Anisotropic Irregularities Research Articles

Statistical Characteristics of the Temporal Spectrum of Scattered Radiation in the Equatorial Ionosphere

On the basis of the solution of the space-time characteristic system by the method of geometric optics using symbolic calculations, analytical and numerical simulation of the propagation of the ordinary and extraordinary radio waves in the conducting equatorial ionospheric plasma was made considering the anisotropy of plasma irregularities and non-stationary nature of propagation medium. Broadening of the spectrum and the displacement of its maximum contain velocity of a turbulent plasma flow and parameters characterizing anisotropic plasmonic structures. Statistical moments of both radio waves do not depend on the absorption sign and are valid for both active and absorptive random media. Temporal pulsations and conductivity of a turbulent ionospheric plasma have an influence on the evaluation of the spectrum-varying propagation distances travelling by these waves. The new double-humped effect in the temporal spectrum has been revealed for the ordinary wave varying anisotropy coefficient and dip angle of stretched plasmonic structures. From a theoretical point of view, the algorithms developed in this work allow effective modelling of the propagation of both radio signals in the equatorial conductive ionospheric plasma, considering the external magnetic field, inhomogeneities of electron density in-homogeneities, as well as non-stationary.

Determining Ionospheric Drift and Anisotropy of Irregularities from LOFAR Core Measurements: Testing Hypotheses behind Estimation

We try to assess the validity of assumptions taken when deriving drift velocity. We give simple formulas for characteristics of the spatiotemporal correlation function of the observed diffraction pattern for the frozen flow and the more general Briggs model. Using Low-Frequency Array (LOFAR) Cassiopeia intensity observation, we compare the experimental velocity scaling factor with a theoretical one to show that both models do not follow observations. We also give a qualitative comparison of our drift velocity estimates with SuperDARN convection maps. The article is essentially an extended version of the conference paper: “Determining ionospheric drift and anisotropy of irregularities from LOFAR core measurements”, Signal Processing Symposium 2021 (SPSympo 2021).

Relationship between the Drift of Medium-Scale Irregularities and the Orientation of Transverse Anisotropy of Small-Scale Irregularities in the F Region of the Midlatitude Ionosphere

Relationship between the Drift of Medium-Scale Irregularities and the Orientation of Transverse Anisotropy of Small-Scale Irregularities in the F Region of the Midlatitude Ionosphere

On the Similarity Between the Orientation of the Cross-Field Anisotropy of Small-Scale Irregularities and the Direction of the Plasma Drift in the Ionosphere Over Moscow

On the Similarity Between the Orientation of the Cross-Field Anisotropy of Small-Scale Irregularities and the Direction of the Plasma Drift in the Ionosphere Over Moscow

On the Similarity Between the Orientation of the Cross-Field Anisotropy of Small-Scale Irregularities and the Direction of the Plasma Drift in the Ionosphere Over Moscow

On the Similarity Between the Orientation of the Cross-Field Anisotropy of Small-Scale Irregularities and the Direction of the Plasma Drift in the Ionosphere Over Moscow

Power Spectrum in the Conductive Terrestrial Ionosphere

Stochastic differential equation of the phase fluctuations is derived for the collision conductive magnetized plasma in the polar ionosphere applying the complex geometrical optics approximation. Calculating second order statistical moments it was shown that the contribution of the longitudinal conductivity substantially exceeds both Pedersen and Hall’s conductivities. Experimentally observing the broadening of the spatial power spectrum of scattered electromagnetic waves which equivalent to the brightness is analyzed for the elongated ionospheric irregularities. It was shown that the broadening of the spectrum and shift of its maximum in the plane of the location of an external magnetic field (main plane) less than in perpendicular plane for plasmonic structures having linear scale tenth of kilometer; and substantially depends on the penetration angle of an incident wave in the conductive collision turbulent magnetized ionospheric plasma. The angle-of-arrival (AOA) in the main plane has the asymmetric Gaussian form while in the perpendicular plane increases at small anisotropy factors and then tends to the saturation for the power-low spectrum characterizing electron density fluctuations. Longitudinal conductivity fluctuations increase the AOAs of scattered radiation than in magnetized plasma with permittivity fluctuations. Broadening of the temporal spectrum containing the drift velocity of elongated ionospheric irregularities in the polar ionosphere allows to solve the reverse problem restoring experimentally measured velocity of the plasma streams and characteristic linear scales of anisotropic irregularities in the terrestrial ionosphere.

Spaceborne P-Band SAR Imaging Degradation by Anisotropic Ionospheric Irregularities: A Comprehensive Numerical Study

There has been a burgeoning prospect in developing a spaceborne P-band synthetic aperture radar (SAR) mission for its stronger penetrability through foliage and subsurface than the higher-frequency system. However, the transionospheric signals operating at P-band are more susceptible to scintillation impacts, which may bring about the decorrelation of the signal amplitude, phase, and frequency introduced by ionospheric irregularities. In this article, a comprehensive numerical model of the generalized ambiguity function (GAF) is established to evaluate SAR imaging deterioration. On the one hand, an improved two-frequency and two-position coherence function (TFTPCF) is integrated in the GAF to include the amplitude scintillation derived from the diffraction. On the other hand, the anisotropic characteristics of the irregular structure is introduced into TFTPCF by adopting the Rino’s 2-D spectrum. Furthermore, the ambiguous resolution is redefined for a more strict numeration. Numerical analyses about the ionospheric coherence and an ambiguous resolution are performed to investigate their sensitivity to scintillation parameters. Results show that this model is capable of depicting more comprehensive effects of the anisotropic irregular ionosphere, which may exhibit a rod-like structure, including elongation by anisotropic scale, rotation by geomagnetic heading, and projection by geomagnetic inclination. At last, the signal-level simulations are operated to verify the effectiveness of numerical conclusions, which further confirm that the structural configuration of anisotropic irregularities has a complicated effect on spaceborne P-band SAR image resolution.

Correlation between the Orientation of Cross-Field Axes of Small-Scale Anisotropic Irregularities in Midlatitude Ionosphere and Drift Direction in the F Region

Introduction. It was previously reported that small-scale irregularities (SSI) in the polar ionosphere are elongated along the magnetic field and anisotropic in its cross-field direction. At the same time, the largest of the SSI cross-field axes tends to orient along the SSI drift direction. This is also indirectly confirmed for the midlatitude ionosphere, however, direct correlations of the SSI anisotropy and ionospheric drift directions in the middle latitudes are absent.Objective. The main objective of this work is a direct comparison of the experimental data of SSI shape with motion parameters of irregularities, which are measured at the same place (Moscow) at the same time. Previously, experimentally obtained values of the SSI cross-field anisotropy orientation in the midlatitude ionosphere were compared only with the neutral winds model.Materials and methods. A tomographic approach is used to determine the SSI anisotropy parameters by processing radio scintillation signals while overfly of several navigation satellites emitting on frequencies of 150 MHz and 400 MHz. Estimations of ratio between the ellipsoids axes and cross-field anisotropy orientations in the framework of SSI model in a form of magnetic field oriented ellipsoids with three different dimensions along and across the Earth's magnetic field are obtained. Irregularities parameters are obtained by selecting the model parameters when the calculated logarithm dispersion of the satellite signals relative amplitude while they orbiting is the closest to the experimentally obtained curve. Estimations of the velocity and drift direction of medium-scale irregularities (MSI) by using DPS-4 ionosonde data acquired while decameter-wave radar studies of ionosphere from the Earth's surface are obtained. The socalled "sky maps" presenting the energy distribution of scattered radio waves on incident angles are used. Simultaneous measurements of Doppler frequency shifts and incident angles of scattered waves allow obtaining estimations of three components of the medium-scale irregularities drift velocity.Results. A good correlation between the drift direction of medium-scale irregularities and cross-field anisotropy orientation of small-scale irregularities is found.Conclusion. The correlation between the cross-field anisotropy orientation of the elongated irregularities and their drift direction can be useful under conditions of the lack of information on ionospheric irregularities.

Effect of Anisotropy on Ionospheric Scintillations Observed by SAR

Studies pertaining to small scale structures producing scintillations using synthetic aperture radars (SARs) have predominantly been conducted at low-latitude regions. The high-latitude region (auroral belt and polar caps) is highly dynamic and varies in response to stimuli from solar winds and the magnetosphere in complex ways. In this paper, the authors have shown the capability of SAR for scintillation observation in the auroral region. An attempt has been made to fit an irregularity anisotropy model to SAR measurements for characterizing the ionospheric irregularities in the auroral regions. The dependency of anisotropy irregularity model on parameters, such as irregularity structure (axial ratio), their orientation with respect to magnetic field lines, and the ionospheric plasma drift, is closely studied using Advanced Land Observing Satellite (ALOS)-2 datasets acquired over Alaska. Geomagnetic indices and total electron content data are consistent with the occurrence of the scintillation event under study. Drift velocity measurements from high-frequency radars in the super dual array radar network (SuperDARN) showed that the anisotropy is independent of the magnitude and the azimuth angle of the plasma drift. The typical range of orientation angle suitable for the high latitude regions probed by ALOS-2 is demonstrated to be between 120°–135°. This paper explores the idea of inferring irregularity anisotropy by comparing the amplitude scintillation (S4) index measured in SAR data pairs using two well-established techniques. The image contrast technique heavily relies on the accurate modeling of anisotropy, whereas the radar cross-sectional enhancement method is independent of it. This feature has been exploited in the $S_{4}$ comparison to finally fit the choice of irregularity axial ratio and conclude that the sheet-like structures best describe the ionospheric irregularity structure in the region under observation.

Anisotropy of stratospheric irregularities inferred from stellar scintillation measurements by GOMOS/ENVISAT

Abstact. In this paper, we discuss the methods for evaluation of the anisotropy of small-scale atmospheric irregularities using satellite stellar occultation measurements. These methods are based on coherency analysis of bi-chromatic scintillation observed in occultations of double stars and in tangential (horizontal) occultations. These methods are applied to scintillation measurements by two fast photometers of the GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board ENVISAT. The estimates of the anisotropy of air density irregularities generated by saturated gravity waves are obtained for altitudes 30-45 km. The anisotropy coefficient varies from 50 down to 10 with decreasing vertical scales from 60 m to 10 m.

Modeling and Analyzing Impacts of Drifting Anisotropic Ionospheric Irregularities on Inclined Geosynchronous SAR

The sub-satellite track of geosynchronous synthetic aperture radar (GEO SAR) presents the figure 8 or O, which causes the great changes of platform motion direction and the different projection of anisotropic irregularities along the line-of-sight (LOS) direction. Due to the almost equal angle velocity to that of Earth, the GEO SAR has smaller ionospheric penetration point (IPP) scanning velocity which is much smaller to the counterpart of the low earth orbit SAR (LEO SAR) while is comparable to the drifting velocity of irregularities, which will affect the effective azimuthal velocity. These facts lead to the consequence that the satellite signals from the GEO SAR would become more vulnerable when they are transmitted in the environment where the ionospheric scintillation occurs. However, few works are focused on these mentioned issues towards the GEO SAR system. In this paper, the impacts of ionospheric scintillation on GEO SAR imaging will be analyzed considering the anisotropy and drifting velocity of irregularities. The anisotropy and drifting velocity effects can both originate from the effect on power spectral density (PSD) of phase screen which is used to model the ionospheric scintillation effects. Based on the data from international geomagnetic reference field (IGRF) and satellite tool kit (STK), the GEO SAR imaging simulations for different GEO SAR orbital configurations and positions are carried out. The simulation results demonstrate that the anisotropy and the drifting velocity of irregularities will cause the changes of stripe direction and affect the quality of GEO SAR images.

Statistical Characteristics and Scintillation Level of Scattered Radio Waves in the Magnetized Plasma

Second order statistical moments of scattered radiation in the magnetized plasma is considered using modified smooth perturbation method taking into account both diffraction effects and polarization coefficients of both the ordinary and extraordinary waves. The broadening of the spatial spectrum and displacement of its maximum are obtained. Scintillation level of scattered waves is analyzed for different parameters characterizing anisotropic irregularities for the ionospheric F-region.

Correlation between the horizontal wind direction and orientation of cross-field anisotropy of small-scale irregularities in the F region of midlatitude ionosphere

Radio sounding of midlatitude ionosphere shows that natural small-scale electron density irregularities in the F region are cross-field anisotropic. The orientation of the cross-field anisotropy is different under different geophysical conditions. The cross-field anisotropy orientation is matched with the horizontal wind direction calculated within the HWM07 model for each event. It is ascertained that natural irregularities in a plane perpendicular to the magnetic field are stretched along the horizontal wind direction under different geophysical conditions.

Extension of Hybrid Scintillation Propagation Model to the case of field propagation in the ionosphere with highly anisotropic irregularities

AbstractThe detailed effects of the high‐frequency radio wave propagation in a stochastic transionospheric propagation channel are discussed. This is a special case of propagation along or almost along the lines of the Earth's magnetic field. In this case, the ionospheric random irregularities of the electron density of the ionosphere may have very high values of the aspect ratio, which stands beyond the range of validity of the traditional analytical approaches utilized to treat the appropriate transionospheric propagation problems. In the consideration presented here, new analytical results are obtained for treating the problem under consideration. Based on the analytical results obtained, the previously developed Hybrid Scintillation Propagation Model is further extended, which also includes the software simulator of the signals propagating on the transionospheric paths with the random electron density irregularities highly elongated in the direction of the Earth's magnetic field.

Modelling ionospheric scintillation under the crest of the equatorial anomaly

WAM is realized making use of the plasma density data collected via the retarding potential analyser on board the Dynamics Explorer 2 spacecraft, capable to model the scintillation climatology over the northern hemisphere high latitude ionosphere. More recently, WAM has been tuned to model the ionospheric scintillations also over the equatorial latitudes. The effort has been done to support the CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project in the assessment of the scintillations climatology over Latin America.The concept of the new release of WAM is the same already adopted for the high latitudes: the in situ measurements, supplemented with an ionospheric model and with the irregularity anisotropy model, are treated to describe the morphology of scintillation, provided a suitable propagation model is used. Significant differences have been included in the low latitudes release to account for the anisotropy of the irregularities and for strong scattering regime. The paper describes the new WAM formulation and presents comparisons of the model predictions with the actual measurements collected in Brazil.

Second Order Statistical Moments of the Power Spectrum of Ionospheric Scintillation

Paper considers some aspects of the relationship between scintillation level of scattered radiation and anisotropic plasma irregularities in the collision turbulent magnetized plasma. Analytical calculations are carried out using modify smooth perturbation method taking into account both diffraction effects and polarization coefficients of both ordinary and extraordinary waves. Correlation function and the wave structure function of the phase fluctuations are obtained for arbitrary correlation function of electron density fluctuations. Analytical and numerical calculations of the second order statistical moments are carried out for the anisotropic Gaussian correlation function taking into account both anisotropy factor and the inclination angle of the elongated plasma irregularities with respect to the external magnetic field. Scintillation level and the power spectrum of the intensity fluctuations are analyzed for different parameters characterizing anisotropic irregularities.

Cubic Phase Distortion and Irregular Degradation on SAR Imaging Due to the Ionosphere

The interest in the use of spaceborne synthetic aperture radar (SAR) for collecting earth bio/geophysical information and detecting foliage-obscured targets has been increased. However, the signals are inevitably affected by the ionosphere, particularly at very high frequency and ultrahigh frequency. Thus, it is crucial to understand the potential effects of the ionosphere on SAR systems. In this paper, three possible contributions are made to analyze these effects. First, for analyzing range resolution, in addition to linear and quadratic phase errors due to the background ionosphere, the cubic phase error is considered. The expected mean electron density inferred from the International Reference Ionosphere is used. Second, for analyzing azimuthal resolution, the effects of ionospheric irregularities are evaluated under the conditions of oblique incidence and anisotropic irregularities. The model is presented on the basis of the multiple phase screen (MPS) method. Compared with the previous model, the MPS method can give results in good agreement with both weak and strong scattering theories. Finally, based on the theory of moment equation, range resolution degradation caused by the multiple scattering is also studied in the case of anisotropic irregularities. By using the range Doppler algorithm, a number of degraded point responses due to these ionospheric effects are shown, and then, some evaluation results are listed.

Effects of Anisotropic Ionospheric Irregularities on Space-Borne SAR Imaging

This paper discusses the degradation of space-borne synthetic aperture radar (SAR) imaging due to anisotropic ionospheric irregularities. In the case of the oblique incidence and anisotropy of ionospheric irregularities, two types of potential ionospheric effects on SAR imaging are derived from an improved transverse correlation function. First, in the range direction, the image shift due to both multiple scattering and dispersion of irregularities is considered. In addition, pulse broadening due to multiple scattering of the irregularities is also studied, which leads to degradation of the range resolution. Second, in the azimuthal direction, the decorrelation distance is obtained by the second moment of the generalized ambiguity function. Using the range Doppler algorithm (RDA), several degraded point target responses due to the ionospheric irregularities are evaluated. The simulation results show that the adverse effects of ionospheric irregularities on SAR imaging are worsened at lower frequencies and stronger ionospheric fluctuations.

Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions

Abstract. Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

First evidence of anisotropy of GPS phase slips caused by the mid-latitude field-aligned ionospheric irregularities

The mid-latitude field-aligned irregularity (FAI) along the magnetic field line is a common phenomenon in the ionosphere. However, few data reveal the field-aligned ionospheric irregularities. They are insufficient to identify FAIs effects so far, particularly effect on global positioning system (GPS) signals. In this paper, the mid-latitude FAIs by line-of-sight angular scanning relative to the local magnetic field vector are investigated using the denser GPS network observations in Japan. It has been the first found that total GPS L2 phase slips over Japan, during the recovery phase of the 12 Feb 2000 geomagnetic storm were caused by GPS signal scattering on FAIs both for the lines-of-sight aligned to the magnetic field line (the field of aligned scattering, FALS) and across the magnetic field line (the field of across scattering, FACS). The FALS results are also in a good agreement with the data of the magnetic field orientation control of GPS occultation observations of equatorial scintillation during thorough low earth orbit (LEO) satellites measurements, e.g. Challenging Minisatellite Payload (CHAMP) and Satellite de Aplicaciones Cientificas-C (SAC-C). The role of large-angle scattering almost along the normal to the magnetic field line in GPS scintillation is determined by attenuation of the irregularity anisotropy factor as compared with the other factors.