Vorticity and magnetic dynamo from subsonic expansion waves

Abstract

This work concentrates on the effect of an irrotational forcing on a magnetized flow in the presence of rotation, baroclinicity, shear, or a combination of them. By including magnetic field in the model we can evaluate the occurrence of dynamo on both small and large scales. We aim at finding what are the minimum ingredients needed to trigger a dynamo instability and what is the relation between dynamo and the growth of vorticity. We use the Pencil code to run resistive MHD direct numerical simulations. We report no dynamo in all cases where only rotation is included, regardless on the equation of state. Conversely, the inclusion of a background sinusoidal shearing profile leads to an hydrodynamic instability that produces an exponential growth of the vorticity at all scales, starting from small ones. This is know as vorticity dynamo. The onset of this instability occurs after a rather long temporal evolution of several thousand turbulent turnover times. The vorticity dynamo in turn drives an exponential growth of the magnetic field, first at small scales, then also at large one. The instability then saturates and the magnetic field approximately reaches equipartition with the turbulent kinetic energy. During the saturation phase we can observe a winding of the magnetic field in the direction of the shearing flow. By varying the intensity of the shear we see that the growth rates of this instability change. The inclusion of the baroclinic term delays the onset of the vorticity dynamo but leads to a more rapid growth. We demonstrate how in the presence of shear, even a purely irrotational forcing amplifies the field to equipartition. At the same time, we confirm how this forcing alone does not lead to vorticity nor magnetic field growth, and this picture does not change in the presence of rotation or baroclinicity up to $256^3$ meshpoints.

arXiv comment14 pages, 8 figures, submitted for publication, comments welcome

References in corpus (38)

• Astrophysical magnetic fields and nonlinear dynamo theory
• Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing
• A New Model of the Galactic Magnetic Field
• Mach Number Dependence of Turbulent Magnetic Field Amplification: Solenoidal versus Compressive Flows
• Dynamo theories
• The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained
• Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
• Electromotive Force and Large-Scale Magnetic Dynamo in a Turbulent Flow with a Mean Shear
• Direct simulations of a supernova-driven galactic dynamo
• The Distribution of Pressures in a Supernova-Driven Interstellar Medium. I. Magnetized Medium
• Simulations of magnetized multiphase galactic disk regulated by supernovae explosions
• The supernova-regulated ISM. I. The multi-phase structure
• Mach number dependence of the onset of dynamo action
• The supernova-regulated ISM. II. The mean magnetic field
• Vorticity, Shocks and Magnetic Fields in Subsonic, ICM-like Turbulence
• The mean electromotive force due to turbulence of a conducting fluid in the presence of mean flow
• Turbulence from localized random expansion waves
• Galactic Dynamos
• Turbulent dynamo in the two-phase interstellar medium
• Numerical experiments on dynamo action in sheared and rotating turbulence
• The small-scale dynamo: Breaking universality at high Mach numbers
• Generation of Large-Scale Vorticity in a Homogeneous Turbulence with a Mean Velocity Shear
• Vorticity production through rotation, shear and baroclinicity
• Small-Scale Dynamo in Supernova-Driven Interstellar Turbulence
• Numerical study of large-scale vorticity generation in shear-flow turbulence
• The alpha effect in rotating convection with sinusoidal shear
• Efficient highly-subsonic turbulent dynamo and growth of primordial magnetic fields
• The supernova-regulated ISM. III. Generation of vorticity, helicity and mean flows
• The small-scale dynamo in a multiphase supernova-driven medium
• The role of the turbulence driving mode for the Initial Mass Function
• A dynamo amplifies the magnetic field of a Milky-Way-like galaxy
• A Vorticity-Magnetic Field Dynamo Instability
• Numerical studies of dynamo action in a turbulent shear flow - I
• Magnetic fields in massive spirals: The role of feedback and initial conditions
• Witnessing History: Rates and Detectability of Naked-Eye Milky-Way Supernovae
• On large-scale dynamos with stable stratification and the application to stellar radiative zones
• Magnetized stars with differential rotation and a differential toroidal field
• A closer look at supernovae as seeds for galactic magnetization

Cited by in corpus (4)

• Magnetic field amplification during a turbulent collapse
• Vorticity and magnetic dynamo from subsonic expansion waves II: Dependence on magnetic Prandtl number, forcing scale, cooling time
• Magnetically assisted vorticity production in decaying acoustic turbulence
• Large-Scale Dynamos Driven by Shear-Flow-Induced Jets