Optimum parameter for estimating phase fluctuations on transionospheric signals at high latitudes

Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S₄ and the standard deviation of the received phase σ_? typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project ( De Franceschi et al., 2006) are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S_?. This parameter is shown to better correlate with S₄ being much less detrending dependent than σ_?. The couple (S₄, S_?) should be then considered a more physical proxy of radio scintillation than the couple (S₄, σ_?).     

Using GPS-SCINDA observations to study the correlation between scintillation,total electron content enhancement and depletions over the Kenyan region

This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S₄ index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.     

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.     

Investigation of ionospheric irregularities and scintillation using TEC at high latitude

The occurrence of ionospheric irregularities at high latitudes, with dimensions of several kms down to decameter scale size shows strong correlation with geomagnetic disturbance, season and solar activity. Transionospheric radio waves propagating through these irregularities experience rapid random fluctuations in phase and/or amplitude of the signal at the receiver, termed scintillation, which can degrade GNSS services. Thus, investigation and prediction of this scintillation effect is very important. To investigate such scintillation effects, a GISTM (GPS Ionospheric Scintillation and TEC Monitoring) NovAtel dual frequency (L1/L2) GPS receiver has been installed at Trondheim, Norway (63.41°$63.41°$ N, 10.4°$10.4°$ E), capable of collecting scintillation indices at a 1 min rate as well as the raw data (phase and intensity) of the satellite signals at a 50 Hz sampling rate and TEC (Total Electron Content) at a 1 Hz rate. Many researchers have reported that both phase and amplitude scintillation is closely associated with TEC fluctuations or associated with a significant developing enhancement or depletion in the TEC. In this study, a novel analogous phase index is developed which provides samples at a 1 min rate. Generally the scintillation indices can help in estimating the irregularity scintillation effect at a one minute rate, but such procedures are time consuming if DFTs of the phase and/or amplitude at a 50 Hz data are required. In this study, instead, this analogous phase index is estimated from 1 Hz rate TEC values obtained from the raw signals and is then compared for weak, moderate and strong scintillation at Trondheim for one year of data collected from the installed GPS receiver. The spectral index of the irregularities (that is the inverse power law of their spatial spectrum) is determined from the resultant phase scintillation psd. The correlations of the scintillation indices and spectral indices with the analogous phase index have been investigated under different geomagnetic conditions (represented by the Kp index) and an approximate linear correlation of phase scintillation with the analogous phase index was found. Then a principal advantage of this index is that it achieves this correlation without requiring a high sampling data rate and the need for DFTs. Thus, the index seems a good candidate for developing a simple means of ionospheric scintillation prediction which could also be utilized in the development of alerts using regional mappings.     

A study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station

The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.16°N, 72.78°E; Geomagnetic: 12.90°N, 147.35°E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S₄ index_, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70–96% scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S₄ ? 0.2) and enhancement of ROTI with (ROTI ? 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.     

Case study of inclined sporadic E layers in the Earth’s ionosphere observed by CHAMP/GPS radio occultations: Coupling between the tilted plasma layers and internal waves

We have used the radio occultation (RO) satellite data CHAMP/GPS (Challenging Minisatellite Payload/Global Positioning System) for studying the ionosphere of the Earth. A method for deriving the parameters of ionospheric structures is based upon an analysis of the RO signal variations in the phase path and intensity. This method allows one to estimate the spatial displacement of a plasma layer with respect to the ray perigee, and to determine the layer inclination and height correction values. In this paper, we focus on the case study of inclined sporadic E (E_s) layers in the high-latitude ionosphere based on available CHAMP RO data. Assuming that the internal gravity waves (IGWs) with the phase-fronts parallel to the ionization layer surfaces are responsible for the tilt angles of sporadic plasma layers, we have developed a new technique for determining the parameters of IGWs linked with the inclined E_s structures. A small-scale internal wave may be modulating initially horizontal E_s layer in height and causing a direction of the plasma density gradient to be rotated and aligned with that of the wave propagation vector k. The results of determination of the intrinsic wave frequency and period, vertical and horizontal wavelengths, intrinsic vertical and horizontal phase speeds, and other characteristics of IGWs under study are presented and discussed.     

Ionospheric scintillations at Guilin detected by GPS ground-based and radio occultation observations

The occurrence of ionospheric scintillations with S4 ? 0.2 was studied using GPS measurements at Guilin, China (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of the equatorial anomaly. The results are presented for data collected from January 2009 to March 2010. The results show that nighttime amplitude scintillations only took place in February and March of the considered years, while daytime amplitude scintillations occurred in August and December of 2009. Nighttime amplitude scintillations, observed in the south of Guilin, always occurred with phase scintillations, TEC (Total Electron Content) depletions, and ROT (Rate Of change of TEC) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations always took place simultaneously for most of the GPS satellites which appeared over Guilin in different azimuth directions. Ground-based GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio (SNR) measurements obtained from GPS-COSMIC radio occultation indicate that nighttime and daytime scintillations are very likely caused by ionospheric F region irregularities and sporadic E, respectively. Moreover, strong daytime amplitude scintillations may be associated with the plasma density enhancements in ionospheric E region caused by the Perseid and Geminid meteor shower activities.     

Simulation and analysis of chemical release in the ionosphere

Ionospheric inhomogeneous plasma produced by single point chemical release has simple space-time structure, and cannot impact radio wave frequencies higher than Very High Frequency (VHF) band. In order to produce more complicated ionospheric plasma perturbation structure and trigger instabilities phenomena, multiple-point chemical release scheme is presented in this paper. The effects of chemical release on low latitude ionospheric plasma are estimated by linear instability growth rate theory that high growth rate represents high irregularities, ionospheric scintillation occurrence probability and high scintillation intension in scintillation duration. The amplitude scintillations and the phase scintillations of 150?MHz, 400?MHz, and 1000?MHz are calculated based on the theory of multiple phase screen (MPS), when they propagate through the disturbed area.     

Statistical properties of variability of the quiet ionosphere F2-layer maximum parameters over Irkutsk under low solar activity

The statistical analysis of the quiet ionosphere F2-layer maximum parameters variability (deviations of NmF2 and hmF2 from the quiet medians, δn and Δh) under solar minimum at day (10–16 LT) and night (22–04 LT) hours based on data of Irkutsk station for 2007–2010 is presented. It is found that the experimental distribution (histogram) of δn can be approximated by a mixture of two normal distributions. The first and second components of the mixture characterize, mainly, relatively weak and strong fluctuations of δn which are presumably associated with the ionospheric effects of the atmospheric gravity waves and of the planetary waves and tides correspondingly. Deviation of the δn histogram from a single normal distribution is most considerable at night hours in winter and equinoxes. For these conditions the weak fluctuations of δn are mainly negative and the strong ones are mainly positive. The Δh histogram is a normal distribution except day hours in winter and equinoxes when a weak deviation of the histogram from the normal distribution occurs.     

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.      

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

Transition radiation mechanism for broad-band component of stimulated electromagnetic emission

As is well known in the F-region of the ionosphere modified by high power HF radio waves broad-band electromagnetic stimulated emission (SEE) is observed. It was discovered both the beams of superthermal electrons and intensive small-scale irregularities in modified region. These magnetic field aligned irregularities have caviton shape — deep electron density holes. The presence of such irregularities and superthermal electrons create conditions for generation of transition emission. We consider this radiation mechanism for interpretation the SEE broad-band component.     

Geomagnetic storm effects on the occurrences of ionospheric irregularities over the African equatorial/low-latitude region

The study investigated the effects of intense geomagnetic storms of 2015 on the occurrences of large scale ionospheric irregularities over the African equatorial/low-latitude region. Four major/intense geomagnetic storms of 2015 were analyzed for this study. These storms occurred on 17th March 2015 (?229?nT), 22nd June 2015 (?204?nT), 7th October 2015 (?124?nT), and 20th December 2015 (?170?nT). Total Electron Content (TEC) data obtained from five African Global Navigation Satellite Systems (GNSS) stations, grouped into eastern and western sectors were used to derive the ionospheric irregularities proxy indices, e.g., rate of change of TEC (ROT), ROT index (ROTI) and ROTI daily average (ROTI_AVE). These indices were characterized alongside with the disturbance storm time (Dst), the Y component of the Interplanetary Electric Field (IEFy), polar cap (PC) index and the H component of the Earth’s magnetic field from ground-based magnetometers. Irregularities manifested in the form of fluctuations in TEC. Prompt penetration of electric field (PPEF) and disturbance dynamo electric field (DDEF) modulated the behaviour of irregularities during the main and recovery phases of the geomagnetic storms. The effect of electric field over both sectors depends on the local time of southward turning of IMF Bz. Consequently, westward electric field inhibited irregularities during the main phase of March and October 2015 geomagnetic storms, while for the June 2015 storm, eastward electric field triggered weak irregularities over the eastern sector. The effect of electric field on irregularities during December 2015 storm was insignificant. During the recovery phase of the storms, westward DDEF suppressed irregularities.     

Scale analysis of pre- and post-midnight ESF bubbles at storm time and quiet time

This paper adopts a scale analysis technique to investigate the properties of intermediate-scale plasma structures observed by ROCSAT-1 in the equatorial F-region. A procedure of scale analysis that is developed via the empirical mode decomposition (EMD) method of Hilbert–Huang transform (HHT) technique allows the mutually correlated components in velocity, density and relative density gradient to be identified and extracted. Comparing the three parameters, good match in wave form is found for density and velocity in the scales between kilometers and hundred meters (few kilometers to 300 m). It implies that there are electric fields proportional to density fluctuation −δn/n in the form similar to what is expected for the generalized Rayleigh–Taylor instability. We find that such a one-to-one match holds for various pre- and post-midnight ESF bubbles during quiet and storm times. It, therefore, means that spatial structures of electric field in the intermediate-scale (300 m to few kilometers) correlates to the density structures in a manner of δE ∝  −δn/n that is not necessarily depending on the driving mechanism of ESF bubbles, although it is known that ESF bubbles can be driven by different mechanisms under different space weather conditions. In smaller scales (300–50 m), fluctuation patterns of density and velocity do not correlate to each other any more, the good match is then found in the density gradient ∇_xn/n and velocity. It is known as the manifestation of the Boltzmann relation. We note that the GRT instability related relationship δV_z ∝  −δn/n for irregularities in scale of kilometers holds only for ESF bubbles that occur within ±5 dip latitude, while the Boltzmann relation (δV_z proportional to ∇_xn/n) holds for small-scale irregularities without such a limitation.     

Stochastic modelling considering ionospheric scintillation effects on GNSS relative and point positioning

Global Navigation Satellite Systems (GNSS), in particular the Global Positioning System (GPS), have been widely used for high accuracy geodetic positioning. The Least Squares functional models related to the GNSS observables have been more extensively studied than the corresponding stochastic models, given that the development of the latter is significantly more complex. As a result, a simplified stochastic model is often used in GNSS positioning, which assumes that all the GNSS observables are statistically independent and of the same quality, i.e. a similar variance is assigned indiscriminately to all of the measurements. However, the definition of the stochastic model may be approached from a more detailed perspective, considering specific effects affecting each observable individually, as for example the effects of ionospheric scintillation. These effects relate to phase and amplitude fluctuations in the satellites signals that occur due to diffraction on electron density irregularities in the ionosphere and are particularly relevant at equatorial and high latitude regions, especially during periods of high solar activity. As a consequence, degraded measurement quality and poorer positioning accuracy may result.     

Characterization of the low latitude plasma density irregularities observed using C/NOFS and SCINDA data

Complex electrodynamic processes over the low latitude region often result in post sunset plasma density irregularities which degrade satellite communication and navigation. In order to forecast the density irregularities, their occurrence time, duration and location need to be quantified. Data from the Communication/Navigation Outage Forecasting System (C/NOFS) satellite was used to characterize the low latitude ion density irregularities from 2011 to 2013. This was supported by ground based data from the SCIntillation Network Decision Aid (SCINDA) receivers at Makerere (Geographic coordinate 32.6°E, 0.3°N, and dip latitude ?9.3°N) and Nairobi (Geographic coordinate 36.8°E, ?1.3°N, and dip latitude ?10.8°N). The results show that irregularities in ion density have a daily pattern with peaks from 20:00 to 24:00 Local Time (LT). Scintillation activity at L band and VHF over East Africa peaked in 2011 and 2012 from 20:00 to 24:00 LT, though in many cases scintillation at VHF persisted longer than that at L band. A longitudinal pattern in ion density irregularity occurrence was observed with peaks over 135–180°E and 270–300°E. The likelihood of ion density irregularity occurrence decreased with increasing altitude. Analysis of C/NOFS zonal ion drift velocities showed that the largest nighttime and daytime drifts were in 270–300°E and 300–330°E longitude regions respectively. Zonal irregularity drift velocities over East Africa were for the first time estimated from L-band scintillation indices. The results show that the velocity of plasma density irregularities in 2011 and 2012 varied daily, and hourly in the range of 50–150 m s^?1. The zonal drift velocity estimates from the L-band scintillation indices had good positive correlation with the zonal drift velocities derived from VHF receivers by the spaced receiver technique.     

Fading in the HF ionospheric channel and the role of irregularities

It is well known that the ionosphere affects radio wave propagation especially in the high frequency (HF) range. HF radio waves reflected by the ionosphere can reach considerable distances, often with changes in amplitude, phase, and frequency. The ionosphere is a dispersive in frequency and time, bi-refractive, absorbing medium, in which multipath propagation due to traveling irregularities is very frequent. The traveling irregularities undulate the reflecting ionospheric layer, introducing variations in signal amplitude (fading). In this multipath time variant channel fading is mainly considered, even though it is not the sole effect. Echo signals from a single reflection, as in ionospheric vertical sounding (VIS) techniques, are affected by a certain degree of variability even in quiet ionospheric conditions. In this work the behavior of the ionospheric channel is studied and characterized by observing the power variation of received echoes using the VIS technique. Multipath fading was analyzed quantifying the power variation of the signal echo due to irregularities on a temporal scale from 0.5 to 256 s. An experimental set-up derived from an ionosonde was implemented and the analysis was performed employing a special numerical algorithm operating off-line on the acquired time sequence of the signal. The gain-loss of the irregularity shapes are determined in some special cases.     

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.     

基于小波分解与重构方法研究电离层偶发E层

电离层不规则体对卫星导航、通信、雷达系统等有重要影响.通过数值模拟及与实测数据的对比,论证基于小波分解与重构方法实现利用掩星数据反演电离层不规则体的可行性.以电离层偶发E层为例,利用国际参考电离层(IRI)模拟背景电离层电子密度分布,利用掩星探测的水平电子密度总含量δht反演不规则体信息,并与模拟数据进行比较.对200...     

The propagation features of LF radio waves at topside ionosphere and their variations possibly related to Wenchuan earthquake in 2008

The artificial low frequency (LF) signals recorded at topside ionosphere from DEMETER satellite were analyzed in this paper, and the typical diurnal and seasonal variations were illustrated around 162?kHz in electric field spectra. The larger power spectrum density (PSD) values in electric field at local nighttime and in winter season all demonstrate the correlation feature of lower plasma content to higher penetration of LF waves into ionosphere. Around Wenchuan earthquake, the comparison of signal-noise-ratio (SNR) values in electric field with each half month during January to May in 2008 and the same half month in May from 2005 to 2007 revealed their lowest values and small covering area around the preparation region of Wenchuan earthquake in 2008. Combined with other researches in VLF radio waves and geochemical observations from satellite, the interaction of ion accumulation and upward movement from gas-water release at surface might be a key factor to disturb the ionospheric plasma density, and then possibly leading to the decrease of low energy penetration of LF radio waves from the artificial transmitted source at ground.