| Based on a large number of international and domestic electromagnetic science satellites,scientists have made a large number of detailed observations,carried out theoretical and simulation studies,and confirmed that the geophysical field and geochemical multi-parameter signals released during earthquake pregnancy,such as electromagnetic anomalies,matter,energy,and geochemical signals,will pass through the lithosphere-atmosphere-ionospheric coupling.It travels to the ionosphere in nearEarth space and even as far as the magnetosphere.This provides technical support for the identification and extraction of earthquake precursor information and risk prediction,and brings new solutions for space environment monitoring and natural disaster monitoring such as earthquakes.In order to explore new methods and technologies for earthquake prediction,China’s first self-developed test satellite for seismic electromagnetic monitoring,Zhangheng-1 satellite,was successfully launched into orbit on February 2,2018,with an orbital altitude of 507 km and an inclination of 97.4°.The satellite has been in orbit for more than five years and has accumulated detailed observation data of electromagnetic fields,plasma parameters and energy particles.The orbital space of the Zhangheng-1 electromagnetic satellite is full of various types of disturbances,especially the ionospheric hiss phenomenon in the ELF band is very common,even in the quiet condition of the geomagnetic field ELF hiss fluctuation intensity is much more active than other types of electromagnetic fluctuations.ELF hiss fluctuations can be considered as background electromagnetic fluctuations in the ionospheric/plasmonic space.Based on the study of seismic ionospheric anomalies observed by satellites in this space,it is found that earthquakes can also induce electromagnetic disturbances similar to hiss in ELF frequency band.Therefore,how to extract earthquake anomaly information objectively and accurately in ELF frequency band needs to carry out in-depth research on ELF ionospheric hiss in this orbital space.The ionospheric hissing phenomenon in ELF frequency band is studied by using the electromagnetic wave and spectral data of Zhangheng1 electromagnetic satellite.This paper focuses on using all the orbital data accumulated by the Zhangheng-01 satellite during the strong earthquakes in inland China since 2018,and calculates and studies the wave propagation parameters of the ionospheric hiss event in the seismo-induced space-time window during the strong earthquakes by spectral signature analysis and singular value wave vector decomposition(SVD),and statistically analyzes the propagation characteristics of the seismo-ionospheric hiss event.Secondly,this paper attempts to simulate the wave propagation process by the finite-difference time-domain(FDTD)method.The specific research results are as follows:1.An observational data set for the study of seismo-ionospheric hissing is constructedIn this paper,all the seismo-ionospheric hiss related electromagnetic anomalies of Zhangheng-1 satellite in China are selected by data preprocessing method,and the seismo-ionospheric spatiotemporal correlation and the influence of space weather events are analyzed.In this paper,all the orbital data related to seismo-ionospheric hiss events are stored together.In order to reduce the cost of subsequent data calculation,we construct a dataset of seismo-ionospheric hiss events observation by intercepting the specific orbits of all the orbital hiss structures according to time and frequency.2.Propagation and distribution characteristics of downward ionospheric hissIn this paper,it is found that the spectrum of the ionospheric hiss is unstructured,most of the wave vector features propagate downward,and most of the normal polar angles of the waves appear at-180°.In particular,the study on the characteristics of PCBEs along the proton cyclotron shows that the bandwidth of PCBEs decreases with the decrease of magnetic latitude,and there is obvious seasonal regularity.Among the many downgoing ionospheric hiss events,we find a small number of upgoing ionospheric hiss events,and the two are often mixed together.3.The statistical characteristics of ELF ionospheric hiss propagation parameters during earthquakesIn this paper,the key propagation parameters such as azimuth,polar Angle,flatness and elliptic polarizability are calculated by SVD algorithm for each event in the data set.In this paper,the propagation direction of the wave is judged according to the magnitude of the propagation azimuth.If the azimuthal angle is close to 0°,the wave propagates upward from the earth direction.Among the most waves traveling down toward Earth,this paper finds a small number of waves traveling up,and the two are often mixed together.In this paper,the propagation parameters of all upward propagating waves are statistically analyzed,and it is found that most of the ionospheric hiss during the earthquake occurs in the frequency band of 300 Hz to 800 Hz,the wave normal angle of most waves is between 40° and 60°,the azimuthal angle for the wave vector is mainly near 0° and about 60°,and the elliptic polarizability shows more complex characteristics.The ellipticity is about ±0.5.The planarity is mainly between 0.5 and 1,and the peak concentration is between 0.8 and 0.9.4.The ELF band wave propagation process using FDTD methodIn this paper,a simple Earth-ionosphere simulation model of ELF wave propagation is established based on the combination of Maxwell equation and plasma fluid equation.Based on a simple model,the propagation process of the vertical incident wave from the ground to the ionospheric height is simulated.By using the iteration of FDTD,the electromagnetic field distribution with the change of time and space height is tentatively calculated. |
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