VLF电波渗透到卫星高度电离层传播的全波计算

考虑斜向地磁场的影响将电离层设为多层水平分层各向异性有耗介质, 利用传播矩阵法求解全波方程, 进而研究分析VLF频段电离层反射系数随电波频率的变化, 电离层中两种特征极化波的折射和极化特性, 两特征波的电磁场水平分量以及坡印廷能流密度随传播高度的变化. 数值计算结果表明, 地—电离层波导中的垂直极化波比平行极化波易渗透进入电离层; 电离层中两种特征极化波可分为左旋和右旋圆极化波, 左旋分支由于D层强吸收作用表现为速衰减模, 而右旋分支表现为可传播模, 在传播过程中电磁波的能量主要存储在磁场中; 电波频率越低, 其在电离层中的传播损耗越小. 由数值模拟结果发现, 卫星监测VLF频段的低频部分及更低频段的水平磁场变化对于发现地震电离层电磁前兆异常可能更为有效.      

VLF波渗透电离层传播计算研究进展

3~30kHz的甚低频（Very-Low-Frequency,VLF）电磁波对近地空间的高能粒子分布具有非常重要的作用.闪电和地面VLF通信台等VLF波主要辐射源产生的VLF波能够渗透进入电离层,并以哨声波模式继续传播至磁层与高能粒子发生相互作用.本文从VLF电磁波渗透电离层传播计算方法的发展、计算模型验证以及模型在电离层现象研究中的应用等方面对VLF波渗透进电离层之后的传播计算的研究进展进行综述,并对未来研究进行初步展望.      

DEMETER卫星记录到的电离层加热现象

在法国DEMETER卫星运行期间, 地基电离层加热装置SURA和HAARP开展了一系列加热试验, 记录到各类电离层异常信息, 发现电离层加热过程中卫星观测的电离层扰动信号包括HF发射泵波及边带泵波、VLF人工源增强及频谱拓展、ULF/ELF/VLF调制波、ELF电磁扰动、ULF谐振波、等离子体特征参量扰动及高能粒子沉降等. 由于传播及耦合机制的差异, 发射泵波可以穿透电离层直达卫星高度, 其观测概率最高达到68%以上, 其他扰动受发射调制模式及当地电磁环境等影响,观测概率相对较低, 均在40%左右, 有些甚至是某次试验中出现的个别事例. 结合中国地震电磁监测试验卫星飞行轨道设计及载荷配置等,对未来开展加热试验进行了分析论证, 并参考DEMETER卫星试验结果给出了一些建议.      

多频GNSS电离层折射误差修正方法CSCD

针对电离层折射误差较大的特点,分别对GPS(Global Positioning System)和BDS(Bei Dou Navigation Satellite System)的单一频率的电离层折射误差进行了分析,并将不同频率进行线性组合,计算出组合后的电离层折射误差。此方法修正了双频一阶项、三频一阶项和三频二阶项电离层折射误差。由于电离层延迟修正的同时会放大观测噪声,为此分析比较了不同频率组合修正后的观测噪声,为最佳频率组合的选取提供了理论方法。     

海口站电离层闪烁强度功率谱分析与建模应用

基于中国海口站(20.00°N, 110.33°E)获取的2013全年UHF 频段电离层闪烁信号强度数据, 利用Welch算法, 分析给出了四类典型电离层闪 烁强度功率谱. 利用功率谱时序图分析方法, 发现电离层不均匀体存在东西向水 平周期性变化结构. 提取全年功率谱指数p, 通过统计分析发现谱指数p存在规律变化, 据此建立一个谱指数经验模型, 该谱指数模型已用于中国低纬 区域电离层闪烁发生预报概率模型(CMSOP), 进而对海口站UHF频段电离层闪烁发生 概率观测数据和预报结果进行了对比分析.      

新疆于田7.2 级地震前的电离层电磁扰动

利用法国DEMETER 卫星观测的电磁、等离子体等参量的观测数据分析了2008 年3 月21日新疆于田7.2 级地震前离子温度Ti、离子密度Ni、甚低频(VLF) 磁场等出现的异常变化. Ti分析显示震中区上空出现的突变信号在更大空间多次重复出现, 作为地震异常的信度不高. 通过多次重访轨道对比发现H+ 离子密度整体变化形态一致, 但夏秋季节的峰值变化幅度是冬春季节的1 倍. 通过把2008 年的数据与2007 年和2006 年同期的重访轨道数据进行对比, 发现2008 年在于田地震前1 个月左右Ni 在峰值区呈现了显著增强的变化趋势, 并一直持续到地震前, 2008 年2 月以后的峰值幅度是其他两年同期的~1 倍多. 对于磁场频谱数据, 抽取了震中区域上空2000 km 范围内多个轨道的单频(400 Hz)信息, 并在研究区域统一纬度空间(0~60°N) 进行了时间序列对比, 结果发现地震前2 日内震中区域上空400 Hz 谱密度增加了近一个数量级, 反映空间存在明显的VLF 电磁辐射现象. 对于3 月20 日极低频段(ELF) 三分量电磁场的矢量分析显示观测时段内存在一些左旋极化电磁信号, 反映了空间电离层离子密度变化伴随ELF 电磁辐射信息. 综合分析认为, 这次地震前各参量的变化过程与目前地震孕育及电离层耦合机理相吻合.      

预加热时间对低电离层预加热幅度调制的影响

基于电离层连续方程和能量方程，建立预加热模式下低电离层幅度调制加热理论模型，模拟大功率电波幅度调制电离层电导率和电流随时间的变化，分析预加热时间对幅度调制产生的极低频/甚低频（ELF/VLF）强度的影响.结果表明，低电离层中的电流能够被大功率电波有效调制，可以作为ELF/VLF电波的辐射源.预加热幅度调制模式在预加热时间较长时效果更明显.在考虑能耗的情况下，当有效辐射功率为200MW及高频波的调制频率f_ELF/VLF为1kHz时，预加热幅度调制模式辐射ELF/VLF加热0.1s时效果最好，强度可提高约7.95dB.      

大气行星波对LF和VLF无线电波传播的影响

本文采用最大熵谱估计方法,对1985-1987年三个冬天,在东半球(20°S—65°N、13°E—141°E)白天观测的LF和VLF电波相位、幅度、卫星观测的Ly-α射线通量及高纬上空的行星波活动等大量资料,进行了熵谱分析。观测及分析结果表明:(1)LF和VLF电波的相位具有2—2.2天、3—4天、6—12天、12—20天、20—32天周期范围的行星尺度扰动.Ly-α射线辐射通量主要具有20—32天周期范围内的波动.(2)发现在中纬地区冬天观测的LF电波幅度大的扰动与高纬60°N上空观测的行星波H_1的变化规律非常一致. 波形结构的主要峰和谷几乎完全对应,仅在时间上前者约滞后3—4天.计算给出两者的相关系数在0.65—0.85之间.根据观测事实和谱分析结果对比,作者认为LF和VLF电波相位、幅度周期在20天以上的扰动主要受太阳Ly-α辐射通量变化的控制.冬天其周期在2—20天范围内的扰动,主要受来自对流层和下平流层中激发的大气行星波的影响.     

电离层Es对HF和VHF传播信号的影响

Es层是存在于电离层中的电子密度非常高的偶发E层（Sporadic E）,其电子密度可达常规E层的100倍.电离层Es能够反射原本穿透F层的VHF低频段（30~150MHz）无线电波,而且对HF高频段（10~30MHz）无线电波传播具有显著影响.运用垂测和斜测观测数据,研究HF频段Es层电波传播特征,得到了不同类型及不同高度Es层的衰减系数.根据f₀E_s的日变化规律,可得HF频段衰减系数的日变化规律,进而分析并得到Es层对短波传播的影响.不存在电离层Es时,通常无法通过电离层实现VHF超远距离通信.为了对VHF链路通过电离层Es的传播衰减进行定量分析,根据EBU多条链路的观测结果,拟合并建立了电离层Es衰减模型.将该模型、ITU模型和观测数据进行对比,发现本文建立的模型准确度更高.利用建立的模型,对电离层Es不同临频f₀E_s条件下接收信号场强和电压随传播距离的变化进行了计算,结果可为VHF链路设计及建立提供参考.      

E10.7指数在软X射线谱段的修正应用

表征EUV辐射通量的E10.7指数在越来越多的研究和应用中被用来代替传统的F10.7指数.X射线对地球D层和E层的电离起着重要作用,但由于D层观测数据的不足和E层电离源的多样性,难以被用来考虑X射线对电离层的影响.火星电离层下层的电离源几乎是单一的软X射线,这为研究X射线对电离层的作用提供了可能性.通过研究火星电离层下层的峰值电子浓度对E10.7的依赖关系,发现即便经过必要的修正,这种关系对不同的观测时段并不具备一致性.通过理论推导和数据分析,得到了一种特别用于描述太阳软X射线辐射通量的新指数,即Xs指数,用来替代E10.7指数.Xs指数在描述火星电离层下层对太阳辐射的依赖关系时,不同的观测时段有很好的一致性,表明Xs指数在表征太阳软X射线辐射强度方面比E10.7指数更加合适.      

Appearance of VLF and ELF-noises in topside ionosphere under the action of high power radio wave from data of satellite intercosmos-24

Results of a satellite experiment are presented on detection of VLF and ELF-waves excited by irradiation of the night ionosphere F-region by the field of a nonmodulated high-power radio wave. The excited VLF and ELF-waves have been detected at the topside ionosphere heights h=500–1000 km in the frequency bands 8 kHz相似文献   

Effects of man on geomagnetic activity and pulsations

Until recently the only known large-scale effects of man on geomagnetic activity and pulsations were those produced by high-altitude nuclear explosions. However, in just the last few years, measurements have indicated that changes can also be produced by (1) moderately-powered pulsed HF radio (1–20 MHz) transmissions into the ionosphere, (2) high-powered pulsed VLF radio transmissions (3–15 kHz) into the magnetosphere, and (3) by the ULF magnetic noise (frequencies < 5 Hz) from modern dc electric powered mass transit systems. Further, experiments reported by U.S. and Soviet scientists support the suggestion that ULF geomagnetic changes in the ionosphere and magnetosphere can be generated by the passage of a large ULF current around a peninsula in the sea. As a result of these experimental activities, it appears that controlled experiments using artificially generated ULF signals in the ionized upper atmosphere are now feasible. Finally, the report of a “weekend effect” in geomagnetic activity (as measured by the geomagnetic activity index Ap, for example) suggests that man's activities may already have been subtly influencing geomagnetic activity for many years, possibly because of the radiation from electric power distribution systems into the magnetosphere.     

Ionospheric response to the June 2015 geomagnetic storm in the South American region

The ionospheric dynamics in the South America (SA) sector during geomagnetic disturbed period from 21 to 24 June 2015 is investigated through ground ionosonde stations and Global Navigation Satellite System (GNSS) receivers, supported by Very Low Frequency (VLF) and magnetometer data. These disturbances were caused by 3 interplanetary shocks (IS) derived from 3 consecutives coronal mass ejections (CME) from the same solar active region; the first two CME were caused by filament eruptions, and the third was a much larger full halo CME, associated with a M2.6 solar flare. The first 2 shocks were compressive and did not cause an immediate response to the ionosphere in the analyzed region, while the third shock increased considerably the electron density from low to high-latitudes, triggering the second strongest geomagnetic storm of the 24th solar cycle. It was possible to observe the expansion of the crest of equatorial ionospheric anomaly (EIA) at midlatitudes and high-latitudes mainly due to prompt penetration electric field (PPEF) during the main phase and the recovery phase of the geomagnetic storm during the day.     

ULF wave index as magnetospheric and space-weather parameters

Waves in the Ultra Low Frequency (ULF) band owe their existence to solar wind turbulence and transport momentum and energy from the solar wind to the magnetosphere and farther down. Therefore an index based on ULF wave power could better characterize solar wind–magnetosphere interaction than K_P, Dst, AE, etc. indices which described mainly quasi-study state condition of the system. We have shown that the ULF wave index accurately characterize relativistic electron dynamics in the magnetosphere as these waves are closely associated with circulation, diffusion and energization of relativistic electrons in the magnetosphere. High speed solar wind streams also act as a significant driver of activity in the Earth’s magnetosphere co-rotating interaction region and are responsible for geomagnetic activities. In the present paper, we have analyzed various cases related with very weak (quiet) days, weak days, storm days and eclipse events and discussed the utility of the ULF wave index to explain the magnetospheric dynamics and associated properties. We have tried to explain that the ULF wave index can equally be useful as a space weather parameter like the other indices.     

Ionospheric precursors of earthquakes recorded by VLF receiver at Tashkent IHY station

Tashkent International Heliophysical Year (IHY) station is a member of Atmospheric Weather Electromagnetic System for Observation, Modeling and Education (AWESOME) network being operated globally to study the ionosphere and the magnetosphere with the help of electromagnetic waves in Very Low Frequency (VLF) band. Regular monitoring of the D- and F-layers of ionosphere over Central Asia territory is being performed on the permanent basis starting year 2008. We have studied VLF amplitude anomalies related to the EQs occurred in 2008–2009 years with magnitude more than 5 on the path way from the VLF transmitters to the Tashkent station assuming that propagation of VLF ground-based transmitters signals can be perturbed by EQ preparation detectable from the ground-based measurements in the VLF bands. For analyzing narrowband data we have used the nighttime fluctuation (NF) method paying attention to the data obtained during the local nighttime (20:00 LT–04:00 LT) in Tashkent where the VLF receiver is operating. The mean nighttime amplitude (or trend) and nighttime fluctuation are found to increase significantly before the EQ occurred on the path way from the transmitters to the receiver. The obtained results have revealed an agreement with VLF amplitude anomalies observed in Tashkent VLF station during the strong EQs occurred on the path way from the transmitters to the receiver. Some results are presented to show the probing potentiality of VLF waves to predict short term EQs with high magnitude.