Modelling the Evolution of Solar Coronal Magnetic Fields and its Heliospheric Consequences

Dash, Soumyaranjan (2022) Modelling the Evolution of Solar Coronal Magnetic Fields and its Heliospheric Consequences. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Soumyaranjan Dash (16RS002)) 16RS002.pdf - Submitted Version Restricted to Repository staff only Download (38MB)

Abstract

The impact of the magnetic field dynamics of our middle-aged host star, the Sun, sitting hundreds of millions of kilometers away, is manifested in various forms. The particle flux, electromagnetic radiation, and thermal radiation from the Sun create space weather. The dynamic space weather and climate conditions are caused by variations in solar magnetic activity across different timescales. Solar activity typically follows an 11-year cycle. Similarly, magnetic field mediated phenomena, e.g. solar flares and coronal mass ejections (CMEs), follow roughly the same 11-year cycle periodicity. Solar flares and CMEs are hazardous when directed towards Earth as they inject energies of the order of 10²⁶ joules into the interplanetary medium. The resulting geomagnetic storm impacts our telecommunication system, navigation near polar routes, health of astronauts, and space-based technologies. Apart from such transient eruptive events, solar magnetic activity also influences the propagation of cosmic ray particles, which are relevant for space climate studies as well as for deciphering past solar activity. The physical process of magnetic field generation and evolution is complex, and non-linear. Hence prediction of solar magnetic field dynamics is challenging. It is difficult to measure the magnetic fields in the solar corona due to its low density and the bright surface radiation in the background, requiring extremely sensitive polarization measurements. However, it is important to constrain coronal magnetic fields because they govern the flow of the solar wind, the dynamics of eruptive events, and coronal heating. When the radiation from the solar surface is occulted during a total solar eclipse, we observe the multiple radii long streamers and the million-degree hot solar corona. Eclipse observations are utilized to constrain the predictive capability of our numerical models to simulate the coronal magnetic field. In this thesis, we utilize numerical models to compute the large-scale coronal magnetic fields and study their impact on the heliosphere. The significance of solar coronal magnetic field dynamics is described in Chapter 1 with an overview of observations of the solar corona. This is followed by a discussion on different coronal structures associated with the large-scale magnetic fields and their impacts. We also briefly introduce theoretical concepts that are utilized to understand the coronal field configuration. A brief discussion on the models used for our research is provided in Chapter 2. The essential theoretical background for the models and a few examples are provided to understand the formalism in detail. Accurate prediction of solar surface magnetic field distribution is essential to model the large-scale coronal magnetic fields with reasonable confidence. We present our predictive approach for modeling the coronal magnetic field configuration in Chapter 3 and discuss its application to the 02 July 2019 eclipse. We also demonstrate the importance of accurate polar field constraints on the structuring of the global corona along with polarization characteristics computation based on the predicted field distribution. Cosmic rays which are an important driver of state of the heliosphere plays a role in modulating space climate. Cosmic ray flux is also important for uncovering past solar activity. The flux of cosmic rays at earth is governed by open flux variations which is in turn governed by coronal magnetic field structures. In Chapter 4 we present a novel work which demonstrates how long-term variations in the solar magnetic output as modeled by the solar dynamo mechanism reflects in variations in the coronal structure, solar open flux and the cosmic ray modulation potential. We believe such a study will help us in establishing causality between variations of cosmic rays and the solar open flux and the dynamics of the magnetic fields in the Sun’s interior. Magnetic fields from the solar corona extend out into the heliosphere and is manifested as open flux. They interact with CME flux tubes during their passage in the interplanetary medium. Our research on understanding such interaction and its impact on their geoeffectiveness is provided in Chapter 5. We have described the component for calculating the large-scale coronal magnetic fields for this project. We discovered a solar cycle link between eroded CME flux (erosion due to their interaction with the large-scale solar magnetic fields) mediated via the solar open flux. In Chapter 6 we end with a summary of our work and discuss future applications that may be explored with the modeling approaches discussed here. Individual chapters (except the introduction, model details and concluding summary chapter) are written in the form of research publications, some of which are already published. They are self-contained with independent introductions and conclusions.

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