Toroidal Magnetic Component Research Articles

Magnetic field evolution in magnetar crusts through three-dimensional simulations

Current models of magnetars require extremely strong magnetic fields to explain their observed quiescent and bursting emission, implying that the field strength within the star's outer crust is orders of magnitude larger than the dipole component inferred from spin-down measurements. This presents a serious challenge to theories of magnetic field generation in a proto-neutron star. Here, we present detailed modeling of the evolution of the magnetic field in the crust of a neutron star through 3D simulations. We find that, in the plausible scenario of equipartition of energy between global-scale poloidal and toroidal magnetic components, magnetic instabilities transfer energy to nonaxisymmetric, kilometer-sized magnetic features, in which the local field strength can greatly exceed that of the global-scale field. These intense small-scale magnetic features can induce high-energy bursts through local crust yielding, and the localized enhancement of Ohmic heating can power the star's persistent emission. Thus, the observed diversity in magnetar behavior can be explained with mixed poloidal-toroidal fields of comparable energies.

Magnetic field topology ofτScorpii

The early B-type star tau Sco exhibits an unusually complex, relatively weak surface magnetic field. Its topology was previously studied with the Zeeman Doppler imaging (ZDI) modelling of high-resolution circular polarisation (Stokes V) observations. Here we assess the robustness of the Stokes V ZDI reconstruction of the magnetic field geometry of tau Sco and explore the consequences of using different parameterisations of the surface magnetic maps. We succeeded in reproducing previously published magnetic field maps of tau Sco using both general harmonic expansion and a direct, pixel-based representation of the magnetic field. These maps suggest that the field topology of tau Sco is comprised of comparable contributions of the poloidal and toroidal magnetic components. At the same time, we also found that available Stokes V observations can be successfully fitted employing restricted harmonic expansions, by either neglecting the toroidal field altogether or linking the radial and horizontal components of the poloidal field as required by the widely used potential field extrapolation technique. These alternative modelling approaches lead to a stronger and topologically more complex surface field structure. The field distributions recovered with different ZDI options differ significantly, yielding indistinguishable Stokes V profiles but different linear polarisation (Stokes Q and U) signatures. Our investigation underscores the well-known problem of non-uniqueness of the Stokes V ZDI inversions. For the magnetic stars with properties similar to tau Sco (relatively complex field, slow rotation) the outcome of magnetic reconstruction depends sensitively on the adopted field parameterisation. Stokes Q and U spectropolarimetric observations represent the only way of breaking the degeneracy of surface magnetic field models.

Comparison of 8-kW CCTT IM and Discrete Inductor Interleaved Boost Converter for Renewable Energy Applications

This paper presents a comparison of two magnetic component topologies for use in a high-power high-current dc-dc boost preregulator for renewable applications. The industry-standard two-phase (2L) interleaved dc-dc boost converter consisting of two discrete toroid magnetic components is considered as the baseline design. A 3C92 CCTT-core split-winding integrated magnetic (CCTT IM) is developed and compared for similar conditions. The topologies are compared for the same worst case phase-current ripple conditions. First, the baseline industry-standard 2L design is presented, which consists of a toroidal magnetic component along with stranded copper conductors, which are used to reduce the effects of ac copper loss. The CCTT IM component is designed for the same worst case phase-current ripple as this allows for a size saving with respect to the baseline design. The CCTT IM boxed volume is investigated as the number of turns is varied, but for a like-for-like comparison, the final CCTT IM design has the same number of turns and copper cross-sectional area as the 2L baseline design. A 2D finite element analysis (FEA) is used in order to validate and optimize the designs. The 8-kW experimental results are presented that indicate that the CCTT IM option allows for an approximate reduction of 50% in both magnetic mass and boxed volume with respect to the 2L toroid inductors. Critically, this size saving does not come at the expense of reduced efficiency, and the CCTT IM exhibits greater efficiency than the 2L baseline design. The IM does require additional input capacitance compared to the 2L design. The overall $LC$ filter of the IM design, comprising the boost magnetics and input capacitance, is reduced by approximately 20% in volume and 44% in mass compared to the 2L design.

Effect of the toroidal magnetic field on the runaway instability of relativistic tori

Runaway instability operates in fluid tori around black holes. It affects systems close to the critical (cusp overflowing) configuration. The runaway effect depends on the radial profile l(R) of the angular momentum distribution of the fluid, on the dimension-less spin a of the central black hole, and other factors, such as self-gravity. Previously it was demonstrated that, for the power-law dependence of the radial angular momentum profile, non-magnetized tori always become runaway stable for a sufficiently high positive value of q. Here we discuss the role of runaway instability within a framework of an axially symmetric model of perfect fluid endowed with a purely toroidal magnetic field. The gradual accretion of material over the cusp transfers the mass and angular momentum into the black hole, thereby changing the intrinsic parameters of the Kerr metric. We studied the effect of the ratio of gas to magnetic pressure and other parameters of the model on the evolution of critical configurations that are just on the verge of cusp overflow. We show that the toroidal magnetic component inside an accretion torus does not change the frequency of its oscillations significantly. We identify these oscillations as the radial epicyclic mode. These weak effects can trigger the runaway instability even in situations when the purely hydrodynamical regime of the torus is stable. On the other hand, in most cases the stable configuration remains unaffected, and the initial deviations gradually decay after several orbital periods. We show examples of the torus evolution depending on the initial magnetization beta, the slope q, and the spin a. The toroidal magnetic field plays a more important role in the early phases of the accretion process until the perturbed configuration finds a new equilibrium or disappears because of the runaway instability.

ON THE MODE OF DYNAMO ACTION IN A GLOBAL LARGE-EDDY SIMULATION OF SOLAR CONVECTION

In this paper, we examine the mode of dynamo action in the implicit large-eddy magnetohydrodynamical simulation of solar convection reported upon in Ghizaru et al. Motivated by the presence of a strong and well-defined large-scale axisymmetric magnetic component undergoing regular polarity reversals, we define the fluctuating component of the magnetic field as the difference between the total field and its zonal average. The subsequent analysis follows the physical logic and mathematical formulation of mean-field electrodynamics, whereby a turbulent electromotive force (EMF) is computed by the suitable averaging of cross-correlations between fluctuating flow and field components and expressed in terms of the mean field via a linear truncated tensorial expansion. We use singular value decomposition to perform a linear least-squares fit of the temporal variation of the EMF to that of the large-scale magnetic component, which yields the components of the full α-tensor. Its antisymmetric component, describing general turbulent pumping, is also extracted. The α-tensor so calculated reproduces a number of features already identified in local, Cartesian simulations of magnetohydrodynamical rotating convection, including an αϕϕ component positive in the northern solar hemisphere, peaking at high latitudes, and reversing sign near the bottom of the convection zone; downward turbulent pumping throughout the convecting layer; and significant equatorward turbulent pumping at mid latitudes, and poleward at high latitudes in subsurface layers. We also find that the EMF contributes significantly to the regeneration of the large-scale toroidal magnetic component, which from the point of view of mean-field dynamo models would imply that the simulation operates as an α2Ω dynamo. We find little significant evidence of α-quenching by the large-scale magnetic field. The amplitude of the magnetic cycle appears instead to be regulated primarily by a magnetically driven reduction of the differential rotation.

MAGNETIC CYCLES IN GLOBAL LARGE-EDDY SIMULATIONS OF SOLAR CONVECTION

We report on a global magnetohydrodynamical simulation of the solar convection zone, which succeeds in generating a large-scale axisymmetric magnetic component, antisymmetric about the equatorial plane and undergoing regular polarity reversals on decadal timescales. We focus on a specific simulation run covering 255 years, during which 8 polarity reversals are observed, with a mean period of 30 years. Time-latitude slices of the zonally averaged toroidal magnetic component at the base of the convecting envelope show a well-organized toroidal flux system building up in each solar hemisphere, peaking at mid-latitudes and migrating toward the equator in the course of each cycle, in remarkable agreement with inferences based on the sunspot butterfly diagram. The simulation also produces a large-scale dipole moment, varying in phase with the internal toroidal component, suggesting that the simulation may be operating as what is known in mean-field theory as an αΩ dynamo.

A polarity reversal in the large-scale magnetic field of the rapidly rotating sun HD 190771

Aims. We investigate the long-term evolution of the large-scale photospheric magnetic field geometry of the solar-type star HD 190771. With fundamental parameters very close to those of the Sun except for a shorter rotation period of 8.8 d, HD 190771 provides us with a first insight into the specific impact of the rotation rate in the dynamo generation of magnetic fields in 1 $M_\odot$ stars. Methods. We use circularly polarized, high-resolution spectra obtained with the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute cross-correlation line profiles with high signal-to-noise ratio to detect polarized Zeeman signatures. From three phase-resolved data sets collected during the summers of 2007, 2008, and 2009, we model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler imaging and follow its temporal evolution. Results. The comparison of the magnetic maps shows that a polarity reversal of the axisymmetric component of the large-scale magnetic field occurred between 2007 and 2008, this evolution being observed in both the poloidal and toroidal magnetic components. Between 2008 and 2009, another type of global evolution occured, characterized by a sharp decrease of the fraction of magnetic energy stored in the toroidal component. These changes were not accompanied by significant evolution in the total photospheric magnetic energy. Using our spectra to perform radial velocity measurements, we also detect a very low-mass stellar companion to HD 190771.

Gravitational radiation from collapsing magnetized dust. II. Polar parity perturbation

Using gauge-invariant perturbation theory, we study the effects of stellar magnetic fields on polar gravitational waves emitted during the collapse of homogeneous dust. We found that the emitted energy in gravitational waves depends strongly on the initial stellar radius as well as on the ratio between the poloidal and toroidal magnetic components. The polar gravitational-wave output of such a collapse can easily be up to a few orders of magnitude larger than what we get from the nonmagnetized collapse. The changes due to the existence of a magnetic field could be helpful in extracting some information about inner magnetic profiles of progenitors from the detection of the gravitational waves radiated during black hole formation, which results from the stellar collapse.

The Effect of Neutrino Radiation on Magnetorotational Instability in Proto–Neutron Stars

Neutrino radiation takes a major role in the momentum, heat, and lepton transports in proto-neutron stars (PNSs). These diffusive processes affect the growth of the magnetorotational instability (MRI) in PNSs. We perform a local linear analysis for the axisymmetric and nonaxisymmetric MRI including the effects of neutrino transport and ohmic dissipation. We find that the MRI can grow even in the multidiffusive situations that are realized in neutrino-loaded PNSs. When the toroidal magnetic component dominates over the poloidal one, nonaxisymmetric MRI modes grow much faster than axisymmetric modes. These results suggest the importance of the nonaxisymmetric MRI in PNSs. Thus, understanding the three-dimensional nonlinear evolution of the MRI is necessary for revealing the explosion mechanism of core-collapse supernovae.

Three‐dimensional Magnetohydrodynamic Simulations of Radiatively Cooling, Pulsed Jets

We here investigate, by means of fully three-dimensional smoothed particle magnetohydrodynamic numerical simulations, the effects of different initial magnetic field configurations on the evolution of overdense, radiatively cooling, pulsed jets using the following different initial magnetic field topologies: (1) longitudinal, (2) helical geometry permeating both the jet and the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as different values of the magnetic field strength (? 0.1-? or B 260-0 ?G). The presence of a helical or toroidal field tends to affect the global characteristics of the fluid more than a longitudinal field. However, the relative differences that have been previously detected in two-dimensional simulations involving distinct magnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head, inhibiting its fragmentation in the early evolution of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex morphology at the jet head (as required by the observations of Herbig-Haro [HH] jets) even in the MHD jet models with helical or toroidal configurations. The detailed structure and emission properties of the internal working surfaces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of ?) improves the jet collimation and amplifies the density (by factors up to 1.4 and 4) and the H? intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and ? 1-0.1 relative to a nonmagnetic jet. As a consequence, the corresponding I[S II]/IH? ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to nonmagnetic calculations by about the same amounts, although the intensity estimates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical magnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional jet simulations with initial toroidal magnetic fields is found in the three-dimensional flows or even in the ? 0.1 case. The implications of our results for HH jets are briefly discussed.

Solar spin-down with internal magnetic fields

view Abstract Citations (15) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Solar Spin-down with Internal Magnetic Fields Charbonneau, P. ; MacGregor, K. B. Abstract We investigate the rotational evolution of a solar-type star containing a large-scale poloidal magnetic field in its radiative core, in response to the torque applied to it by magnetically coupled wind. Our model takes into account both the generation of a toroidal magnetic component via shearing of the existing poloidal component by differential rotation, as well as the back-reaction on the differential rotation due to Lorentz forces associated with the toroidal field. Our computations demonstrate the existence of classes of large-scale poloidal magnetic fields allowing rapid spin-down of the surface layers shortly after the arrival on the zero-age main sequence, while producing weak internal differential rotation in the radiative core by the solar age. This indicates that the constraints brought about by rotational evolution of solar analogs in young clusters and by helioseismology are not incompatible with the existence of large-scale magnetic fields in stellar radiative interiors. The present surface solar rotation rate is also shown to be a poor indicator of the strength and geometry of hypothetical poloidal magnetic fields pervading the solar radiative interior. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/186545 Bibcode: 1992ApJ...397L..63C Keywords: Late Stars; Spin Dynamics; Stellar Evolution; Stellar Magnetic Fields; Stellar Rotation; Magnetic Field Configurations; Stellar Interiors; Stellar Models; Stellar Winds; Astrophysics; STARS: INTERIORS; STARS: MAGNETIC FIELDS; STARS: ROTATION full text sources ADS | Related Materials (1) Erratum: 1993ApJ...403L..87C

Angular momentum transport in magnetized stellar radiative zones. I - Numerical solutions to the core spin-up model problem

The present paper investigates the time evolution of the angular momentum and induced toroidal magnetic field distribution in an initially nonrotating radiative stellar envelope containing a large-scale poloidal magnetic field, following the impulsive spin-up of the underlying core. A large set of numerical calculations pertaining to monopolar, dipolar, and quadrupolar magnetic configurations, with and without density gradients across the envelope, as well as a set of solutions for which the poloidal field is only partially anchored on the core is presented. It is demonstrated that in moderate to high Reynolds-number systems, any global magnetic dissipation time scale constructed using length scales of order of the stellar radius greatly overestimates the true dissipation time scale of the toroidal magnetic component.