Exploring the Origin and Dynamics of Solar Magnetic Fields

TL;DR

The thesis investigates the origin and dynamics of solar magnetic fields by combining magnetohydrodynamic theory, mean‑field dynamo concepts, and data‑driven analyses. It develops low‑order time‑delay dynamo models and explores surface flux transport mechanisms, highlighting the necessity of a weak‑field mean‑field α‑effect for recovering from grand minima and showing turbulent pumping can replace deep meridional flows in flux transport dynamos. The work also blends observations with theory to connect photospheric magnetic structure to coronal heating and eruptive activity, demonstrating that total magnetic flux largely governs coronal X‑ray brightness and that kink instability explains only a subset of flares. Overall, the study advances a nuanced view of solar dynamos, flux transport, and atmospheric heating, with implications for long‑term solar activity forecasting and stellar dynamo understanding. The results underscore the importance of multiple poloidal sources, turbulent pumping, and stochastic fluctuations in shaping the solar cycle’s amplitude, parity, and recovery from low activity phases.

Abstract

The Sun is a magnetically active star and is the source of the solar wind, electromagnetic radiation and energetic particles which affect the heliosphere and the Earths atmosphere. The magnetic field of the Sun is responsible for most of the dynamic activity of the Sun. This thesis research seeks to understand solar magnetic field generation and the role that magnetic fields play in the dynamics of the solar atmosphere. Specifically, this thesis focuses on two themes: in the first part, we study the origin and behaviour of solar magnetic fields using magnetohydrodynamic dynamo theory and modelling, and in the second part, utilizing observations and data analysis we study two major problems in solar physics, namely, the coronal heating problem and initiation mechanisms of solar flares.

Figures (46)

• Figure 1: A cartoon image of the structure of the Sun.
• Figure 2: SDO-HMI magnetogram image recorded on May 11, 2015 showing bipolar sunspot pairs within active region structures. In the image, white signifies positive polarity while black signifies negative polarity sunspots.
• Figure 3: Top panel: Plot of international sunspot number as a function of time in years. Bottom panel: Latitude vs time plot from recent high resolution observations. Background shows weak, diffuse radial field on the photosphere. This plot is widely known as the butterfly diagram. Image credit: Hathaway/NASA/MSFC.
• Figure 4: Parker's turbulent dynamo: (a) The $\omega$-effect: Poloidal field lines are stretched by differential rotation in the solar interior and produces the toroidal component of magnetic field. (b) $\alpha$-effect: At the time of rise through the convection zone, toroidal flux tubes are twisted due to helical turbulence and produces magnetic field components in the poloidal plane. Image credit: Hathaway/NASA/MSFC
• Figure 5: A cartoon image of the Babcock-Leighton mechanism: (a) Newly emerged bipolar magnetic regions with opposite leading/ following polarity patterns obeying Hale's polarity law. (b) Decaying bipolar magnetic regions, Trailing polarity goes to higher latitude while leading components reconnect across the equator. Image credit: Paul Charbonneau