Signal Processing Of Artificial VLF Wave And Study On Electromagnetic Disturbance Characteristics In Space

In 2004, DEMETER satellite of France was launched successfully, in which 6-year operation accumulated abundant basic information for space electromagnetic observation. Many countries in the world have been carrying out the application research of in-orbit satellite monitoring data, and the extraction and identification of the ionospheric perturbations around earthquakes have become a hot spot in earthquake monitoring and prediction research. Based on the data collection of power spectral density(PSD) in VLF electric field recorded by DEMETER satellite during January 2005 to November 2011 at 19.8kHz from the Austrilian VLF transmitter NWC, its spatial distribution and attenuation features at topside ionosphere has been analyzed by using statistical, interpolation and exponential fitting methods. Furthermore, case studies have been done to distinguish the earthquake-related ionospheric disturbances. All the researches provides reference frame for understanding the spatial distribution from VLF transmitters on ground and extracting the abnormal disturbance features.The innovation of the thesis is that by using multi data processing methods, aiming at satellite observation, the spatial distribution and attenuation features at different directions in electric field at satellite altitude have been analyzed and achieved from artificial VLF transmitters, whcih lays the foundation for further earthquake-induced disturbance extraction. The main results of the thesis are as follows:(1) The spatial propagation characteristics from the VLF transmitter: The VLF electric field exhibits strong relations in the ionosphere over the transmitter and its magnetic conjugate area, presenting two electric field vortex centers at southern and northern hemisphere respectively; The longitude and latitude of VLF electric field center in ionosphere has an offset to the actual location of the transmitter, in which the averaged longitudinal deviation at dayside is greater than that at nightside, while the latitudinal deviation at dayside is less than that at nightside; The annual period is revealed in dayside VLF electric field intensity variations; Within 10° scale around the VLF electric field center, the electric field intensity decreases quickly with the increase of distance, and the averaged fitted attenuation coefficient b values(-0.004) is steady in six years.(2)The attenuation features at different directions by using statistical analysis and linear fitting methods: At the topside ionosphere over NWC, the electric field recorded by satellite exhibits elliptical dispersion; the attenuation gradient at north direction in studied region is the smallest in four directions, and the east one is the largest; at its magnetic conjugate region in Northern Hemisphere, the attenuation gradient at south direction is the smallest, but the north one is the highest. The comprehensive analysis illustrates that, to the equatorial direction, the VLF electromagnetic wave attenuates slowly relative to the high latitude direction; the attenuation gradient over the transmitter is larger than that over its conjugate region, which illustrates that the electric field at conjugate region is not only from the ground-ionosphere waveguide, but may be overlapped by other electromagnetic energies from other magnetic fluxes.(3)The analysis of ionospheric effect from Wenchuan earthquake: the epicenter of Wenchuan earthquake is located at the south of the Magnetic conjugate electric field center, where its intensity is stronger than the other three orientations; during 2005 to 2010, the southern attenuation gradient of the electric field is the smallest among other three orientations(east, west and north) in Apirl to June; in 2008, the southern attenuation of the electric field had dropped to minimize 2 months before the earthquake, while the spectrum of the electric field gradually increased form February to April, and the maximum value was reached in May the electric field intensity maximum, which is possibly linked to the earthquake electromagnetic emissions.