subject:(Dynamo theory)

University of Rochester

1. Park, Kiwan. Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition.

Degree: PhD, 2013, University of Rochester

URL: http://hdl.handle.net/1802/27870

► Many astrophysical phenomena depend on the underlying dynamics of magnetic fields. The observations of accretion disks and their jets, stellar coronae, and the solar corona… (more)

▼ Many astrophysical phenomena depend on the underlying dynamics of magnetic fields. The observations of accretion disks and their jets, stellar coronae, and the solar corona are all best explained by models where magnetic fields play a central role. Understanding these phenomena requires studying the basic physics of magnetic field generation, magnetic energy transfer into radiating particles, angular momentum transport, and the observational implications of these processes. Each of these topics comprises a large enterprise of research. However, more practically speaking, the nonlinearity in large scale dynamo is known to be determined by magnetic helicity(⟨A · B⟩), the topological linked number of knotted magnetic field. Magnetic helicity, which is also observed in solar physics, has become an important tool for observational and theoretical study. The first part of my work addresses one aspect of the observational implications of magnetic fields, namely Faraday rotation. It is shown that plasma composition affects the interpretation of Faraday rotation measurements of the field, and in turn how this can be used to help constrain unknown plasma composition. The results are applied to observations of astrophysical jets. The thesis then focuses on the evolution of magnetic fields. In particular, the dynamo amplification of large scale magnetic fields is studied with an emphasis on the basic physics using both numerical simulations and analytic methods. In particular, without differential rotation, a two and three scale mean field (large scale value + fluctuation scales) dynamo theory and statistical methods are introduced. The results are compared to magnetohydrodynamic (MHD) simulations of the Pencil code, which utilizes high order finite difference methods. Simulations in which the energy is initially driven into the system in the form of helical kinetic energy (via kinetic helicity) or helical magnetic energy (via magnetic helicity) reveal the exponential growth of seed magnetic fields by a mechanism known as “alpha effect.” The generalized theory systematically explains the simulation results, showing how magnetic energy is inversely cascaded from small to large scales, and how the large scale field growth saturates. In addition to work on the nonlinear saturation of large scale magnetic fields, the thesis also includes a study of the influence of the magnitude and distribution of the magnetic energy on the large scale field growth rate in the last chapter. Since the large scale dynamos of most astrophysical objects are likely not yet in a resistively saturated state (due to the high conductivity of astrophysical plasmas), the evolution of the magnetic field in the pre-saturation regime is most important. The results show that, within the limitations of the present study, the effect of the initial field distribution on the large scale field growth is limited only to the early growth regime, not the saturated time regime.

Subjects/Keywords: Dynamo; Faraday Rotation; MHD; Simulation; Theory; Turbulence

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APA (6^th Edition):

Park, K. (2013). Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition. (Doctoral Dissertation). University of Rochester. Retrieved from http://hdl.handle.net/1802/27870

Chicago Manual of Style (16^th Edition):

Park, Kiwan. “Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition.” 2013. Doctoral Dissertation, University of Rochester. Accessed January 27, 2021. http://hdl.handle.net/1802/27870.

MLA Handbook (7^th Edition):

Park, Kiwan. “Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition.” 2013. Web. 27 Jan 2021.

Vancouver:

Park K. Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition. [Internet] [Doctoral dissertation]. University of Rochester; 2013. [cited 2021 Jan 27]. Available from: http://hdl.handle.net/1802/27870.

Council of Science Editors:

Park K. Theory and simulation of magnetohydrodynamic dynamos and Faraday rotation for plasmas of general composition. [Doctoral Dissertation]. University of Rochester; 2013. Available from: http://hdl.handle.net/1802/27870

University of Colorado

2. Featherstone, Nicholas Andrew. Exploring Convection and Dynamos in the Cores and Envelopes of Stars.

Degree: PhD, Astrophysical & Planetary Sciences, 2011, University of Colorado

URL: https://scholar.colorado.edu/astr_gradetds/5

► We present theoretical studies in two complementary areas dealing with convection, rotation, and dynamos in A-type stars, and with local helioseismology in the Sun.… (more)

▼ We present theoretical studies in two complementary areas dealing with convection, rotation, and dynamos in A-type stars, and with local helioseismology in the Sun. Our studies begin with the main-sequence A stars (stars of about 2 solar masses) that possess a radiative envelope overlying a convective core. Using 3-D simulations with the Anelastic Spherical Harmonic (ASH) code to study full spherical domains, we examine the effects of a primordial magnetic field on the dynamo action realized in the turbulent core. Dynamo activity realized in the presence of such a field is significantly more efficient than in its absence, yielding magnetic energies that are roughly tenfold those of the kinetic energy associated with the convective motions. Both convective motions and magnetic fields assume a decidedly global-scale topology in this regime, with convective downdrafts from one side of the core streaming freely across the rotation axis, advecting and stretching magnetic fields across distant portions of the core in the process. We examine the topology of these strong magnetic fields and aspects of their generation in this super-equipartition dynamo. We next develop a 3-D inversion method for helioseismic measurements of horizontal flows obtained using ring-diagram analysis. Helioseismology uses the broad range of acoustic oscillations observed at the solar surface to study properties deep within the Sun. Our inversion method (called ARRDI) incorporates measurements of the wavefield made at multiple horizontal resolutions to discern the subsurface structure of horizontal flows within the star. We adopt a regularized least squares (RLS) approach for these inversions and develop a novel iterative extension to the RLS scheme wherein the flow field across the entire solar disk may be efficiently recovered. We have calculated the set of 3-D sensitivity kernels necessary for the application of our inversion technique to MDI data. We explore the horizontal- and depth-averaging properties of these sensitivity kernels, and find they differ substantially between measurements made at different horizontal resolutions. After characterizing the errors and averaging properties of our inversion algorithm, we examine the subsurface flows around sunspots. We find that sunspots possess outflows which extend to a depth of 10 Mm. These outflows possess a noticeable two-component structure, characterized by a near-surface moat outflow and another deeper outflow at 5 Mm. Our 3-D inversion procedure should be very useful in interpreting the vast helioseismic data sets now becoming available. Advisors/Committee Members: Juri Toomre, Bradley W Hindman, Allan Sacha Brun.

Subjects/Keywords: Convection; Dynamo Theory; Helioseismology; Astrophysics and Astronomy

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APA (6^th Edition):

Featherstone, N. A. (2011). Exploring Convection and Dynamos in the Cores and Envelopes of Stars. (Doctoral Dissertation). University of Colorado. Retrieved from https://scholar.colorado.edu/astr_gradetds/5

Chicago Manual of Style (16^th Edition):

Featherstone, Nicholas Andrew. “Exploring Convection and Dynamos in the Cores and Envelopes of Stars.” 2011. Doctoral Dissertation, University of Colorado. Accessed January 27, 2021. https://scholar.colorado.edu/astr_gradetds/5.

MLA Handbook (7^th Edition):

Featherstone, Nicholas Andrew. “Exploring Convection and Dynamos in the Cores and Envelopes of Stars.” 2011. Web. 27 Jan 2021.

Vancouver:

Featherstone NA. Exploring Convection and Dynamos in the Cores and Envelopes of Stars. [Internet] [Doctoral dissertation]. University of Colorado; 2011. [cited 2021 Jan 27]. Available from: https://scholar.colorado.edu/astr_gradetds/5.

Council of Science Editors:

Featherstone NA. Exploring Convection and Dynamos in the Cores and Envelopes of Stars. [Doctoral Dissertation]. University of Colorado; 2011. Available from: https://scholar.colorado.edu/astr_gradetds/5

University of Cambridge

3. Valeria Shumaylova, Valeria. Scale selection in hydromagnetic dynamos.

Degree: PhD, 2019, University of Cambridge

URL: https://www.repository.cam.ac.uk/handle/1810/290138

► One of the extraordinary properties of the Sun is the observed range of motion scales from the convection granules to the cyclic variation of magnetic… (more)

▼ One of the extraordinary properties of the Sun is the observed range of motion scales from the convection granules to the cyclic variation of magnetic activity. The Sun’s magnetic field exhibits coherence in space and time on much larger scales than the turbulent convection that ultimately powers the dynamo. Motivated by the scale separation considerations, in this thesis we study the parametric scale selection of dynamo action. Although helioseismology has made a lot of progress in the study of the solar interior, the precise motions of plasma are still unknown. In this work, we assume that the model flow is forced with helical viscous body forces acting on different characteristic scales and weak and strong large-scale shear flows that are believed to be present near the base of the convection zone. In this thesis, we look for numerical evidence of a large-scale magnetic field relative to the characteristic scale of the model flow. The investigation is based on the simulations of incompressible MHD equations in elongated triply-periodic domains. To commence the investigation, a linear stability analysis of the coarsening instability in a one-dimensional periodic system is performed to study the stability threshold in the mean-field limit that assumes large scale separation in the system. The simulations are used to discriminate between different forms of the mean-field α -effect and domain aspect ratio. The notion of scale selection refers to methods for estimating characteristic scales. We define the dynamo scale through the characteristic scales of the underlying model flow, forcing and the realised magnetic field. The aspect ratio of the elongated domains plays a crucial role in all considered cases. In Part II, we examine the dynamo generated by the imposed model flows. The transition from large-scale dynamo at the onset to small-scale dynamo as we increase Rm is smooth and takes place in two stages: a fast transition into a predominantly small-scale magnetic energy state and a slower transition into even smaller scales. The long wavelength perturbation imposed on the ABC flow in the modulated case is not preserved in the eigenmodes of the magnetic field. In the presence of the linear (semi-linear shearing-box approximation) and the sinusoidal shearing motions, the field again undergoes a smooth transition at the slow non-sheared rate, which is associated with the balance of the advection and diffusion terms in the induction equation. Part III considers the nonlinear extension of the analysis in Part II, where the incompressible cellular and sheared flows interact with the exponentially growing magnetic field via the Lorentz force in the dynamical regime. Both sheared and non-sheared helical cellular flows become unstable to large-scale perturbations even in the limit of high viscosity. Due to the helical properties of the imposed forcing, the inverse cascade of helicity leads to energy accumulation in the largest scales of the domain, albeit the characteristic lengthscale exhibits the transitional nature at a…

Subjects/Keywords: Dynamo theory; MHD; ABC flow; Mean-field theory

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Valeria Shumaylova, V. (2019). Scale selection in hydromagnetic dynamos. (Doctoral Dissertation). University of Cambridge. Retrieved from https://www.repository.cam.ac.uk/handle/1810/290138

Chicago Manual of Style (16^th Edition):

Valeria Shumaylova, Valeria. “Scale selection in hydromagnetic dynamos.” 2019. Doctoral Dissertation, University of Cambridge. Accessed January 27, 2021. https://www.repository.cam.ac.uk/handle/1810/290138.

MLA Handbook (7^th Edition):

Valeria Shumaylova, Valeria. “Scale selection in hydromagnetic dynamos.” 2019. Web. 27 Jan 2021.

Vancouver:

Valeria Shumaylova V. Scale selection in hydromagnetic dynamos. [Internet] [Doctoral dissertation]. University of Cambridge; 2019. [cited 2021 Jan 27]. Available from: https://www.repository.cam.ac.uk/handle/1810/290138.

Council of Science Editors:

Valeria Shumaylova V. Scale selection in hydromagnetic dynamos. [Doctoral Dissertation]. University of Cambridge; 2019. Available from: https://www.repository.cam.ac.uk/handle/1810/290138

University of Cambridge

4. Valeria Shumaylova, Valeria. Scale selection in hydromagnetic dynamos.

Degree: PhD, 2019, University of Cambridge

URL: https://doi.org/10.17863/CAM.37367 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767874

► One of the extraordinary properties of the Sun is the observed range of motion scales from the convection granules to the cyclic variation of magnetic… (more)

▼ One of the extraordinary properties of the Sun is the observed range of motion scales from the convection granules to the cyclic variation of magnetic activity. The Sun's magnetic field exhibits coherence in space and time on much larger scales than the turbulent convection that ultimately powers the dynamo. Motivated by the scale separation considerations, in this thesis we study the parametric scale selection of dynamo action. Although helioseismology has made a lot of progress in the study of the solar interior, the precise motions of plasma are still unknown. In this work, we assume that the model flow is forced with helical viscous body forces acting on different characteristic scales and weak and strong large-scale shear flows that are believed to be present near the base of the convection zone. In this thesis, we look for numerical evidence of a large-scale magnetic field relative to the characteristic scale of the model flow. The investigation is based on the simulations of incompressible MHD equations in elongated triply-periodic domains. To commence the investigation, a linear stability analysis of the coarsening instability in a one-dimensional periodic system is performed to study the stability threshold in the mean-field limit that assumes large scale separation in the system. The simulations are used to discriminate between different forms of the mean-field α -effect and domain aspect ratio. The notion of scale selection refers to methods for estimating characteristic scales. We define the dynamo scale through the characteristic scales of the underlying model flow, forcing and the realised magnetic field. The aspect ratio of the elongated domains plays a crucial role in all considered cases. In Part II, we examine the dynamo generated by the imposed model flows. The transition from large-scale dynamo at the onset to small-scale dynamo as we increase Rm is smooth and takes place in two stages: a fast transition into a predominantly small-scale magnetic energy state and a slower transition into even smaller scales. The long wavelength perturbation imposed on the ABC flow in the modulated case is not preserved in the eigenmodes of the magnetic field. In the presence of the linear (semi-linear shearing-box approximation) and the sinusoidal shearing motions, the field again undergoes a smooth transition at the slow non-sheared rate, which is associated with the balance of the advection and diffusion terms in the induction equation. Part III considers the nonlinear extension of the analysis in Part II, where the incompressible cellular and sheared flows interact with the exponentially growing magnetic field via the Lorentz force in the dynamical regime. Both sheared and non-sheared helical cellular flows become unstable to large-scale perturbations even in the limit of high viscosity. Due to the helical properties of the imposed forcing, the inverse cascade of helicity leads to energy accumulation in the largest scales of the domain, albeit the characteristic lengthscale exhibits the transitional nature at a…

Subjects/Keywords: Dynamo theory; MHD; ABC flow; Mean-field theory

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Valeria Shumaylova, V. (2019). Scale selection in hydromagnetic dynamos. (Doctoral Dissertation). University of Cambridge. Retrieved from https://doi.org/10.17863/CAM.37367 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767874

Chicago Manual of Style (16^th Edition):

Valeria Shumaylova, Valeria. “Scale selection in hydromagnetic dynamos.” 2019. Doctoral Dissertation, University of Cambridge. Accessed January 27, 2021. https://doi.org/10.17863/CAM.37367 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767874.

MLA Handbook (7^th Edition):

Valeria Shumaylova, Valeria. “Scale selection in hydromagnetic dynamos.” 2019. Web. 27 Jan 2021.

Vancouver:

Valeria Shumaylova V. Scale selection in hydromagnetic dynamos. [Internet] [Doctoral dissertation]. University of Cambridge; 2019. [cited 2021 Jan 27]. Available from: https://doi.org/10.17863/CAM.37367 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767874.

Council of Science Editors:

Valeria Shumaylova V. Scale selection in hydromagnetic dynamos. [Doctoral Dissertation]. University of Cambridge; 2019. Available from: https://doi.org/10.17863/CAM.37367 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.767874

Indian Institute of Science

5. Hazra, Gopal. Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow.

Degree: PhD, Faculty of Science, 2018, Indian Institute of Science

URL: http://etd.iisc.ac.in/handle/2005/3791

► Our Sun is a variable star. The magnetic fields in the Sun play an important role for the existence of a wide variety of phenomena… (more)

▼ Our Sun is a variable star. The magnetic fields in the Sun play an important role for the existence of a wide variety of phenomena on the Sun. Among those, sunspots are the slowly evolving features of the Sun but solar ares and coronal mass ejections are highly dynamic phenomena. Hence, the solar magnetic fields could affect the Earth directly or indirectly through the Sun's open magnetic flux, solar wind, solar are, coronal mass ejections and total solar irradiance variations. These large scale magnetic fields originate due to Magnetohydrodynamic dynamo process inside the solar convection zone converting the kinetic energy of the plasma motions into the magnetic energy. Currently the most promising model to understand the large scale magnetic fields of the Sun is the Flux Transport Dynamo (FTD) model. FTD models are mostly axisymmetric models, though the non-axisymmetric 3D FTD models are started to develop recently. In these models, we assume the total magnetic fields of the Sun consist of poloidal and toroidal components and solve the magnetic induction equation kinematicaly in the sense that velocity fields are invoked motivated from the observations. Differential rotation stretches the poloidal field to generate the toroidal field. When toroidal eld near the bottom of the convection zone become magnetically buoyant, it rises through the solar convection zone and pierce the surface to create bipolar sunspots. While rising through the solar convection zone, the Coriolis force keeps on acting on the flux tube, which introduces a tilt angle between bipolar sunspots. Since the sunspots are the dense region of magnetic fields, they diffuse away after emergence. The leading polarity sunspots (close to equator) from both the hemisphere cancel each other across the equator and trailing polarity sunspots migrate towards the pole to generate effective poloidal fields. This mechanism for generation of poloidal field from the decay of sunspots is known as Babcock-Leighton process. After the poloidal field is generated, the meridional flow carries this field to the pole and further to the bottom of the convection zone where differential rotation again acts on it to generate toroidal field. Hence the solar dynamo goes on by oscillation between the poloidal field and toroidal field, where they can sustain each other through a cyclic feedback process. Just like other physical models, FTD models have various assumptions and approximations to incorporate these different processes. Some of the assumptions are observationally verified and some of them are not. Considering the availability of observed data, many approximations have been made in these models on the theoretical basis. In this thesis, we present various studies leading to better understanding of the different processes and parameters of FTD models, which include magnetic buoyancy, meridional circulation and Babcock-Leighton process. In the introductory Chapter 1, we first present the observational features of the solar magnetic fields, theoretical background of the FTD… Advisors/Committee Members: Choudhuri, Arnab Rai (advisor), Banerjee, Dipankar (advisor).

Subjects/Keywords: Solar Cycle; Sunspots; Solar Magnetic Field; Dynamo Theory; Flux Transport Dynamo Model; Meridional Circulation; Solar Polar Field; Flux Transport Solar Dynamo; Solar Cycles; Sun’s Polar Magnetic Field; Dynamo Model; 3D Babcock-Leighton Solar Dynamo Model; Astrophysics

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Hazra, G. (2018). Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow. (Doctoral Dissertation). Indian Institute of Science. Retrieved from http://etd.iisc.ac.in/handle/2005/3791

Chicago Manual of Style (16^th Edition):

Hazra, Gopal. “Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow.” 2018. Doctoral Dissertation, Indian Institute of Science. Accessed January 27, 2021. http://etd.iisc.ac.in/handle/2005/3791.

MLA Handbook (7^th Edition):

Hazra, Gopal. “Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow.” 2018. Web. 27 Jan 2021.

Vancouver:

Hazra G. Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow. [Internet] [Doctoral dissertation]. Indian Institute of Science; 2018. [cited 2021 Jan 27]. Available from: http://etd.iisc.ac.in/handle/2005/3791.

Council of Science Editors:

Hazra G. Understanding the Behavior of the Sun's Large Scale Magnetic Field and Its Relation with the Meridional Flow. [Doctoral Dissertation]. Indian Institute of Science; 2018. Available from: http://etd.iisc.ac.in/handle/2005/3791

6. Chen, Long. Optimization of Kinematic Dynamos Using Variational Methods.

Degree: 2018, ETH Zürich

URL: http://hdl.handle.net/20.500.11850/227237

► The Earth possesses a magnetic field that is generated by the fluid motion in a conducting outer core. This system that converts kinetic energy into… (more)

▼ The Earth possesses a magnetic field that is generated by the fluid motion in a conducting outer core. This system that converts kinetic energy into long lasting magnetic energy is called a dynamo. Not only found on the Earth, a dynamo is a fundamental mechanism that also exists in astrophysical bodies, and various research groups have reproduced dynamos with computer simulations and experiments. Despite extensive studies there is no general recipe to guarantee dynamo action. One important question is therefore: how to generate a dynamo most efficiently? In this thesis, we adapt a variational method to search numerically for the most efficient dynamos and the corresponding optimal flow fields. This method covers a large parameter space that in theory represents infinitely many field configurations, something conventional methods cannot achieve. Our optimization scheme combines existing dynamo models with adjoint modelling and subsequent updates using variational derivatives. We start with a kinematic dynamo model and update iteratively the initial conditions of both a steady flow field and a magnetic field. We use the enstrophy based magnetic Reynolds number (Rm) as an input parameter. For a given Rm, the asymptotic growth of the magnetic energy needs to be non-negative in order to maintain a dynamo. When the asymptotic growth is precisely zero in an optimized model, we identify the corresponding value of Rm as the lower bound for dynamo action, denoted by the minimal critical magnetic Reynolds number Rm_c,min. For some non-dynamo configurations the magnetic energy can grow during a transient period but eventually decays. The critical transient magnetic Reynolds number for which the magnetic energy cannot grow in any time window, even a very narrow one, is denoted by Rm_t. Using this method, we study kinematic dynamos in three main categories: unconstrained dynamos in a cube, unconstrained dynamos in a full sphere and dynamos with symmetries in a full sphere. All models are implemented numerically using a spectral Galerkin method. In the cubic model, we study optimized dynamos at Rm_c,min with four sets of magnetic boundary conditions: NNT, NTT, NNN and TTT (T denotes superconducting boundary conditions and N denotes pseudo-vacuum boundary conditions on opposite sides of the cube), meanwhile keeping the flow field satisfying impermeable boundary conditions. Numerically swapping the magnetic boundary conditions from T to N leaves the magnetic energy growth nearly unchanged, and if \mathbf{u} is an optimal flow field, then - \mathbf{u} is the new optimum after swapping. For the mixed cases, we can represent the dominant optimal flow field at Rm_c,min with three Fourier modes that each describe a 2D flow field. In the unconstrained spherical models, we impose electrically insulating boundary conditions on the magnetic field while we let the flow field satisfy either no-slip or free-slip boundary conditions. For the no-slip case, we find the optimal flow at Rm_c,min is spatially… Advisors/Committee Members: Jackson, Andrew, Noir, Jérõme André Roland, Willis, Ashley.

Subjects/Keywords: Dynamo theory; Variational methods; Kinematic dynamo; Optimization; info:eu-repo/classification/ddc/550; info:eu-repo/classification/ddc/530; Earth sciences; Physics

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Chen, L. (2018). Optimization of Kinematic Dynamos Using Variational Methods. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/227237

Chicago Manual of Style (16^th Edition):

Chen, Long. “Optimization of Kinematic Dynamos Using Variational Methods.” 2018. Doctoral Dissertation, ETH Zürich. Accessed January 27, 2021. http://hdl.handle.net/20.500.11850/227237.

MLA Handbook (7^th Edition):

Chen, Long. “Optimization of Kinematic Dynamos Using Variational Methods.” 2018. Web. 27 Jan 2021.

Vancouver:

Chen L. Optimization of Kinematic Dynamos Using Variational Methods. [Internet] [Doctoral dissertation]. ETH Zürich; 2018. [cited 2021 Jan 27]. Available from: http://hdl.handle.net/20.500.11850/227237.

Council of Science Editors:

Chen L. Optimization of Kinematic Dynamos Using Variational Methods. [Doctoral Dissertation]. ETH Zürich; 2018. Available from: http://hdl.handle.net/20.500.11850/227237

UCLA

7. Cheng, Jonathan. Characterizing convection in geophysical dynamo systems.

Degree: Geophysics & Space Physics, 2015, UCLA

URL: http://www.escholarship.org/uc/item/69z5f5gz

► The Earth’s magnetic field is produced by a fluid dynamo in the molten iron outer core. This geodynamo is driven by fluid motions induced by… (more)

▼ The Earth’s magnetic field is produced by a fluid dynamo in the molten iron outer core. This geodynamo is driven by fluid motions induced by thermal and chemical convection and strongly influenced by rotational and magnetic field effects. While frequent observations are made of the morphology and time-dependent field behavior, flow dynamics in the core are all but inaccessible to direct measurement. Thus, forward models are essential for exploring the relationship between the geomagnetic field and its underlying fluid physics. The goal of my PhD is to further our understanding of the fluid physics driving the geodynamo.In order to do this, I have performed a suite of nonrotating and rotating convection laboratory experiments and developed a new experimental device that reaches more extreme values of the governing parameters than previously possible. In addition, I conduct a theoretical analysis of well-established results from a suite of dynamo simulations by Christensen and Aubert (2006). These studies are conducted at moderate values of the Ekman number (ratio between viscosity and Coriolis forces, ~ 10^−4), as opposed to the the extremely small Ekman numbers in planetary cores (~ 10^−15). At such moderate Ekman values, flows tend to take the form of large-scale, quasi-laminar axial columns. These columnar structures give the induced magnetic field a dipolar morphology, similar to what is seen on planets. However, I find that some results derived from these simulations are fully dependent on the fluid viscosity, and therefore are unlikely to reflect the fluid physics driving dynamo action in the core. My findings reinforce the need to understand the turbulent processes that arise as the governing parameters approach planetary values. Indeed, my rotating convection experiments show that, as the Ekman number is decreased beyond ranges currently accessible to dynamo simulations, the regime characterized by laminar columns is found to dwindle. We instead find a large variety of behavioral regimes ranging from axial columns to fully three-dimensional turbulence. By comparing these to direct numerical simulations and asymptotically-reduced models, we find broad agreement in both the heat transfer scaling properties and flow morphologies in these separate regimes. In particular, large, multi-scale axial vortices emerge consistently in numerical and asymptotic simulations. Such multi-scale structures in the core may be related to the Earth’s dipolar magnetic field structure. I have designed and fabricated a novel laboratory experimental device capable of characterizing these flow regimes in great detail using accurate heat transfer and velocity measurements and high-resolution flow imaging.

Subjects/Keywords: Geophysics; Core dynamics; Dynamo theory; Geophysical fluid dynamics; Rotating convection

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APA (6^th Edition):

Cheng, J. (2015). Characterizing convection in geophysical dynamo systems. (Thesis). UCLA. Retrieved from http://www.escholarship.org/uc/item/69z5f5gz

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16^th Edition):

Cheng, Jonathan. “Characterizing convection in geophysical dynamo systems.” 2015. Thesis, UCLA. Accessed January 27, 2021. http://www.escholarship.org/uc/item/69z5f5gz.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7^th Edition):

Cheng, Jonathan. “Characterizing convection in geophysical dynamo systems.” 2015. Web. 27 Jan 2021.

Vancouver:

Cheng J. Characterizing convection in geophysical dynamo systems. [Internet] [Thesis]. UCLA; 2015. [cited 2021 Jan 27]. Available from: http://www.escholarship.org/uc/item/69z5f5gz.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Cheng J. Characterizing convection in geophysical dynamo systems. [Thesis]. UCLA; 2015. Available from: http://www.escholarship.org/uc/item/69z5f5gz

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

8. Eraso, Gustavo Andres Guerrero. Estudos numéricos do dínamo solar.

Degree: PhD, Astronomia, 2009, University of São Paulo

URL: http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12082009-065953/ ;

► O ciclo solar é um dos fenômenos magnéticos mais interessantes do Universo. Embora ele tinha sido descoberto há mais de 150 anos, até agora permanece… (more)

▼ O ciclo solar é um dos fenômenos magnéticos mais interessantes do Universo. Embora ele tinha sido descoberto há mais de 150 anos, até agora permanece um problema em aberto para a Astrofísica. Há diferentes tipos de observações que sugerem que o ciclo solar corresponde a um processo de dínamo operando em algum lugar do interior solar. Parker foi o primeiro a tentar explicar o dínamo solar como um processo hidro-magnético acerca de 50 anos atrás. Desde então, embora tenha havido avanços significativos nas observações e investigações teóricas e numéricas, uma resposta definitiva para o dínamo solar ainda não existe. Acredita-se que no caso do Sol, pelo menos dois processos são necessários para completar o ciclo magnético observado: a transformação de um campo poloidal inicial em um campo toroidal, um processo conhecido como efeito , o qual se deve ao cisalhamento em grande escala ocasionado pela rotação diferencial; e a transformação do campo toroidal em um novo campo poloidal de polaridade oposta ao inicial. Esse segundo processo é menos conhecido e motivo de intensas discussões e pesquisas. Duas hipóteses principais foram formuladas para explicar a natureza deste processo, usualmente conhecido como efeito : a primeira, baseada na idéia de Parker de um mecanismo turbulento onde os campos poloidais resultam de movimentos convectivos ciclônicos operando em tubos de fluxo toroidais em pequena escala. Esses modelos se depararam, no entanto, com um serio inconveniente: na fase não-linear, i.e., quando a reação dinâmica do campo magnético ao fluido torna-se importante, o efeito pode ser amortecido de forma catastrófica, levando a um dínamo pouco efetivo. A segunda hipótese é baseada nas idéias de Babcock (1961) e Leighton (1969) (BL), que propuseram que o campo poloidal forma-se devido à emergência e decaimento posterior das regiões bipolares ativas. Neste modelo a circulação meridional tem um papel fundamental pois atua como mecanismo de transporte do fluxo magnético, de tal forma que a escala de tempo advectivo deve dominar sobre a escala de tempo difusiva. Por essa razão essa classe de modelos é comumente conhecida como modelo de dínamo dominado pelo transporte de fluxo, ou dínamo advectivo. Os modelos de dínamo dominados pelo transporte de fluxo são relativamente bem sucedidos em reproduzir as características em grande escala do ciclo solar, tornando-se populares entre a comunidade de Física solar, no entanto, também apresentam vários problemas amplamente discutidos na literatura. O objetivo principal deste trabalho foi identificar as principais limitações dessa classe de modelos e explorar as suas possíveis soluções. Para tal, construímos um modelo numérico bi-dimensional de dínamo cinemático baseado na teoria de campo médio e investigamos primeiro os efeitos da geometria e da espessura da tacoclina solar na amplificação do dínamo. Depois, consideramos o processo de bombeamento magnético turbulento como um mecanismo alternativo de transporte de fluxo magnético, e finalmente,… Advisors/Committee Members: Pino, Elisabete Maria de Gouveia Dal.

Subjects/Keywords: Ciclo solar; dynamo theory; MHD; MHD; solar cycle; teoría dínamo

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APA (6^th Edition):

Eraso, G. A. G. (2009). Estudos numéricos do dínamo solar. (Doctoral Dissertation). University of São Paulo. Retrieved from http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12082009-065953/ ;

Chicago Manual of Style (16^th Edition):

Eraso, Gustavo Andres Guerrero. “Estudos numéricos do dínamo solar.” 2009. Doctoral Dissertation, University of São Paulo. Accessed January 27, 2021. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12082009-065953/ ;.

MLA Handbook (7^th Edition):

Eraso, Gustavo Andres Guerrero. “Estudos numéricos do dínamo solar.” 2009. Web. 27 Jan 2021.

Vancouver:

Eraso GAG. Estudos numéricos do dínamo solar. [Internet] [Doctoral dissertation]. University of São Paulo; 2009. [cited 2021 Jan 27]. Available from: http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12082009-065953/ ;.

Council of Science Editors:

Eraso GAG. Estudos numéricos do dínamo solar. [Doctoral Dissertation]. University of São Paulo; 2009. Available from: http://www.teses.usp.br/teses/disponiveis/14/14131/tde-12082009-065953/ ;

McMaster University

9. Jackel, Benjamin. Magnetic Dynamos: How Do They Even Work?.

Degree: PhD, 2015, McMaster University

URL: http://hdl.handle.net/11375/18299

►

The origin of cosmic magnetic fields is a important area of astrophysics. The process by which they are created falls under the heading of dynamo… (more)

▼

The origin of cosmic magnetic fields is a important area of astrophysics. The process by which they are created falls under the heading of dynamo theory, and is the topic of this thesis. Our focus for the location of where these magnetic fields operate is one the most ubiquitous objects in the universe, the accretion disk. By studying the accretion disk and the dynamo process that occurs there we wish to better understand both the accretion process and the dynamo process in stars and galaxies as well. We analyse the output from a stratified zero net flux shearing box simulation performed using the ATHENA MHD code in collaboration with Shane Davis. The simulation has turbulence which is naturally forced by the presence of a linear instability called the magnetorotational instability (MRI). We utilise Fourier filtering and the tools of mean field dynamo theory to establish a connection between the calculated EMF and the model predictions of the dynamically quenched alpha model. We find a positive correlation for both components parallel to the large scale magnetic field and the azimuthal components. We have explored many aspects of the theory including additional contributions from magnetic buoyancy and an effect arising from the large scale shear and the current density. We also directly measure the turbulent correlation time for the velocity and magnetic fields both large scale and small. We can also observe the effects of the dynamo cycle, with the azimuthal component of the large scale magnetic field flipping sign in this analysis. We find a positive correlation between the divergence of the eddy scale magnetic helicity flux and the component of the electromotive force parallel to the large scale magnetic field. This correlation directly links the transfer of magnetic helicity to the dynamo process in a system with naturally driven turbulence. This highlights the importance of magnetic helicity and its conservation even in a system with triply periodic boundary conditions.

Thesis

Doctor of Philosophy (PhD)

Advisors/Committee Members: Vishniac, Ethan, Physics and Astronomy.

Subjects/Keywords: Accretion Disk; Magnetic Dynamo; magnetohydrodynamics; Mean Field Theory

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APA (6^th Edition):

Jackel, B. (2015). Magnetic Dynamos: How Do They Even Work?. (Doctoral Dissertation). McMaster University. Retrieved from http://hdl.handle.net/11375/18299

Chicago Manual of Style (16^th Edition):

Jackel, Benjamin. “Magnetic Dynamos: How Do They Even Work?.” 2015. Doctoral Dissertation, McMaster University. Accessed January 27, 2021. http://hdl.handle.net/11375/18299.

MLA Handbook (7^th Edition):

Jackel, Benjamin. “Magnetic Dynamos: How Do They Even Work?.” 2015. Web. 27 Jan 2021.

Vancouver:

Jackel B. Magnetic Dynamos: How Do They Even Work?. [Internet] [Doctoral dissertation]. McMaster University; 2015. [cited 2021 Jan 27]. Available from: http://hdl.handle.net/11375/18299.

Council of Science Editors:

Jackel B. Magnetic Dynamos: How Do They Even Work?. [Doctoral Dissertation]. McMaster University; 2015. Available from: http://hdl.handle.net/11375/18299

University of Cambridge

10. Donnelly, Cara. Shearing waves and the MRI dynamo in stratified accretion discs.

Degree: PhD, 2014, University of Cambridge

URL: https://www.repository.cam.ac.uk/handle/1810/246452https://www.repository.cam.ac.uk/bitstream/1810/246452/3/thesis.pdf.txt ; https://www.repository.cam.ac.uk/bitstream/1810/246452/4/thesis.pdf.jpg

► Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet… (more)

▼ Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet formation. We discuss two different methods of angular momentum transport: first, generation of acoustic waves by mixing of inertial waves, and second, the generation of a self-sustaining magnetic field via the magnetorotational instability (MRI) which would be a source of dissipative turbulence. Previous local simulations of the MRI have shown that the dynamo changes character on addition of vertical stratification. We investigate numerically 3D hydrodynamic shearing waves with a conserved Hermitian form in an isothermal disc with vertical gravity, and describe the associated symplectic structure. We continue with a numerical investigation into the linear evolution of the MRI and the undular magnetic buoyancy instability in isolated flux regions and characterise the resultant quasi-linear EMFs as a function of height above the midplane. We combine this with an analytic description of the linear modes under an assumption of a poloidal-toroidal scale separation. Finally, we use RAMSES to perform full MHD simulations in a zero net flux shearing box, followed by spatial and a novel temporal averaging to reveal the essential structure of the dynamo. We find that inertial modes may be efficiently converted into acoustic modes for "bending waves", despite a fundamental ambiguity in the inertial mode structure. With our linear MRI and the undular magnetic buoyancy modes we find the localisation of the instability high in the atmosphere becomes determined by magnetic buoyancy rather than field strength for small enough azimuthal wavenumber, and that the critical Alfven speed below which the dynamo can operate increases with increasing distance from the midplane. We calculate analytically quasi-linear EMFs which predict both a vertical propagation of toroidal field and a method for creation of radial field. From our fully nonlinear calculations we find an electromotive force in phase with the toroidal field, which is itself 3π/2 out of phase with the radial (sheared) field at the midplane, and good agreement with our quasi-linear analytics. We have identified an efficient mechanism for generating acoustic waves in a disc. In our investigation of the accretion disc dynamo, we have reproduced analytically the EMFs calculated in our simulations, given arguments based on the phase of relevant quantities, several correlation integrals and the scalings suggested by our analytic work. Our analysis contributes significantly to an explanation for the dynamo in an accretion disc.

Subjects/Keywords: Protoplanetary disks; Angular momentum; Theory of Wave-motion; Shear flow; Dynamo theory (Cosmic physics)

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APA (6^th Edition):

Donnelly, C. (2014). Shearing waves and the MRI dynamo in stratified accretion discs. (Doctoral Dissertation). University of Cambridge. Retrieved from https://www.repository.cam.ac.uk/handle/1810/246452https://www.repository.cam.ac.uk/bitstream/1810/246452/3/thesis.pdf.txt ; https://www.repository.cam.ac.uk/bitstream/1810/246452/4/thesis.pdf.jpg

Chicago Manual of Style (16^th Edition):

Donnelly, Cara. “Shearing waves and the MRI dynamo in stratified accretion discs.” 2014. Doctoral Dissertation, University of Cambridge. Accessed January 27, 2021. https://www.repository.cam.ac.uk/handle/1810/246452https://www.repository.cam.ac.uk/bitstream/1810/246452/3/thesis.pdf.txt ; https://www.repository.cam.ac.uk/bitstream/1810/246452/4/thesis.pdf.jpg.

MLA Handbook (7^th Edition):

Donnelly, Cara. “Shearing waves and the MRI dynamo in stratified accretion discs.” 2014. Web. 27 Jan 2021.

Vancouver:

Donnelly C. Shearing waves and the MRI dynamo in stratified accretion discs. [Internet] [Doctoral dissertation]. University of Cambridge; 2014. [cited 2021 Jan 27]. Available from: https://www.repository.cam.ac.uk/handle/1810/246452https://www.repository.cam.ac.uk/bitstream/1810/246452/3/thesis.pdf.txt ; https://www.repository.cam.ac.uk/bitstream/1810/246452/4/thesis.pdf.jpg.

Council of Science Editors:

Donnelly C. Shearing waves and the MRI dynamo in stratified accretion discs. [Doctoral Dissertation]. University of Cambridge; 2014. Available from: https://www.repository.cam.ac.uk/handle/1810/246452https://www.repository.cam.ac.uk/bitstream/1810/246452/3/thesis.pdf.txt ; https://www.repository.cam.ac.uk/bitstream/1810/246452/4/thesis.pdf.jpg

McMaster University

11. Cridland, Alex J. Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos.

Degree: MSc, 2013, McMaster University

URL: http://hdl.handle.net/11375/13636

►

Here we present direct numerical simulations of a shearing box which models the MHD turbulence in astrophysical systems with cylindrical geometries. The purpose of… (more)

Subjects/Keywords: Astrophysics; Magnetohydrodynamics; MHD; dynamo theory; direct numerical simulation; Physical Processes; Physical Processes

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APA (6^th Edition):

Cridland, A. J. (2013). Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos. (Masters Thesis). McMaster University. Retrieved from http://hdl.handle.net/11375/13636

Chicago Manual of Style (16^th Edition):

Cridland, Alex J. “Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos.” 2013. Masters Thesis, McMaster University. Accessed January 27, 2021. http://hdl.handle.net/11375/13636.

MLA Handbook (7^th Edition):

Cridland, Alex J. “Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos.” 2013. Web. 27 Jan 2021.

Vancouver:

Cridland AJ. Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos. [Internet] [Masters thesis]. McMaster University; 2013. [cited 2021 Jan 27]. Available from: http://hdl.handle.net/11375/13636.

Council of Science Editors:

Cridland AJ. Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos. [Masters Thesis]. McMaster University; 2013. Available from: http://hdl.handle.net/11375/13636

University of Colorado

12. Augustson, Kyle C. Convection and Dynamo Action in Massive Stars.

Degree: PhD, Astrophysical & Planetary Sciences, 2013, University of Colorado

URL: https://scholar.colorado.edu/astr_gradetds/26

► Contact between numerical simulations and observations of stellar magnetism is sought, with an emphasis on those stars that are the most readily observed and… (more)

▼ Contact between numerical simulations and observations of stellar magnetism is sought, with an emphasis on those stars that are the most readily observed and those that may have magnetic activity cycles: the Sun, F-type, and B-type stars. Two approaches are taken in studying stellar dynamos and dynamics, utilizing three-dimensional MHD simulations run on massively parallel supercomputers with the full spherical geometry and employing a new compressible code in the spherical wedge geometry. A 3D MHD simulation of the solar dynamo that utilizes the Anelastic Spherical Harmonic (ASH) code is presented. This simulation self-consistently exhibits four prominent aspects of solar magnetism: activity cycles, polarity cycles, the equatorward field migration, and grand minima. The ASH framework and this simulation’s ability to capture many aspects of the solar dynamo represent a first step toward a more complete model of the Sun’s global-scale magnetic activity and its cycles. The dynamics and dynamos of F-type stars are studied through global-scale ASH simulations, with significant contact made between the observed differential rotation and magnetic cycle periods of these stars and those achieved in the simulations. Separately, ASH simulations of core convection in the massive B-type stars show that generation of superequipartition magnetic fields with peak strengths above 1 MG is possible within their cores, which has implications for the evolution of these stars as well as for the properties of their remnants. The internal waves excited by overshooting convection and rotation in these stars radiative exteriors are assessed for their asteroseismic signatures. The results of 3D compressive MHD simulations of the solar near-surface shear layer with the Compressible Spherical Segment (CSS) code are shown, with such layers arising in the coupled dynamics of ASH and CSS as well as in a more rapidly rotating, thin convective envelope of an F-type star. Advisors/Committee Members: Juri Toomre, Bradley W. Hindman, Mark S. Miesch, Mark P. Rast, Jeffrey B. Weiss.

Subjects/Keywords: Dynamo Theory; Fluid Dynamics; Stellar Convection; Stellar Magnetism; Astrophysics and Astronomy; Physical Processes; Stars, Interstellar Medium and the Galaxy

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APA (6^th Edition):

Augustson, K. C. (2013). Convection and Dynamo Action in Massive Stars. (Doctoral Dissertation). University of Colorado. Retrieved from https://scholar.colorado.edu/astr_gradetds/26

Chicago Manual of Style (16^th Edition):

Augustson, Kyle C. “Convection and Dynamo Action in Massive Stars.” 2013. Doctoral Dissertation, University of Colorado. Accessed January 27, 2021. https://scholar.colorado.edu/astr_gradetds/26.

MLA Handbook (7^th Edition):

Augustson, Kyle C. “Convection and Dynamo Action in Massive Stars.” 2013. Web. 27 Jan 2021.

Vancouver:

Augustson KC. Convection and Dynamo Action in Massive Stars. [Internet] [Doctoral dissertation]. University of Colorado; 2013. [cited 2021 Jan 27]. Available from: https://scholar.colorado.edu/astr_gradetds/26.

Council of Science Editors:

Augustson KC. Convection and Dynamo Action in Massive Stars. [Doctoral Dissertation]. University of Colorado; 2013. Available from: https://scholar.colorado.edu/astr_gradetds/26

University of Exeter

13. Zhan, Xiaoya. A study of convection and dynamo in rotating fluid systems.

Degree: PhD, 2010, University of Exeter

URL: https://ore.exeter.ac.uk/repository/handle/10036/100074 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517199

► Convection in a Boussinesq fluid confined by a annular channel fast rotating about a vertical axis and uniformly heated from below, is one of our… (more)

▼ Convection in a Boussinesq fluid confined by a annular channel fast rotating about a vertical axis and uniformly heated from below, is one of our concerns in this thesis. An assumption that the channel has a sufficiently large radius in comparison with its gap-width is employed, so that the curvature effect can be neglected. The aspect ratio of the channel has great influence on the convective flow in it. Guided by the result of the linear stability analysis, we perform three-dimensional numerical simulations to investigate the convective flows under three different types of aspect ratios, which are namely the moderate or large aspect ratios, the very small aspect ratios and the moderately small aspect ratios. Also, we numerically study how convection in the channel is affected by inhomogeneous heat fluxes on sidewalls, which is a simple simulation of the thermal interaction between the Earth's core and mantle. Convection and dynamo action in a rapidly rotating, self-gravitating, Boussinesq fluid sphere is the other concern. We develop a finite element model for the dynamo problem in a whole sphere. This model is constructed by incorporating dynamo equations with globally implemented magnetic boundary conditions to a whole sphere convection model, which is also presented here. The coordinate singularity at the center usually encountered when applying the spectral method is no longer an obstacle and no nonphysical assumptions (i.e. hyper-diffusivities) are used in our model. A large effort has been made to efficiently parallelize the model. Consequently, it can take the full advantage of modern massively parallel computers. Based on this dynamo model, we investigate the dynamo process in a sphere and find that self-sustaining dynamos are more difficult to obtain in a sphere than in a spherical shell. They are activated at relatively high Rayleigh numbers. Moreover, the magnetic fields generated are not dipole-dominant, different from those generated in most dynamo simulations.

Subjects/Keywords: 620.106; convection; dynamo theory; spherical geometry; channel

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APA (6^th Edition):

Zhan, X. (2010). A study of convection and dynamo in rotating fluid systems. (Doctoral Dissertation). University of Exeter. Retrieved from https://ore.exeter.ac.uk/repository/handle/10036/100074 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517199

Chicago Manual of Style (16^th Edition):

Zhan, Xiaoya. “A study of convection and dynamo in rotating fluid systems.” 2010. Doctoral Dissertation, University of Exeter. Accessed January 27, 2021. https://ore.exeter.ac.uk/repository/handle/10036/100074 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517199.

MLA Handbook (7^th Edition):

Zhan, Xiaoya. “A study of convection and dynamo in rotating fluid systems.” 2010. Web. 27 Jan 2021.

Vancouver:

Zhan X. A study of convection and dynamo in rotating fluid systems. [Internet] [Doctoral dissertation]. University of Exeter; 2010. [cited 2021 Jan 27]. Available from: https://ore.exeter.ac.uk/repository/handle/10036/100074 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517199.

Council of Science Editors:

Zhan X. A study of convection and dynamo in rotating fluid systems. [Doctoral Dissertation]. University of Exeter; 2010. Available from: https://ore.exeter.ac.uk/repository/handle/10036/100074 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517199

University of Arizona

14. BOYER, DARRYL WILLIAM. MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS.

Degree: 1982, University of Arizona

URL: http://hdl.handle.net/10150/185249

► A fossil magnetic field embedded in the radiative core of the Sun has been thought possible for some time now. However, such a fossil magnetic… (more)

▼ A fossil magnetic field embedded in the radiative core of the Sun has been thought possible for some time now. However, such a fossil magnetic field has, a priori, not been considered a visible phenomenon due to the effects of turbulence in the solar convection zone. Since a well developed theory (referred to herein as magnetohydrodynamic dynamo theory) exists for describing the regeneration of magnetic fields in astrophysical objects like the Sun, it is possible to quantitatively evaluate the interaction of a fossil magnetic field with the magnetohydrodynamic dynamo operating in the solar convection zone. In this work, after a brief description of the basic dynamo equations, a spherical model calculation of the solar dynamo is introduced. First, we calculate the interaction of a fossil magnetic field with a dynamo in which the regeneration mechanisms of cyclonic convection and large-scale, nonuniform rotation are confined to spherical shells. It is argued that the amount of amplification or suppression of a fossil magnetic field will be smallest for a uniform distribution of cyclonic convection and nonuniform rotation, as expected in the Sun. Secondly, we calculate the interaction of a fossil magnetic field with a dynamo having a uniform distribution of cyclonic convection and large-scale, nonuniform rotation. We find that the dipole or quadrupole moments of a fossil magnetic field are suppressed by factors of -0.35 and -0.37, respectively. The dynamo modified fossil field, superimposed on the theoretically calculated magnetic fields of the solar magnetic cycle, are compared with the actual sunspot cycle and solar magnetic fields as observed by others, indicating that a fossil magnetic field may be responsible for asymmetries in the sunspot cycle and an observed solar magnetic quadrupole moment. Further observations and reduction of the data are required before the presence of a fossil magnetic field can be established. A discussion is given of the implications for the Sun if a fossil magnetic field is observed and identified. It is considered most likely that a fossil magnetic field would be a remnant of the possible Hayashi phase of a fully convective, protosun. Other possibilities also exist. Advisors/Committee Members: Levy, Eugene (advisor).

Subjects/Keywords: Solar magnetic fields.; Magnetohydrodynamics.; Dynamo theory (Cosmic physics); Astrophysics.

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APA (6^th Edition):

BOYER, D. W. (1982). MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS. (Doctoral Dissertation). University of Arizona. Retrieved from http://hdl.handle.net/10150/185249

Chicago Manual of Style (16^th Edition):

BOYER, DARRYL WILLIAM. “MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS. ” 1982. Doctoral Dissertation, University of Arizona. Accessed January 27, 2021. http://hdl.handle.net/10150/185249.

MLA Handbook (7^th Edition):

BOYER, DARRYL WILLIAM. “MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS. ” 1982. Web. 27 Jan 2021.

Vancouver:

BOYER DW. MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS. [Internet] [Doctoral dissertation]. University of Arizona; 1982. [cited 2021 Jan 27]. Available from: http://hdl.handle.net/10150/185249.

Council of Science Editors:

BOYER DW. MAGNETOHYDRODYNAMIC DYNAMOS IN THE PRESENCE OF FOSSIL MAGNETIC FIELDS. [Doctoral Dissertation]. University of Arizona; 1982. Available from: http://hdl.handle.net/10150/185249

Indian Institute of Science

15. Chatterjee, Piyali. Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability.

Degree: PhD, Faculty of Science, 2010, Indian Institute of Science

URL: http://etd.iisc.ac.in/handle/2005/657

► The Sun, by the virtue of its proximity to Earth, serves as an excellent astrophysical laboratory for testing our theoretical ideas. The Sun displays a… (more)

▼ The Sun, by the virtue of its proximity to Earth, serves as an excellent astrophysical laboratory for testing our theoretical ideas. The Sun displays a plethora of visually awe-inspiring phenomena including flares, prominences, sunspots, corona, CMEs and uncountable others. It is now known that it is the magnetic field of the Sun which governs all these and also the geomagnetic storms at the Earth, which owes its presence to the interaction between the geomagnetic field and the all-pervading Solar magnetic field in the interplanetary medium. Since the solar magnetic field affects the interplanetary space around the Earth in a profound manner, it is absolutely essential that we develop a comprehensive understanding of the generation and manifestation of magnetic fields of the Sun. This thesis aims at developing a state-of-the-art dynamo code SURYA1taking into account important results from helioseismology and magnetohydrodynamics. This dynamo code is then used to study various phenomenon associated with solar activity including evolution of solar parity, response to stochastic fluctuations, helicity of active regions and prediction of future solar cycles. Within last few years dynamo theorists seem to have reached a consensus on the basic characteristics of a solar dynamo model. The solar dynamo is now believed to be comprised of three basic processes: (i)The toroidal field is produced by stretching of poloidal field lines primarily inside the tachocline – the region of strong radial shear at the bottom of the convection zone. (ii) The toroidal field so formed rises to the surface due to magnetic buoyancy to form active regions. (iii) Poloidal field is generated at the surface due to decay of tilted active regions – an idea attributed to Babcock (1961) & Leighton (1969). The meridional circulation then carries the poloidal field produced near the surface to the tachocline. The profile of the solar differential rotation has now been mapped by helioseismology and so has been the poleward branch of meridional circulation near the surface. The model I describe in this thesis is a two-dimensional kinematic solar dynamo model in a full sphere. Our dynamo model Surya was developed over the years in stages by Prof. Arnab Rai Choudhuri, Dr. Mausumi Dikpati, Dr. Dibyendu Nandy and myself. We provide all the technical details of our model in Chap. 2 of this thesis. In this model we assume the equatorward branch of the meridional circulation (which hasn’t been observed yet), to penetrate slightly below the tachocline (Nandy & Choudhuri 2002, Science, 296, 1671). Such a meridional circulation plays an important role in suppressing the magnetic flux eruptions at high latitudes. The only non-linearity included in the model is the prescription of magnetic buoyancy. Our model is shown to reproduce various aspects of observational data, including the phase relation between sunspots and the weak, efficient. An important characteristic of our code is that it displays solar-like dipolar parity (anti-symmetric toroidal fields across… Advisors/Committee Members: Choudhuri, Arnab Rai (advisor).

Subjects/Keywords: Solar Magnetic Field; Magnetism (Astrophysics); Helioseismology; Solar Dynamo Models; Solar Oscillations; Helicity; Solar Dynamo Theory; Stochastic Fluctuations; Mean Field Dynamo Model; Magnetic Flux Tube; Poloidal Field Accretion; Dynamo Model SURYA; Astrophysics

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APA (6^th Edition):

Chatterjee, P. (2010). Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability. (Doctoral Dissertation). Indian Institute of Science. Retrieved from http://etd.iisc.ac.in/handle/2005/657

Chicago Manual of Style (16^th Edition):

Chatterjee, Piyali. “Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability.” 2010. Doctoral Dissertation, Indian Institute of Science. Accessed January 27, 2021. http://etd.iisc.ac.in/handle/2005/657.

MLA Handbook (7^th Edition):

Chatterjee, Piyali. “Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability.” 2010. Web. 27 Jan 2021.

Vancouver:

Chatterjee P. Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability. [Internet] [Doctoral dissertation]. Indian Institute of Science; 2010. [cited 2021 Jan 27]. Available from: http://etd.iisc.ac.in/handle/2005/657.

Council of Science Editors:

Chatterjee P. Understanding The Solar Magnetic Fields :Their Generation, Evolution And Variability. [Doctoral Dissertation]. Indian Institute of Science; 2010. Available from: http://etd.iisc.ac.in/handle/2005/657

16. Donnelly, Cara. Shearing waves and the MRI dynamo in stratified accretion discs.

Degree: PhD, 2014, University of Cambridge

URL: https://doi.org/10.17863/CAM.16134 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633480

► Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet… (more)

▼ Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet formation. We discuss two different methods of angular momentum transport: first, generation of acoustic waves by mixing of inertial waves, and second, the generation of a self-sustaining magnetic field via the magnetorotational instability (MRI) which would be a source of dissipative turbulence. Previous local simulations of the MRI have shown that the dynamo changes character on addition of vertical stratification. We investigate numerically 3D hydrodynamic shearing waves with a conserved Hermitian form in an isothermal disc with vertical gravity, and describe the associated symplectic structure. We continue with a numerical investigation into the linear evolution of the MRI and the undular magnetic buoyancy instability in isolated flux regions and characterise the resultant quasi-linear EMFs as a function of height above the midplane. We combine this with an analytic description of the linear modes under an assumption of a poloidal-toroidal scale separation. Finally, we use RAMSES to perform full MHD simulations in a zero net flux shearing box, followed by spatial and a novel temporal averaging to reveal the essential structure of the dynamo. We find that inertial modes may be efficiently converted into acoustic modes for "bending waves", despite a fundamental ambiguity in the inertial mode structure. With our linear MRI and the undular magnetic buoyancy modes we find the localisation of the instability high in the atmosphere becomes determined by magnetic buoyancy rather than field strength for small enough azimuthal wavenumber, and that the critical Alfven speed below which the dynamo can operate increases with increasing distance from the midplane. We calculate analytically quasi-linear EMFs which predict both a vertical propagation of toroidal field and a method for creation of radial field. From our fully nonlinear calculations we find an electromotive force in phase with the toroidal field, which is itself 3π/2 out of phase with the radial (sheared) field at the midplane, and good agreement with our quasi-linear analytics. We have identified an efficient mechanism for generating acoustic waves in a disc. In our investigation of the accretion disc dynamo, we have reproduced analytically the EMFs calculated in our simulations, given arguments based on the phase of relevant quantities, several correlation integrals and the scalings suggested by our analytic work. Our analysis contributes significantly to an explanation for the dynamo in an accretion disc.

Subjects/Keywords: 523.01; Protoplanetary disks; Angular momentum; Theory of Wave-motion; Shear flow; Dynamo theory (Cosmic physics)

…magnetic buoyancy 1.8.2 Dynamo theory . . . . . . . . . . 1.9 Thesis outline… …nothing which is the outcome of work done in collaboration. Shearing Waves and the MRI Dynamo… …of the MRI have shown that the dynamo changes character on addition of vertical… …spatial and a novel temporal averaging to reveal the essential structure of the dynamo. We find… …azimuthal wavenumber, and that the critical Alfv´en speed below which the dynamo can operate…

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APA (6^th Edition):

Donnelly, C. (2014). Shearing waves and the MRI dynamo in stratified accretion discs. (Doctoral Dissertation). University of Cambridge. Retrieved from https://doi.org/10.17863/CAM.16134 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633480

Chicago Manual of Style (16^th Edition):

Donnelly, Cara. “Shearing waves and the MRI dynamo in stratified accretion discs.” 2014. Doctoral Dissertation, University of Cambridge. Accessed January 27, 2021. https://doi.org/10.17863/CAM.16134 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633480.

MLA Handbook (7^th Edition):

Donnelly, Cara. “Shearing waves and the MRI dynamo in stratified accretion discs.” 2014. Web. 27 Jan 2021.

Vancouver:

Donnelly C. Shearing waves and the MRI dynamo in stratified accretion discs. [Internet] [Doctoral dissertation]. University of Cambridge; 2014. [cited 2021 Jan 27]. Available from: https://doi.org/10.17863/CAM.16134 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633480.

Council of Science Editors:

Donnelly C. Shearing waves and the MRI dynamo in stratified accretion discs. [Doctoral Dissertation]. University of Cambridge; 2014. Available from: https://doi.org/10.17863/CAM.16134 ; https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633480

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