| Summary: | 博士 === 國立中央大學 === 太空科學研究所 === 102 === Precisely knowing the current state of the rapidly changing ionospheric weather is important for warning about its impact on modern telecommunication and navigation systems. The effect of the ionosphere on radio waves transmitting from a satellite to a receiver can be used to estimate the plasma density along a signal path with measurements of the modulations on carrier phases and code pseudoranges recorded by dual-frequency receivers. Therefore, this thesis presents applications of global measurements of electron density from the ground- and space-based GPS observational systems to the in-depth investigation of the regional and global ionospheric weather events.
The ground-based GPS receivers, which are distributed widely over the world’s continents, provide us with an efficient and economic way to monitor the total electron contents (TECs, 1 TEC unit (TECU)=1016 el/m2) with high temporal resolutions. While the radio occultation (RO) observational systems, such as FORMOSAT-3/COSMIC (F3/C), measures the three-dimensional ionospheric electron density globally, including the oceans, deserts, and polar regions, where ground-based observatories are scarce. Since the ionospheric weather-generated signals measured by observational instruments are usually sparse and complicated (nonlinear and non-stationary), simple data process procedures and traditional harmonic and stationary assumptions may not be sufficient to expose the full aspects of the weather phenomena. Therefore, the objective of this thesis is to demonstrate how the analysis of GPS data can be aided with more advanced methods, such as statistical tools, nonlinear and non-stationary mathematical methods, data assimilation techniques, and a physical-based model.
After generally introducing the Earth’s ionosphere and ionospheric weather (Chapter 1) as well as the ground- and space-based GPS observational systems (Chapter 2), four events are investigated in detail. Chapter 3 shows the solid evidence of the eclipse-triggered bow wave with the help of the nonlinear and non-stationary time frequency data analysis method, the Hilbert-Huang transform (HHT). Chapters 4 and 5 are studies of ionospheric plasma density irregularities at low latitude and midlatitude, respectively, during geomagnetic storms. The results can help us to hypothesize the possible behavior of irregularities after storm onsets. In Chapter 5, the TEC maps are constructed by using the non-stationary wavelet-based covariance to study the finer-scale TEC structures near the auroral oval. Chapter 6 proposes a model neutral wind bias correction scheme to improve the electron density of the global physics-based Ionosphere Plasmasphere Electrodynamics (IPE) model at midlatitude. The agreement between the F3/C observations and the model simulations reveals that the eastward movement of the Southern Hemisphere Midlatitude Summer Nighttime Anomaly (southern MSNA) / Weddell Sea Anomaly (WSA) in the local time coordinate is primarily caused by the field-aligned projection of thermospheric neutral winds. The four studies shown in this thesis reveal that the GPS observational systems are capable of recording fully the features of the fast changing ionospheric weather on both regional and global scales. The physical mechanisms behind the ionospheric weather features are further examined and discussed.
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