scholarly journals Kinematic dynamos in spheroidal geometries

2017 ◽  
Vol 473 (2206) ◽  
pp. 20170432 ◽  
Author(s):  
D. J. Ivers
Keyword(s):  
Anisotropic Diffusion ◽  
Solution Method ◽  
Spherical Geometry ◽  
Strong Magnetic Fields ◽  
Free Condition ◽  
Kinematic Dynamo ◽  
Aspect Ratios ◽  
Oblate Spheroids ◽  
Harmonic Techniques ◽  
The Magnetic Field

The kinematic dynamo problem is solved numerically for a spheroidal conducting fluid of possibly large aspect ratio with an insulating exterior. The solution method uses solenoidal representations of the magnetic field and the velocity by spheroidal toroidal and poloidal fields in a non-orthogonal coordinate system. Scaling of coordinates and fields to a spherical geometry leads to a modified form of the kinematic dynamo problem with a geometric anisotropic diffusion and an anisotropic current-free condition in the exterior, which is solved explicitly. The scaling allows the use of well-developed spherical harmonic techniques in angle. Dynamo solutions are found for three axisymmetric flows in oblate spheroids with semi-axis ratios 1≤ a / c ≤25. For larger aspect ratios strong magnetic fields may occur in any region of the spheroid, depending on the flow, but the external fields for all three flows are weak and concentrated near the axis or periphery of the spheroid.

Kinematic dynamo problem in a linear velocity field

1984 ◽  
Vol 144 ◽  
pp. 1-11 ◽  
Author(s):  
Ya. B. Zel'Dovich ◽  
A. A. Ruzmaikin ◽  
S. A. Molchanov ◽  
D. D. Sokoloff
Keyword(s):  
Magnetic Field ◽  
Spatial Distribution ◽  
Velocity Field ◽  
Random Matrix ◽  
Magnetic Energy ◽  
Linear Velocity ◽  
Finite Conductivity ◽  
Kinematic Dynamo ◽  
Linear Velocity Field ◽  
The Magnetic Field

A magnetic field is shown to be asymptotically (t → ∞) decaying in a flow of finite conductivity with v = Cr, where C = Cζ(t) is a random matrix. The decay is exponential, and its rate does not depend on the conductivity. However, the magnetic energy increases exponentially owing to growth of the domain occupied by the field. The spatial distribution of the magnetic field is a set of thin ropes and (or) layers.

scholarly journals On the stability of a general magnetic field topology in stellar radiative zones

2019 ◽  
Vol 82 ◽  
pp. 365-371
Author(s):  
K. Augustson ◽  
S. Mathis ◽  
A. Strugarek
Keyword(s):  
Magnetic Field ◽  
Global Change ◽  
Magnetic Fields ◽  
Potential Energy ◽  
Massive Stars ◽  
Spherical Geometry ◽  
Stable Region ◽  
Magnetic Field Topology ◽  
The Stability ◽  
The Magnetic Field

This paper provides a brief overview of the formation of stellar fossil magnetic fields and what potential instabilities may occur given certain configurations of the magnetic field. One such instability is the purely magnetic Tayler instability, which can occur for poloidal, toroidal, and mixed poloidal-toroidal axisymmetric magnetic field configurations. However, most of the magnetic field configurations observed at the surface of massive stars are non-axisymmetric. Thus, extending earlier studies in spherical geometry, we introduce a formulation for the global change in the potential energy contained in a convectively-stable region for both axisymmetric and non-axisymmetric magnetic fields.

scholarly journals Photons in a cold axion background and strong magnetic fields: Polarimetric consequences

2015 ◽  
Vol 30 (17) ◽  
pp. 1550099 ◽  
Author(s):  
Domènec Espriu ◽  
Albert Renau
Keyword(s):  
Magnetic Field ◽  
Dark Matter ◽  
Galactic Halo ◽  
Local Density ◽  
Polarization Plane ◽  
Strong Magnetic Fields ◽  
Two Photon ◽  
Main Effect ◽  
Optical Table ◽  
The Magnetic Field

In this work, we analyze the propagation of photons in an environment where a strong magnetic field (perpendicular to the photon momenta) coexists with an oscillating cold axion background with the characteristics expected from dark matter in the galactic halo. Qualitatively, the main effect of the combined background is to produce a three-way mixing among the two photon polarizations and the axion. It is interesting to note that in spite of the extremely weak interaction of photons with the cold axion background, its effects compete with those coming from the magnetic field in some regions of the parameter space. We determine (with one plausible simplification) the proper frequencies and eigenvectors as well as the corresponding photon ellipticity and induced rotation of the polarization plane that depend both on the magnetic field and the local density of axions. We also comment on the possibility that some of the predicted effects could be measured in optical table-top experiments.

Study on Synthesis of the High Aspect Ratios Nesquehonite Whiskers

2011 ◽  
Vol 239-242 ◽  
pp. 1118-1122 ◽  
Author(s):  
Ping Ke Yan ◽  
Bin Wang ◽  
Yu Juan Gao
Keyword(s):  
Aqueous Solution ◽  
Reaction Time ◽  
Low Temperature ◽  
Growth Mechanism ◽  
Reaction Temperature ◽  
Solution Method ◽  
Temperature Reaction ◽  
Aspect Ratios ◽  
Whisker Length ◽  
Aqueous Solution Method

In this paper, nesquehonite whiskers were synthesized by low-temperature aqueous solution method, and the impacts of reaction temperature, reaction time and surfactant dosage and other factors on the maximum whisker length and high aspect ratios of nesquehonite whiskers were also investigated. Results showed that under the conditions that the reaction temperature was 40 – 50 °C the reaction time was 50 – 60min and the amount of surfactant dosage was 1% (by mass), high aspect ratios nesquehonite whisker products can be synthesized. On this basis, growth mechanism of the nesquehonite whiskers was discussed.

scholarly journals Oscillatory convection in strong magnetic fields and origin of active regions

1968 ◽  
Vol 35 ◽  
pp. 127-130 ◽  
Author(s):  
S. I. Syrovatsky ◽  
Y. D. Zhugzhda
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Active Regions ◽  
Oscillatory Convection ◽  
Strong Magnetic Fields ◽  
Various Boundary Conditions ◽  
Convective Cells ◽  
The Magnetic Field ◽  
Polytropic Atmosphere

The convection in a compressible inhomogeneous conducting fluid in the presence of a vertical uniform magnetic field has been studied. It is shown that a new mode of oscillatory convection occurs, which exists in arbitrarily strong magnetic fields. The convective cells are stretched along the magnetic field, their horizontal dimensions are determined by radiative cooling. Criteria for convective instability in a polytropic atmosphere are obtained for various boundary conditions in the case when the Alfvén velocity is higher compared with the velocity of sound.The role of oscillatory convection in the origin of sunspots and active regions is discussed.

scholarly journals Topology optimization for three-dimensional electromagnetic waves using an edge element-based finite-element method

2016 ◽  
Vol 472 (2189) ◽  
pp. 20150835 ◽  
Author(s):  
Yongbo Deng ◽  
Jan G. Korvink
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Topology Optimization ◽  
Electromagnetic Waves ◽  
Three Dimensional ◽  
Free Condition ◽  
Edge Element ◽  
Divergence Free ◽  
The Magnetic Field ◽  
Element Method

This paper develops a topology optimization procedure for three-dimensional electromagnetic waves with an edge element-based finite-element method. In contrast to the two-dimensional case, three-dimensional electromagnetic waves must include an additional divergence-free condition for the field variables. The edge element-based finite-element method is used to both discretize the wave equations and enforce the divergence-free condition. For wave propagation described in terms of the magnetic field in the widely used class of non-magnetic materials, the divergence-free condition is imposed on the magnetic field. This naturally leads to a nodal topology optimization method. When wave propagation is described using the electric field, the divergence-free condition must be imposed on the electric displacement. In this case, the material in the design domain is assumed to be piecewise homogeneous to impose the divergence-free condition on the electric field. This results in an element-wise topology optimization algorithm. The topology optimization problems are regularized using a Helmholtz filter and a threshold projection method and are analysed using a continuous adjoint method. In order to ensure the applicability of the filter in the element-wise topology optimization version, a regularization method is presented to project the nodal into an element-wise physical density variable.

scholarly journals The study of the specific resistance of bismuth crystals and its change in strong magnetic fields and some allied problems

1928 ◽  
Vol 119 (782) ◽  
pp. 358-443 ◽  
Keyword(s):  
Magnetic Field ◽  
Specific Resistance ◽  
Cleavage Plane ◽  
Time Lag ◽  
General View ◽  
Time Lags ◽  
Strong Magnetic Fields ◽  
First Moment ◽  
The Magnetic Field ◽  
A Current

It is well known that in a magnetic field bismuth shows a greater change of resistance than any other substance, and it is also known that in the case of a crystal this phenomenon varies very much with the orientation of the crystal. A great deal of literature exists on this subject. The general view of the phenomenon is that the increase of resistance is largest when the cleavage plane of the crystal is parallel to the magnetic field, and when the current is flowing perpendicular to it. It is also known that the resistance in a magnetic field increases very rapidly with decreasing temperature. A complication in all these phenomena arises through certain time lags. When a current is passed through bismuth placed in a magnetic field, the resistance at the first moment is large, and then gradually decreases to its final value. This time lag accounts for the fact, first discovered by Lenard, that bismuth has a larger resistance for alternating currents than for direct currents. This phenomenon also depends on the crystal state of the bismuth.

The Effect of a Magnetic Field on Individual Convective Cells

1974 ◽  
Vol 2 (5) ◽  
pp. 267-269
Author(s):  
J. O. Murphy
Keyword(s):  
Magnetic Field ◽  
Close Association ◽  
Cell Structure ◽  
Longitudinal Field ◽  
Strong Magnetic Fields ◽  
Convective Processes ◽  
Granulation Pattern ◽  
Convective Cells ◽  
The Magnetic Field

The darkness of sunspots has been attributed by many authors (Biermann 1941; Danielson 1961) to the inhibition of the normal solar convective processes by the presence of strong magnetic fields. Observations of the solar photospheric granulation pattern have also shown that a weak longitudinal field exists outside the activity regions. Although these observations have not revealed any close association between the magnetic field and individual granules, nor the exact reasons for the darker cell boundaries, it must be accepted that, overall, the role of the magnetic field must be such as to influence the cell structure and reduce the normal heat transfer by convection.

A "SELF-CONSISTENT SOLUTION" METHOD FOR DETERMINING THE ELECTRICAL CONDUCTIVITY IN THE SUBSURFACE REGIONS OF THE EARTH

1964 ◽  
Vol 1 (3) ◽  
pp. 206-210
Author(s):  
Tomiya Watanabe
Keyword(s):  
Electrical Conductivity ◽  
Boundary Conditions ◽  
Solution Method ◽  
Magnetic Method ◽  
Entire Surface ◽  
Conductivity Distribution ◽  
The Earth ◽  
Self Consistent ◽  
Consistent Solution ◽  
The Magnetic Field

The assumptions on which the so-called magneto-telluric method to determine the subsurface conductivity of the earth is based are examined and it is shown how the method can be revised to get rid of those assumptions which are not necessarily legitimate. The principle of this revised or generalized magneto-telluric method is that the magnetic and telluric field components which observation can provide over the entire surface of the earth are more than sufficient viewed as boundary conditions to determine the electromagnetic field inside the earth with a prescribed conductivity distribution and, therefore, the extra boundary conditions can be consistent with each other only by the correctly prescribed (or chosen) distribution of electrical conductivity. The purely magnetic method to determine the conductivity, which relies on the assumption that the magnetic field in space above the surface of the earth, is a potential field, is also revised to free it of the assumption which, does not hold true unconditionally.

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