Temporal behaviour of artificial small-scale ionospheric irregularities: Review of experimental results

Features of artificial small-scale ionospheric irregularities (ASI) induced at F-region heights by powerful HF waves are discussed. The investigations presented here were performed during the past two decades at the Zimenki and Sura heating facilities, located at middle latitudes near Nizhniy Novgorod, Russia, as well as at the Gissar facility located at a lower latitude near Dushanbe, Tadzhikistan. The measurements were made by a variety of diagnostic methods employing artificial field-aligned scattering of HF and VHF radio waves, sounding of the disturbed region by means of low-power probing waves and testing of the artificial turbulence by means of stimulated electromagnetic emission (SEE). The dependence of ASI on such parameters as transverse scale length of the artificial irregularities, their location in the disturbed region, the power of the heating wave, the duration of HF radiation, geophysical conditions, aftereffects of the preceding modification, and the schedule of heater operation is considered for both the development stage after pump turn-on and the decay stage after pump turn-off. The temporal evolution of ASI spectral characteristics during the heater period is discussed. An empirical model for the ASI, based on the data available, was elaborated and verified by computer simulation of SEE generation.     

Linear modelling in vertical ionospheric sounding

Summary Applying the methods of computing N(h) profiles to scalar product spaces provides a more general view of the differences between the individual ionospheric models, which enables a better selection of the optimum model.

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u n¶rt; m¶rt;uu ama N(h) nu nmama a nu¶rt;u anauam u ¶rt; a au ¶rt; m¶rt;u uu ¶rt;u, m nm nmm uam nmua ¶rt;.

     

EISCAT verification in the development of ionospheric tomography

This paper highlights the important role played by the EISCAT radar for verification in the development of tomographic techniques to produce images of ionospheric electron density. A brief review is given of some of the stages in the application of tomographic reconstruction techniques to the ionosphere. Results are presented to illustrate the effectiveness of the method in imaging ionospheric structures at high latitudes. In addition, the results include the first tomographic image of the ionosphere for a region extending from mid-latitudes over mainland Scandinavia to high latitudes above Svalbard.     

Modeling of vertical sounding ionograms for nonmonotonous ionospheric layers

The problem of radiosignal reflection from a plane layered isotropic ionosphere has been considered. The vertical sounding ionograms for ionospheric layers with a complicated structure have been modeled. It has been demonstrated that the structure of reflected signals depends on the degree of ionospheric irregularities. Diffuse reflection of radiosignals, caused by the vertical irregularity of the ionosphere, has been detected.     

Model results for the ionospheric E region: solar and seasonal changes

A new, empirical model for NO densities is developed, to include physically reasonable variations with local time, season, latitude and solar cycle. Model calculations making full allowance for secondary production, and ionising radiations at wavelengths down to 25 Å, then give values for the peak density N _mE that are only 6% below the empirical IRI values for summer conditions at solar minimum. At solar maximum the difference increases to 16%. Solar-cycle changes in the EUVAC radiation model seem insufficient to explain the observed changes in N _mE, with any reasonable modifications to current atmospheric constants. Hinteregger radiations give the correct change, with results that are just 2% below the IRI values throughout the solar cycle, but give too little ionisation in the E-F valley region. To match the observed solar increase in N _mE, the high-flux reference spectrum in the EUVAC model needs an overall increase of about 20% (or 33% if the change is confined to the less well defined radiations at <150 Å). Observed values of N _mE show a seasonal anomaly, at mid-latitudes, with densities about 10% higher in winter than in summer (for a constant solar zenith angle). Composition changes in the MSIS86 atmospheric model produce a summer-to-winter change in N _mE of about–2% in the northern hemisphere, and +3% in the southern hemisphere. Seasonal changes in NO produce an additional increase of about 5% in winter, near solar minimum, to give an overall seasonal anomaly of 8% in the southern hemisphere. Near solar maximum, reported NO densities suggest a much smaller seasonal change that is insufficient to produce any winter increase in N _mE. Other mechanisms, such as the effects of winds or electric fields, seem inadequate to explain the observed change in N _mE. It therefore seems possible that current satellite data may underestimate the mean seasonal variation in NO near solar maximum. A not unreasonable change in the data, to give the same 2:1 variation as at solar minimum, can produce a seasonal anomaly in NmE that accounts for 35–70% of the observed effect at all times.     

Estimation of ionospheric radiowave absorption based on one vertical sounding ionogram

A method for estimating absorption value on the basis on one ionogram of a vertical sounding dynasonde was proposed. The main idea is that averaging the amplitudes of the signals reflected from the ionosphere is replaced over time by frequency averaging of the amplitude factor introduced by the authors. The amplitude factor was determined. The regularities of daily variations in the amplitude factor are discussed.     

An experimental study of wave packets in travelling ionospheric disturbances

The complex demodulation method was employed to analyse observations of TIDs carried out by means of multifrequency HF Doppler sounding at vertical incidence over Almaty (76°55′E, 43°15′N). Two types of behaviour of the amplitude and phase in the band-pass filtered Doppler shift oscillations were revealed. These were packet-like wave structures and quasi-stochastic TIDs. A study of the phase behaviour inside wave packets showed that most of them had a quasi-monochromatic structure, some of them had oscillations with a period which increased with time, and none had oscillations with a period which decreased with time. Phase jumps between nearby packets were explained as a result of interference between two adjacent packets having similar oscillation periods. A method of calculating the correlation coefficient for the wave packets propagating on a background of quasi-stochastic TIDs was developed. This showed that wave packets (having periods between 7 and 50 min) travelled in the vertical plane practically without any loss of coherence, while quasi-stochastic TIDs had a short coherence length; the velocity of the quasi-stochastic TIDs was noticeably larger than that of the wave packets.     

Use of the index of TEC vertical variation disturbance in studying ionospheric effects of the Chelyabinsk meteorite

The results of an analysis of the ionospheric effects accompanying fall of the Chelyabinsk meteorite on February 15, 2013 are presented using a method of calculating the index of the disturbance of total electron content vertical variations (Wtec) according to data from the GPS receiver network. A substantial increase (by a factor of 2–3) in the Wtec index with a duration of ~1.5 h was observed in the studied region after the main height explosion accompanying the meteorite fall at 0320 UT. The ionospheric response in Wtec was most significant statistically registered at the radio rays “receiver–satellite” for the GPS located southward from the place of explosion.     

利用精密单点定位求解电离层延迟

近年来,高时空分辨率的全球导航卫星系统(GNSS)观测信号已成为电离层研究的重要资源.利用GNSS研究电离层,需首先将观测资料转换成包含电离层信息的可观测量(Ionospheric Observables,称之为"电离层观测值").目前,最常用的电离层观测值一般采用联合无几何影响组合的码和相位观测,利用相位平滑伪距方法计算得到(称之为"平滑电离层观测值"),但该过程易受平滑弧段长度和与测站有关的误差(如多路径效应和观测噪声)的影响.本文提出利用精密单点定位(Precise Point Positioning, PPP)提取电离层观测值(称之为"PPP电离层观测值",形式与平滑电离层观测值相同).与相位平滑伪距相比,IGS发布的卫星轨道、钟差产品可被PPP合理利用,从而有效减少了待估参数,使得电离层观测值的估计精度得到改善.基于短基线和零基线实验,通过考察两类电离层观测值的站间单差结果在各卫星弧段间的离散程度,验证了PPP电离层观测值的可靠性:以某两天的短基线实验结果为例,与测站有关的误差对PPP电离层观测值的影响分别为对平滑电离层观测值影响的44.4%和35.7%,表明PPP电离层观测值更利于高精度电离层建模、预报等研究.     

轨道高低不平顺谱分析

基于国内外轨道高低不平顺功率谱密度拟合函数,通过编程数值计算分别对比研究了普通线路谱和高速线路谱对行车平稳舒适性、安全性、轮轨动力效应的影响.结果表明,铁科院干线谱和原长沙铁道学院谱激励下列车的平稳舒适性略优于美国六级谱,而前者的轮轨动力效应介于美国六级谱和美国五级谱之间,后者则与美国六级谱相当;时速120 km等级普通线路谱和时速160 km等级提速线路谱引起的列车行驶平稳性介于美国五级谱和六级谱之间,轮轨动力效应与美国六级谱较一致;铁科院郑武线高速谱和时速200 km等级提速线路谱引起的列车平稳舒适性介于德国高干扰谱和低干扰谱之间,而前者引起的轮轨力大于德国轨道谱,后者则与德国低干扰谱相当.同时采用三角级法给出各轨道谱的时域样本,作为车辆-轨道垂向耦合动力分析模型的轮轨激励输入,仿真计算了青藏客车YZ25T在普通轨道谱激扰下以时速90 km/h行驶和高速轨道谱激励下以时速200 km/h行驶时的轮轨竖向作用力,较好地验证了基于轨道谱密度函数的轮轨力效应分析结果.研究成果可为列车行驶振动反应分析中轮-轨不平顺激励谱的选择提供参考.     

Comparison of ionospheric equivalent slab thickness with bottomside digisonde profile over Wuhan

A comparison of the diurnal and seasonal variations in the ionospheric equivalent slab thickness (τ) and bottomside slab thickness (B0) is presented based on the observation during high solar activities at a mid-latitude station—Wuhan (114.4°E, 30.6°N). The investigated data include foF2, hmF2, B0, B1, and TEC, and are derived from the measured ionogram and GPS receiver over Wuhan from April 1999 to March 2000. The results show that τ and B0 are highly/weakly correlated during the day/night, respectively. Furthermore, a comprehensive discussion of the relation between τ, B0, and hmF2 for geomagnetic storm events is provided in this paper.     

电离层不规则结构漂移的GPS测量及其初步结果

本文阐述了利用GPS接收机台阵测量到的闪烁和TEC变化率ROT快速起伏图样估计F层不规则结构漂移的原理和方法,并利用实测数据估计了静日和暴时电离层不规则结构的水平漂移速度.短间距台网和超短间距台链观测实例的计算结果表明,暴时武汉地区引起TEC快速起伏的电离层不规则结构沿纬圈向西漂移,21∶30至03∶00 LT,西向漂移速度在约40 m/s至130 m/s的范围内变化;在桂林地区,磁静日午夜前后引起L波段电波闪烁的电离层不规则结构沿纬圈向东漂移,漂移速度从约70 m/s下降到约55 m/s,磁扰日午夜前不规则结构向西漂移,速度从约150 m/s下降到约50 m/s,午夜后转为向东漂移,速度从约25 m/s上升到约65 m/s.文中还提出了由单站多卫星观测估计F层不规则结构漂移的设想.实例分析与计算结果表明,利用单站多卫星观测估计电离层不规则结构漂移是一种合理可行的方法.     

Analysis of ionospheric vertical total electron content before the 1 April 2014 Mw 8.2 Chile earthquake

This paper studies ionospheric vertical total electron content (VTEC) variations before the 1 April 2014 Mw 8.2 Chile earthquake. VTEC derived from 14 global positioning system (GPS) stations and global ionospheric map (GIM) were used to analyze ionospheric variations before the earthquake using the sliding interquartile range method, and the results showed that significant positive VTEC anomalies occurred on 28 March. To explore possible causes of these anomalies, effects of solar and geomagnetic activities were examined, and VTEC variations during 17 March to 31 March in 2009–2013 were cross-compared. Also, VTEC for a full year before the earthquake was investigated. The results indicated that the anomalies were weakly associated with high solar activities and geomagnetic storms and that the anomalies were not normal seasonal and diurnal variations. An analysis of the spatial distribution of the observed anomalies was also presented, and it demonstrated that the anomalies specifically appeared around the epicenter on 28 March. We suggest that the observed anomalies may be associated with the subsequent Chile earthquake. Equatorial anomaly variations were analyzed to discuss the possible physical mechanism, and results showed that the equatorial anomaly unusually increased on 28 March, which indicates that anomalous electric fields generated in the earthquake preparation area and the meridional wind are possible causes of the observed ionospheric anomalies.     

VSP正反演综述

垂直地震剖面法是一种发展很迅速的地球物理方法,由于垂直地震剖面法是在介质内部点上直接观测,因而能够避开或减弱剖面上部低速带的干扰及外界噪声干扰,可以更直接、更有效地研究波的运动学和动力学特征,解决其地质问题,因此在地球物理勘探领域中最为活跃.本文从垂直地震剖面正反演的角度,并结合国内外实际的例子来介绍垂直地震剖面,同时讨论了垂直地震剖面未来发展的方向.     

Validation of the stream function method used for reconstruction of experimental ionospheric convection patterns

In this study we test a stream function method suggested by Israelevich and Ershkovich for instantaneous reconstruction of global, high-latitude ionospheric convection patterns from a limited set of experimental observations, namely, from the electric field or ion drift velocity vector measurements taken along two polar satellite orbits only. These two satellite passes subdivide the polar cap into several adjacent areas. Measured electric fields or ion drifts can be considered as boundary conditions (together with the zero electric potential condition at the low-latitude boundary) for those areas, and the entire ionospheric convection pattern can be reconstructed as a solution of the boundary value problem for the stream function without any preliminary information on ionospheric conductivities. In order to validate the stream function method, we utilized the IZMIRAN electrodynamic model (IZMEM) recently calibrated by the DMSP ionospheric electrostatic potential observations. For the sake of simplicity, we took the modeled electric fields along the noon-midnight and dawn-dusk meridians as the boundary conditions. Then, the solution(s) of the boundary value problem (i.e., a reconstructed potential distribution over the entire polar region) is compared with the original IZMEM/DMSP electric potential distribution(s), as well as with the various cross cuts of the polar cap. It is found that reconstructed convection patterns are in good agreement with the original modeled patterns in both the northern and southern polar caps. The analysis is carried out for the winter and summer conditions, as well as for a number of configurations of the interplanetary magnetic field.     

弹性介质中垂直地震剖面有限差分正演

垂直地震剖面法可以通过认识反射和透射过程,深入了解地震波传播特性,又可以改善地表地震资料关于构造、地层和岩性的解释.本文是在基于垂直地震剖面的基本原理基础上,采用高阶交错网格有限差分数值求解二维弹性波方程,然后对常见的层状介质和岩溶介质模型分别进行模拟,根据模型合成了相关的垂直地震剖面记录,期间采用的是完全匹配层法吸收...     

Using tomography of GPS TEC to routinely determine ionospheric average electron density profiles

This paper introduces a technique that calculates average electron density (N_e) profiles over a wide geographic area of coverage, using tomographic ionospheric N_e profiles. These N_e profiles, which can provide information of the N_e distribution up to global positioning system (GPS) orbiting altitude (with the coordination of space-based GPS tomographic profiles), can be incorporated into the next generation of the international reference ionosphere (IRI) model. An additional advantage of tomography is that it enables accurate modeling of the topside ionosphere. By applying the tomographic reconstruction approach to ground-based GPS slant total electron content (STEC), we calculate 3-h average N_e profiles over a wide region. Since it uses real measurement data, tomographic average N_e profiles describe the ionosphere during quiet and disturbed periods. The computed average N_e profiles are compared with IRI model profiles and average N_e profiles obtained from ground-based ionosondes.