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Magnetic helicity and force- free properties of astrophysical magnetic fields (Record no. 19486)

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005 - DATE AND TIME OF LATEST TRANSACTION
control field 20211110120233.0
008 - FIXED-LENGTH DATA ELEMENTS--GENERAL INFORMATION
fixed length control field 211028s9999 xx 000 0 eng d
040 ## - CATALOGING SOURCE
Transcribing agency IIA Library
100 ## - MAIN ENTRY--PERSONAL NAME
Personal name Prasad, Avijeet
9 (RLIN) 37253
Relator term Author
245 #0 - TITLE STATEMENT
Title Magnetic helicity and force- free properties of astrophysical magnetic fields
Statement of responsibility, etc. Avijeet Prasad
Medium [Ph.D Thesis]
260 ## - PUBLICATION, DISTRIBUTION, ETC.
Place of publication, distribution, etc. Bangalore
Name of publisher, distributor, etc. Indian Institute of Astrophysics
Date of publication, distribution, etc. 2015
300 ## - PHYSICAL DESCRIPTION
Extent xxvii, 265p.
502 ## - DISSERTATION NOTE
Degree type Doctor of Philosophy
Name of granting institution Indira Gandhi National Open University
Year degree granted 2016
520 ## - SUMMARY, ETC.
Summary, etc. "Magnetic fields are ubiquitous in the universe and play an important role in<br/>variety of astrophysical phenomenon. It is thus very important to understand<br/>the origin, structure and strength of these astrophysical magnetic fields. In<br/><br/>this Thesis, we use the concept of magnetic helicity conservation and prop-<br/>erties of force-free magnetic fields to investigate the topological properties<br/><br/>of magnetic fields in the solar corona and the amplification and nonlinear<br/>saturation of dynamo generated field in disc galaxies.<br/>For the case of solar corona, we solve the linear and nonlinear force-free<br/><br/>field equation using photospheric boundary conditions to obtain simple ax-<br/>isymmetric magnetic field configurations in spherical geometry. We show<br/><br/>that the condition of separability of solutions in the radial and angular vari-<br/>ables leads to two classes of solutions: linear and nonlinear force-free fields<br/><br/>(NLFF). We extended the set of NLFF solutions with radial power law index<br/>n = p/q, for all cases of odd p and cases of q > p for even p. We apply these<br/>solutions to simulate photospheric vector magnetograms obtained using the<br/>spectro-polarimeter on board Hinode and search for best-fit configurations.<br/>The effectiveness of our search strategy is demonstrated on test inputs of<br/>dipolar, axisymmetric, and non axisymmetric linear force-free fields. Using<br/>the best fit, we build three- dimensional axisymmetric field configurations<br/>and calculate the energy and relative helicity with two independent meth-ods. The magnetic helicity and free energy content of these fields are useful<br/>indicators of energy available for release during eruptive events like solar<br/><br/>flares. We analyze five magnetograms for active regions (AR) 10930 span-<br/>ning a period of three days during which two X-class flares occurred and<br/><br/>calculate the free energy and relative helicity of the active region before and<br/>after the flare. Our analysis indicates a peak in these quantities before the<br/>flare events, which is consistent with the previous results. We also analyze<br/>single-polarity regions AR 10923 and 10933, which showed very good fits to<br/><br/>potential fields. This method provides useful reconstruction of NLFF and in-<br/>put fields for other numerical techniques. We also apply the NLFF solutions<br/><br/>to calculate the amount of braiding in coronal magnetic fields using the con-<br/>cept of mean crossing number. This is then used to estimate the free energy<br/><br/>content in solar active regions. We find that the free energy estimates ob-<br/>tained from calculation of magnetic braiding is in good agreement with those<br/><br/>obtained by exact calculations of NLFF fields. We then apply the model of<br/>self-organized criticality (SOC) to these braided field lines and calculate the<br/>distribution of coherent braid sequences and flare energies. We find find good<br/>agreement in the flare energy distributions obtained using SOC model and<br/><br/>NLFFF extrapolation. These results provide useful information on the coro-<br/>nal loop structure and also imply that the coronal heating can be supplied<br/><br/>by the braiding in the case of the active sun.<br/>We provide a new formulation for relative helicity in arbitrary geometries<br/>using the toroidal-poloidal representation of the magnetic field iand discuss<br/>the special cases of planar and spherical geometry. In a general astrophysical<br/><br/>application, the fields penetrate the generation region and extend to a sur-<br/>rounding corona. It is important to develop gauge-free form for Helicity that<br/><br/>can be readily used in different geometries without involving integrals over external volumes. The further extension of the ideas here can be formalized<br/>through use of differential geometry.<br/>Magnetic fields correlated on kiloparsec scales are seen in disc galaxies.<br/><br/>The origin could be due to amplification of small scale seed fields by a tur-<br/>bulent dynamo. Helicity conservation imposes constraints on dynamo action<br/><br/>and one can study the minimal field strength of the large scale magnetic field<br/><br/>that could arise despite the constraint. The calculation of helicity is tech-<br/>nically complicated because of open boundaries and the usual form for the<br/><br/>magneto-hydrodynamic (MHD) invariant needs to be modified to take this<br/>into account. We then present a global semi-analytic axisymmetric model<br/>for a turbulent dynamo operating in a galaxy with a corona. Here, we show<br/>that the supernovae (SNe) and magneto-rotational instability (MRI) driven<br/>turbulence parameters have nearly the same radial dependence and can be<br/>treated in a common formalism; however we assume the main contribution<br/><br/>from SNe. The general toroidal-poloidal representation is then used to cal-<br/>culate the global gauge invariant relative magnetic helicity in cylindrical ge-<br/>ometry. We present the analytic steady-state solutions within the disc that<br/><br/>are matched to force-free fields in the corona. A dynamical solution for the<br/>dynamo is then obtained by expanding the time-dependent field in the basis<br/>obtained using the steady-state solutions. The non-linear quenching of the<br/><br/>dynamo is alleviated by inclusion of small-scale advective and diffusive mag-<br/>netic helicity fluxes, which allow the helicity to be transferred outside the<br/><br/>disc and consequently build up a corona during the course of dynamo action.<br/>We find quadrupolar solutions for in the galactic disc that extend out into<br/>the corona and show oscillations radially. The mean field is found to reach<br/>saturation within a timescale of 1 Gyr with a strength which is of the order<br/>of equipartition magnetic energy (~Beq ). The following is the arrangement of the Thesis. Chapter 1 gives an<br/>overview of astrophysical magnetic fields with special focus on observations<br/>of solar and galactic magnetic fields. Chapter 2 outlines the basics concepts<br/>of MHD and describes the processes relating to magnetic field generation and<br/>dissipation. We also discuss the topological properties of magnetic field using<br/>magnetic helicity and provide a novel prescription for calculating magnetic<br/>helicity in arbitrary geometries. Chapter 3 presents a description of potential<br/>and force-free fields and outlines their important properties. We then discuss<br/>analytical and numerical techniques for solving potential and force-free fields<br/>equations for determining coronal magnetic fields. In Chapter 4, we present<br/>an overview of various coronal heating mechanisms and discuss the statistical<br/>properties of solar flares. We then discuss braiding in coronal magnetic fields<br/><br/>and calculate the free energy in these configurations due to braiding. Chap-<br/>ter 5 gives an introduction to large-scale turbulent dynamos and discusses<br/><br/>various closure approximations used in mean field MHD. We then present its<br/>application to disc galaxies, discuss the basic analytic solutions and give an<br/>overview of current problems in dynamo theory. In Chapter 6, we present<br/><br/>new solutions to the nonlinear force-free field equation and discuss its appli-<br/>cation for determining the topological properties of coronal magnetic fields,<br/><br/>such as their free-energy and relative helicity. We then apply the solutions to<br/>a time sequence of vector magnetograms to estimate the energy released in<br/>a solar flare due to change in magnetic field configuration. In Chapter 7, we<br/>use the NLFF field solutions obtained in Chapter 6 and estimate the amount<br/>of free-energy due to braiding in these configurations. We then apply a model<br/>of SOC to this field and calculate the power-law distribution of flare energies<br/>which is then compared with observations. In Chapter 8, we present a model<br/>of nonlinear turbulent dynamo applied to a disc galaxy having a force-free corona. We discuss the significance of small-scale magnetic helicity fluxes<br/>with regards nonlinear saturation of the dynamo. Chapter 9 then presents a<br/>summary of the results from all chapters, highlight the novel aspects of this<br/>Thesis with its impact. Then, we present future work which includes papers<br/>under preparation."
650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM
Topical term or geographic name entry element Astrophysical magnetic fields
9 (RLIN) 37254
650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM
Topical term or geographic name entry element Magnetic helicity
9 (RLIN) 37255
700 ## - ADDED ENTRY--PERSONAL NAME
Personal name Arun Mangalam
9 (RLIN) 37029
Relator term Supervisor
856 ## - ELECTRONIC LOCATION AND ACCESS
Uniform Resource Identifier <a href="http://prints.iiap.res.in/handle/2248/7551">http://prints.iiap.res.in/handle/2248/7551</a>
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942 ## - ADDED ENTRY ELEMENTS (KOHA)
Koha item type Thesis & Dissertations
Source of classification or shelving scheme Universal Decimal Classification
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    Universal Decimal Classification     IIA Library-Bangalore IIA Library-Bangalore General Stacks 24/12/2019 20408 05/11/2021 24/12/2019 Thesis & Dissertations

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