Ionospheric Scatter Research Articles

Symbol Shortening Transmission Algorithm for SC-IFDMA System in Ionospheric Scatter Channel

Symbol Shortening Transmission Algorithm for SC-IFDMA System in Ionospheric Scatter Channel

Application of Wide‐Beam Transmission for Advanced Operations of SuperDARN Borealis Radars in Monostatic and Multistatic Modes

AbstractThe Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 monostatic high‐frequency (HF, 8–20 MHz) radars to study dynamic processes in the ionosphere. SuperDARN provides maps of global‐scale ionospheric plasma drift circulation from the mid‐latitudes to the poles. The conventional SuperDARN radars consecutively scan through 16 beam directions with a lower limit of 1 minute to sample the entire field of view. In this work, we use the advanced capabilities of the recently developed Borealis digital SuperDARN radar system. Combining a wide transmission beam with multiple narrow reception beams allows us to sample all conventional beam directions simultaneously and to speed up scanning of the entire field‐of‐view by up to 16 times without noticeable deterioration of the data quality. The wide‐beam emission also enabled the implementation of multistatic operations, where ionospheric scatter signals from one radar are received by other radars with overlapping viewing areas. These novel operations required the development of a new model to determine the geographic location of the source of the multistatic radar echoes. Our preliminary studies showed that, in comparison with the conventional monostatic operations, the multistatic operations provide a significant increase in geographic coverage, in some cases nearly doubling it. The multistatic data also provide additional velocity vector components, increasing the likelihood of reconstructing full plasma drift velocity vectors. The developed operational modes can be readily implemented at other fully digital SuperDARN radars.

The Distribution of Small‐Scale Irregularities in the E‐Region, and Its Tendency to Match the Spectrum of Field‐Aligned Current Structures in the F‐Region

AbstractWe associate new data from icebear, a coherent scatter radar located in Saskatchewan, Canada, with scale‐dependent physics in the ionosphere. We subject the large‐scale icebear 3D echo patterns (treated as 2D point clouds) to a data analysis technique hitherto never applied to the ionosphere, a technique that is widely applied in cosmological red‐shift surveys to characterize the spatial clustering of galaxies. The technique results in a novel method to calculate the spatial power spectral density of the greater ionospheric irregularity field. We compare results from this method to in‐situ plasma density and magnetic field observations from the Swarm mission. We show that there is a remarkable similarity between echo clustering spectra in the E‐region and the field‐aligned current structuring spectrum observed in the F‐region: a clear and characteristic preferred scale (5 km) both in the E‐ and F‐region spectra. We discuss the possibility that this represents evidence of an energy injection into the ionospheric irregularity field via energetic particle precipitation, but offer alternative interpretations with wider connotations for the ionosphere‐magnetosphere system. These findings open new and promising avenues of research for the study of the location of ionospheric scatter echoes with 3D information. It constitutes a novel way to consider the pattern of ionospheric irregularities over wide fields of view when there is an abundance of radar echoes, which allows for the analysis of radar data as point clouds.

An Examination of SuperDARN Backscatter Modes Using Machine Learning Guided by Ray‐Tracing

AbstractThe Super Dual Auroral Radar Network (SuperDARN) is a network of High Frequency (HF) radars that are typically used for monitoring plasma convection in the Earth's ionosphere. A majority of SuperDARN backscatter can broadly be divided into three categories: (a) ionospheric scatter due to reflections from plasma irregularities in the E and F regions of the ionosphere, (b) ground scatter caused by reflections from the ground/sea surface following reflection in the ionosphere, and (c) backscatter from meteor trails left by meteoroids as they enter the Earth's atmosphere. Due to the complex nature of HF propagation and mid‐latitude electrodynamics, it is often not straightforward to distinguish between different modes of backscatter observed by SuperDARN. In this study, we present a new two‐stage machine learning algorithm for identifying different backscatter modes in SuperDARN data. In the first stage, a neural network that “mimics” ray‐tracing is used to predict the probability of ionospheric and ground scatter occurring at a given location along with parameters like the elevation angles, reflection heights etc. The inputs to the network include parameters that control HF propagation, such as signal frequency, season, UT time, and geomagnetic activity levels. In the second stage, the output probabilities from the neural network and actual SuperDARN data are clustered together to determine the category of the backscatter. Our model can distinguish between meteor scatter, 1/2 hop E‐/F‐region ionospheric as well as ground/sea scatter. We validate our model by comparing predicted elevation angles with those measured at a SuperDARN radar.

Virtual Height Characteristics of Ionospheric and Ground Scatter Observed by Mid‐Latitude SuperDARN HF Radars

AbstractPropagation of high‐frequency (HF) radio signals is strongly dependent on the ionospheric electron density structure along a communications link. The ground‐based, HF space weather radars of the Super Dual Auroral Radar Network (SuperDARN) utilize the ionospheric refraction of transmitted signals to monitor the global circulation of E‐ and F‐region plasma irregularities. Previous studies have assessed the propagation characteristics of backscatter echoes from ionospheric irregularities in the auroral and polar regions of the Earth's ionosphere. By default, the geographic location of these echoes are found using empirical models which estimate the virtual backscattering height from the measured range along the radar signal path. However, the performance of these virtual height models has not yet been evaluated for mid‐latitude SuperDARN radar observations or for ground scatter (GS) propagation modes. In this study, we derive a virtual height model suitable for mid‐latitude SuperDARN observations using 5 years of data from the Christmas Valley East and West radars. This empirical model can be applied to both ionospheric and GS observations and provides an improved estimate of the ground range to the backscatter location compared to existing high‐latitude virtual height models. We also identify a region of overlapping half‐hop F‐region ionospheric scatter and one‐hop E‐region GS where the measured radar parameters (e.g., velocity, spectral width, elevation angle) are insufficient to discriminate between the two scatter types. Further studies are required to determine whether these backscatter echoes of ambiguous origin are observed by other mid‐latitude SuperDARN radars and their potential impact on scatter classification schemes.

A Novel Weighted LLR Algorithm for Non-Coherent OFDM-MFSK System in Ionospheric Scatter Channel

A novel weighted log likelihood ratio (LLR) algorithm is proposed for the non-coherent orthogonal frequency division multiplexing with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${M}$ </tex-math></inline-formula> -ary frequency shift keying (OFDM-MFSK) system in an ionospheric scatter channel (ISC) with serious frequency-selective fading. In the algorithm, the bit LLR is derived according to the probability distribution of non-coherent received signals in the MFSK symbol and is approximately simplified to a concise expression, which reveals that LLRs cannot indicate the effect of the frequency-selective fading channel on different MFSK symbols. The proposed weighted LLR algorithm maximizes the signal-to-noise ratio (SNR) of the metric combined with LLRs in channel decoding. The weighted factors were estimated blindly from the received MFSK signals. Simulation results show that the bit error rate (BER) of OFDM-MFSK systems with the proposed LLR-weighted algorithm is superior than that of conventional LLR algorithms, and has approximately 1.6 dB SNR gain when BER is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${10^{-4}}$ </tex-math></inline-formula> .

Multiple E-Region Radar Propagation Modes Measured by the VHF SIMONe Norway System During Active Ionospheric Conditions

Multiple propagation modes between different bistatic radar links were measured during the operations of a very high frequency (VHF) 32.55 MHz radar system in northern Norway. The Spread Spectrum Interferometric Multistatic meteor radar Observing Network (SIMONe) Norway system detected meteor trails, direct transmitter to receiver signal propagation, over-the-horizon signal propagation from the SIMONe Germany system, ground and/or sea scatter, and ionospheric scatter on 27 August 2021 between 16:30–20:00 UT. These simultaneous detections were during an active ionospheric period with multiple occurrences of energetic charged particle precipitation. The SIMONe systems used continuous-wave (CW) pseudo-random phase modulated transmit signals and interferometry to make it possible to isolate each of these propagation modes and examine their characteristics. Different multistatic links at three receiver locations were analyzed, providing multistatic measurements of the regions with spatial and temporal resolutions on the order of 1.5 km and 2 s. The analysis techniques are described, with characteristics of the radar signal presented for each propagation mode and multistatic link. This study serves to highlight the capabilities of the SIMONe Norway system to research multiple aspects of ionospheric phenomena, specifically in the lower thermosphere-mesosphere boundary region.

Echo occurrence in the southern polar ionosphere for the SuperDARN Dome C East and Dome C North radars

In this paper, echo occurrence rates for the Dome C East (DCE) and the new Dome C North (DCN) radars are studied. We report the ionospheric and ground scatter echo occurrence rates for selected periods around equinoxes and solstices in the final part of the solar cycle XXIV. The occurrence maps built in Altitude Adjusted Corrected Geomagnetic latitude and Magnetic Local Time coordinates show peculiar patterns highly variable with season. The comparisons of the radar observations with the International Reference Ionosphere model electron density and with ray tracing simulations allow us to explain the major features of observed patterns in terms of electron density variations. The study shows the great potential of the DCE and DCN radar combination to the Super Dual Auroral Radar Network (SuperDARN) convection mapping in terms of monitoring key regions of the high-latitude ionosphere critical for understanding of the magnetospheric dynamics.

Determination of the Azimuthal Extent of Coherent E‐Region Scatter Using the ICEBEAR Linear Receiver Array

AbstractThe Ionospheric Continuous‐wave E‐region Bistatic Experimental Auroral Radar (ICEBEAR) is a VHF coherent scatter radar that operates with a field‐of‐view centered on 58°N, 106°W and measures characteristics of ionospheric E‐region plasma density irregularities. The initial operations of ICEBEAR utilized a wavelength‐spaced linear receiving array to determine the angle of arrival of the ionospheric scatter at the receiver site. Initially only the shortest baselines were used to determine the angle of arrival of the scatter. This publication uses this linear antenna array configuration and expands on the initial angle of arrival determination by including all the cross‐spectra available from the antenna array to determine both the azimuthal angle of arrival and the azimuthal extent of the incoming ionospheric scatter. This is accomplished by fitting Gaussian distributions to the complex coherence of the signal between different antennas and deriving the azimuthal angle and extent based on the best fit. Fourteen hours of data during an active ionospheric period (March 10, 2018, 0–14 UT) were analyzed to investigate the Gaussian fitting procedure and determine its feasibility for implementation with ICEBEAR. A comparison between mapped scatter, both neglecting azimuthal extent and including azimuthal extent is presented. It demonstrates that the azimuthal extent of the ionospheric E‐region scatter is very important for accurately portraying and analyzing the ICEBEAR measurements.

An Improved Estimation of SuperDARN Heppner‐Maynard Boundaries Using AMPERE Data

AbstractSuper Dual Auroral Radar Network (SuperDARN) ionospheric convection maps are a powerful tool for the study of solar wind‐magnetosphere‐ionosphere interactions. SuperDARN data have high temporal (approximately minutes) and spatial (∼45 km) resolution, meaning that the convection can be mapped on fine time scales that show more detail than the large‐scale changes in the pattern. The Heppner‐Maynard boundary (HMB) defines the low‐latitude limit of the convection region, and its identification is an essential component of the standard SuperDARN convection mapping technique. However, the estimation of the latitude of this boundary is dependent on ionospheric scatter availability. Consequentially it is susceptible to nonphysical variations as areas of scatter in different latitude and local time regions appear and disappear, often due to changing propagation conditions. In this paper, the HMB is compared to an independent field‐aligned current data set from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). A linear trend is found between the HMB and the boundary between the AMPERE Region 1 and Region 2 field‐aligned currents in the Northern Hemisphere, at both solar minimum and solar maximum. The use of this trend and the AMPERE current data set to predict the latitude position of the HMB is found to improve the interpretation of the SuperDARN measurements in convection mapping.

Identifying ground scatter and ionospheric scatter signals by using their fine structure at Ekaterinburg decametre coherent radar

The analysis of the scattered signal was carried out in the cases of ground scatter and ionospheric scatter. The analysis is based on the data of the decameter coherent EKB ISTP SB RAS radar. In the paper the signals scattered in each sounding run were analyzed before their statistical averaging. Based on the analysis, a model is constructed for ionospheric scatter and ground scatter signal, based on previously studied mechanisms. Within the framework of the Bayesian approach and based on large number of the data, the technique for identifying the two types of signals is constructed based on their different nature. The technique works without using traditional SuperDARN methods for estimating scattered signal parameters - spectral width, Doppler drift velocity or ray-tracing. The statistical analysis of the results was carried out. The total error produced by our IQ algorithm over the selected data was $13.3\%$, that is about two times less than total error produced by traditional algorithms.

Automated Ionospheric Scattering Layer Hypothesis Generation for Detected and Classified Auroral Global Positioning System Scintillation Events

AbstractWe describe a method to detect and classify global positioning system (GPS) scintillation, and then hypothesize a possible ionospheric layer scattering the signal. The objective is to routinely identify events of interest to investigate in detail in future work. Scintillation types include amplitude, phase, or both amplitude and phase. A scintillation event is one for which a scintillation index remains above a threshold across a majority of closely spaced receivers viewing a single satellite. Events are categorized by signal frequency and scintillation type. An event is then hypothesized to be due to the E or F layer using an independent data source. Data from the scintillation auroral GPS array located in Poker Flat Research Range, Alaska, are used to analyze L1 and L2C frequencies in 2014 and 2015. The irregularity layer associated with each scintillation event is hypothesized to be due to the activity in the E layer of the ionosphere (below 150 km) or in the F layer (above 195 km) using collocated Poker Flat Incoherent Scatter Radar electron density measurements. Events in a transition layer (150–195 km) and inconclusive results are also recorded. We find that nearly all of the over 4,000 events are phase scintillations. The majority of the events are hypothesized to occur when the peak density is at E‐layer altitudes. This indicates that E‐layer‐related scintillation may be quite common at auroral latitudes and that GPS receivers are sensitive to the irregularities occurring both there and at F layer altitudes.

Design and Application of the Distributed Ionospheric Coherent Scatter Radar

In this letter, a newly designed distributed coherent scatter radar for localization of ionospheric irregularities is presented. It is composed of a detection network that can detect ionospheric irregularities with the help of a time synchronization module. To achieve a fairly narrow beam with high directive gain, an antenna array is used in this transmitter module. The frequency band is from high frequency (HF) to very HF to detect irregularities at different scales. In addition, the radar uses a universal serial bus to reduce its size, which allows it to be easily moved to different areas. An iterative ray tracing method is also applied to localize the ionospheric irregularities. The results indicate that the radar can effectively track ionospheric irregularity in 3-D space.

Space weather impact on radio device operation

This paper reviews the space weather impact on operation of radio devices. The review is based on recently published papers, books, and strategic scientific plans of space weather investigations. The main attention is paid to ionospheric effects on propagation of radiowaves, basically short ones. Some examples of such effects are based on 2012–2016 ISTP SB RAS EKB radar data: attenuation of ground backscatter signals during solar flares, effects of traveling ionospheric disturbances of different scales in ground backscatter signals, effects of magnetospheric waves in ionospheric scatter signals.

Влияние факторов космической погоды на работу радиосредств

This paper reviews the space weather impact on operation of radio devices. The review is based on recently published papers, books, and strategic scientific plans of space weather investigations. The main attention is paid to ionospheric effects on propagation of radiowaves, basically short ones. Some examples of such effects are given based on 2012–2016 ISTP SB RAS EKB radar data: attenuation of ground backscatter signals during solar flares, effects of travelling ionospheric disturbances of different scales in ground backscatter signals, effects of magnetospheric waves in ionospheric scatter signals.

First observation of the anomalous electric field in the topside ionosphere by ionospheric modification over EISCAT

AbstractWe have developed an active ground‐based technique to estimate the steady state field‐aligned anomalous electric field (E*) in the topside ionosphere, up to ~600 km, using the European Incoherent Scatter (EISCAT) ionospheric modification facility and UHF incoherent scatter radar. When pumping the ionosphere with high‐power high‐frequency radio waves, the F region electron temperature is significantly raised, increasing the plasma pressure gradient in the topside ionosphere, resulting in ion upflow along the magnetic field line. We estimate E* using a modified ion momentum equation and the Mass Spectrometer Incoherent Scatter model. From an experiment on 23 October 2013, E* points downward with an average amplitude of ~1.6 μV/m, becoming weaker at higher altitudes. The mechanism for anomalous resistivity is thought to be low‐frequency ion acoustic waves generated by the pump‐induced flux of suprathermal electrons. These high‐energy electrons are produced near the pump wave reflection altitude by plasma resonance and also result in observed artificially induced optical emissions.

Determination of ionospheric parameters in real time using SuperDARN HF Radars

AbstractA technique for determining key ionospheric parameters from high‐frequency (HF) over‐the‐horizon radar ground scatter data is investigated using two Southern Hemisphere SuperDARN radars and also a Northern Hemisphere SuperDARN radar with reliable elevation angle‐of‐arrival capability. Ground scatter data are analyzed over a range of frequencies from 8 to 18 MHz to determine the maximum usable frequency and the vertical critical frequency over a wide geographical area within the radar field of view. The technique is shown to be well suited to middle to high latitudes where backscatter echoes from the ground dominate over those from ionospheric scattering targets. However, the technique is shown to break down during the winter months and away from solar maximum. It is shown that the use of reliable elevation angles can greatly enhance such methods allowing discrimination between ground scatter propagating via the E and F regions. It is also shown that contamination from very low velocity ionospheric scatter and ground scatter originating from the back lobe of the radar can be effectively filtered out, with the use of reliable elevation angles. This greatly improves the reliability of the ionospheric data products and allows for a high degree of automation of the process.

On the spatial distribution of decameter‒scale subauroral ionospheric irregularities observed by SuperDARN radars

AbstractThe midlatitude Super Dual Auroral Radar Network (SuperDARN) radars regularly observe nighttime low‒velocity Sub‒Auroral Ionospheric Scatter (SAIS) from decameter‒scale ionospheric density irregularities during quiet geomagnetic conditions. To establish the origin of the density irregularities responsible for low‒velocity SAIS, it is necessary to distinguish between the effects of high frequency (HF) propagation and irregularity occurrence itself on the observed backscatter distribution. We compare range, azimuth, and elevation data from the Blackstone SuperDARN radar with modeling results from ray tracing coupled with the International Reference Ionosphere assuming a uniform irregularity distribution. The observed and modeled distributions are shown to be very similar. The spatial distribution of backscattering is consistent with the requirement that HF rays propagate nearly perpendicular to the geomagnetic field lines (aspect angle ≤1°). For the first time, the irregularities responsible for low‒velocity SAIS are determined to extend between 200 and 300 km altitude, validating previous assumptions that low‒velocity SAIS is an F‒region phenomenon. We find that the limited spatial extent of this category of ionospheric backscatter within SuperDARN radars' fields‒of‒view is a consequence of HF propagation effects and the finite vertical extent of the scattering irregularities. We conclude that the density irregularities responsible for low‒velocity SAIS are widely distributed horizontally within the midlatitude ionosphere but are confined to the bottom‒side F‒region.

Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density

AbstractWe examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour‐long period of geomagnetic activity driven by a coronal mass ejection, a channel of high‐density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small‐scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.

An adjusted location model for SuperDARN backscatter echoes

Abstract. The radars that form the Super Dual Auroral Radar Network (SuperDARN) receive scatter from ionospheric irregularities in both the E- and F-regions, as well as the Earth's surface, either ground or sea. For ionospheric scatter, the current SuperDARN standard software considers a straight-line propagation from the radar to the scattering zone with an altitude assigned by a standard height model. The knowledge of the group delay to a scatter volume is not sufficient for an exact determination of the location of the irregularities. In this study, the difference between the locations of the backscatter echoes determined by SuperDARN standard software and by ray tracing has been evaluated, using the ionosonde data collected at Sodankylä, which is in the field-of-view of Hankasalmi SuperDARN radar. By studying elevation angle information of backscattered echoes from the data sets of Hankasalmi radar in 2008, we have proposed an adjusted fitting location model determined by slant range and elevation angle. To test the reliability of the adjusted model, an independent data set is selected in 2009. The result shows that the difference between the adjusted model and the ray tracing is significantly reduced and the adjusted model could provide a more accurate location for backscatter targets.