The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequen...The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequency in the ionosphere.The ionosphere refractive index is also analyzed.The ionosphere above 70 km is considered to be homogeneous and anisotropic,and the reflection and transmission coefficient matrix is calculated using matrix method.Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere.The results show that when the wave frequency is higher,it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller.TE（traverse electric） wave and TM（traverse magnetic） wave can both penetrate into the ionosphere with a small incident angle,while TE wave can hardly transmit into the ionosphere when the incident angle is large.The transmission coefficient decreases as the geomagnetic inclination increases.TE and TM wave cannot penetrate into the ionosphere at magnetic equator.When the electron collision frequency is higher,it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker,which may be caused by the energy transportation between the waves and the particles.The ordinary（O） wave experiences severer attenuation than extraordinary（X） wave,and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere.All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude.It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation.It may transmit into the ionosphere with a small incident angle due to the nonlinear effect,for example,the interaction between the waves and the particles or cross modulation,and then propagate along the whistle duct with small attenuation.This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.展开更多
超大浮式结构(very large floating structure,简称VLFS)是集空港和海港为一体的大型海上多功能浮式结构,现有研究大多关注规则波下刚性模块柔性连接模型的动力响应,忽略了台风浪极端环境下海上机场自身柔性引起的非线性振动特性。针对...超大浮式结构(very large floating structure,简称VLFS)是集空港和海港为一体的大型海上多功能浮式结构,现有研究大多关注规则波下刚性模块柔性连接模型的动力响应,忽略了台风浪极端环境下海上机场自身柔性引起的非线性振动特性。针对此问题,提出了一种新型多柔-刚性混合模块建模方法,采用Jonswap谱特征参数对台风“鲇鱼”过境实测海浪谱开展了精细化仿真模拟,分析了台风浪下海上机场VLFS整体和局部非线性动态响应特性,揭示了海上机场与环境荷载之间的能量转换机理。结果表明:海上机场多柔-刚性混合模块模型可以较好地反映此类VLFS结构动力响应特性;海上机场超长柔性及台风浪场不均匀性使其结构呈现显著非线性,位移、转角和水弹性变形分别以沿波向、绕展向和沿垂向为主,极值应力主要分布于撑杆附近;环境荷载能量和结构重力势能在初始阶段主要转换为系泊势能,稳定阶段则主要转换为结构动能和弹性势能。展开更多
There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evo...There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evolution of VLF propagation anomalies with special attention to one particular propagation path. The most important advantage of this paper is the simultaneous use of several propagation paths. A succession of earthquakes (EQs) happened in the Kumamoto area in Kyusyu Island;two strong foreshocks with magnitude of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15 April (UT). Because the EQ epicenters are not far from the VLF transmitter (with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously 8 observing stations of our network all over Japan. Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs. It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock. When the perturbation is most disturbed, the maximum change in vertical direction is depletion in the VLF effective ionospheric height of the order of 10 km, and its horizontal scale (or its radius) is about 1000 km. These spatio-temporal changes of the seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe with the same magnitude and of the same fault-type, and a brief discussion on the generation mechanism of seismo-ionospheric perturbation is finally made.展开更多
在星载甚低频（VLF）通信技术中,研究甚低频信号在电离层中的传输特性具有重要意义。现有的利用时域有限差分（FDTD）算法研究电离层传输特性大多基于高频脉冲信号,且传播模型相对粗略,计算存在一定误差。为此,建立了更为精确的电离层...在星载甚低频（VLF）通信技术中,研究甚低频信号在电离层中的传输特性具有重要意义。现有的利用时域有限差分（FDTD）算法研究电离层传输特性大多基于高频脉冲信号,且传播模型相对粗略,计算存在一定误差。为此,建立了更为精确的电离层各向异性传播模型,对常规的FDTD算法引入变步长因子,并用该算法对VLF波在电离层中的场强衰减进行了数值计算,研究了昼夜变化及频率对其传输特性的影响。仿真结果与卫星实测数据对比表明：与常规的FDTD算法以及全波分析方法相比,引入变步长因子的FDTD算法精度更高;在VLF频段,降低频率有利于电磁波穿透电离层,且白天的衰减明显大于夜间;VLF波在射入电离层并穿透D层时衰减严重（30-40 d B）,约为F1层中衰减的3倍。展开更多
This paper introduces a new approach for the determination of the source region of beat wave(BW)modulation.This type of modulation is achieved by transmitting high-frequency(HF)continuous waves with a frequency differ...This paper introduces a new approach for the determination of the source region of beat wave(BW)modulation.This type of modulation is achieved by transmitting high-frequency(HF)continuous waves with a frequency difference f,where f is the frequency of modulated ELF/VLF(extremely low frequency/very low frequency)waves from two sub-arrays of a high power HF transmitter.Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency,there exists a controversy on the physical mechanism of BW and its source region.In this paper,the two controversial theories,i.e.,BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously.According to the analysis and simulation results presented in this paper,it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions.Therefore,this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.展开更多
基金supported by Chinese National Science and Technology Support Pro-gram (2008BAC35B01)Basic Research Project from Institute of Earthquake Science,China Earthquake Ad-ministration (02092408)
文摘The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequency in the ionosphere.The ionosphere refractive index is also analyzed.The ionosphere above 70 km is considered to be homogeneous and anisotropic,and the reflection and transmission coefficient matrix is calculated using matrix method.Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere.The results show that when the wave frequency is higher,it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller.TE（traverse electric） wave and TM（traverse magnetic） wave can both penetrate into the ionosphere with a small incident angle,while TE wave can hardly transmit into the ionosphere when the incident angle is large.The transmission coefficient decreases as the geomagnetic inclination increases.TE and TM wave cannot penetrate into the ionosphere at magnetic equator.When the electron collision frequency is higher,it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker,which may be caused by the energy transportation between the waves and the particles.The ordinary（O） wave experiences severer attenuation than extraordinary（X） wave,and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere.All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude.It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation.It may transmit into the ionosphere with a small incident angle due to the nonlinear effect,for example,the interaction between the waves and the particles or cross modulation,and then propagate along the whistle duct with small attenuation.This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.
文摘超大浮式结构(very large floating structure,简称VLFS)是集空港和海港为一体的大型海上多功能浮式结构,现有研究大多关注规则波下刚性模块柔性连接模型的动力响应,忽略了台风浪极端环境下海上机场自身柔性引起的非线性振动特性。针对此问题,提出了一种新型多柔-刚性混合模块建模方法,采用Jonswap谱特征参数对台风“鲇鱼”过境实测海浪谱开展了精细化仿真模拟,分析了台风浪下海上机场VLFS整体和局部非线性动态响应特性,揭示了海上机场与环境荷载之间的能量转换机理。结果表明:海上机场多柔-刚性混合模块模型可以较好地反映此类VLFS结构动力响应特性;海上机场超长柔性及台风浪场不均匀性使其结构呈现显著非线性,位移、转角和水弹性变形分别以沿波向、绕展向和沿垂向为主,极值应力主要分布于撑杆附近;环境荷载能量和结构重力势能在初始阶段主要转换为系泊势能,稳定阶段则主要转换为结构动能和弹性势能。
文摘There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evolution of VLF propagation anomalies with special attention to one particular propagation path. The most important advantage of this paper is the simultaneous use of several propagation paths. A succession of earthquakes (EQs) happened in the Kumamoto area in Kyusyu Island;two strong foreshocks with magnitude of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15 April (UT). Because the EQ epicenters are not far from the VLF transmitter (with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously 8 observing stations of our network all over Japan. Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs. It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock. When the perturbation is most disturbed, the maximum change in vertical direction is depletion in the VLF effective ionospheric height of the order of 10 km, and its horizontal scale (or its radius) is about 1000 km. These spatio-temporal changes of the seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe with the same magnitude and of the same fault-type, and a brief discussion on the generation mechanism of seismo-ionospheric perturbation is finally made.
文摘在星载甚低频（VLF）通信技术中,研究甚低频信号在电离层中的传输特性具有重要意义。现有的利用时域有限差分（FDTD）算法研究电离层传输特性大多基于高频脉冲信号,且传播模型相对粗略,计算存在一定误差。为此,建立了更为精确的电离层各向异性传播模型,对常规的FDTD算法引入变步长因子,并用该算法对VLF波在电离层中的场强衰减进行了数值计算,研究了昼夜变化及频率对其传输特性的影响。仿真结果与卫星实测数据对比表明：与常规的FDTD算法以及全波分析方法相比,引入变步长因子的FDTD算法精度更高;在VLF频段,降低频率有利于电磁波穿透电离层,且白天的衰减明显大于夜间;VLF波在射入电离层并穿透D层时衰减严重（30-40 d B）,约为F1层中衰减的3倍。
基金supported by the National Natural Science Foundation of China(Grant No.41804149)China Scholarship Council。
文摘This paper introduces a new approach for the determination of the source region of beat wave(BW)modulation.This type of modulation is achieved by transmitting high-frequency(HF)continuous waves with a frequency difference f,where f is the frequency of modulated ELF/VLF(extremely low frequency/very low frequency)waves from two sub-arrays of a high power HF transmitter.Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency,there exists a controversy on the physical mechanism of BW and its source region.In this paper,the two controversial theories,i.e.,BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously.According to the analysis and simulation results presented in this paper,it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions.Therefore,this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.
