Power variation analysis of echo signals from ionospheric reflectors

A series of Power Virtual Height measurements (PVH) of radio echoes reflected from the ionosphere were acquired at a given frequency during the period 3–22 January 2008 with the purpose of studying the slow fading variations through time of the ionospheric channel. To obtain PVH data, an ionospheric vertical sounding system was suitably adapted to work at a single fixed frequency. PVH measurements were recorded between two routine ionospheric vertical soundings, providing a data type that enables evaluation of fading fluctuation through time. The time stability of the ionospheric layers is determined by analyzing the level of the received signal power within a chosen threshold. In this paper the fading behaviour and its characteristics are described, considering only temporal periodicity above 0.5 s. In a further analysis a relation is demonstrated between the recorded fading and the time stability of the signal within a fixed interval of values.     

Multiple-site investigation of the properties of an HF radio channel and the ionosphere using Digital Radio Mondiale broadcasting

The Digital Radio Mondiale (DRM), one of the new digital radio broadcasting standards, has been designed to overcome typical short wave radio channel difficulties, such as the multipath propagation and fast temporal changes of the received signal level, both related to the properties of the ionosphere along the path of propagation. In particular, some of the RF carriers used in the applied COFDM transmission technique serve to estimate the current state of the radio channel to enable the proper demodulation of the received signal.We have been detecting such RF carriers on select frequency channels (standard DRM broadcast) using a network of recording stations located in different parts of Poland in order to collect data on the HF radio channel. We have been also evaluating the usefulness of this procedure in providing information on the current state of the ionosphere in the refraction region between the transmitter and receivers. When the DRM system becomes more widespread, this method can supplement data that comes from the ionosondes, since it does not require much financial resources and the receivers can be easily scattered over a large area. This paper presents a set of experimental data and its analysis.     

高频电波在背景备向异性和不均匀的随机起伏电离层中的传播

运用波动理论,讨论背景为各向异性、不均匀的随机起伏电离层中高频电波的传播和散射问题,给出了电离层不均匀体散射引起的反射回波波场的起伏,并在垂测条件下计算了接收波场的相位与振幅起伏的谱分布。      

Method of the HF wave absorption evaluation based on GIRO network data

High frequency (HF) communication is strongly dependent on the state of the ionosphere, which specifies the mode structure of the radio wave propagating in ionosphere. Another core factor defining the strength of the HF signal at the receiving site is the ionospheric absorption. Accurate modelling the effect of absorption is an essential part of many studies of the HF propagation in the ionosphere.This paper proposes a method for estimating the absorption. The method is based on analysis of vertical sounding ionograms. The main idea of the approach is to compare the main parameters retrieved from measured and simulated ionograms. The combination of Global Ionospheric Radio Observatory (GIRO, http://giro.uml.edu) data and ionograms modelling allows for developing the empiric absorption model available at near real-time. The ionogram simulation taking into account absorption utilizes the NIM-RT (North Ionospheric Model and Ray Tracing) software. As a result, the proposed technique provides more reliable and accurate evaluation of minimum frequency at which echoes are observed in vertical incidence ionosonde soundings. The values of these frequencies should be used in the following simulation to optimize parameters in the empirical formulae for defining absorption HF wave in ionosphere.The ultimate objective of this work is the designing the method, which allows the simulating of HF radio channel accounting for regular absorption due to UV radiation of the Sun. Eventually it could be considered as some kind of the HF propagation forecasting.     

An experiment to study solar flare effects on radio-communication signals

The occurrence of radio signal fading events caused by ionospheric absorption plays an important role in the performance of radio-communication systems. It is necessary to know the magnitude and time-scale of such events in order to specify technical parameters of the communication system to be used. Generally, fading events are associated with solar flares, which are characterized by sudden increase in the solar X-ray flux that causes an increase in the ionization in the lower ionosphere. The abrupt increase of ionization causes the absorption of radio waves propagating in the Earth–ionosphere wave-guide and is reported as radio signal fading events. A simple experiment to monitor the behavior of lower ionosphere has been carried out at the Southern Space Observatory-SSO/INPE (29.43°S, 53.8°W), located in southern Brazil. The experiment is basically a computer controlled radio receiver that records the received signal strength of Amplitude Modulated (AM) radio signals in the HF (High Frequencies) range. We analyzed data of the 6 MHz beacon signal that has been transmitted by a broadcasting radio station located about 400 km from the observation site. In this work we present initial results of daily variation of the received signal strength and fading events associated with solar flares observed in the 6 MHz signal monitored by the experiment during 2001. X-ray solar flux data from the GOES-8 satellite were used to identify X-ray solar bursts associated with solar flares. Based on the one-year data collected by the experiment, a statistical summary of fading occurrences and their correlation with solar flares, as well as the distributions of time-scales and magnitudes of such events are presented.     

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.      

大功率高频无线电波对赤道E区双流不稳定性的影响--PIC静电粒子模拟研究

地面入射的大功率高频无线电波(泵波)和电离层等离子体之间的参数相互作用，能够引起静电波的激发，在一定条件下，产生不稳定性．本文用PIC静电粒子模拟方法，研究泵波与赤道电离层E区等离子体的相互作用．研究结果表明，泵波能够控制双流不稳定性的发生，在不同条件下，泵波对双流不稳定性起着稳定与不稳作用，模拟结果定性地与理论研究结果相符合，这为我们对不规则体产生的地面人工控制提供了依据．     

暴时密度增强区附近电离层不均匀体研究

基于IGS提供的TEC数据, 研究了2003年10月大磁暴期间的暴时密度增强(Storm Enhanced Density, SED)现象; 利用GPS观测数据, 计算出ROTI (Standard deviations of ROT)指数, 分析了SED边界附近电离层小尺度不均匀体结构的时间和空间演变. 研究表明, 在磁暴主相期间SED边界附近不均匀体随着磁暴的发展逐渐增多; 在主相的中后期不均匀体的分布密集度达到最大; 在恢复相期间, 不均匀体分布很少; 随着磁暴的发展, 不均匀体开始主要集中在40~45oN范围内, 随后向高纬漂移, 主要集中在45~55oN范围内.      

Impact of heliogeophysical disturbances on ionospheric HF channels

The article presents the results of the observation of a strong magnetic storm and two X-ray flares during the summer solstice in 2015, and their impact on the HF signals characteristics in ionospheric oblique sounding. It was found that the negative phase of the magnetic storm led to a strong degradation of the ionospheric channel, ultimately causing a long blackout on paths adjacent to subauroral latitudes. On mid-latitude paths, the decrease in 1FMOF reached ～50% relative to the average values for the quiet ionosphere. It is shown that the propagation conditions via the sporadic Es layer during the magnetic storm on a subauroral path are substantially better than those for F-mode propagation via the upper ionosphere. The delay of the sharp decrease in 1FMOF during the main phase of the magnetic storm allowed us to determine the propagation velocity of the negative phase disturbances (～100 m/s) from subauroral to mid-latitude ionosphere along two paths: Lovozero – Yoshkar-Ola and Cyprus – Nizhny Novgorod. It is shown that both the LOF and the signal/noise ratio averaged over the frequency band corresponding to the propagation mode via the sporadic Es layer correlate well with the auroral AE index. Using an over-the-horizon chirp radar with a bistatic configuration on the Cyprus – Rostov-on-Don path, we located small-scale scattering irregularities responsible for abnormal signals in the region of the equatorial boundary of the auroral oval.     

Physical and model interpretation of HF radio propagation on the St. Petersburg–Longyearbyen (Svalbard) path

HF radio wave observations have been carried out with an oblique ionospheric sounding (OIS) method on the radio path from St. Petersburg to Longyearbyen (Svalbard), and experimental ionograms were obtained for December 2001. These ionograms have been analysed to investigate the impact of the main ionospheric trough (MIT) and magnetic disturbances on the signals on this path. The observations during weakly disturbed (K_р = 2) magnetic conditions on 14–15 December 2001 were compared with predictions from ray-tracing through a numerical model of the ionosphere. The ray-tracing computer program synthesizes the OIS ionograms by means of the “shooting method”. This method calculates trajectories of HF radio waves for different values of elevation angle and transmission frequency. There was a variety of calculated trajectories, from which we choose those which reach the receiver, and the selected paths provide a synthesis of the oblique ionograms. To simulate HF radio wave propagation, we apply a three-dimensional distribution of the electron density calculated with the mathematical model of the high-latitude ionosphere developed in the Polar Geophysical Institute (PGI). These numerical simulations permit us to interpret specific peculiarities of the OIS data such as abnormal propagation modes, increased delays of signals, enhanced MOF (maximum observed frequency) values etc. New results of the study are summarised as follows. (1) An unusual feature of the propagation along the path is the change of propagation mechanism during substorms on entering a path midpoint (or 1-hop reflection point) to the MIT. (2) Even weak substorms, having the distinguished intensities, lead to the appearance of different types of irregularities observed by the CUTLASS radar and therefore to the different propagation modes and F2MOF values. (3) The PGI model of the ionosphere was first used for ray-tracing at high latitudes. The model results are basically in a good qualitative agreement with experimental observations. This model provides the satisfactory agreement between the calculated and experimental F2MOF values while not correctly representing the fine structure of the experimental OIS ionograms at night. An agreement between the calculated and experimental data is better for day and evening hours than at night.     

Effects of a “day-time” substorm on the ionosphere and radio propagation

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.     

Application of small-size antennas for estimation of angles of arrival of HF signals scattered by ionospheric irregularities

A modification of the Doppler Interferometry Technique is suggested to enable estimating angles of arrival of comparatively broadband HF signals scattered by random irregularities of the ionospheric plasma with the use of small-size weakly directional antennas. The technique is based on the measurements of cross-spectra phases of the probe radiation recorded at least in three spatially separated points. The developed algorithm has been used to investigate the angular and frequency-time characteristics of HF signals propagating at frequencies above the maximum usable one (MUF) for the direct radio path Moscow-Kharkiv. The received signal spectra show presence of three families of spatial components attributed, respectively, to scattering by plasma irregularities near the middle point of the radio path, ground backscatter signals and scattering of the sounding signals by the intense plasma turbulence associated with auroral activations. It has been shown that the regions responsible for the formation of the third family components are located well inside the auroral oval. The drift velocity and direction of the auroral ionosphere plasma have been determined. The obtained estimates are consistent with the classical conception of the ionospheric plasma convection at high latitudes and do not contradict the results of investigations of the auroral ionosphere dynamics using the SuperDARN network.     

Enhancement of VLF-ducts under the action of high power radio wave on the ionospheric F-region

Wave effects are discussed pointing to improvement of whistler propagation in the ionosphere illuminated by a powerful radio wave. The large scale irregularities (ducts) responsible for these effects are formed in the illuminated ionospheric region by the process of electron heating by the fields of the pump wave and excited plasma oscillations. These irregularities may also be created in the ionosphere and plasmasphere by fluxes of suprathermal electrons accelerated by the plasma turbulence in the reflection region of the pump wave.     

Studying characteristics of traveling ionospheric disturbances using U-shaped traces on vertical incidence ionograms

This paper presents a case study when due to the descending additional U-shaped trace on vertical incidence ionograms, increased critical frequency stabilizes. This corresponds to an ionospheric disturbance that moves toward the ionosonde and then stays overhead.Within a 2D model, traveling ionospheric disturbances (TIDs) are superimposed on the inverted background ionosphere. So ray tracing is used to obtain propagation paths through non-stratified ionosphere thus synthesizing the disturbed ionogram traces. Investigated are changes in the cusp shape caused by varying TID parameters. A cusp-fitting method to determine the TID amplitude, spatial scale, and horizontal drift velocity are shown.     

Anthropogenic trigger of substorms and energetic particles precipitations

The high-frequency (HF) emission in near-Earth space from various powerful transmitters (radio communications, radars, broadcasting, universal time and navigation stations, etc.) form an integral part of the modern world that it cannot do without. In particular, special-purpose research facilities equipped with powerful HF transmitters are used successfully for plasma experiments and local modification of the ionosphere. In this work, we are using the results of a complex space-ground experiment to show that exposure of the subauroral region to HF emission can not only cause local changes in the ionosphere, but can also trigger processes in the magnetosphere–ionosphere system that result in intensive substorm activity (precipitations of high-energy particles, aurorae, significant variations in the ionospheric parameters and, as a consequence, in radio propagation conditions).     

Near real-time PPP-based monitoring of the ionosphere using dual-frequency GPS/BDS/Galileo data

Ionosphere delay is very important to GNSS observations, since it is one of the main error sources which have to be mitigated even eliminated in order to determine reliable and precise positions. The ionosphere is a dispersive medium to radio signal, so the value of the group delay or phase advance of GNSS radio signal depends on the signal frequency. Ground-based GNSS stations have been used for ionosphere monitoring and modeling for a long time. In this paper we will introduce a novel approach suitable for single-receiver operation based on the precise point positioning (PPP) technique. One of the main characteristic is that only carrier-phase observations are used to avoid particular effects of pseudorange observations. The technique consists of introducing ionosphere ambiguity parameters obtained from PPP filter into the geometry-free combination of observations to estimate ionospheric delays. Observational data from stations that are capable of tracking the GPS/BDS/GALILEO from the International GNSS Service (IGS) Multi-GNSS Experiments (MGEX) network are processed. For the purpose of performance validation, ionospheric delays series derived from the novel approach are compared with the global ionospheric map (GIM) from Ionospheric Associate Analysis Centers (IAACs). The results are encouraging and offer potential solutions to the near real-time ionosphere monitoring.     

Characterization of ionospheric amplitude scintillations using wavelet entropy detrended GNSS data

The extensive monitoring networks of Global Navigation Satellite System (GNSS) ionospheric scintillation have been established to continuously log observation data. Further, the amplitude scintillation index and the phase scintillation index, which are derived from scintillation observations, are anticipated to accommodate the accuracy requirement of both the user level and the monitoring station level. However, raw scintillation observations essentially measure superposed waveform impairments of GNSS signals propagating through ionosphere and troposphere. It implies that fluctuations of raw scintillation observations are caused by multiple factors from the entire radio propagation environment. Hence, it is crucial to characterize ionospheric scintillations from GNSS observation data. And the characterization is implemented through extracting fluctuations of raw observations merely induced by ionospheric scintillations. Designed to address this problem by means of Fourier filtering detrending, the present work investigates the influence of varying detrending cutoff frequencies on wavelet statistical energy and wavelet entropy distributions of scintillation data. It consequently derives criteria on the optimum detrending cutoff frequency for three types of raw amplitude scintillation data, which are classified by their wavelet energy distributions. Results of the present work verify that detrending with specific optimum cutoff frequencies rather than the fixed and universally applicable one renders the validity and credibility of characterizing ionospheric scintillations as the part of GNSS observation fluctuations purely induced by ionosphere electron density irregularities whose scale sizes are comparable with or smaller than the Fresnel scale.     

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.     

电离层不规则结构对GPS性能的影响

电离层不规则结构的存在可引起无线电信号的幅度和相位发生随机起伏, 这 种电离层闪烁现象会影响全球定位系统(Global Positioning System, GPS)的 性能, 降低定位精度, 严重时导致信号失锁. 电离层不规则结构对GPS性能的 影响涉及电离层物理、接收机设计和表征卫星几何分布的精度衰减因子(Dilution of Positioning, DOP)等多方面因素. 本文通过对表征电离层不规则结构参数 的分析, 根据GPS接收机跟踪环路和闪烁信号模型, 综合研究了电离层闪烁对 GPS接收机载波跟踪环和码跟踪环跟踪误差的影响; 结合实际观测, 评述 了电离层不规则结构对单频和双频GPS接收机定位性能的影响, 在此基础上 提出了有待深入研究的问题及具体建议.      

Comparison between HF propagation and DEMETER satellite measurements within the mid-latitude trough

The morphology of the auroral, sub-auroral and mid-latitude trough region of the ionosphere is strongly dependent on the interplanetary magnetic field and the level of geomagnetic activity. Changes in the morphology impact on the characteristics of HF signals propagating through these regions of the ionosphere. In order to develop a better understanding of these effects, a number of experiments have recently been undertaken in which the time of flight and direction of arrival of HF signals have been measured over several paths aligned along the mid-latitude trough. In addition, observations made by the DEMETER satellite of the mid-latitude trough electron density structure, dynamics and wave activity were used in order to investigate the effect of the fine structure of the ionosphere on HF signals. For two types of relatively common night time HF time of flight and azimuth of arrival behaviour (referred to here and elsewhere as ‘Type 1’ and ‘Type 2’ propagation), the signal behaviour is consistent with scatter from irregularities in the auroral region in the one case, and from irregularities present on the floor of the trough in the other.