Space Weather Drivers and their Geoeffectiveness

Pal, Sanchita (2020) Space Weather Drivers and their Geoeffectiveness. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sanchita Pal (15RS036)) 15RS036.pdf - Submitted Version Restricted to Repository staff only Download (22MB)

Abstract

Understanding the Earth's natural environment for forecasting day to day variability of the planetary atmospheric state is of utmost importance and constitutes one of the oldest human endeavors that began in the 18th century. With the advent of the space age in the 20th century, the effects of space weather were gradually uncovered, and subsequently, its exploration was initiated. Space weather originates at the Sun and is primarily driven by intense solar activities, which often manifest as large-scale magnetized plasma being ejected into the heliosphere. Such a phenomenon is known as a Coronal Mass Ejection (CME). CMEs often inject energetic particles and large amounts of energy into the Earth's magnetosphere thereby resulting in geomagnetic storms. Their impact on the planetary space environments is capable of causing severe harm to satellites, space-based technologies, health of astronauts involved in long-duration space missions, global communication and navigation systems, air-traffic on polar routes and high-voltage power grids. The ability of CMEs to drive geomagnetic storms is referred to as geoeffectiveness. The geoeffectiveness of CMEs depend on their kinematics and magnetic properties, which might evolve in the course of their interplanetary propagation. Forecasting space weather through prior estimation of the geoeffectiveness of CMEs is quite a challenging task as it has to be made in a dynamic and complex solar-terrestrial system with considerable accuracy, reliability and timeliness. This research presented in this dissertation investigates and thereby, improves the current understanding of the probable origin and Sun-Earth evolution of the properties determining the geoeffectiveness of CMEs. With an intention to predict the geoeffectiveness of CMEs, we attempt to constrain their magnetic structures before they arrive at Earth. We begin in Chapter 1 with an overview of space weather, its origins, drivers and potential effects on Earth, and subsequently, provide an outline of the connections between the properties of one of the significant space weather drivers, i.e., CMEs, and their geoeffectiveness. CMEs are born in solar magnetic structures, whose properties ascertain their associated eruption characteristics. Although the ambient solar wind infuences interplanetary CMEs (ICMEs), the inherent features of ICMEs are determined by the properties of their associated CMEs and solar sources. The near-Sun kinematics of CMEs determine the severity and arrival time of the resulting geomagnetic storms. We investigate the relationship between the deprojected speed and kinetic energy of CMEs and magnetic measures of their solar sources and intrinsic flux rope characteristics. The near-Sun velocity and kinetic energy of CMEs are found to be well correlated with the associated magnetic reconnection flux. On the contrary, the correlation between CME speed and their source active region size & global nonpotentiality is found to be comparatively weak. A statistically significant empirical relationship is found between the CME speed and reconnection flux which may be utilized for prediction purposes. Apart from this, we find that CME kinematics are related to the axial magnetic field intensity and relative magnetic helicity of their intrinsic flux ropes. These results constrain processes related to the origin and propagation of CMEs and may lead to better empirical forecasting of their arrival speed, time and geoeffectiveness. This research is reported in Chapter 2. The proper understanding of the origin and evolution of the magnetic properties is necessary for determining the geoeffectiveness of Earth-directed interplanetary coronal mass ejections (ICMEs). We compare their magnetic properties, specifically magnetic flux and helicity with those of their solar sources with the aim of understanding the origin of magnetic properties of flux ropes. The magnetic helicity describes the twisting, writhing and linking of ICME flux ropes at 1 AU. It is observed that the poloidal flux and helicity of 1-AU flux ropes, i.e., magnetic clouds (MCs) are highly relevant to low-corona magnetic reconnection at the associated eruption site. In contrast to the above, the toroidal flux of MC flux ropes is a fraction of the total magnetic reconnection flux. These results indicate that CMEs are formed due to low-coronal magnetic reconnection at their solar sources, a process that transfers magnetic properties to the flux ropes. This research is reported in Chapter 3. While interacting with the ambient solar wind magnetic fields (i.e., heliospheric open flux) during interplanetary passages, MCs may lose a substantial amount of their initial magnetic flux via magnetic reconnection, which in some cases, reduce their geoeffectiveness. A linear correlation is found between the eroded flux of MCs and solar open flux, which is consistent with the scenario that MC erosion is mediated via the local heliospheric magnetic field draping around an MC during its interplanetary propagation. The solar open flux is governed by the sunspot cycle. Thus, we uncover a hitherto unknown pathway for solar cycle modulation of the properties of MCs. This research is reported in Chapter 4. The MCs having prolonged southward magnetic field components are bound to expose the Earth's atmosphere to the heliospheric environment via magnetic reconnection. Thus, it is essential to predict the magnetic profile of Earth-directed MCs to estimate their geomagnetic responses. The configuration of a flux-rope CME can be approximated as a radially expanding force-free cylindrical structure. Near-Sun geometrical, magnetic and kinetic properties of CMEs are combined with the self-similarly expanding force-free cylindrical model to forecast the magnetic vectors within the Earth-directed segments of MCs. A proper estimation of near- Sun CME properties can lead to prediction of MC magnetic profiles with minimum deviations from their in situ observations. We employ this approach and devise a methodology that is quite successful in predicting near-Earth magnetic profile of MCs. This research is reported in Chapter 5. The ultimate goal of this thesis is to investigate the key factors governing the geoeffectiveness of CMEs and devise improved methodologies for predicting their geoeffectiveness - thereby contributing significantly to understanding and forecasting space weather.

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