▪ Abstract We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in disks halos spiral galaxies. Among other topics, we consider enhancement global fields interarm regions, arms, representations as superpositions azimuthal modes, emphasizing a number unresolved questions. It is argued that turbulent hydromagnetic dynamo some kind an inverse cascade energy gives most plausible explanation for regular galactic fields. Primordial...

view Abstract Citations (724) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamo-generated Turbulence and Large-Scale Magnetic Fields in a Keplerian Shear Flow Brandenburg, Axel ; Nordlund, Ake Stein, Robert F. Torkelsson, Ulf The nonlinear evolution of magnetized shear flows is simulated local, three-dimensional model, including the effects compressibility stratification. Supersonic are initially generated by Balbus-Hawley magnetic...

A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which essentially the prototype α2 dynamo mean field theory, produces strong action an additional large-scale on scale box (at wavenumber k = 1; at 5). This nearly force free and exceeds equipartition value. As magnetic Reynolds number Rm increases, saturation strength growth rate increase. However, time it takes to build up from its final superequipartition value increases number....

Arguments for and against the widely accepted picture of a solar dynamo being seated in tachocline are reviewed alternative ideas concerning dynamos operating bulk convection zone, or perhaps even near-surface shear layer, discussed. Based on angular velocities magnetic tracers it is argued that observations compatible with distributed may be strongly shaped by layer. Direct simulations action slab turbulence presented to discuss filling factor tilt angles bipolar regions such model.

The decay of a turbulent magnetic field is slower with helicity than without. Furthermore, the correlation length grows faster for helical non-helical field. Both and laws involve conserved quantities: mean density Hosking integral. Using direct numerical simulations in triply periodic domain, we show quantitatively that fractionally case energy are approximately given by maximum values purely cases. time switchover from one to other law can be obtained on dimensional grounds...

Nonhelical hydromagnetic forced turbulence is investigated using large scale simulations on up to 256 processors and 1024(3) mesh points. The magnetic Prandtl number varied between 1/8 30, although in most cases it unity. When the Reynolds based inverse forcing wave number, critical value for dynamo action shown be around 35 of For small numbers we find increase with decreasing number. Kazantsev k(3/2) spectrum energy confirmed kinematic regime, i.e., when nonlinear effects are still...

We further explore nondimensional relationships between the magnetic dynamo cycle period Pcyc, rotational Prot, activity level (as observed in Ca II HK), and other stellar properties by expanding sample studied first paper this series. do adding photometric cycles seen active stars secondaries of CV systems selectively less certain from Mount Wilson HK survey; evolved stars, long-term trends secondary Pcyc are also considered. confirm that most with age t ≳ 0.1 Gyr occupy two roughly...

We investigate hydromagnetic turbulence of primordial magnetic fields using magnetohydrodynamics (MHD) in an expanding universe. present the basic, covariant MHD equations, find solutions for waves early universe, and equations numerically random two spatial dimensions. formation structures at larger scales as time goes on. In three dimensions we use a cascade (shell) model, that has been rather successful study certain aspects hydrodynamic turbulence. Using such model after ${\cal O}(10^9)$...

We use three-dimensional simulations to study compressible convection in a rotating frame with magnetic fields and overshoot into surrounding stable layers. The, initially weak, field is amplified maintained by dynamo action becomes organized flux tubes that are wrapped around vortex tubes. also observe buoyancy which causes upward flows the cores of extended downdraughts. An analysis angles between various vector shows there tendency for be parallel or antiparallel vorticity vector,...

view Abstract Citations (229) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamo Action in Stratified Convection with Overshoot Nordlund, Ake ; Brandenburg, Axel Jennings, Richard L. Rieutord, Michel Ruokolainen, Juha Stein, Robert F. Tuominen, Ilkka Results are presented from direct simulations of turbulent compressible hydromagnetic convection above a stable overshoot layer. Spontaneous dynamo action occurs followed by saturation, most...

Magnetohydrodynamic simulations of fully convective, rotating spheres with volume heating near the center and cooling at surface are presented. The dynamo-generated magnetic field saturates equipartition strength surface. In interior, is dominated by small-scale structures, but outside sphere, global scale. Azimuthal averages reveal a large-scale smaller amplitude also inside star. internal angular velocity shows some tendency to be constant along cylinders "antisolar" (fastest poles slowest...

We report on simulations of turbulent, rotating, stratified, magnetohydrodynamic convection in spherical wedge geometry. An initially small-scale, random, weak-amplitude magnetic field is amplified by several orders magnitude the course simulation to form oscillatory large-scale fields saturated state dynamo. The differential rotation solar-like (fast equator), but neither coherent meridional poleward circulation nor near-surface shear layer develop these runs. In addition a branch activity...

The turbulent magnetic diffusivity tensor is determined in the presence of rotation or shear. question addressed whether dynamo action from shear-current effect can explain large-scale field generation found simulations with For this purpose a set evolution equations for response to imposed test fields solved and mean motions calculated momentum continuity equations. corresponding results electromotive force are used calculate transport coefficients. diagonal components be very close...

In the presence of magnetic helicity, inverse transfer from small to large scales is well known in magnetohydrodynamic (MHD) turbulence and has applications astrophysics, cosmology, fusion plasmas. Using high resolution direct numerical simulations magnetically dominated self-similarly decaying MHD turbulence, we report a similar even absence helicity. We compute for first time spectral energy rates show that this about half as strong with but both cases gain at results velocity interacting...

We consider the evolution of primordial magnetic fields generated during cosmological, electroweak, or QCD phase transitions. assume that field generation can be described as an injection energy to cosmological plasma at a given scale determined by moment generation. A high Reynolds number ensures strong coupling between and fluid motions. The subsequent is governed decaying hydromagnetic turbulence. Both our numerical simulations phenomenological description allow us recover ``universal''...

We perform direct numerical simulations of magnetohydrodynamic turbulence in the early universe and numerically compute resulting stochastic background gravitational waves relic magnetic fields. These do not make simplifying assumptions earlier analytic work. If is assumed to have an energy-carrying scale that about a hundredth Hubble radius at time generation, as expected first-order phase transition, peak wave power will be mHz frequency range for signal produced electroweak scale. The...

We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts the latitudinal and longitudinal extents a star. Here we consider set runs for which density stratification is varied, keeping Reynolds Coriolis numbers at similar values. In case weak stratification, find quasi-steady dynamo solutions moderate rotation oscillatory ones with poleward migration activity belts more rapid rotation. For stronger growth rate tends to...

We perform numerical simulations of decaying hydrodynamic and magnetohydrodynamic turbulence. classify our time-dependent solutions by their evolutionary tracks in parametric plots between instantaneous scaling exponents. find distinct classes evolving along specific trajectories toward points on a line self-similar solutions. These are determined the underlying physics governing individual cases, while infrared slope initial conditions plays only limited role. In helical case, even for...

Kepler data from G-, K-, and M-type stars are used to study conditions that lead superflares with energies above 1034 erg. From the 117,661 included, 380 show a total of 1690 such events. We whether parameters, like effective temperature or rotation rate, have any effect on superflare occurrence rate energy. With increasing we observe decrease in which is analogous previous findings dynamo activity temperature. For slowly rotating stars, find quadratic increase mean up critical point, after...

We present new simulations of decaying hydromagnetic turbulence for a relativistic equation state relevant to the early universe. compare helical and nonhelical cases either with kinetically or magnetically dominated initial fields. Both kinetic magnetic helicities lead maximally fields after some time, but different temporal decay laws. are universe, although no mechanisms have yet been identified that produce helicity strengths comparable big bang nucleosynthesis limit at scales Hubble...

Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and anti-solar (AS) otherwise. Hydrodynamic simulations show a transition from SL AS as influence of on reduced, but opposite occurs at different point parameter space. system bistable, i.e., profiles can both be stable. We study effect dynamo-generated magnetic field large-scale flows, particularly possibility bistable...