| dc.description.abstract | Solar flares and Coronal Mass Ejections (CMEs) are among the most violent and energetic phenomena observed in the solar atmosphere, resulting from the sudden release of immense amounts of energy. A typical solar flare is characterized by a rapid increase in light emission across a wide range of the electromagnetic spectrum, whereas a CME is defined as the expulsion of vast quantities of plasma and high-energy particles from the Sun into space.
Both solar flares and CMEs play crucial roles in shaping space weather, which can have profound effects on Earth. The intense radiation from solar flares can disrupt satellite communications, navigation systems, and power grids, while CMEs can cause geomagnetic storms that impact Earth's magnetosphere, leading to auroras and potential damage to satellites and other space-based technologies. Understanding these phenomena is vital for both scientific research and technological advancements. Solar flares and CMEs both provide insights into the fundamental processes of energy release and particle acceleration in the Sun's atmosphere and offer a unique perspective on the Sun's magnetic field dynamics. Studying the mechanisms behind these events helps scientists develop models to predict solar activity and mitigate its impacts on space weather. This knowledge is essential for improving space weather forecasting and developing strategies to protect Earth's technological infrastructure from solar disturbances.
In addition to their scientific importance, solar flares and CMEs are also of great interest for space exploration. Understanding the behavior of these solar phenomena is critical for ensuring the safety of astronauts and spacecraft. As human space exploration extends beyond Earth's orbit, predicting and preparing for the effects of solar flares and CMEs will be essential for the success of long-duration missions. Thus continued research into these dynamic solar events will enhance our ability to predict and mitigate their impacts, contributing to the advancement of space science and the protection of Earth's technological systems.
In the thesis, we provide an overview of solar eruptions, alongside a discussion of the numerical equations that govern the magneto-hydrodynamic (MHD) simulations used in our study. We also describe the observational instruments utilized to gather data, allowing us to compare our simulation results with observational insights.
Next we provide our study regarding the photospheric magnetic imprints of solar flares associated with coronal mass ejections (CMEs). Solar flares often leave distinct imprints on the magnetic field at the photosphere, typically observed as abrupt and permanent changes in the downward-directed Lorentz force within localized regions of the active region. Our study aims to differentiate eruptive and confined solar flares by analyzing the variations in the vertical Lorentz force. We focus on 26 eruptive and 11 confined major solar flares, all stronger than the GOES M5 class, observed between 2011 and 2017. For this analysis, we utilize SHARP vector-magnetograms obtained from NASA's Helioseismic and Magnetic Imager (HMI).
In addition to observational data, we incorporate data from two synthetic flares derived from a $\delta$--sunspot simulation as reported by \cite{chatterjee2016repeatedflare}. Our methodology involves estimating changes in the horizontal magnetic field and the total Lorentz force integrated over areas around the polarity inversion line (PIL), which encompasses the flare locations. To achieve this, we developed a semi-automatic contouring algorithm that delineates the region near the Polarity Inversion Line (PIL) where the most significant magnetic changes occur.
Our findings indicate a rapid increase in the horizontal magnetic field along the flaring PIL, coinciding with significant changes in the downward-directed Lorentz force in the same vicinity. A crucial aspect of our results is the identification of a threshold in Lorentz force changes. All confined flares in our study exhibit total Lorentz force changes of less than $1.8 \times 10^{20} dyne$. This threshold proves to be a significant factor in effectively distinguishing between eruptive and confined flares.
Moreover, for eruptive events where the change in Lorentz force is below the threshold, we noticed a significantly higher ribbon distance between the parallel flare ribbons, typically exceeding 15 Mm at the onset time of the flare. This indicates a potential implication between the ribbon separation and the magnitude of the Lorentz force change during eruptive events. Therefore, ribbon separation could serve as an additional factor to consider when studying the magnetic imprints associated with the solar flares. We applied the similar procedure to the B \& C class synthetic flare events and noticed a remarkable resemblance in the temporal evolution with the observational data. Our observation indicates that the Lorentz force propagates from the reconnection site towards the photosphere. This provides valuable insights into understanding the mechanisms of flare-related upward impulse transmission, which is crucial for the associated coronal mass ejection (CME) dynamics. Our study not only enhances the understanding of the magnetic and dynamic characteristics of solar flares but also has significant implications for predicting the potential impact of these solar events on space weather. The ability to distinguish between eruptive and confined flares based on Lorentz force changes could lead to better understanding of the relation between the sunspot topology and the ejective flaring.
Finally, we shed light on the changes in reconnection flux throughout the evolution of CMEs, from their onset to eruption. Additionally, we correlate these reconnection flux changes with the velocity of the ejected material. Coronal mass ejections (CMEs) are among the most powerful drivers of space weather, with magnetic flux ropes (MFRs) widely considered their primary precursors. However, the three-dimensional variation in reconnection flux during the evolution of MFRs throughout CME eruptions remains insufficiently understood. Here, we present a detailed study utilizing a realistic three-dimensional magneto-hydrodynamic (3D MHD) model to explore the temporal evolution of reconnection flux during MFR evolution. Our approach integrates both numerical simulations and observational data to provide a comprehensive analysis.
We begin our investigation with an initial coronal configuration characterized by an isothermal atmosphere and a potential arcade magnetic field, beneath which an MFR emerges at the lower boundary. Our model incorporates radiative cooling and a coronal heating function. Additionally, we have included field-aligned Spitzer thermal conduction. However, our model does not account for solar wind. As the MFR rises, we observe significant stretching and compression of the overlying magnetic field. This dynamic process leads to the formation of a current sheet, initiating magnetic reconnection. The reconnection process gradually intensifies, eventually resulting in the impulsive expulsion of the flux rope.
Our simulation generates two homologous CME eruptions, each characterized by an impulsive increase in kinetic energy and a corresponding release of magnetic energy. The peak velocities of the CMEs in our simulation are approximately 224 $\rm km \, s^{-1}$ and 213 $\rm km \, s^{-1}$. In the first eruption, the magnetic flux rope exhibits only torus instability, while in the second eruption, it demonstrates both torus and kink instabilities. Our analysis focuses on the temporal evolution of reconnection fluxes during these two successive MFR eruptions, with the twisted flux continuously emerging through the lower boundary. To complement our simulations, we perform a parallel analysis using observational data from NASA's Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) along with Solar TErrestrial RElations Observatory (STEREO-A) spacecraft for a specific eruptive event.
Our findings indicate that changes in reconnection flux play a crucial role in determining CME dynamics. Specifically, the acceleration of CMEs are linearly correlated (CC = 0.58 and 0.81) to the amount of the reconnection flux, highlighting the importance of reconnection dynamics in the overall process of CME initiation and propagation.
This nearly realistic simulation of a solar eruption offers significant insights into the complex dynamics of CME initiation and progression. The ability to model and understand the temporal evolution of reconnection flux in three dimensions provides a more accurate and detailed picture of the mechanisms driving CMEs. Consequently, our study enhances the understanding of how the reconnection flux changes over time during the solar eruption processes and demonstrates the vital role of reconnection flux and velocity of CMEs from the onset to the eruption.
Finally, we provide a brief overview of our future goals. In this thesis, we have presented realistic magneto-hydrodynamic (MHD) simulations of solar eruptions alongside observed space-based data. By comparing the simulation results with observational data from instruments, we aim to enhance our understanding to better represent the solar eruptions and their impacts. Our research not only closes the gap between theoretical models and observational data but also underscores the crucial role of concurrent observation and simulation in understanding solar eruptions. By leveraging both observational data and modeling efforts, this thesis lays the groundwork for further improvements of such lower corona models and techniques to a border range of solar eruptions study and their implications for solar physics. | en_US |
