Study Of The Transionospheric Propagation Of Terrestrial VLF Radiations And Up To The Magnetosphere

The VLF signal from terrestrial VLF transmitters can propagate away from the transmitters reflected between the Earth-ionospheric wave-guide with some radiation continually leaking upward through the ionosphere and up to the magnetosphere with small attenuation along the whistler duct,which can be detected by satellites.Study of the penetration of VLF radiations into the ionosphere and up to the magnetosphere will not only help us to understand and find the new effects and phenomena in the ionosphere,but also deepen the understanding of the physical process during the magnetosphere-ionosphere-atmosphere coupling which is helpful for promoting the forecasting and research of space weather.It has certain significant values in the practical application;for example,providing the electromagnetic field distribution excited by ground-based VLF transmitters at the altitude of satellites which cannot only provide certain references for the data validation and quality evaluation of the electromagnetic field data of China seismo-electromagnetic satellite,but also provide certain theoretical support for study the signal-to-noise ratio changes of VLF transmitters observing on the satellite before the earthquakes.This study may be also useful to the study of lightning related phenomenon in the ionosphere and manual-controlled high-energy particles precipitation in the radiation belt.While the VLF waves penetrating into the ionosphere,the energy of the waves will meet attenuation because of the collision of the particles in the D region of the ionosphere.In the 1960s,Helliwell has given an ensemble averaged ionosphere electron-density model to calculate the absorption of wave power for 2 kHz and 20 kHz waves during daytime and nighttime as a function of latitude,which has been widely used.The absorption curves must be modified because the electron densities in the D region varying with latitude and seasons.With the development of the observation technology and the improvement of computing capabilities,it has been found the previous theoretical models of estimating VLF waves loss while penetrate the ionosphere all overestimate the loss of the ionospheric absorption.The previous models becomes short at explaining the observed more complex phenomenon in the ionosphere,so the more accurate numerical calculation method is needed to solve the transionospheric propagation of VLF waves and its propagation in the magnetosphere.The penetration of VLF radiations from the ground into the ionosphere and up to the magnetosphere will be studied from three respects,theoretical analysis,numerical simulation as well as observation data analysis.The main research results of this paper are as follows:1.The full-wave infinite method is used to construct the propagation model of VLF waves at lower ionosphere the calculation processes of which has been deduced and introduced carefully.The VLF full-wave infinite model has been applied to calculate the spatial distribution of the electromagnetic response in the ionosphere excited by two VLF transmitters-NWC and GBZ,meanwhile according the satellite observation data the temporal and spatial distribution characteristics of electromagnetic response in the ionosphere excited by the two transmitters have been analyzed.The diurnal,seasonal and annual variation of the electromagnetic field in the ionosphere excited by NWC as well as the spatial distribution of electromagnetic field in the ionosphere excited by NWC and GBZ at different latitudes with the different radiation powers but the same frequency have been studied.The calculation results also have good consistency with satellite data analysis results which means that the full-wave calculation model we have construct has been verified meanwhile we get the temporal and spatial distribution of electromagnetic field in the ionosphere excited by ground-based VLF transmitters.The calculation results and the data analysis results all have shown that:(1)The temporal changes of electric field are more obvious than magnetic field.The electron density below 250 km plays a more important role on the attenuation of the energy of electromagnetic wave.The more active the sun,the lower the electromagnetic field at the satellite altitude.The simulation results have good consistency with satellite data analysis results.(2)According to the simulation results of the spatial distribution of electromagnetic response excited by NWC and GBZ in the ionosphere,it can be concluded that the spatial distribution of the electromagnetic response is to present a set of concentric rings during day and night time,which may be caused by the mode interference in the wave-guide mapping into the ionosphere.The center of the concentric ring projected to the ground has a longitudinal deviation relative to the source.The concentric ring is a north south asymmetry,the side that magnetic force line points is larger which may be due to the influence of the geomagnetic dip angle.The simulation results also have good consistency with satellite data analysis results.2.Using the validated finite element calculation mode,the VLF full wave numerical simulation experiment has been done to study the spatial distributions of the electromagnetic field energy in the ionosphere excited by the VLF transmitters on the ground with different radiation source parameters with different magnetic field parameters,and ionospheric parameters.The simulation results show that:(1)The field energy in the earth-ionosphere wave-guide excited by vertical electric dipole shows clearly mode interference phenomenon,the change trends of field energy at upper and lower boundary of the wave-guide are contrary the difference of phase is 180°.While the electromagnetic radiation propagates from the ground surface into the ionosphere,the energy attenuation is mainly in the earth-ionosphere wave-guide(0-65 km)and the lower ionosphere under 120 km.The energy of the electromagnetic wave which penetrates through the D region of the ionosphere has not weaken while flow into the upper ionosphere and magnetosphere along the geomagnetic field lines with the energy gathered at the side of the pointing direction of the magnetic force lines.(2)The frequency of the radiation source determines the energy distribution in the earth-ionosphere wave guide,while the magnetic field parameters and ionospheric parameters have little impact on the distribution of energy in the wave guide.The lower the frequency,the energy in the wave-guide and the ionosphere reduce significantly,while the smaller the D region absorption instead.The geomagnetic intensity is greater the D region absorption is smaller.When the geomagnetic field increases to a certain value,the region D absorption does not continue to decrease.Geomagnetic inclination determines the direction of the electromagnetic energy beam in the ionosphere,the smaller the geomagnetic inclination,the energy maximum points excited in the ionosphere is farer from the source location.The attenuation of the poynting flux is not a linear relationship with the increase of electron density and collision frequency.3.The simple introduction to the ray tracing theory has been carried in this article.The reason of the magnetosphere reflection is that the refractive index surface including ions is very different from that including electrons only.The wave can not be reflected,because the refractive index surface only including electrons is not closed which has the resonance cone.The surface considering ions is closed at lower frequency so that the magnetospheric reflection can happen while the ray direction changes.The ray tracing method is used to simulate the ray path of VLF wave in the magnetosphere,and get the L value that different VLF signal can reach which is instructive to study the high-energy particle precipitation caused by VLF signals.The propagation characteristics of VLF wave in the magnetosphere at middle-and high-latitude has been simulated.The simulation results show that it is easier for VLF waves to form MR whistlers in the magnetosphere.For the signals with different frequency starting at the same latitude,the frequency higher,the L-shell the waves can propagate becomes smaller.The signals can propagate far away from the earth while be reflected between the north and south hemispheres the L-value become a constant in the end.The wave normal angle becomes smaller,which means that the VLF waves starting from the higher latitude tend to propagate along the geomagnetic field as PL mode whistlers.4.In this paper,the full wave model has been expanded from VLF to ULF frequency to study the ULF electric field anomaly in the ionosphere due to the changes of plasma density causing by the earthquakes.The gyro-frequency of an ion is comparable to the wave frequency at a ULF band,so the effect of an ion density cannot be ignored in ULF wave propagation model.Full-wave model considering the effect of ion density has been used to construct a new ULF wave model in the ionosphere to study the effects of plasma parameters(Ne and Ni)on the propagating waves at different frequencies at ULF band.The variation of electric field related with two earthquakes at different latitude has been simulated.The results show that the effect of electron density on the electromagnetic field is larger than ion density.The attenuation of ULF wave is smaller with lower frequency which means the electromagnetic response of lower frequency wave can be detected easier in the topside ionosphere.The attenuation of electromagnetic field is smaller at middle latitude compared with the result at equatorial region.It illustrates that it is easier for the ULF electromagnetic wave to penetrate into the topside of the ionosphere at higher latitude than equatorial.