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Title: Probe the ionospheric D region with lightning sferics based on a ray-tracing earth-ionosphere wave guide model
Authors: Qin, Zilong
Degree: Ph.D.
Issue Date: 2019
Abstract: The lowermost part of the ionosphere, D region (or D layer), has an extremely low electron density, is dubbed the "ignorosphere" due to the difficulty of systematic measurement. On the other hand, the electromagnetic wave emitted by lightning discharge has a spectrum ranging from several Hz through serval GHz with the power concentrating at bands from several kHz through several 10 kHz at distances over 100 km, which highly interferes with the lower electron density region of the ionosphere - D region. With the development of lightning detection techniques, the lightning sferic has been becoming a valuable tool for probing the ionospheric D region at high temporal and spatial resolutions. This thesis aims to develop a comprehensive method that can accurately retrieve the electron density profile of the ionospheric D region with lightning sferic measurements. The main contributions are listed below. 1) A comprehensive Ray-Theory and Transfer-Matrix-Method based model for a lightning electromagnetic pulse (LEMP) propagating in the Earth-Ionosphere waveguide (EIWG) has been proposed, i.e., RT-EIWG model. The model involves the representation of a lightning source, parameterization of the lower ionosphere, derivation of a transfer function representing all effects of EIWG on LEMP skywave and determination of attenuation mode of the LEMP ground wave. The lightning source is simplified as an electric point-dipole standing on the Earth surface with finite conductance. The transfer function for the skywave is derived based on the Ray-Theory and Transfer-Matrix-Method. The attenuation mode for the ground wave is solved from the Fock's diffraction equations. 2) The RT-EIWG model has then been applied to several lightning sferics observed in central China during the day and night times within 1000 km. The results show that the model can precisely predict the skywave for all these observed lightning sferics. Both simulations and observations indicate that the lightning sferics in nighttime has a more complicated waveform than in the daytime. Particularly, when a LEMP propagates from east to west (Φ = 270°) and in the nighttime, its skywave tends to be a double-peak waveform (dispersed skywave) rather than a single-peak one. Such a dispersed skywave in nighttime may be attributed to the Faraday rotation phenomenon in the lower ionosphere. All these show that the RT-EIWG model works well in the interpretation of the interaction between the lightning sferic and the lower ionosphere during both day and night times. 3) For improving the model accuracy at large distances evaluating the model performance quantitatively, the RT-EIWG model has been further modified and compared with the full-wave FDTD model. The results show that this modified RT-EIWG is in close agreement with the full-wave FDTD method in modeling the lightning sferic in frequencies bands lower to 3 kHz, 5 kHz and 7 kHz for distances up to 500 km, 800 km and 1000 km, respectively. All these prove that the RT-EIWG model provides us an efficient way for retrieving the electron density profile of the lower ionosphere, and hence a way for monitoring the spatial and temporal variations of the lower ionosphere via lightning sferics.
4) Based on the above modified RT-EIWG model, we have developed a new inversion technique for retrieving the daytime Wait's Electron Density Profile (WEDP) of the ionospheric D region. The technique consists of the RT- EIWG model and a two-step curve fitting method. With this technique and numerous sferic waveforms recorded, we have analyzed over ten days' WEDP variation of the ionospheric D region over the southern coastal area of China. By correlating the source peak current (SPC) of lightning return strokes (RS) with the steepness β and height h of the retrieved WEDP, an explicit dependency between the SPC and the retrieved WEDP is found. Further analysis indicates that such a dependency might be caused by either the direct LEMP heating of the ionosphere or the potential connection between the RS source vertical extension and the SPC. With a large number of observed lightning sferics whose SPC <25 kA, a reasonable daytime variation of the WEDP of the ionospheric D region over the southern coastal area of China (a subtopic area) has been obtained and analyzed against the solar zenith angle (SZA). The SZA directly modules the vertical flux of Solar X-ray radiation during the daytime, thus closely related to the status of the ionospheric D region. With the present technique, we have retrieved successfully the daytime WEDP from noon-time to the moment of SZA = 90°. The complete curve of WEDP for SZA = 0° to 90° shows that there is an fast decrease and recovery of the steepness β of the electron density profile around the time of SZA = 75°. Such a variation curve of WEDP including the twilight period may deepen our understanding of the mechanism of formation and dissipation of the ionospheric D region. 5) Additionally, we have proposed and practiced a GPU-based parallel computing algorithm that can be implemented in real time in a ground-based multi-station lightning location network. This is because that the real-time accurate location information of the lightning sferics is necessary for real-time monitoring of the ionospheric D region with the above new WEDP inversion technique. In this study, we have introduced a GPU-based parallel computing algorithm that can extensively benefit lightning geolocation networks. One can build up a geolocation database based on numerical solutions of the certain complete objective function in advance, lightning geolocations can then be readily determined with a grid searching technique in real-time. One such a grid searching technique we called the grid traverse algorithm (GTA) for the traditional time of arrival (TOA) technique. By running GPU-based GTA in a 6-station-2D and a 5-station 3D networks, it has been shown that high efficiency can be achieved, with a processing speed of about 2700 times faster than CPU-based GTA. The location accuracy of GPU-GTA is examined with Monte Carlo simulations, showing that GPU-GTA can locate a lightning source in real-time with high accuracy. We also find that when the grid step is comparable with the inherent time uncertainty of a network, the location accuracy cannot be improved further with a finer grid step. In summary, a ray-theory based earth-ionosphere waveguide model (RT-EIWG) that can well interpret the interaction of the lightning sferic with the ionospheric D region has been proposed and examined. Based on this RT-EIWG model, a new technique for the inversion of Wait's electron density profile (WEDP) of the ionospheric D region from lightning sferics has been developed. With this inversion technique, the complete WEDP evolution pattern, including the twilight period has been obtained. Furthermore, we have proposed and practiced a GPU-based algorithm that can accurately locate a lighting sferic source over even a very complicated ground, which can benefit the monitoring of the ionospheric D region through lightning sferics in real-time with high accuracy.
Subjects: Hong Kong Polytechnic University -- Dissertations
Ionospheric electron density
Electrons -- Measurement
Pages: xxv, 152 pages : color illustrations
Appears in Collections:Thesis

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