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

MARC details
fixed length control field 08874nam a2200217Ia 4500
control field IN-BaIIA
control field 20211110120233.0
fixed length control field 211028s9999 xx 000 0 eng d
Transcribing agency IIA Library
Personal name Prasad, Avijeet
9 (RLIN) 37253
Relator term Author
Title Magnetic helicity and force- free properties of astrophysical magnetic fields
Statement of responsibility, etc. Avijeet Prasad
Medium [Ph.D Thesis]
Place of publication, distribution, etc. Bangalore
Name of publisher, distributor, etc. Indian Institute of Astrophysics
Date of publication, distribution, etc. 2015
Extent xxvii, 265p.
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."
Topical term or geographic name entry element Astrophysical magnetic fields
9 (RLIN) 37254
Topical term or geographic name entry element Magnetic helicity
9 (RLIN) 37255
Personal name Arun Mangalam
9 (RLIN) 37029
Relator term Supervisor
Uniform Resource Identifier <a href="http://prints.iiap.res.in/handle/2248/7551">http://prints.iiap.res.in/handle/2248/7551</a>
Link text Click Here to Access eThesis
Koha item type Thesis & Dissertations
Source of classification or shelving scheme Universal Decimal Classification
Withdrawn status Lost status Source of classification or shelving scheme Damaged status Not for loan Home library Current library Shelving location Date acquired Barcode Date last seen Price effective from Koha item type
    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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