Understanding the behavior of the Sun's large scale magnetic field and its relation with the meridional flow

TL;DR

This thesis advances understanding of the solar magnetic cycle by combining observational analysis with improved flux transport dynamo models, including 3D simulations that incorporate realistic physical mechanisms.

Contribution

It introduces a 3D flux transport dynamo model that better captures magnetic buoyancy, meridional flow variations, and non-axisymmetric effects, enhancing theoretical understanding of solar magnetic behavior.

Findings

Irregular solar cycle properties explained by magnetic buoyancy treatments.

Meridional flow variations linked to the solar cycle via Lorentz force feedback.

3D models reveal effects of non-axisymmetric turbulence on magnetic field evolution.

Abstract

In this thesis, various studies are performed leading to better understanding of the 11-year solar cycle and its theoretical modeling with the flux transport dynamo model. Although this is primarily a theoretical thesis, there is a part dealing with the analysis of observational data. The various proxies of solar activity from various observatory including the sunspot area records of Kodaikanal Observatory have been analyzed to study the irregular aspects of solar cycles and an analysis has been carried out on the correlation between the decay rate and the next cycle amplitude. Theoretical analysis starts with explaining how the magnetic buoyancy has been treated in the flux transport dynamo models, and advantages and disadvantages of different treatments. It is found that some of the irregular properties of the solar cycle in the decaying phase can only be well explained using a particular treatment of the magnetic buoyancy. Next, the behavior of the dynamo with different spatial structures of the meridional flow based on recent helioseismology results has been studied. A theoretical model is constructed considering the back reaction due to the Lorentz force on the meridional flows which explains the observed variation of the meridional flow with the solar cycle. Finally, some results with 3D FTD models are presented. This 3D model is developed to handle the Babcock-Leighton mechanism and magnetic buoyancy more realistically than previous 2D models and can capture some important effects connected with the subduction of the magnetic field in polar regions, which are missed in 2D surface flux transport models. This 3D model is further used to study the evolution of the magnetic fields due to a turbulent non-axisymmetric velocity field and to compare the results with the results obtained by using a simple turbulent diffusivity coefficient.