scholarly journals Hall effect in protostellar disc formation and evolution

2020 ◽  
Vol 492 (3) ◽  
pp. 3375-3395 ◽  
Author(s):  
Bo Zhao ◽  
Paola Caselli ◽  
Zhi-Yun Li ◽  
Ruben Krasnopolsky ◽  
Hsien Shang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Large Population ◽  
Outer Part ◽  
Cosmic Ray ◽  
Rotating Disc ◽  
Ohmic Dissipation ◽  
Outward Diffusion ◽  
The Magnetic Field ◽  
Disc Formation

ABSTRACT The Hall effect is recently shown to be efficient in magnetized dense molecular cores and could lead to a bimodal formation of rotationally supported discs (RSDs) in the first core phase. However, how such Hall dominated systems evolve in the protostellar accretion phase remains unclear. We carry out 2D axisymmetric simulations including Hall effect and ohmic dissipation, with realistic magnetic diffusivities computed from our equilibrium chemical network. We find that Hall effect only becomes efficient when the large population of very small grains (VSGs: ≲100 Å) is removed from the standard Mathis–Rumpl–Nordsieck size distribution. With such an enhanced Hall effect, however, the bimodality of disc formation does not continue into the main accretion phase. The outer part of the initial ∼40 au disc formed in the anti-aligned configuration ($\boldsymbol {\Omega \cdot B}\lt 0$) flattens into a thin rotationally supported Hall current sheet as Hall effect moves the poloidal magnetic field radially inward relative to matter, leaving only the inner ≲10–20 au RSD. In the aligned configuration ($\boldsymbol {\Omega \cdot B}\gt 0$), disc formation is suppressed initially but a counter-rotating disc forms subsequently due to efficient azimuthal Hall drift. The counter-rotating disc first grows to ∼30 au as Hall effect moves the magnetic field radially outward, but only the inner ≲10 au RSD is long lived like in the anti-aligned case. Besides removing VSGs, cosmic ray ionization rate should be below a few 10−16 s−1 for Hall effect to be efficient in disc formation. We conclude that Hall effect produces small ≲10–20 au discs regardless of the polarity of the magnetic field, and that radially outward diffusion of magnetic fields remains crucial for disc formation and growth.

scholarly journals Magnetic Field Evolution in Neutron Star Crusts: Beyond the Hall Effect

Symmetry ◽  
2022 ◽  
Vol 14 (1) ◽  
pp. 130
Author(s):  
Konstantinos N. Gourgouliatos ◽  
Davide De Grandis ◽  
Andrei Igoshev
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Neutron Stars ◽  
Hall Effect ◽  
Thermal Emission ◽  
Ohmic Dissipation ◽  
Field Evolution ◽  
The Universe ◽  
The Magnetic Field ◽  
Maxwell Stresses

Neutron stars host the strongest magnetic fields that we know of in the Universe. Their magnetic fields are the main means of generating their radiation, either magnetospheric or through the crust. Moreover, the evolution of the magnetic field has been intimately related to explosive events of magnetars, which host strong magnetic fields, and their persistent thermal emission. The evolution of the magnetic field in the crusts of neutron stars has been described within the framework of the Hall effect and Ohmic dissipation. Yet, this description is limited by the fact that the Maxwell stresses exerted on the crusts of strongly magnetised neutron stars may lead to failure and temperature variations. In the former case, a failed crust does not completely fulfil the necessary conditions for the Hall effect. In the latter, the variations of temperature are strongly related to the magnetic field evolution. Finally, sharp gradients of the star’s temperature may activate battery terms and alter the magnetic field structure, especially in weakly magnetised neutron stars. In this review, we discuss the recent progress made on these effects. We argue that these phenomena are likely to provide novel insight into our understanding of neutron stars and their observable properties.

Kinematic dynamos and the Earth’s magnetic field

1973 ◽  
Vol 275 (1251) ◽  
pp. 425-461 ◽  
Keyword(s):  
Magnetic Field ◽  
Inner Core ◽  
Magnetic Energy ◽  
Liquid Core ◽  
Outer Part ◽  
Ohmic Dissipation ◽  
Tidal Dissipation ◽  
Magnetic Dipoles ◽  
Magnetic Energy Density ◽  
The Magnetic Field

The Bullard—Gellman formalism is applied to investigate the existence of convergent solutions for steady kinematic dynamos. It is found that the solutions for the Bullard—Gellman dynamo, as well as for Lilley’s modification of it, do not converge. Convergent solutions have been found for a class of spherical convective cells which would be stationary in a perfect fluid in the absence of rotation and of the magnetic field. By calibrating the theoretical magnetic dipole so as to fit the observed value at the Earth’s surface, one can find a dynamo in the above class which also matches the observed equatorial magnetic dipoles. There is a dynamo which has a rate of total ohmic dissipation of only 1.8 x 1016 erg s-1 for an assumed electrical conductivity of 3 x 10~6 e.m.u.'f This is one thousandth the rate of tidal dissipation, and one hundred thousandth the rate of heat outflow from the surface of the Earth. The required velocities are of the order of 10~3 cm s_1, and the average magnetic energy density is 4 erg cm-3. The internal structure of the magnetic field in this model shows a dynamo mechanism situated in the outer part of the liquid core and is thus insensitive to possible rigidity of the material in the * inner core.

scholarly journals The Origin and Dynamical Effects of the Magnetic Fields and Cosmic Rays in the Disk of the Galaxy

1970 ◽  
Vol 39 ◽  
pp. 168-183
Author(s):  
E. N. Parker
Keyword(s):  
Magnetic Field ◽  
Cosmic Rays ◽  
Magnetic Fields ◽  
Dynamical Behavior ◽  
Cosmic Ray ◽  
Interested Reader ◽  
Written Text ◽  
The Galaxy ◽  
Basic Ideas ◽  
The Magnetic Field

The topic of this presentation is the origin and dynamical behavior of the magnetic field and cosmic-ray gas in the disk of the Galaxy. In the space available I can do no more than mention the ideas that have been developed, with but little explanation and discussion. To make up for this inadequacy I have tried to give a complete list of references in the written text, so that the interested reader can pursue the points in depth (in particular see the review articles Parker, 1968a, 1969a, 1970). My purpose here is twofold, to outline for you the calculations and ideas that have developed thus far, and to indicate the uncertainties that remain. The basic ideas are sound, I think, but, when we come to the details, there are so many theoretical alternatives that need yet to be explored and so much that is not yet made clear by observations.

Magnetostrictive Self-Diagnosing Smart Bolts

2014 ◽  
Author(s):  
Vladislav Sevostianov
Keyword(s):  
Magnetic Field ◽  
Plastic Deformation ◽  
Magnetic Fields ◽  
Hall Effect ◽  
Stress Level ◽  
Experimental Validation ◽  
Rock Bolts ◽  
Three Point Bending ◽  
Bending Loading ◽  
The Magnetic Field

The paper presents the concept of self-diagnosing smart bolts and its experimental validation. In the present research such bolts are designed, built, and experimentally tested. As a key element of the design, wires of Galfenol (alloy of iron and gallium) are used. This material shows magnetostrictive properties, and, at the same time, is sufficiently ductile to follow typical deformation of rock bolts, and is economically affordable. Two types of Galfenol were used: Ga10Fe90 and Ga17Fe83. The wires have been installed in bolts using two designs — in a drilled central hole or in a cut along the side — and the bolts were tested for generation of the magnetic field under three-point bending loading. To measure the magnetic field in the process of deformation, a magnetometer that utilizes the GMR effect was designed, built, and compared with one utilizing the Hall effect. It is shown that (1) magnetic field generated by deformation of the smart bolts at the stress level of plastic deformation is sufficient to be noticed by the proposed magnetometer; however, the magnetometer using Hall effect is insufficient; (2) Ga10Fe90 produces higher magnetic fields than Ga17Fe83; (3) the magnetic field in plastically bended bolts is relatively stable with time.

scholarly journals The hall effect in liquid electrolytes

1913 ◽  
Vol 88 (606) ◽  
pp. 588-604 ◽  
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Electric Current ◽  
Metal Plate ◽  
Satisfactory Explanation ◽  
Thermo Electric ◽  
Field Independent ◽  
Set Up ◽  
The Magnetic Field ◽  
Thin Metal Plate

The distortion of the lines of flow of an electric current in a thin metal plate by the action of a magnetic field was discovered in 1879. Hall attributed this to the action of the magnetic field on the molecular currents in the metal film, which were deflected to one side or the other and accompanied by a corresponding twist of the equipotential lines. This explanation did not pass without criticism, and another theory of the effect found by Hall was published in 1884. In that paper the author seeks to explain the effect by assuming a combination of certain mechanical strains and Peltier effects, a thermo-electric current being set up between the strained and the unstrained portions. The effect of such strain was to produce a reverse effect in some metals, and these were precisely the metals for which the Hall effect was found to reverse. Aluminium was the only exception. In other respects, however, as shown by Hall in a later paper, Bidwell's theory did not stand the test of experiment, and the results lend no support to his theory, while they are in complete accordance withe the explanation that the molecular currents are disturbed by the action of the magnetic field. On the electron theory of metallic conduction, the mechanism of the Hall effect is more obvious, but at present no satisfactory explanation of the reversal found in some metals is known. Further experiments have made it clear that there is a real deflection of the elementary currents, due to the application of the magnetic field, independent of any effect due to strain.

Quantum Hall effects

Quantum 20/20 ◽  
2019 ◽  
pp. 303-322
Author(s):  
Ian R. Kenyon
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Energy Gap ◽  
Quantum Hall ◽  
Free Electrons ◽  
Electron Gases ◽  
Hall Conductance ◽  
Hall Effects ◽  
The Stability ◽  
The Magnetic Field

It is explained how plateaux are seen in the Hall conductance of two dimensional electron gases, at cryogenic temperatures, when the magnetic field is scanned from zero to ~10T. On a Hall plateau σ‎xy = ne 2/h, where n is integral, while the longitudinal conductance vanishes. This is the integral quantum Hall effect. Free electrons in such devices are shown to occupy quantized Landau levels, analogous to classical cyclotron orbits. The stability of the IQHE is shown to be associated with a mobility gap rather than an energy gap. The analysis showing the topological origin of the IQHE is reproduced. Next the fractional QHE is described: Laughlin’s explanation in terms of an IQHE of quasiparticles is presented. In the absence of any magnetic field, the quantum spin Hall effect is observed, and described here. Time reversal invariance and Kramer pairs are seen to be underlying requirements. It’s topological origin is outlined.

scholarly journals Estimation of Electrical Conductivity and Magnetization Parameter of Neutron Star Crusts and Applied to the High-Braking-Index Pulsar PSR J1640-4631

Universe ◽  
2020 ◽  
Vol 6 (5) ◽  
pp. 63
Author(s):  
Hui Wang ◽  
Zhi-Fu Gao ◽  
Huan-Yu Jia ◽  
Na Wang ◽  
Xiang-Dong Li
Keyword(s):  
Magnetic Field ◽  
Neutron Star ◽  
Neutron Stars ◽  
General Expression ◽  
Equations Of State ◽  
Rotation Period ◽  
Ohmic Dissipation ◽  
Hartree Fock ◽  
Induction Equation ◽  
The Magnetic Field

Young pulsars are thought to be highly magnetized neutron stars (NSs). The crustal magnetic field of a NS usually decays at different timescales in the forms of Hall drift and Ohmic dissipation. The magnetization parameter ω B τ is defined as the ratio of the Ohmic timescale τ O h m to the Hall drift timescale τ H a l l . During the first several million years, the inner temperature of the newly born neutron star cools from T = 10 9 K to T = 1.0 × 10 8 K, and the crustal conductivity increases by three orders of magnitude. In this work, we adopt a unified equations of state for cold non-accreting neutron stars with the Hartree–Fock–Bogoliubov method, developed by Pearson et al. (2018), and choose two fiducial dipole magnetic fields of B = 1.0 × 10 13 G and B = 1.0 × 10 14 G, four different temperatures, T, and two different impurity concentration parameters, Q, and then calculate the conductivity of the inner crust of NSs and give a general expression of magnetization parameter for young pulsars: ω B τ ≃ ( 1 − 50 ) B 0 / ( 10 13 G) by using numerical simulations. It was found when B ≤ 10 15 G, due to the quantum effects, the conductivity increases slightly with the increase in the magnetic field, the enhanced magnetic field has a small effect on the matter in the low-density regions of the crust, and almost has no influence the matter in the high-density regions. Then, we apply the general expression of the magnetization parameter to the high braking-index pulsar PSR J1640-4631. By combining the observed arrival time parameters of PSR J1640-4631 with the magnetic induction equation, we estimated the initial rotation period P 0 , the initial dipole magnetic field B 0 , the Ohm dissipation timescale τ O h m and Hall drift timescale τ H a l l . We model the magnetic field evolution and the braking-index evolution of the pulsar and compare the results with its observations. It is expected that the results of this paper can be applied to more young pulsars.

scholarly journals Accretion Disc Formation in Intermediate Polars

1996 ◽  
Vol 158 ◽  
pp. 161-164
Author(s):  
G. A. Wynn ◽  
A. R. King
Keyword(s):  
Magnetic Field ◽  
Accretion Disc ◽  
Local Magnetic Field ◽  
Magnetic Systems ◽  
X Ray ◽  
Accretion Flow ◽  
Intermediate Polars ◽  
X Ray Binaries ◽  
The Magnetic Field ◽  
Disc Formation

The large-scale accretion flow in the intermediate polars (IPs) is still a matter of vigorous debate. It is known that the magnetic field of the white dwarf (WD) controls the accretion flow close to the surface, channeling the plasma onto the polecaps and giving rise to X-ray emission modulated at the WD spin period (Pspin). After their discovery it was assumed that IPs were the WD analogues of the pulsing X-ray binaries, where a magnetic neutron star accretes from a disrupted accretion disc. However, a number of authors have pointed out that the criteria for disc formation in IPs are less certain than those for the X-ray binaries.The simplest possible criterion for disc formation in a binary is that the accretion flow should be able to orbit freely about the primary star (see Frank, King & Raine 1991 for a review). In non-magnetic systems this is merely the condition that the minimum approach distance of the free stream (Rmin) should exceed the radius of the primary. The situation in magnetic systems is more complex, as the magnetic field of the primary presents an obstacle to the infalling accretion stream. In many treatments of IPs it is assumed that the plasma stream is able to orbit freely about the WD until the ram pressure of the stream is of the same order as the magnetic pressureρv2~B2/8π, whereρis the stream density,vthe stream velocity andBthe local magnetic field strength. This condition fixes the magnetospheric radius,Rmag, inside which the magnetic field is assumed to thread the stream material and direct the accretion flow along the fieldlines.

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