scholarly journals An Analytic Study of the Reversal of Hartmann Flows by Rotating Magnetic Fields

2012 ◽  
Vol 2012 ◽  
pp. 1-15 ◽  
Author(s):  
Manuel Núñez
Keyword(s):  
Magnetic Field ◽  
Rotation Speed ◽  
Rotating Magnetic Field ◽  
Rotating Magnetic Fields ◽  
Boundary Wall ◽  
Stationary Version ◽  
Field Lines ◽  
The Magnetic Field ◽  
Distance To The Boundary ◽  
Transversal Magnetic

The effects of a background uniform rotating magnetic field acting in a conducting fluid with a parallel flow are studied analytically. The stationary version with a transversal magnetic field is well known as generating Hartmann boundary layers. The Lorentz force includes now one term depending on the rotation speed and the distance to the boundary wall. As one intuitively expects, the rotation of magnetic field lines pushes backwards or forwards the flow. One consequence is that near the wall the flow will eventually reverse its direction, provided the rate of rotation and/or the magnetic field are large enough. The configuration could also describe a fixed magnetic field and a rotating flow.

scholarly journals On the role of rotation in the outflows of the Crab pulsar

2015 ◽  
Vol 24 (06) ◽  
pp. 1550042
Author(s):  
Gudavadze Irakli ◽  
Osmanov Zaza ◽  
Rogava Andria
Keyword(s):  
Magnetic Field ◽  
Rotation Axis ◽  
Three Dimensional ◽  
Rotating Magnetic Field ◽  
Crab Pulsar ◽  
X Ray ◽  
Field Lines ◽  
The Magnetic Field ◽  
Component Parallel

In order to study constraints imposed on kinematics of the Crab pulsar's jet, we consider motion of particles along co-rotating field lines in the magnetosphere of the Crab pulsar. It is shown that particles following the co-rotating magnetic field lines may attain velocities close to observable values. In particular, we demonstrate that if the magnetic field lines are within the light cylinder (LC), the maximum value of the velocity component parallel to the rotation axis is limited by 0.5c. This result in the context of the X-ray observations performed by Chandra X-ray Observatory seems to be quite indicative and useful to estimate the density of field lines inside the jet. Considering the three-dimensional (3D) field lines crossing the LC, we found that for explaining the force-free regime of outflows the magnetic field lines must asymptotically tend to the Archimedes spiral configuration. It is also shown that the 3D case may explain the observed jet velocity for appropriately chosen parameters of magnetic field lines.

scholarly journals Magnetic nulls in interacting dipolar fields

2021 ◽  
Vol 87 (2) ◽  
Author(s):  
Todd Elder ◽  
Allen H. Boozer
Keyword(s):  
Magnetic Field ◽  
Current Density ◽  
Magnetic Flux Tube ◽  
Electron Inertia ◽  
Characteristic Spatial Scale ◽  
Field Lines ◽  
Dipolar Fields ◽  
Time Required ◽  
Magnetic Null ◽  
The Magnetic Field

The prominence of nulls in reconnection theory is due to the expected singular current density and the indeterminacy of field lines at a magnetic null. Electron inertia changes the implications of both features. Magnetic field lines are distinguishable only when their distance of closest approach exceeds a distance $\varDelta _d$ . Electron inertia ensures $\varDelta _d\gtrsim c/\omega _{pe}$ . The lines that lie within a magnetic flux tube of radius $\varDelta _d$ at the place where the field strength $B$ is strongest are fundamentally indistinguishable. If the tube, somewhere along its length, encloses a point where $B=0$ vanishes, then distinguishable lines come no closer to the null than $\approx (a^2c/\omega _{pe})^{1/3}$ , where $a$ is a characteristic spatial scale of the magnetic field. The behaviour of the magnetic field lines in the presence of nulls is studied for a dipole embedded in a spatially constant magnetic field. In addition to the implications of distinguishability, a constraint on the current density at a null is obtained, and the time required for thin current sheets to arise is derived.

Magnetic Field Spectroheliograms from the San Fernando Observatory

1971 ◽  
Vol 43 ◽  
pp. 329-339 ◽  
Author(s):  
Dale Vrabec
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spiral Structure ◽  
Longitudinal Component ◽  
Solar Telescope ◽  
Solar Magnetic Fields ◽  
San Fernando ◽  
Field Lines ◽  
The Magnetic Field ◽  
Field Network

Zeeman spectroheliograms of photospheric magnetic fields (longitudinal component) in the CaI 6102.7 Å line are being obtained with the new 61-cm vacuum solar telescope and spectroheliograph, using the Leighton technique. The structure of the magnetic field network appears identical to the bright photospheric network visible in the cores of many Fraunhofer lines and in CN spectroheliograms, with the exception that polarities are distinguished. This supports the evolving concept that solar magnetic fields outside of sunspots exist in small concentrations of essentially vertically oriented field, roughly clumped to form a network imbedded in the otherwise field-free photosphere. A timelapse spectroheliogram movie sequence spanning 6 hr revealed changes in the magnetic fields, including a systematic outward streaming of small magnetic knots of both polarities within annular areas surrounding several sunspots. The photospheric magnetic fields and a series of filtergrams taken at various wavelengths in the Hα profile starting in the far wing are intercompared in an effort to demonstrate that the dark strands of arch filament systems (AFS) and fibrils map magnetic field lines in the chromosphere. An example of an active region in which the magnetic fields assume a distinct spiral structure is presented.

scholarly journals Efficiency of thermal conduction in a magnetized circumgalactic medium

2021 ◽  
Vol 502 (1) ◽  
pp. 1263-1278
Author(s):  
Richard Kooij ◽  
Asger Grønnow ◽  
Filippo Fraternali
Keyword(s):  
Magnetic Field ◽  
Thermal Conduction ◽  
Large Temperature ◽  
Gas Condensation ◽  
Circumgalactic Medium ◽  
Field Lines ◽  
Gas Accretion ◽  
Cloud Evolution ◽  
The Magnetic Field ◽  
Cold Gas

ABSTRACT The large temperature difference between cold gas clouds around galaxies and the hot haloes that they are moving through suggests that thermal conduction could play an important role in the circumgalactic medium. However, thermal conduction in the presence of a magnetic field is highly anisotropic, being strongly suppressed in the direction perpendicular to the magnetic field lines. This is commonly modelled by using a simple prescription that assumes that thermal conduction is isotropic at a certain efficiency f < 1, but its precise value is largely unconstrained. We investigate the efficiency of thermal conduction by comparing the evolution of 3D hydrodynamical (HD) simulations of cold clouds moving through a hot medium, using artificially suppressed isotropic thermal conduction (with f), against 3D magnetohydrodynamical (MHD) simulations with (true) anisotropic thermal conduction. Our main diagnostic is the time evolution of the amount of cold gas in conditions representative of the lower (close to the disc) circumgalactic medium of a Milky-Way-like galaxy. We find that in almost every HD and MHD run, the amount of cold gas increases with time, indicating that hot gas condensation is an important phenomenon that can contribute to gas accretion on to galaxies. For the most realistic orientations of the magnetic field with respect to the cloud motion we find that f is in the range 0.03–0.15. Thermal conduction is thus always highly suppressed, but its effect on the cloud evolution is generally not negligible.

ORIENTATION DYNAMICS OF MAGNETORHEOLOGICAL FLUIDS SUBJECT TO ROTATING EXTERNAL FIELDS

2001 ◽  
Vol 15 (06n07) ◽  
pp. 758-766 ◽  
Author(s):  
SONIA MELLE ◽  
MIGUEL A. RUBIO ◽  
GERALD G. FULLER
Keyword(s):  
Magnetic Field ◽  
Light Scattering ◽  
Simple Model ◽  
Critical Frequency ◽  
Optical Methods ◽  
Phase Lag ◽  
Rotating Magnetic Fields ◽  
Mr Fluids ◽  
Small Angle Light Scattering ◽  
The Magnetic Field

The formation and orientation of field-induced structures in magnetorheological (MR) fluids subject to rotating magnetic fields have been studied using two optical methods: scattering dichroism and small angle light scattering (SALS). The SALS patterns show how these chain-like aggregates follow the magnetic field with the same frequency but with a retarded phase angle for all the frequencies measured. Using scattering dichroism two different behaviors for both, dichroism and phase lag, are found below or above a critical frequency. Experimental results have been reproduced by a simple model considering the torques balance on the chain-like aggregates.

scholarly journals The development of magnetic field line wander by plasma turbulence

2017 ◽  
Vol 83 (4) ◽  
Author(s):  
Gregory G. Howes ◽  
Sofiane Bourouaine
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Magnetic Field Line ◽  
Large Scale ◽  
Magnetic Energy ◽  
Plasma Turbulence ◽  
Alfven Wave ◽  
Astrophysical Plasmas ◽  
Field Lines ◽  
The Magnetic Field

Plasma turbulence occurs ubiquitously in space and astrophysical plasmas, mediating the nonlinear transfer of energy from large-scale electromagnetic fields and plasma flows to small scales at which the energy may be ultimately converted to plasma heat. But plasma turbulence also generically leads to a tangling of the magnetic field that threads through the plasma. The resulting wander of the magnetic field lines may significantly impact a number of important physical processes, including the propagation of cosmic rays and energetic particles, confinement in magnetic fusion devices and the fundamental processes of turbulence, magnetic reconnection and particle acceleration. The various potential impacts of magnetic field line wander are reviewed in detail, and a number of important theoretical considerations are identified that may influence the development and saturation of magnetic field line wander in astrophysical plasma turbulence. The results of nonlinear gyrokinetic simulations of kinetic Alfvén wave turbulence of sub-ion length scales are evaluated to understand the development and saturation of the turbulent magnetic energy spectrum and of the magnetic field line wander. It is found that turbulent space and astrophysical plasmas are generally expected to contain a stochastic magnetic field due to the tangling of the field by strong plasma turbulence. Future work will explore how the saturated magnetic field line wander varies as a function of the amplitude of the plasma turbulence and the ratio of the thermal to magnetic pressure, known as the plasma beta.

scholarly journals Mapping of steady-state electric fields and convective drifts in geomagnetic fields – Part 1: Elementary models

2016 ◽  
Vol 34 (1) ◽  
pp. 55-65 ◽  
Author(s):  
A. D. M. Walker ◽  
G. J. Sofko
Keyword(s):  
Magnetic Field ◽  
Steady State ◽  
Electric Field ◽  
Electric Fields ◽  
Magnetic Field Line ◽  
Geomagnetic Field ◽  
Geomagnetic Field Models ◽  
Spatial Derivatives ◽  
Field Lines ◽  
The Magnetic Field

Abstract. When studying magnetospheric convection, it is often necessary to map the steady-state electric field, measured at some point on a magnetic field line, to a magnetically conjugate point in the other hemisphere, or the equatorial plane, or at the position of a satellite. Such mapping is relatively easy in a dipole field although the appropriate formulae are not easily accessible. They are derived and reviewed here with some examples. It is not possible to derive such formulae in more realistic geomagnetic field models. A new method is described in this paper for accurate mapping of electric fields along field lines, which can be used for any field model in which the magnetic field and its spatial derivatives can be computed. From the spatial derivatives of the magnetic field three first order differential equations are derived for the components of the normalized element of separation of two closely spaced field lines. These can be integrated along with the magnetic field tracing equations and Faraday's law used to obtain the electric field as a function of distance measured along the magnetic field line. The method is tested in a simple model consisting of a dipole field plus a magnetotail model. The method is shown to be accurate, convenient, and suitable for use with more realistic geomagnetic field models.

scholarly journals Cusp-like plasma in high altitudes: a statistical study of the width and location of the cusp from Magion-4

2002 ◽  
Vol 20 (3) ◽  
pp. 311-320 ◽  
Author(s):  
J. Mĕrka ◽  
J. Šafránková ◽  
Z. Nĕmeček
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
High Altitudes ◽  
Data Set ◽  
Latitudinal Dependence ◽  
Merging Process ◽  
Low Altitude ◽  
Field Lines ◽  
The Magnetic Field ◽  
Dipole Tilt

Abstract. The width of the cusp region is an indicator of the strength of the merging process and the degree of opening of the magnetosphere. During three years, the Magion-4 satellite, as part of the Interball project, has collected a unique data set of cusp-like plasma observations in middle and high altitudes. For a comparison of high- and low-altitude cusp determination, we map our observations of cusp-like plasma along the magnetic field lines down to the Earth’s surface. We use the Tsyganenko and Stern 1996 model of the magnetospheric magnetic field for the mapping, taking actual solar wind and IMF parameters from the Wind observations. The footprint positions show substantial latitudinal dependence on the dipole tilt angle. We fit this dependence with a linear function and subtract this function from observed cusp position. This process allows us to study both statistical width and location of the inspected region as a function of the solar wind and IMF parameters. Our processing of the Magion-4 measurements shows that high-altitude regions occupied by the cusp-like plasma (cusp and cleft) are projected onto a much broader area (in magnetic local time as well as in a latitude) than that determined in low altitudes. The trends of the shift of the cusp position with changes in the IMF direction established by low-altitude observations have been confirmed.Key words. Magnetospheric physics (magnetopause, cusp and boundary layer; solar wind – magnetosphere interactions)

scholarly journals Magnetotaxis as a Means for Nanofabrication

2014 ◽  
Vol 23 (01n02) ◽  
pp. 1450008
Author(s):  
Isaac Macwan ◽  
Zihe Zhao ◽  
Omar Sobh ◽  
Jinnque Rho ◽  
Ausif Mahmood ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Geomagnetic Field ◽  
Semiconductor Manufacturing ◽  
Magnetic Field Intensity ◽  
Magnetotactic Bacteria ◽  
Semiconductor Substrate ◽  
Field Lines ◽  
A Chain ◽  
Magnetospirillum Magneticum ◽  
The Magnetic Field

Magnetotactic bacteria (MTB), discovered in early 1970s contain single-domain crystals of magnetite ( Fe 3 O 4) called magnetosomes that tend to form a chain like structure from the proximal to the distal pole along the long axis of the cell. The ability of these bacteria to sense the magnetic field for displacement, also called magnetotaxis, arises from the magnetic dipole moment of this chain of magnetosomes. In aquatic habitats, these organisms sense the geomagnetic field and traverse the oxic-anoxic interface for optimal oxygen concentration along the field lines. Here we report an elegant use of MTB where magnetotaxis of Magnetospirillum magneticum (classified as AMB-1) could be utilized for controlled navigation over a semiconductor substrate for selective deposition. We examined 50mm long coils made out of 18AWG and 20AWG copper conductors having diameters of 5mm, 10mm and 20mm for magnetic field intensity and heat generation. Based on the COMSOL simulations and experimental data, it is recognized that a compound semiconductor manufacturing technology involving bacterial carriers and carbon-based materials such as graphene and carbon nanotubes would be a desirable choice in the future.

An experimental investigation of current production by means of rotating magnetic fields

1981 ◽  
Vol 26 (3) ◽  
pp. 465-480 ◽  
Author(s):  
W. N. Hugrass ◽  
I. R. Jones ◽  
M. G. R. Phillips
Keyword(s):  
Magnetic Field ◽  
Experimental Investigation ◽  
Threshold Value ◽  
Rotating Magnetic Field ◽  
Electron Current ◽  
Rotating Magnetic Fields ◽  
Theta Pinch ◽  
Current Production ◽  
Made In ◽  
Rotating Field

An investigation of current production by means of a rotating magnetic field is made in an experiment in which the technique is used to generate a theta-pinch- like distribution of field and plasma. Detailed measurements are made of both the generated unidirectional azimuthal electron current and the penetration of the rotating field into the plasma. The experimental results support the theoretical prediction that a threshold value of the amplitude of the applied rotating field exists for setting the electrons into rotation.

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