Investigation of equatorial ionosphere scintillations using GNSS observations from Hong Kong region

Abstract

Transionospheric radio waves may experience rapid fluctuations in signal amplitude and/or phase when random electron density irregularities exist in the ionosphere. This phenomenon is usually termed as ionospheric scintillation. Ionospheric scintillation has gained significant attentions in the community of Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS), as it can result in cycle slips or even complete loss of lock of GNSS signals. While being affected by ionospheric scintillations, GNSS signals effectively provide data sources to investigate the morphology of ionospheric scintillations and the formation and evolution of ionospheric irregularities that cause scintillations. The regions most likely to have ionospheric scintillation occurrences are the equatorial bands within ± 20° of magnetic equator and the auroral and polar zones. Hong Kong is a typical low-latitude region with geographic latitude around 22.33° N and longitude: around 114.20° E (magnetic latitude around 15.7° N). Ionospheric scintillations of GNSS signals can be observed frequently in Hong Kong. This thesis focuses on the investigation of equatorial ionospheric scintillations using GNSS observations from Hong Kong during the 24th solar cycle. The GNSS observations are collected by ground-based GNSS ionospheric scintillation monitors and GPS receivers onboard Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites. Several studies are conducted based on ionospheric scintillation indices (S4/σφ) and ionospheric rate of change of Total Electron Content index (ROTI) that are derived from those GNSS observations. Firstly, a detailed description of temporal-spatial characteristics of ionospheric scintillations as well as the variability of ionospheric plasma irregularities associated with scintillation are presented. Ionospheric plasma irregularities of a few hundred meters in size are quantified by 1-s sampled ROTI. The study is conducted to elucidate ionospheric scintillation occurrences over Hong Kong as a typical equatorial low-latitude region. Secondly, a correlation analysis of ionospheric scintillation indices (S4/σφ) and ROTI for low latitude regions is carried out, to demonstrate the feasibility of utilizing ROTI derived from 1-sec sampled GPS legacy observations of common non-scintillation GNSS receivers to quantify and describe ionospheric scintillation activity. Through the analysis, a high correlation is observed between S4/σφ and ROTI. It is suggested that the common geodetic GNSS network would provide valuable data source for ionospheric scintillation studies. Thirdly, an investigation of exploiting new/multi-GNSS signals (i.e., GPS, GLONASS, Galileo and BeiDou) to characterize ionospheric plasma irregularities and associated scintillations, as well as characterizing ionospheric scintillations of multi-GNSS signals is conducted, after demonstrating the correlation between ionospheric scintillation indices (S4/σφ) and ROTI. The results for the first time show that there is an inconsistency in 1-sec sampled ROTI derived from multi-GNSS observations, and demonstrate that diverse tracking techniques of current GNSS receivers may contribute to the inconsistency. Lastly, an observational study of ionospheric responses in terms of the occurrence of ionospheric irregularities and scintillations to a tropical cyclone over Hong Kong is presented. This case study for the first time observes the possible sign of coupling between the ionosphere and the lower atmosphere, and indicates the passage of tropical cyclones may degrade the quality of GNSS signals though the ionospheric responses. The findings presented in the thesis extend current knowledge of ionospheric irregularities and scintillations of GNSS signals over Hong Kong, for developing efficient tool to forecast and predict ionospheric scintillations in future. Meanwhile, the findings at this typical low-latitude region would be contributed to both GNSS community and ionosphere space weather research.