Exploring the Origin and Dynamics of Solar Magnetic Fields

Hazra, Soumitra (2016) Exploring the Origin and Dynamics of Solar Magnetic Fields. PhD thesis, Indian Institute of Science Education and Research Kolkata.

PDF (PhD thesis of Soumitra Hazra (09RS028)) Soumitra_Hazra.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

It is expected that during grand minima phases, the Babcock-Leighton mechanism would not be able to produce poloidal field as this mechanism relies on the presence of sunspots on the solar surface. This leads to the following fundamental question: How does the solar cycle recover every time from these episodes? We address this question through diverse means. In chapter 2 of this thesis, we develop a mathematical, low order time delay dynamo model (based on delay differential equations) removing all spatial dependence terms from the magnetic induction equation and mimic flux transport through the introduction of finite time delays in the system. By introducing fluctuations in the Babcock-Leighton source term of this low order dynamo model, we, for the first time explicitly demonstrate that a solar cycle model based on the Babcock-Leighton mechanism alone can not recover from a grand minima. We find that an additional poloidal field generation mechanism effective on weak magnetic field is necessary for recovery of the sunspot cycle from grand minima like episodes. In chapter 3 of this thesis, we utilize this state-of-the spatially extended model in a solar like geometry to validate our findings on entry and exit from Maunder minima like episodes. We find that stochastic fluctuations in the Babcock-Leighton mechanism is a possible candidate for triggering entry into grand minima phases. While direct observations over the last fifty years have shown that the solar magnetic cycle exhibits dipolar (odd) parity, our results suggest that the solar cycle has a significant probability to reside in the quadrapolar (even) parity state. Our findings may open the pathway for predicting parity flip in the Sun. In chapter 4 of this thesis, we explore whether flux transport dynamos could function with such a shallow meridional flow and discuss the consequences that this scenario would have on our traditional understanding of magnetic field dynamics in the solar interior. We demonstrate that dynamo models of the solar cycle can produce solar-like solutions with a shallow meridional flow if the effects of turbulent pumping of magnetic flux is taken into account. In the second part of this thesis, we utilize satellite observations to explore the dynamics of magnetic fields in the solar atmosphere. The solar corona, the outer atmosphere of the Sun is very hot compared to the solar surface and can reach millions of degrees. There is controversy regarding the physical processes that heat the solar outer atmosphere to such high temperatures. Such high temperatures result in X-ray emission from the solar corona. In chapter 5, we discuss the observational techniques and the current theoretical understanding developed over time, necessary to explain coronal dynamics. In chapter 6 of this thesis, we explore the relationship between coronal X-ray brightness and sunspot magnetic fields using high resolution observations from the Solar Optical Telescope and X-Ray Telescope onboard the Hinode satellite (a joint JAXA-NASA space mission). We find that the total magnetic flux within active regions sunspot structures is the primary determinant of solar coronal X-ray luminosity suggesting that magnetic flux is the fundamental quantity that determines coronal heating. This result sets important constraints on theories of solar and stellar coronal heating. In chapter 7 of this thesis, a recently developed, flux-tube fitting technique is utilized to measure the non-potentiality (twist) of solar magnetic fields and test whether the kink-instability mechanism (following magnetic helicity conservation) can be a plausible initiation mechanism for solar flares. We demonstrate that those sunspot magnetic field structures in which the twist exceeds the threshold for kink instability are more prone to generate solar flares. This finding may lead to more accurate solar flare prediction schemes based on the kink instability mechanism.

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