LARGE-SCALE COSMOLOGICAL MAGNETIC FIELDS AND MAGNETIC HELICITY

2005 ◽  
Vol 14 (11) ◽  
pp. 1839-1854 ◽  
Author(s):  
V. B. SEMIKOZ ◽  
D. D. SOKOLOFF
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Early Universe ◽  
Galaxy Formation ◽  
Large Scale ◽  
Magnetic Helicity ◽  
Observational Estimate ◽  
Field Production

Role of cosmological magnetic field and cosmological magnetic helicity for astrophysics is considered. We discuss possible mechanisms for cosmological magnetic field production in the early universe as well as upper observational estimate for such field. The general conclusion is that a substantial cosmological field with a non-vanishing magnetic helicity can be generated in the early universe and survive up to the epoch of galaxy formation.

scholarly journals Magnetism in the Early Universe

2018 ◽  
Vol 14 (A30) ◽  
pp. 295-298
Author(s):  
Tina Kahniashvili ◽  
Axel Brandenburg ◽  
Arthur Kosowsky ◽  
Sayan Mandal ◽  
Alberto Roper Pol
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Gravitational Waves ◽  
Early Universe ◽  
Large Scale ◽  
Direct Detection ◽  
Correlation Lengths ◽  
Cosmological Scenario ◽  
Expansion Of The Universe ◽  
The Universe

AbstractBlazar observations point toward the possible presence of magnetic fields over intergalactic scales of the order of up to ∼1 Mpc, with strengths of at least ∼10−16 G. Understanding the origin of these large-scale magnetic fields is a challenge for modern astrophysics. Here we discuss the cosmological scenario, focussing on the following questions: (i) How and when was this magnetic field generated? (ii) How does it evolve during the expansion of the universe? (iii) Are the amplitude and statistical properties of this field such that they can explain the strengths and correlation lengths of observed magnetic fields? We also discuss the possibility of observing primordial turbulence through direct detection of stochastic gravitational waves in the mHz range accessible to LISA.

scholarly journals Magnetic fields in the early Universe

2008 ◽  
Vol 4 (S259) ◽  
pp. 529-538 ◽  
Author(s):  
Eduardo Battaner ◽  
Estrella Florido
Keyword(s):  
Phase Transitions ◽  
Magnetic Fields ◽  
Early Universe ◽  
Galaxy Formation ◽  
Faraday Rotation ◽  
Large Scale ◽  
Large Scale Structure ◽  
Anisotropy Spectra ◽  
The Universe ◽  
Negligible Influence

AbstractThere is increasing evidence that intense magnetic fields exist at large redshifts. They could arise after galaxy formation or in very early processes, such as inflation or cosmological phase transitions, or both. Early co-moving magnetic strengths in the range 1-10 nG could be present at recombination. The possibilities to detect them in future CMB experiments are discussed, mainly considering their impact in the anisotropy spectra as a result of Faraday rotation and Alfven waves. Magnetic fields this magnitude could also have a non-negligible influence in determining the filamentary large scale structure of the Universe.

scholarly journals Magnetic fields in astrophysical objects

2008 ◽  
Vol 366 (1884) ◽  
pp. 4453-4464 ◽  
Author(s):  
L.J Silvers
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Accretion Discs ◽  
Weak Magnetic Fields ◽  
Large Scale Magnetic Field ◽  
Astrophysical Objects ◽  
Recent Developments ◽  
The Magnetic Field

Magnetic fields are known to reside in many astrophysical objects and are now believed to be crucially important for the creation of phenomena on a wide variety of scales. However, the role of the magnetic field in the bodies that we observe has not always been clear. In certain situations, the importance of a magnetic field has been overlooked on the grounds that the large-scale magnetic field was believed to be too weak to play an important role in the dynamics. In this article I discuss some of the recent developments concerning magnetic fields in stars, planets and accretion discs. I choose to emphasize some of the situations where it has been suggested that weak magnetic fields may play a more significant role than previously thought. At the end of the article, I list some of the questions to be answered in the future.

scholarly journals Solar Magnetoconvection

1990 ◽  
Vol 138 ◽  
pp. 191-211
Author(s):  
Å. Nordlund ◽  
R. F. Stein
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Time Evolution ◽  
Large Scale ◽  
Stratified Medium ◽  
Small Scale ◽  
Surface Magnetic Field ◽  
The Hierarchical Structure ◽  
Surface Manifestation

As a prelude to discussing the interaction of magnetic fields with convection, we first review some general properties of convection in a stratified medium. Granulation, which is the surface manifestation of the major energy carrying convection scales, is a shallow phenomenon. Below the surface, the topology changes to one of filamentary cool downdrafts, immersed in a gently ascending isentropic background. The granular downflows merge into more widely separated downdrafts, on scales of mesogranulation and super-granulation.The local topology and time evolution of the small scale, kilo Gauss, network and facular magnetic field elements are controlled by convection on the scale of granulation. The topology and time evolution of larger scale magnetic field concentrations are controlled by the hierarchical structure of the horizontal components of the large scale velocity field. In sunspots, the small scale magnetic field structure determines the energy balance, the systematic flows and the waves. Below the surface, the small scale structure of the magnetic field may change drastically, with little observable effect at the surface. We discuss results of some recent numerical simulations of sunspot magnetic fields, and some mechanisms that may be relevant in determining the topology of the sub-surface magnetic field. Finally, we discuss the role of active region magnetic fields in the global solar dynamo.

The role of magnetic fields in the early stages of star formation

2018 ◽  
Vol 14 (A30) ◽  
pp. 113-114
Author(s):  
Maud Galametz ◽  
Anaëlle Maury ◽  
Valeska Valdivia
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Magnetic Fields ◽  
Large Scale ◽  
Polarized Emission ◽  
Order Of Magnitude ◽  
Low Mass ◽  
Geometrical Effects ◽  
The Magnetic Field

AbstractMagnetic fields are believed to redistribute part of the angular momentum during the collapse and could explain the order-of-magnitude difference between the angular momentum observed in protostellar envelopes and that of a typical main sequence star. The Class 0 phase is the main accretion phase during which most of the final stellar material is collected on the central embryo. To study the structure of the magnetic fields on 50-2000 au scales during that key stage, we acquired SMA polarization observations (870μm) of 12 low-mass Class 0 protostars. In spite of their low luminosity, we detect dust polarized emission in all of them. We observe depolarization effects toward high-density regions potentially due to variations in alignment efficiency or in the dust itself or geometrical effects. By comparing the misalignment between the magnetic field and the outflow orientation, we show that the B is either aligned or perpendicular to the outflow direction. We observe a coincidence between the misalignment and the presence of large perpendicular velocity gradients and fragmentation in the protostar (Galametz et al. 2018). Our team is using MHD simulations combined with the radiative transfer code POLARIS to produce synthetic maps of the polarized emission. This work is helping us understand how the magnetic field varies from the large-scale to the small-scales, quantify beam-averaging biases and study the variations of the polarization angles as a function of wavelength or the assumption made on the grain alignment (see poster by Valdivia).

scholarly journals Mean Field Dynamo Saturation: Toward Understanding Conflicting Results

2002 ◽  
Vol 12 ◽  
pp. 736-738
Author(s):  
Eric G. Blackman ◽  
George B. Field
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Magnetic Energy ◽  
Mean Field ◽  
Magnetic Helicity ◽  
Boundary Terms ◽  
Helicity Conservation ◽  
The Galaxy ◽  
Mean Field Dynamo

AbstractMean field dynamos may explain the origin of large scale magnetic fields of galaxies, but controversy arises over the extent of dynamo quenching by the growing field. Here we explain how apparently conflicting results may be mutually consistent, by showing the role of magnetic helicity conservation and boundary terms usually neglected. We estimate the associated magnetic energy flowing out of the Galaxy but emphasize that the mechanism of field escape needs to be addressed.

scholarly journals An 84-μG Magnetic Field in a Galaxy at Z=0.692?

2008 ◽  
Vol 4 (S254) ◽  
pp. 95-96
Author(s):  
Arthur M. Wolfe ◽  
Regina A. Jorgenson ◽  
Timothy Robishaw ◽  
Carl Heiles ◽  
Jason X. Prochaska
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Large Scale ◽  
Dynamo Model ◽  
Magnetic Field Generation ◽  
Interstellar Gas ◽  
Splitting Technique ◽  
Average Value ◽  
The Magnetic Field

AbstractThe magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars (Beck 2005). The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, i.e., Faraday rotation, yield an average value B ≈ 3 μG (Han et al. 2006). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars (Kronberg et al. 2008) suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain.Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z =0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy (Heiles et al. 2004). This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past, rather than stronger (Parker 1970).The full text of this paper was published in Nature (Wolfe et al. 2008).

scholarly journals The magnetic helicity density patterns from non-axisymmetric solar dynamo

2021 ◽  
Vol 87 (1) ◽  
Author(s):  
Valery V. Pipin
Keyword(s):  
Magnetic Field ◽  
Differential Rotation ◽  
Conservation Law ◽  
Large Scale ◽  
Solar Dynamo ◽  
Magnetic Helicity ◽  
Dynamo Model ◽  
Density Distributions ◽  
Large Scale Magnetic Field ◽  
Helicity Conservation

We study the helicity density patterns which can result from the emerging bipolar regions. Using the relevant dynamo model and the magnetic helicity conservation law we find that the helicity density patterns around the bipolar regions depend on the configuration of the ambient large-scale magnetic field, and in general they show a quadrupole distribution. The position of this pattern relative to the equator can depend on the tilt of the bipolar region. We compute the time–latitude diagrams of the helicity density evolution. The longitudinally averaged effect of the bipolar regions shows two bands of sign for the density distributions in each hemisphere. Similar helicity density patterns are provided by the helicity density flux from the emerging bipolar regions subjected to surface differential rotation.

scholarly journals Modeling of magneto-conductivity of bismuth selenide: a topological insulator

2021 ◽  
Vol 3 (4) ◽  
Author(s):  
Yogesh Kumar ◽  
Rabia Sultana ◽  
Prince Sharma ◽  
V. P. S. Awana
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Single Crystal ◽  
Single Crystals ◽  
X Ray Diffraction ◽  
Field Range ◽  
X Ray ◽  
Bulk Carriers ◽  
Different Temperatures

AbstractWe report the magneto-conductivity analysis of Bi2Se3 single crystal at different temperatures in a magnetic field range of ± 14 T. The single crystals are grown by the self-flux method and characterized through X-ray diffraction, Scanning Electron Microscopy, and Raman Spectroscopy. The single crystals show magnetoresistance (MR%) of around 380% at a magnetic field of 14 T and a temperature of 5 K. The Hikami–Larkin–Nagaoka (HLN) equation has been used to fit the magneto-conductivity (MC) data. However, the HLN fitted curve deviates at higher magnetic fields above 1 T, suggesting that the role of surface-driven conductivity suppresses with an increasing magnetic field. This article proposes a speculative model comprising of surface-driven HLN and added quantum diffusive and bulk carriers-driven classical terms. The model successfully explains the MC of the Bi2Se3 single crystal at various temperatures (5–200 K) and applied magnetic fields (up to 14 T).

scholarly journals No-z Model for Magnetic Fields of Different Astrophysical Objects and Stability of the Solutions

Data ◽  
2021 ◽  
Vol 6 (1) ◽  
pp. 4
Author(s):  
Evgeny Mikhailov ◽  
Daniela Boneva ◽  
Maria Pashentseva
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Point Of View ◽  
Accretion Discs ◽  
Wide Range ◽  
Astrophysical Objects ◽  
Alpha Effect ◽  
The Stability ◽  
The Magnetic Field

A wide range of astrophysical objects, such as the Sun, galaxies, stars, planets, accretion discs etc., have large-scale magnetic fields. Their generation is often based on the dynamo mechanism, which is connected with joint action of the alpha-effect and differential rotation. They compete with the turbulent diffusion. If the dynamo is intensive enough, the magnetic field grows, else it decays. The magnetic field evolution is described by Steenbeck—Krause—Raedler equations, which are quite difficult to be solved. So, for different objects, specific two-dimensional models are used. As for thin discs (this shape corresponds to galaxies and accretion discs), usually, no-z approximation is used. Some of the partial derivatives are changed by the algebraic expressions, and the solenoidality condition is taken into account as well. The field generation is restricted by the equipartition value and saturates if the field becomes comparable with it. From the point of view of mathematical physics, they can be characterized as stable points of the equations. The field can come to these values monotonously or have oscillations. It depends on the type of the stability of these points, whether it is a node or focus. Here, we study the stability of such points and give examples for astrophysical applications.

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