• Title/Summary/Keyword: Magnetic fields

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Cosmic magnetic fields in the large-scale structure of the universe

  • Ryu, Dongsu
    • The Bulletin of The Korean Astronomical Society
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    • v.39 no.2
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    • pp.37-37
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    • 2014
  • Magnetic fields appear to be ubiquitous in astrophysical environments. The existence of magnetic fields in the large-scale structure of the universe has been established through observations of Faraday rotation and synchrotron emission, as well as through recent gamma-ray observations. Yet, the nature and origin of the magnetic fields remains controversial and largely unknown. In this talk, I briefly summarize recent developments in our understanding of the nature and origin of magnetic fields. I also describe a plausible scenario for the origin of the magnetic fields; seed fields were created in the early universe and subsequently amplified during the formation of the large-scale structure of the universe. I then discuss the prospect of observation of magnetic fields in the large-scale structure of the universe.

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Properties of transient horizontal magnetic fields and their implication to the origin of quiet-Sun magnetism

  • Ishikawa, Ryohko
    • The Bulletin of The Korean Astronomical Society
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    • v.37 no.1
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    • pp.84.1-84.1
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    • 2012
  • Recent spectropolarimetric observations with high spatial resolution and high polarization sensitivity have provided us with new insight to better understand the quiet-Sun magnetism. This talk is concerned with the ubiquitous transient horizontal magnetic fields in the quiet-Sun, as revealed by the Solar Optical Telescope (SOT) on board Hinode satellite. Exploiting the SOT data with careful treatment of photon noise, we reveal the enigmatic properties of these horizontal magnetic fields such as lifetime, size, position in terms of granular structure, occurrence rate, three-dimensional structure, total magnetic flux, field strength distribution, relationship with the meso- and super-granulations and so on. Based on these observational consequences, we conjecture that the local dynamo process, which takes place in a relatively shallow layer with the granular size, produces these transient horizontal magnetic fields and that these horizontal magnetic fields contribute to the considerable amount of quiet-Sun magnetic fields. We also estimate the magnetic energy flux carried by these horizontal magnetic fields based on the statistical data, and find that the total magnetic energy is comparable to the total chromospheric and coronal energy loss, implying their important role for the chromospheric heating and dynamism.

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OBTAINING BOUNDARY TANGENTIAL COMPONENTS OF POTENTIAL MAGNETIC FIELDS BY A VARIATIONAL METHOD

  • CHOE G. S.
    • Journal of The Korean Astronomical Society
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    • v.31 no.2
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    • pp.89-93
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    • 1998
  • An attempt is made to find the boundary tangential components of potential magnetic fields without constructing solutions in the entire domain. In our procedure, the magnetic energy is expressed as a functional of tangential and normal magnetic fields at the boundary and is minimized by the variational principle. This paper reports a preliminary study on two dimensional potential fields above a plane.

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On the Association Between Sub-photospheric Flows and Photospheric Magnetic Fields of Solar Active Regions

  • Maurya, Ram Ajor;Chae, Jong-Chul
    • The Bulletin of The Korean Astronomical Society
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    • v.37 no.1
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    • pp.86.2-86.2
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    • 2012
  • We present the study of association between sub-photospheric flow and photospheric magnetic fields of active regions respectively derived from the local helioseismology and observed magnetic fields. It is believed that the energetic transients, e.g., flares and CMES, are caused by changes in magnetic and velocity field topologies in solar atmosphere. These changes are essentially brought about by the magnetic fields that are rooted beneath the photosphere where they interact and get affected by sub-photospheric flows. Therefore, we expect the topology of sub-surface flows to be correlated with the observable topology of magnetic fields at the photosphere and higher layers. In order to examine the correlation, if any, we computed the near photospheric flows and photospheric magnetic fields using the Doppler velocity and magnetic fields observations, respectively, provided by the SDO/HMI. The high resolution Doppler observations from the HMI enabled us to compute the very high p-modes parameters which sample the sub-photosphere shallow near the photosphere. Furthermore, we compute the sub-photospheric flow topology parameters, e.g., vorticity, kinetic helicity, and photospheric magnetic field topology parameters, e.g., magnetic helicity, from the magnetic fields observations to compare their associations. We present the result of the analysis in the paper.

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Study for Exposure Limits of Magnetic Fields in the Transformer Substation (변전소 건설로 인한 자기장 노출범위설정에 관한 연구)

  • Jeon, In-Soo
    • Journal of Environmental Impact Assessment
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    • v.14 no.4
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    • pp.195-202
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    • 2005
  • The health risks from the magnetic fields exposure have been brought out difficulties in the construction of transformer substation. According to several epidemiologic studies and the relevant international organizations, magnetic fields should not exceed the exposure limits of 3mG for the people living near electric power lines. The rigid regulation of the exposure levels for the elementary school and residental areas has been established already in Switzerland and Italy. Since 1998, the environmental impact assessment system in Korea has been reviewed for power-frequency magnetic field by precautionary policies. In this study, the possible application of Prior Environmental Performance Review System for the transformer substation was reviewed from the points of the properties of the powerfrequency magnetic fields. The ranges and survey methods of the assessment for the transformer substation were proposed. The ranges of magnetic fields was between 300m to 500m for the 345kV transformer substation. It is necessary to develop further specific assessment methods for various high-voltage transformer substations.

LARGE SCALE MAGNETOGENESIS THROUGH RADIATION PRESSURE

  • LANGER MATHIEU;PUGET JEAN-LOUP;AGHANIM NABILA
    • Journal of The Korean Astronomical Society
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    • v.37 no.5
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    • pp.553-556
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    • 2004
  • We present a new model for the generation of magnetic fields on large scales occurring at the end of cosmological reionisation. The inhomogeneous radiation provided by luminous sources and the fluctuations in the matter density field are the major ingredients of the model. More specifically, differential radiation pressure acting on ions and electrons gives rise to electric currents which induce magnetic fields on large scales. We show that on protogalactic scales, this process is highly efficient, leading to magnetic field amplitudes of the order of $10^{-1l}$ Gauss. While remaining of negligible dynamical impact, those amplitudes are million times higher than those obtained in usual astrophysical magnetogenesis models. Finally, we derive the relation between the power spectrum of the generated field and the one of the matter density fluctuations. We show in particular that magnetic fields are preferably created on large (galactic or cluster) scales. Small scale magnetic fields are strongly disfavoured, which further makes the process we propose an ideal candidate to explain the origin of magnetic fields in large scale structures.

Generation of Solenoidal Modes in Turbulence Driven by Compressive Driving

  • Lim, Jeonghoon;Cho, Jungyeon;Yoon, Heesun
    • The Bulletin of The Korean Astronomical Society
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    • v.44 no.2
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    • pp.47.3-47.3
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    • 2019
  • In this talk, we present numerical simulations of driven hydrodynamic and magnetohydrodynamic (MHD) turbulence with weak/strong imposed magnetic fields. We mainly focus on turbulence driven compressively (∇ × f = 0). Our main goal is to examine how magnetic fields play a role in generating solenoidal modes in compressive turbulence. From our simulation analysis, we find that solenoidal energy densities in hydrodynamic and weak magnetic field cases are generated up to ~ 30% of total ones. On the other hand, in the case of strong magnetic fields, solenoidal energy densities are excited up to ~ 70%. To interpret the results, we further analyze vorticity (w = ∇ × u) equation and find that magnetic fields directly create solenoidal motions, and magnetic tension is most effective in this sense. In hydrodynamic simulations, however, we find that viscous dissipation provides vorticity seeds at the very early stage and they are amplified via stretching process. Lastly, in weak magnetic fields cases, we find that solenoidal motions are created by the effects of magnetic fields, viscosity, and stretching in conjunction.

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Magnetic fields in clusters of galaxies

  • Roh, Soonyoung;Ryu, Dongsu
    • The Bulletin of The Korean Astronomical Society
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    • v.43 no.1
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    • pp.35.2-35.2
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    • 2018
  • Magnetic fields in clusters of galaxies play a critical role in shaping up the intracluster medium. Their existence has been established through observations of synchrotron emission, especially from radio relics and halos, as well as observations of rotation measure. In the so-called Sausage relic, which is one of Mpc-size giant radio relics detected in the outskirts of merging clusters, for instance, the magnetic fields are believed to have a few ${\mu}G$ strength and a Mpc scale. The observed magnetic fields are conjectured to be produced by the process of small-scale turbulence dynamo. To investigate the dynamo origin, we simulate the development of turbulence and the follow-up amplification of magnetic fields in galaxy clusters using a three-dimensional magnetohydrodynamical(MHD) code. Turbulence is induced in highly stratified backgrounds expected in clusters, and driven sporadically mimicking major mergers. We here present preliminary results, aiming to answer whether the turbulence dynamo scenario can explain observed magnetic fields in clusters of galaxies.

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Measurement of Two Dimensional Magnetic Properties of Electrical Steel Sheets under Rotating Magnetic Fields (전기강판의 회전자계 하에서의 2차원 자계특성 측정)

  • Eum, Young-Hwan;Hong, Sun-Ki;Shin, Pan-Seok;Koh, Chang-Seop
    • The Transactions of the Korean Institute of Electrical Engineers B
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    • v.55 no.12
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    • pp.617-622
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    • 2006
  • It is necessary to measure precisely the magnetic characteristics of electrical steel sheets under rotating magnetic fields, to obtain an accurate numerical performance analysis of electric machines made of electrical steel sheets. In this paper, the two dimensional magnetic characteristics of an electrical steel sheet are measured and explained under rotating magnetic fields using a two-axes-excitation type single sheet tester (SST). Through experiments, the magnetic properties, under rotating magnetic fields, of a non-oriented and grain oriented electrical steel sheet were measured respectively. In addition, the iron losses due to not only the alternating magnetic fields, but also rotating magnetic fields were measured. These experimentally measured results can evidently be applied to the analysis of iron losses in electrical machines.

Occupational Exposure to Steady Magnetic Fields and its Effect on Workers Blood Indices at an Electrolysis Unit

  • Ravandi, Mohammad Reza Ghotbi;Mardi, Hossein;Khanjani, Narges;Barkhordari, Abolfazl
    • Journal of Magnetics
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    • v.21 no.2
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    • pp.255-260
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    • 2016
  • The health effects of environmental and occupational exposure to steady magnetic fields is a matter of concern. The aim of this study was to evaluate the hematologic effects of exposure to steady magnetic fields at the electrolysis unit of a Copper complex. The population under study was the workers of the electrolysis unit of the copper refinery. The average steady magnetic field in the exposure group was 2.5 mT. The blood indices of workers exposure to steady magnetic fields after adjusting for confounders showed decreased white blood cells (except neutrophils) and increase in the number and volume of platelets. Red blood cells did not show any significant difference. Exposure to steady magnetic fields even in proposed safe limits may have hematologic effects on humans. There is a necessity for more research about the safe doses of exposure to magnetic fields.