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McMaster University

1. 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 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)

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

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

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 19, 2021. http://hdl.handle.net/11375/18299.

MLA Handbook (7^{th} Edition):

Jackel, Benjamin. “Magnetic Dynamos: How Do They Even Work?.” 2015. Web. 19 Jan 2021.

Vancouver:

Jackel B. Magnetic Dynamos: How Do They Even Work?. [Internet] [Doctoral dissertation]. McMaster University; 2015. [cited 2021 Jan 19]. 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

McMaster University

2. 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 these simulations is to detect the source of the electromotive force - the driver of large scale magnetic field evolution. This electromotive force is responsible for the large scale dynamo action which builds and maintains the magnetic field against dissipation in plasmas. We compare the estimates of the electromotive force from the kinematic approximation of mean field theory - the most prevalent theory for astrophysical dynamos - with a modified version of mean field theory which restricts the electromotive force by the consideration of magnetic helicity conservation. We will show that in general the kinematic approximation overestimates the observed electromotive force for the majority of the simulation, while the term derived from the helicity conservation estimates the electromotive force very well. We will also illustrate the importance of the shear in the fluid to the growth and strength of the resulting large scale magnetic field. Too strong and the small scale dynamo does not grow enough to properly seed a strong large scale dynamo. Too weak, and no large scale magnetic field is observed after the small scale dynamo has saturated. Finally, we will find that in order to maintain the strength of the emerged large scale magnetic dynamo we require a magnetic Prandtl number (Pr ≡ ν/η) that is at least an order of magnitude above unity.

Master of Science (MSc)

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

Record Details Similar Records

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

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 19, 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. 19 Jan 2021.

Vancouver:

Cridland AJ. Direct Numerical Simulations of Magnetic Helicity Conserving Astrophysical Dynamos. [Internet] [Masters thesis]. McMaster University; 2013. [cited 2021 Jan 19]. 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

McMaster University

3. Jackel, Benjamin. Transport of Magnetic Helicity in Accretion Disks.

Degree: MS, 2010, McMaster University

URL: http://hdl.handle.net/11375/9046

Astrophysical disks are found in many areas of astrophysics, from the protoplanetary disks in which planets are thought to be born, to the accretion disks around white dwarfs, merging stars, and black holes. The key to understanding these disks, is to understand how material overcomes the rotational support and acretes. 'Whatever mechanism is responsible must necessarily explain the transport of angular momentum outward. The current mechanism used to explain this is the magnetorotational instability (MRI). Its ability to transport angular momentum as well as drive a magnetic dynamo, will be discussed in this thesis. The linear equations of motion for a locally Cartesian patch will be solved numerically to get the time evolution of the magnetic and velocity fields. From these solutions, quadratic quantities in the perturbation variables will be calculated, namely the angular momentum and magnetic helicity. The time evolution of these quantities can tell us about the MRI's ability to both transport angular momentum and drive a dynamo through magnetic helicity. By solving the equations of motion in a locally Cartesian patch of a shearing disk, I have calculated the flux of angular momentum and magnetic helicity. The time evolution of these quantities shows that the ability to transport magnetic helicity is very similar the ability to transport angular momentum. This relation is true for a parameter space which corresponds to the asymptotic limit for the MRI.

Master of Science (MS)

Subjects/Keywords: Physics and Astronomy; Astrophysics and Astronomy; Astrophysics and Astronomy

Record Details Similar Records

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

APA (6^{th} Edition):

Jackel, B. (2010). Transport of Magnetic Helicity in Accretion Disks. (Masters Thesis). McMaster University. Retrieved from http://hdl.handle.net/11375/9046

Chicago Manual of Style (16^{th} Edition):

Jackel, Benjamin. “Transport of Magnetic Helicity in Accretion Disks.” 2010. Masters Thesis, McMaster University. Accessed January 19, 2021. http://hdl.handle.net/11375/9046.

MLA Handbook (7^{th} Edition):

Jackel, Benjamin. “Transport of Magnetic Helicity in Accretion Disks.” 2010. Web. 19 Jan 2021.

Vancouver:

Jackel B. Transport of Magnetic Helicity in Accretion Disks. [Internet] [Masters thesis]. McMaster University; 2010. [cited 2021 Jan 19]. Available from: http://hdl.handle.net/11375/9046.

Council of Science Editors:

Jackel B. Transport of Magnetic Helicity in Accretion Disks. [Masters Thesis]. McMaster University; 2010. Available from: http://hdl.handle.net/11375/9046