Kinematic and Morphological Evolution and Dynamics of Coronal Mass Ejections in the Interplanetary Space

Abstract

Studies of Coronal mass ejections (CMEs) are scientifically intriguing and practically important. CMEs are the main driver of space weather that specifies plasma, magnetic and particle conditions in near-Earth space. When CMEs pass through and interact with the Earth’s magnetosphere, they can cause significant disruption in space and produce a variety of harmful effects on human’s technological systems from space to the ground. Many studies have been carried out to understand their evolution. However, their kinematic and morphological evolution as they pass from Sun to Earth is still poorly understood, largely due to the lack of direct observations. Since the launch of the twin-STEREO spacecraft in 2006, tracking of CMEs in interplanetary space was made available for the first time. Further, one could make unprecedented 3-D measurement of CMEs, thanks to the simultaneous observations from two vantage points in space. In this dissertation, I make use of STEREO observations to study the kinematic and morphological evolution of CMEs in interplanetary space. The Raytrace model is utilized as a powerful tool to measure CMEs evolution in 3D. I find that CME leading edge (LE) velocity converges from an initial range between 400 km/s and 1500 km/s at 5 to 10 RS to a narrow range between 500 km/s and 750 km/s at 50 RS. The expansion velocity is also found to converge into a narrow range between 75 km/s and 175 km/s. Both LE and expansion velocities are nearly constant after 50 RS. I further find that the acceleration of CMEs in the inner heliosphere from ∼ 10 to 90 RS can be described by an exponential function, with an initial value as large as ∼ 80 m/s2 but exponentially decreasing to almost zero (more precisely, less than ± 5 m/s2 considering the uncertainty of measurements). These results are important for constructing accurate space weather prediction models. In addition to the observational study, I have used the theoretical flux rope model to explain the observations, and find consistency between theory and observation. The evolution of CMEs can be explained by different forces that act on them: Lorentz force, thermal pressure force, gravity force, aero-dynamic drag force, and magnetic drag force. Based on a set of four events, I find that the drag coefficient from CME to CME is between 2.5 to 3.0, which is much smaller than the factor of twelve suggested by earlier studies. Therefore, we have been able to narrow down the range of drag coefficient, which helps improve the prediction of CME arrival time at the Earth. In the early stage of my Ph.D. study, working with a team, we have identified solar and interplanetary sources of all 88 major geomagnetic storms from 1996 to 2005. We classify the Solar-IP sources into three broad types: (1) S-type, in which the storm is associated with a single ICME and a single CME at the Sun; (2) M-type, in which the storm is associated with a complex solar wind flow produced by multiple interacting ICMEs arising from multiple halo CMEs launched from the Sun in a short period; (3) C-type, in which the storm is associated with a Corotating Interaction Region (CIR) formed at the leading edge of a high-speed stream originating from a solar coronal hole (CH). For the 88 major storms, the S-type, M-type, and C-type events number 53 (60 %), 24 (27 %), and 11 (13 %), respectively. For the 85 events for which the surface source regions could be investigated, 54 (63 %) of the storms originated in solar active regions, 11 (13 %) in quiet Sun regions associated with quiescent filaments or filament channels, and 11 (13 %) were associated with coronal holes. This study improves our understanding of geo-effective CMEs. In conclusion, the dissertation work has improved our understanding about the kinematic and morphologic evolution of CMEs in interplanetary space. In the future, a larger number of events need to be measured and modeled to further constrain CME evolution models, in particular, the drag coefficient and the polytropic index. We are confident with these studies. We are confident that our studies enable us to construct an accurate empirical model to predict the travel times of CMEs from the Sun to the Earth, thus improving our ability to forecast space weather events.