disks where toroidal magnetic fields become dominant.

scholarly journals Three-dimensional MHD simulations of molecular cloud fragmentation regulated by gravity, ambipolar diffusion, and turbulence

2008 ◽  
Vol 4 (S259) ◽  
pp. 115-116
Author(s):  
Takahiro Kudoh ◽  
Shantanu Basu
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Time Scale ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Three Dimensional ◽  
Core Formation ◽  
Mhd Simulations ◽  
Dimensional Simulation

AbstractWe find that the star formation is accelerated by the supersonic turbulence in the magnetically dominated (subcritical) clouds. We employ a fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The time-scale of collapsing core formation in subcritical clouds is a few ×107 years when starting with small subsonic perturbations. However, it is shortened to approximately several ×106 years by the supersonic flows in the clouds. We confirm that higher-spacial resolution simulations also show the same result.

scholarly journals Low-frequency measurements of synchrotron absorbing HII regions and modeling of observed synchrotron emissivity

2019 ◽  
Vol 621 ◽  
pp. A127 ◽  
Author(s):  
I. M. Polderman ◽  
M. Haverkorn ◽  
T. R. Jaffe ◽  
M. I. R. Alves
Keyword(s):  
Synchrotron Radiation ◽  
Magnetic Fields ◽  
Large Scale ◽  
Milky Way ◽  
Galactic Center ◽  
Three Dimensional ◽  
Low Frequency ◽  
Hii Regions ◽  
The Galaxy ◽  
Absorption Measurements

Context. Cosmic rays (CRs) and magnetic fields are dynamically important components in the Galaxy, and their energy densities are comparable to that of the turbulent interstellar gas. The interaction of CRs and Galactic magnetic fields (GMF) produces synchrotron radiation clearly visible in the radio regime. Detailed measurements of synchrotron radiation averaged over the line-of-sight (LOS), so-called synchrotron emissivities, can be used as a tracer of the CR density and GMF strength. Aims. Our aim is to model the synchrotron emissivity in the Milky Way using a three-dimensional dataset instead of LOS-integrated intensity maps on the sky. Methods. Using absorbed HII regions, we measured the synchrotron emissivity over a part of the LOS through the Galaxy, changing from a two-dimensional to a three-dimensional view. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using various simple axisymmetric emissivity models and a number of GMF-based emissivity models, we were able to simulate the synchrotron emissivities and compare them to the observed values in the catalog. Results. We present a catalog of low-frequency absorption measurements of HII regions, their distances and electron temperatures, compiled from literature. These data show that the axisymmetric emissivity models are not complex enough, but the GMF-based emissivity models deliver a reasonable fit. These models suggest that the fit can be improved by either an enhanced synchrotron emissivity in the outer reaches of the Milky Way or an emissivity drop near the Galactic center. Conclusions. Current GMF models plus a constant CR density model cannot explain low-frequency absorption measurements, but the fits improved with slight (ad hoc) adaptations. It is clear that more detailed models are needed, but the current results are very promising.

scholarly journals Three Dimensional MHD Simulations of Parker Instability in Differentially Rotating Disk

1997 ◽  
Vol 163 ◽  
pp. 766-767 ◽  
Author(s):  
T. Matsuzaki ◽  
R. Matsumoto ◽  
T. Tajima ◽  
K. Shibata
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Accretion Disks ◽  
Three Dimensional ◽  
Rotating Disk ◽  
Saturation Level ◽  
Nonlinear Growth ◽  
Magnetic Viscosity ◽  
Mhd Simulations

AbstractNonlinear growth of the Parker instability (PI) and the Balbus & Hawley instability (BHI) in accretion disks have been studied by local three-dimensional magnetohydrodynamic (MHD) simulations. In high-β disks (β = Pgas/Pmag > 1), the PI has only minor effects on the saturation level of BHI. In low β disks (β ≤ 1), the disk stays in a low-β state because magnetic flux cannot escape fast enough to convert the disk into a high-β state. We found that even in low-β disk the BHI generates fluctuating magnetic fields. The effective magnetic viscosity αB(= –⟨BrBΦ/4πP0⟩) is O(0.1) when β ~ 1.

scholarly journals MHD simulations of supernova driven ISM turbulence

2006 ◽  
Vol 2 (S237) ◽  
pp. 415-415
Author(s):  
Oliver Gressel ◽  
Udo Ziegler
Keyword(s):  
Magnetic Fields ◽  
Dynamic Process ◽  
Large Scale ◽  
Three Dimensional ◽  
Rotational Instability ◽  
Dynamic Evolution ◽  
Mhd Simulations ◽  
Mhd Model

AbstractLarge-scale magnetic fields, that can be observed in numerous galaxies, are most likely the outcome of a dynamic process, a so-called dynamo. The favoured mechanisms for driving such a process in the ISM are supernovae (SNe) and/or magneto-rotational instability (MRI). In this work we simulate the dynamic evolution of the turbulent ISM utilising a three-dimensional MHD model.

scholarly journals Three-dimensional MHD simulations of magnetized molecular cloud fragmentation with turbulence and ion-neutral friction

2006 ◽  
Vol 2 (S237) ◽  
pp. 437-437
Author(s):  
T. Kudoh ◽  
S. Basu
Keyword(s):  
Magnetic Field ◽  
Three Dimensional ◽  
Initial Condition ◽  
Dense Region ◽  
Magnetic Diffusion ◽  
Mhd Simulations ◽  
Gas Layer ◽  
Isothermal Gas ◽  
3D Mhd Simulation ◽  
The Magnetic Field

AbstractWe perform a 3D-MHD simulation of a self-gravitating isothermal gas layer that is initially penetrated by a uniform magnetic field. The strength of the initial magnetic field is such that the cloud is slightly subcritical. In this system, we input random supersonic turbulence initially. Ion-neutral friction is also introduced in the magnetized gas so that the magnetic diffusion allows gas to go across the magnetic field and form self-gravitating cores. We found that self-gravitating cores are formed in the dense region enhanced by the shock waves if ion-neutral friction is introduced. The time scale of core formation is on the order of the 106 years, which is faster than the usual magnetic diffusion time (107 years) estimated from the initial condition. Our result is consistent with the results of 2D-MHD simulations by Li & Nakamura (2004).

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