On the mutual interaction between rotation and magnetic fields for axisymmetric bodies

1979 ◽  
Vol 368 (1732) ◽  
pp. 75-97 ◽  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Rotation Rate ◽  
Rotation Axis ◽  
Magnetic Torque ◽  
Rotation Rates ◽  
Driving Torque ◽  
Stable Rotation

As a simple example of the mutual interaction between magnetic fields and material motions, the rotation of an electrically conducting cylinder of solid material in a transverse magnetic field has been investigated. An applied driving torque produces the rotation, which is opposed by friction and the induced magnetic torque. It is well known that when the field is transverse to the rotation axis the magnetic torque rises from zero as the rotation rate Ω is increased, reaches a maximum and tends to zero as Ω → ∞, and the magnetic flux is expelled. We may consider B 0 (the applied magnetic field strength) and Ω 0 (the rotation rate at which the drive is just balanced by friction alone) as control parameters of the system. For sufficiently strong driving torques, the equilibrium surface Ω ( Ω 0 , B 0 ) develops a fold and consists of two branches - ‘ fast friction-dominated ’ and ‘ slow magnetically dominated ’ stable rotation rates. These solutions embrace an unstable intermediate equilibrium, and the system exhibits hysteresis depending on the manner in which the fold is approached. A ‘potential’ function can be introduced in terms of which the equilibria and stability can be analysed, and this potential function indicates that the equilibrium Ω -surfaces display the characteristics of the cusp catastrophe of Thom. One consequence of this folded structure is the existence of a forbidden band of rotation rates for a given driving torque irrespective of magnetic field strength. Similar properties can be shown for spheres, and we speculate that the general features - fold, upper and lower stable branches, forbidden band of stable rotation rates - are generic to all axisymmetric solid bodies and shells rotating about their axes of symmetry in the presence of a magnetic field with a transverse component. These features are absent if the magnetic field is aligned with the rotation axis. The hysteresis should be observable in the laboratory and experimentally verifiable.

scholarly journals roAp stars

1995 ◽  
Vol 10 ◽  
pp. 338-340
Author(s):  
D. Kurtz ◽  
P. Martinez
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Rotation Axis ◽  
Zeeman Effect ◽  
Spectral Lines ◽  
Effective Magnetic Field ◽  
Surface Magnetic Field ◽  
Ap Stars

Among the A stars there is a subclass of peculiar stars, the Ap stars, which show strongly enhanced spectral lines of the Fe peak, rare earth and lanthanide elements. These stars have global surface magnetic fields several orders of magnitude larger than that of the Sun, 0.3 to 30 kGauss is the measured range. For stars with the strongest magnetic fields, the spectral lines are split by the Zeeman Effect and the surface magnetic field strength can be measured. Generally, though, the magnetic fields are not strong enough for the magnetic splitting to exceed other sources of line broadening. In these cases residual polarization differences between the red and blue wings of the spectral lines give a measure of the effective magnetic field strength - the integral of the longitudinal component of the global magnetic field over the visible hemisphere, weighted by limb-darkening. In the Ap stars the effective magnetic field strengths vary with rotation. This is well understood in terms of the oblique rotator model in which the magnetic axis is oblique to the rotation axis, so that the magnetic field is seen from varying aspect with rotation.

Turbulence changes from magnetic fields in a stationary plasma

2010 ◽  
Vol 77 (4) ◽  
pp. 537-545 ◽  
Author(s):  
A. B. ALEXANDER ◽  
C. T. RAYNOR ◽  
D. L. WIGGINS ◽  
M. K. ROBINSON ◽  
C. C. AKPOVO ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Axial Magnetic Field ◽  
Turbulent Energy ◽  
Dominant Mode ◽  
Dc Glow Discharge ◽  
Turbulent Fluctuations ◽  
Stationary Plasma

AbstractWhen the krypton plasma in a DC glow discharge tube is exposed to an axial magnetic field, the turbulent energy and the characteristic dominant mode in the turbulent fluctuations are systematically and unexpectedly reduced with increasing magnetic field strength. When the index measuring the rate of transfer of energy through fluctuation scales is monitored, a lambda-like dependence on turbulent energy is routinely observed in all magnetic fields. From this, a critical turbulent energy is identified, which also decreases with increasing magnetic field strength.

scholarly journals Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?

2020 ◽  
Vol 495 (4) ◽  
pp. 3795-3806 ◽  
Author(s):  
James Wurster ◽  
Benjamin T Lewis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Low Mass Stars ◽  
Rotation Rates ◽  
Ideal Mhd ◽  
Low Mass ◽  
Dominant Process ◽  
Ideal Magnetohydrodynamics ◽  
Disc Formation

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M⊙ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.

Magnetic Fields and Star Formation for the ANS Sample of Galaxies

1990 ◽  
Vol 140 ◽  
pp. 233-234
Author(s):  
J. Stryczynski
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Field Data ◽  
Spiral Galaxies ◽  
Magnetic Field Data ◽  
Uv Range

From the literature we collected radio and magnetic field data for the ANS spiral galaxies. We suggest that the groups of objects, as revealed in the UV range, do not differ in magnetic field strength, although statistics of the sample are very poor.

Planetary Magnetic Fields and Dynamos

2019 ◽  
Author(s):  
Ulrich R. Christensen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Thermal Evolution ◽  
Rotation Axis ◽  
Space Environment ◽  
Conducting Layer ◽  
Iron Core ◽  
Complex Flow ◽  
Dynamo Theory

Since 1973 space missions carrying vector magnetometers have shown that most, but not all, solar system planets have a global magnetic field of internal origin. They have also revealed a surprising diversity in terms of field strength and morphology. While Jupiter’s field, like that of Earth, is dominated by a dipole moderately tilted relative to the planet’s spin axis, the fields of Uranus and Neptune are multipole-dominated, whereas those of Saturn and Mercury are highly symmetric relative to the rotation axis. Planetary magnetism originates from a dynamo process, which requires a fluid and electrically conducting region in the interior with sufficiently rapid and complex flow. The magnetic fields are of interest for three reasons: (i) they provide ground truth for dynamo theory, (ii) the magnetic field controls how the planet interacts with its space environment, for example, the solar wind, and (iii) the existence or nonexistence and the properties of the field enable us to draw inferences on the constitution, dynamics, and thermal evolution of the planet’s interior. Numerical simulations of the geodynamo, in which convective flow in a rapidly rotating spherical shell representing the outer liquid iron core of the Earth leads to induction of electric currents, have successfully reproduced many observed properties of the geomagnetic field. They have also provided guidelines on the factors controlling magnetic field strength and morphology. For numerical reasons the simulations must employ viscosities far greater than those inside planets and it is debatable whether they capture the correct physics of planetary dynamo processes. Nonetheless, such models have been adapted to test concepts for explaining magnetic field properties of other planets. For example, they show that a stable stratified conducting layer above the dynamo region is a plausible cause for the strongly axisymmetric magnetic fields of Mercury or Saturn.

Stabilization of magnetic mirror machine using rotating magnetic field

2018 ◽  
Vol 84 (5) ◽  
Author(s):  
O. Seemann ◽  
I. Be’ery ◽  
A. Fisher
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Collisional Plasma ◽  
Rotating Magnetic Field ◽  
Magnetic Mirror ◽  
Low Density ◽  
Mirror Machine

An increase in symmetry is observed for a low density non-collisional plasma, in a simple magnetic mirror machine, due to the application of external oscillating magnetic fields of 1.5 MHz frequency. The increase in symmetry is attributed to an increase in stability of the flute mode and is dependent on the field’s polarization and trap magnetic field strength.

scholarly journals Quantifying the Benefit of a Dedicated “Magnetoskeleton” in Bacterial Magnetotaxis by Live-Cell Motility Tracking and Soft Agar Swimming Assay

2019 ◽  
Vol 86 (3) ◽  
Author(s):  
Daniel Pfeiffer ◽  
Dirk Schüler
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Strength ◽  
Cell Motility ◽  
Magnetic Field Strength ◽  
Geomagnetic Field ◽  
Soft Agar ◽  
Content Type ◽  
Magnetospirillum Gryphiswaldense ◽  
Weak Magnetic Fields

ABSTRACT The alphaproteobacterium Magnetospirillum gryphiswaldense has the intriguing ability to navigate within magnetic fields, a behavior named magnetotaxis, governed by the formation of magnetosomes, intracellular membrane-enveloped crystals of magnetite. Magnetosomes are aligned in chains along the cell’s motility axis by a dedicated multipart cytoskeleton (“magnetoskeleton”); however, precise estimates of its significance for magnetotaxis have not been reported. Here, we estimated the alignment of strains deficient in various magnetoskeletal constituents by live-cell motility tracking within defined magnetic fields ranging from 50 μT (reflecting the geomagnetic field) up to 400 μT. Motility tracking revealed that ΔmamY and ΔmamK strains (which assemble mispositioned and fragmented chains, respectively) are partially impaired in magnetotaxis, with approximately equal contributions of both proteins. This impairment was reflected by a required magnetic field strength of 200 μT to achieve a similar degree of alignment as for the wild-type strain in a 50-μT magnetic field. In contrast, the ΔmamJ strain, which predominantly forms clusters of magnetosomes, was only weakly aligned under any of the tested field conditions and could barely be distinguished from a nonmagnetic mutant. Most findings were corroborated by a soft agar swimming assay to analyze magnetotaxis based on the degree of distortion of swim halos formed in magnetic fields. Motility tracking further revealed that swimming speeds of M. gryphiswaldense are highest within the field strength equaling the geomagnetic field. In conclusion, magnetic properties and intracellular positioning of magnetosomes by a dedicated magnetoskeleton are required and optimized for bacterial magnetotaxis and most efficient locomotion within the geomagnetic field. IMPORTANCE In Magnetospirillum gryphiswaldense, magnetosomes are aligned in quasi-linear chains in a helical cell by a complex cytoskeletal network, including the actin-like MamK and adapter MamJ for magnetosome chain concatenation and segregation and MamY to position magnetosome chains along the shortest cellular axis of motility. Magnetosome chain positioning is assumed to be required for efficient magnetic navigation; however, the significance and contribution of all key constituents have not been quantified within defined and weak magnetic fields reflecting the geomagnetic field. Employing two different motility-based methods to consider the flagellum-mediated propulsion of cells, we depict individual benefits of all magnetoskeletal constituents for magnetotaxis. Whereas lack of mamJ resulted almost in an inability to align cells in weak magnetic fields, an approximately 4-fold-increased magnetic field strength was required to compensate for the loss of mamK or mamY. In summary, the magnetoskeleton and optimal positioning of magnetosome chains are required for efficient magnetotaxis.

A portable apparatus for measuring the magnetic field strength in an electromagnetic wave

1931 ◽  
Vol 27 (3) ◽  
pp. 481-489
Author(s):  
L. G. Vedy ◽  
A. F. Wilkins
Keyword(s):  
Magnetic Field ◽  
Electromagnetic Wave ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Special Means ◽  
Portable Apparatus ◽  
The Magnetic Field ◽  
Field Strengths

A portable apparatus is described which is capable of measuring directly, by means of a loop aerial, the magnetic field in an electromagnetic wave. Accurate measurements are possible of magnetic fields corresponding to field strengths of 0·2 millivolts per metre. Special means of providing small known calibrating E. M. F. S are described. The apparatus can be used to measure signals over the range 6 microvolts to 300 millivolts. Used in conjunction with a small portable vertical aerial, field strengths down to 2 microvolts per metre can be measured.

scholarly journals Magnetic fields in spiral galaxies

2012 ◽  
Vol 10 (H16) ◽  
pp. 399-399 ◽  
Author(s):  
Marita Krause
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Field Strength ◽  
Radio Emission ◽  
Magnetic Field Strength ◽  
Thin Disk ◽  
Total Magnetic Field ◽  
Galactic Wind ◽  
Plane Parallel

The magnetic field structure in edge-on galaxies observed so far shows a plane-parallel magnetic field component in the disk of the galaxy and an X-shaped field in its halo. The plane-parallel field is thought to be the projected axisymmetric (ASS) disk field as observed in face-on galaxies. Some galaxies addionionally exhibit strong vertical magnetic fields in the halo right above and below the central region of the disk. The mean-field dynamo theory in the disk cannot explain these observed fields without the action of a wind, which also probably plays an important role to keep the vertical scale heights constant in galaxies of different Hubble types and star formation activities, as has been observed in the radio continuum: At λ6 cm the vertical scale heights of the thin disk and the thick disk/halo in a sample of five edge-on galaxies are similar with a mean value of 300 ± 50 pc for the thin disk and 1.8 ± 0.2 kpc for the thick disk (a table and references are given in Krause 2011) with our sample including the brightest halo observed so far, NGC 253, with strong star formation, as well as one of the weakest halos, NGC 4565, with weak star formation. If synchrotron emission is the dominant loss process of the relativistic electrons the outer shape of the radio emission should be dumbbell-like as has been observed in several edge-on galaxies like e.g. NGC 253 (Heesen et al. 2009) and NGC 4565. As the synchrotron lifetime tsyn at a single frequency is proportional to the total magnetic field strength Bt−1.5, a cosmic ray bulk speed (velocity of a galactic wind) can be defined as vCR = hCR/tsyn = 2 hz/tsyn, where hCR and hz are the scale heights of the cosmic rays and the observed radio emission at this freqnency. Similar observed radio scale heights imply a self regulation mechanism between the galactic wind velocity, the total magnetic field strength and the star formation rate SFR in the disk: vCR∝ Bt1.5 ∝ SFR≈ 0.5 (Niklas & Beck 1997).

Sign in / Sign up

Export Citation Format

Share Document