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The ionosphere's dynamic structure affects electromagnetic radiation by altering radio wave propagation, impacting daily communications. The characteristics of the ionosphere are primarily characterized by electron density parameters. This paper proposes a method to construct Three‐Dimensional (3‐D) electron density distributions with arbitrary spatiotemporal resolution in ISR observational regions. The method, termed Artificial Intelligence‐based data assimilation (AI‐Assim), integrates data assimilation directly into a neural network. It assimilates electron density from the IRI‐2020 model to fill ISR observation gaps. Experiments conducted using the Sanya Incoherent Scatter Radar (SYISR) in Hainan, China, successfully constructed a 3‐D electron density structure over the region, with a 0.2° latitude/longitude resolution and 1 km height resolution. The method's effectiveness was validated by calculating the mean square error and comparing the results with digisonde measurements. Plain Language Summary: This study leverages the most powerful ionospheric observation tool, the ISR, to construct 3‐D electron density distributions with arbitrary spatial resolution. Relying solely on empirical models often leads to accuracy issues, while 3‐D electron density models based purely on observational methods typically suffer from low resolution. All observational methods encounter difficulties in achieving continuous, high spatial resolution monitoring of the entire sky, and ISR is one of the most effective techniques available. However, even with interpolation methods, the coverage area of ISR remains limited. Therefore, this study explores a method that uses the neural network to assimilate electron density values from the IRI‐2020 model, aiming to fill the gaps in ISR detection. By assimilating International Reference Ionosphere values to approximate observed values, the accuracy of the 3‐D electron density results is enhanced. Multiple iterations of AI‐ Assim enable the construction of 3‐D electron density distributions with arbitrary spatial resolution. Key Points: We developed a method for constructing 3‐D electron density distributions with arbitrary spatiotemporal resolution at ISR stationsThe method termed AI‐Assim, continuously assimilating electron density from the IRI‐2020 model to fill ISR observation gapsExperiments using SYISR data achieved a 3‐D electron density model with 0.2° map resolution and 1 km height resolution [ABSTRACT FROM AUTHOR]

We discuss an exhaustive search approach to fit the incoherent scatter spectrum (ISS) in the F1‐region for molecular ion fraction (fm), ion temperature (Ti), and electron temperature (Te). The commonly used "full profile" approach for F1‐region measurements parameterizes the molecular ion fraction as a function of altitude and fits all the related heights for the state variables. In our approach, we fit the ISS at each height for fm, Ti, Te, and ion velocity (Vi) independently. Our exhaustive search method finds all the major local minima at each altitude. Although a parameterized function is used to guide the algorithm in finding the best solution, the fitting parameters retain their local characteristics. Despite that fitting fm, Ti, and Te without constraints requires Doppler shift to be accurately determined and the ISS signal‐to‐noise ratio higher than the full‐profile method, simulations show that Ti, Te, and fm can be recovered within a few percent accuracy with a moderate signal‐to‐noise ratio. We apply the exhaustive search approach to the Arecibo high‐resolution incoherent scatter radar data taken on 13 September 2014. The derived ion and electron temperatures are sensitive enough to reveal thermosphere gravity waves commonly seen in the electron density previously. Our method is more robust than previous height‐independent fitting methods. Comparison with another Arecibo program indicates our results are likely more accurate. Simultaneous high‐resolution measurements of Ti, Te, fm, Vi, and electron concentration (Ne) in our approach open new opportunities for synergistic studies of the F1‐region dynamics and chemistry. Plain Language Summary: Incoherent scatter spectra (ISS) can be used to derive a range of ionosphere state variables. Fitting the ISS for ion composition, and ion and electron temperatures is challenging in the ionosphere F1 region because the cost function has only minute differences for multiple solutions. A prevalent approach for F1 region spectral fitting is to assume a parameterized composition profile and fit all heights simultaneously. While such a "full profile" approach allows a general determination of the molecular ion profile, localized variations are smoothed out. In this study, we discuss an exhaustive search approach to derive the state variables that are largely height‐independent. The approach is applied to the Arecibo incoherent scatter radar data taken on 13 September 2014. The derived ion and electron temperatures are sensitive enough to reveal thermospheric gravity waves that are commonly seen in the electron density previously. We discuss the accuracy of our results in the context of previous measurements and other methods. The accuracy and sensitivity of simultaneous measurements of multiple state variables in our approach open new opportunities for synergistic studies related to the F1‐region ionosphere. Key Points: A new exhaustive search method is presented to derive molecular ion fraction and other state variables from incoherent scatter spectraThe method can measure electron and ion temperatures with a few percent accuracy for a moderate signal‐to‐noise ratioApplication to Arecibo data reveals gravity wave structures in electron and ion temperatures, ion velocity, and electron density [ABSTRACT FROM AUTHOR]

To provide new insights into plasma density scale‐sizes in the polar cap, irregularity spectra are developed and tracked relative to magnetic local time (MLT) and solar zenith angle (SZA). A novel Incoherent Scatter Radar (ISR) technique is applied to develop spectra between 20 and 300 km using 2016 to 2018 imaginglp mode data from Resolute Bay ISR‐North. This technique leverages: (a) volumetric plasma density measurements from Advanced Modular ISRs, (b) the slow F‐region cross‐field plasma diffusion at scales greater than 10 km, and (c) that high‐latitude geomagnetic field lines are nearly vertical. The results of this work find that the largest spectral features within periodograms that use sunlit or dayside plasma densities are predominately above 100 km, indicating that structures that are above 100 km are more common than structures below 100 km in dayside/sunlit plasma. However, the opposite is true when plasma is in the dark or on the nightside, where the largest spectral features are predominately below 100 km. This contrast between the dayside and nightside is symptomatic of photoionization generating structures larger than 100 km, highlighting the role of photoionization or E‐region shorting in removing structures less than 100 km or driving larger scale‐structures more strongly, and the role of other mechanisms (such as flows, recombination, precipitation, and instabilities) in generating small‐scale structures. This paper will discuss these findings in detail, as well as discuss forthcoming works. Plain Language Summary: Plasma density variations in the polar cap are challenging to characterize, and it is difficult to discern what mechanisms create what sizes of "structures". This paper is part of a series of papers that use data from the Resolute Bay Incoherent Scatter Radar‐North to examine the drivers of polar cap variations that are between 20 and 300 km. Focusing on the local time and the angle of the sun, this paper finds that plasma density structures above 100 km are more prevalent in the daylight polar cap ionosphere than structures below 100 km. Meanwhile, plasma density structures below 100 km are more prevalent at night than structures above 100 km. This contrast between day and night indicates the effects of mechanisms that generate structuring only on the dayside and mechanisms that generate structuring anywhere in the polar cap. Key Points: Ionospheric polar cap irregularity spectra between 20 and 300 km are calculated using magnetic field mapping and incoherent scatter radarsThe largest dayside spectral features are typically over 100 km due to photoionization and E‐region shorting removing smaller irregularitiesNightside spectral features suggest important role of flows, recombination, precipitation, and instabilities in breaking down structures [ABSTRACT FROM AUTHOR]

This work presents an algorithm for automatic detection of anomalous electron heating (AEH) events in the auroral E‐region ionosphere using data from the Poker Flat Incoherent Scatter Radar (PFISR). The algorithm considers both E‐region electron temperature and magnetically conjugate electric field measurements. Application of this algorithm to 14 years of PFISR data spanning 2010 through 2023 detected 505 AEH events. Measured electron temperatures increase linearly with plasma drift speeds. Statistical trends of AEH occurrence as a function of space weather indices (AE and F10.7) demonstrate correlations with the solar cycle and geomagnetic activity levels. The magnetic local time occurrence rates show preferences for dusk and dawn with most events in the dusk sector. Observed AEH events tend to appear in regions of relatively low electron density and do not appear inside intense auroral arcs with high electron density. Furthermore, AEH detection requires a higher electric field than predicted by the threshold for a positive growth rate of the Farley‐Buneman instability (FBI), according to linear fluid theory. The implications of these findings for kinetic theories of FBI and AEH are discussed. Plain Language Summary: The ionized portion of the upper atmosphere, known as the ionosphere, can conduct electrical currents. In cases where those currents are so large that the relative motion of the electrons and ions exceeds a critical threshold, the ionosphere becomes unstable. The resulting instability rapidly heats electrons in the lower ionosphere, producing what are known as anomalous electron heating (AEH) events. We use ionospheric data from a radar facility in Alaska to detect AEH events. Our detection algorithm identified 505 AEH events in 14 years of data (2010–2023). The occurrence rates of AEH are strongly correlated with the 11‐year solar cycle and indicators of auroral activity. Furthermore, AEH events preferentially occur at times near dawn and dusk, with the highest number of events in the dusk sector. The elevated electron temperatures increase linearly with the magnitude of the ionospheric electric field. Key Points: A novel detection algorithm for identifying anomalous electron heating events primarily based on plasma flow velocities is presentedAnalysis of Poker Flat Incoherent Scatter Radar data from 2010 to 2023 detected 505 eventsStatistical analysis of occurrence rates revealed a strong relationship with geomagnetic activity over the 11‐year solar cycle [ABSTRACT FROM AUTHOR]

Low latitude ionosphere experiences complex dynamical and electrodynamical processes, which make the spatiotemporal variations of the corresponding electron density complicated and therefore influence trans‐ionosphere radio communications. The monitoring of low latitude dynamical drivers, such as neutral wind and ionospheric electric field, is essential for both dynamic mechanism investigations and applications. The Sanya Incoherent Scatter Radar Tristatic System (SYISR‐TS) was proposed with the main objective of low latitude ionospheric monitoring and investigation and has been successfully developed over the past decade. The system consists of the Sanya (18.3°N, 109.6°E) trans‐receiving main station with key parameters of ∼1,600 m2 antenna aperture, >4 MW peak power, <120 K system noise temperature, and ∼46 dBi normal gain, and Danzhou (19.5°N, 109.1°E) and Wenchang (19.6°N, 110.8°E) receiving only stations with key parameters of ∼790 m2 antenna aperture, <130 K system noise temperature, and ∼43 dBi normal gain. Three stations form a quasi‐equilateral triangle at Hainan Island and use Global Navigation Satellite System satellite common view technique to achieve the time synchronization with the uncertainty of the timing and time synchronization less than 50 and 10 ns, respectively. Initial collaborative satellite tracking and ionospheric common volume experiments among three stations have confirmed the detection ability of SYISR‐TS and the feasibility of achieving its scientific goals in the future. Plain Language Summary: Ionosphere is the ionized part (electrons and ions) of upper atmosphere around 60–1,000 km altitude. In the ionosphere, the generation and movement of electrons and ions are jointly determined by the solar radiation, photo‐chemistry, winds and electric fields. Monitoring the density of electrons, and winds and electric fields as well, is important for understanding and predicting the status of ionosphere. Among numerous ionospheric detection methods, Incoherent Scatter Radar (ISR) plays an important role due to its profiling multiple parameters capability. In this paper, we will describe the technical details and initial experiments of the newly built Sanya Incoherent Scatter Radar Tristatic System. It is also the world first tristatic ISR system using phased array technology, which can switch the beam direction quickly therefore increase the time resolution of measurements significantly. Key Points: Sanya Incoherent Scatter Radar Tristatic System (SYISR‐TS) is the world first completed phased array based tristatic ISR systemSYISR‐TS includes 1 trans‐receiving and 2 remote receiving stations and uses Global Navigation Satellite System satellite common view technique to achieve synchronizationInitial collaborative experiments among three stations have confirmed the detection ability of SYISR‐TS [ABSTRACT FROM AUTHOR]

In this paper, we present the results of a special experimental study of quiet-time behavior of the mid-latitude ionosphere over Eastern Europe by synchronously operation of different ground-based facilities. For the first time we used data obtained from Kharkiv incoherent scatter (IS) radar, ionosonde and coherent Doppler HF sounding system to detect and investigate traveling ionospheric disturbances (TIDs). The periods close to winter solstice and autumn equinox in 2018 were analyzed. The dominant periods and horizontal phase velocities of registered TIDs were 45–80 min and 230–460 m s−1, respectively. The strongest signatures were observed in the solstice measurement and represented by large amplitudes. Based on results obtained by all three methods, we found the LS TIDs with the same interval of time and altitude of propagation and similar characteristics (the period of about 50 min, estimated vertical 80 m/s and horizontal 460 m/s velocity and horizontal spatial scale size about 1360 km) for winter measurement. Such observational findings confirm the reliability of these TID detection techniques. Possible sources of TIDs generation were considered including solar terminator. [ABSTRACT FROM AUTHOR]

This study presents a first simultaneous observation of N 2 + Meinel (0,0) band (hereafter, N 2 + (M)) aurora by cutting-edge short-wavelength infrared imaging spectrograph (NIRAS-2) and monochromatic camera (NIRAC) installed at the Kjell Henriksen Observatory (78 ∘ N, 16 ∘ E). On January 21 2023, N 2 + (M) intensification that is associated with a band-shape aurora structure was observed by the NIRAS-2 and the NIRAC having temporal resolutions of 30 s and 20 s, respectively. In addition, the European incoherent scatter Svalbard Radar also observed electron density variations at the same time. Electron density measured at altitude range from 100 km 120 km shows similar variations as of N 2 + (M) intensity, which implies that a primary source of N 2 + (M) emissions is direct collisions of N 2 by precipitating electrons penetrating down to around 100 km altitude (up to 10 keV). However, the observation also demonstrated moderate correlations between N 2 + (M) intensity and electron density above 140 km, which implies that different N 2 + (M) generation process, N 2 charge exchange with O + , may work up to near 160 km and make a non-negligible contribution to N 2 + (M) emissions. This hypothesis would be verified with further radar observations or stereo imaging observations useful to estimate the vertical distribution of the emission layers. The N 2 + (M) is a very promising target wavelength for aurora observation because the quality of sensors is highly expected to improve further and further. Continuous observations with our new instruments will undoubtedly provide an important information of N 2 + (M) characteristics, for future missions of both balloon-borne and satellite-borne imaging. [ABSTRACT FROM AUTHOR]

This paper introduces the Peak Density Thickness (PDT) formalism, a novel approach to representing the F2 layer's vertical electron density profile in the ionosphere. It diverges from the conventional "pointed‐peak" model by suggesting a "broad‐peak" or "flat‐nose" profile where plasma density remains constant within an altitude interval χ. This theory is backed by independent observations, including a comprehensive data set from the Millstone Hill Incoherent Scatter Radar at the MIT Haystack observatory, spanning from 1993 to 2023, which illustrates the presence and diurnal variation of PDT. A single‐day intensive cross‐verification using Digisonde Portable Sounder DPS4D soundings of the sub‐peak ionosphere has shown remarkable agreement in the measurements of the lower boundary of the χ interval and the peak density. This study suggests incorporating the flat‐nose section χ into the F‐region profile formalism. Such a shift could improve the accuracy of topside specifications derived from ground‐based ionosonde measurements, enhancing our understanding of ionospheric plasma dynamics. Key Points: Peak Density Thickness (PDT) formalism that allows the plasma density at the F2 layer profile peak to remain constant with height for tens of kmCombined Incoherent Scatter Radar (ISR) and Digisonde measurements at mid‐latitudeUsing Millstone Hill ISR measurements from 1993 to 2023 to study the ionosphere Peak Plasma Density Thickness (PDT) [ABSTRACT FROM AUTHOR]

We present an analysis of 6 h oscillations in the thermosphere ranging from 150 km to 400 km. The analysis applies 134 days of data from an incoherent scatter radar located at Arecibo Observatory (18.3°N, 66.7°W) from 1984 to 2015. To our knowledge, the climatological and seasonal characteristics of the 6 h oscillations in the thermosphere were investigated for the first time over Arecibo. The climatological mean amplitude of the 6 h oscillation in the thermosphere is about 11 m/s, and it increases slowly with altitude above 225 km. The climatological mean amplitude of the 6 h oscillation is comparable with semidiurnal and terdiurnal tides at Arecibo above 250 km. The climatological mean phase exhibits limited vertical variation. The 6 h oscillation is the most prominent in autumn, with amplitudes reaching around 20 m/s compared to approximately 10 m/s in other seasons. The phase structure in all seasons exhibits weak vertical variations. The responses of the thermospheric 6 h oscillation to solar and geomagnetic activities are also analyzed in this study. Our results indicate that at low latitude, solar activities have a small impact on the variation in the thermospheric 6 h oscillation, while it appears that the amplitude of the 6 h oscillation increases with increasing geomagnetic activity. Above 250 km, the amplitude of the 6 h oscillation reaches ~20 m/s during strong geomagnetic activity, which is almost twice of that occurring during weak geomagnetic activity. [ABSTRACT FROM AUTHOR]

Incoherent scatter radars (ISRs) represent the only instrument (both ground and space based) capable of making high temporal and spatial resolution measurements of multiple atmospheric parameters—such as densities, temperatures, particle velocities, mass flux—over an altitude range covering the entire mesosphere/lower thermosphere/ionosphere (MLTI) system on a quasi-continuous basis. The EISCAT Svalbard incoherent scatter radar (ESR), located just outside Longyearbyen (78.15 ∘ N) on Svalbard, is the only currently operating facility capable of making such measurements inside the polar cusp—an area of significant energy input into the atmosphere and characterized by heating instabilities and turbulence. The ESR was built in the mid-1990s and has provided valuable data for the international experimental and modelling communities. New radar technologies are now available, in the form of phased array systems, which offer new data products and operational flexibility. This paper outlines the achievements and current research focus of the ESR and provides scientific arguments, compiled from inputs across the international scientific community, for a new phased array ISR facility on Svalbard. In addition to the fundamental scientific arguments, the paper discusses additional benefits of continued ISR observations on Svalbard, building on the key findings of the ESR. Svalbard has a large network of complementary instrumentation both focused on the MLTI system (e.g. the Kjell Henriksen auroral Observatory, the Svalbard SuperDARN radar and the Svalrak sounding rocket launch facility) with synergies to other research fields, such as meteorology and oceanography. As a further holistic system science view of the Earth becomes more important, a new ISR on Svalbard will be important also in this respect with its ability to provide datasets with a wide range of scientific applications. Increased activity in space has highlighted problematic issues such as space debris. A changing Arctic has also seen increased human activity via the opening up of new shipping routes, which are reliant on GNSS technology that is effected by severe turbulence in the MLTI system. As such, societal applications of a future ISR are also presented. The accessibility and logistical support for such a facility is also briefly discussed. [ABSTRACT FROM AUTHOR]

The claim that incoherent scatter radar data show electron density enhancements of 50–80% during some ionospheric heating experiments in a recent paper is questioned. The backscatter from the monostatic radar can indeed be enhanced during these experiments, but the conclusion that a large electron density increase is the cause is almost certainly wrong. Some natural plasma line data are presented in support of our claim. Previously published studies of similar events and a possible explanation for the observed increases in backscattered power are pointed out. [ABSTRACT FROM AUTHOR]

The ionosphere is an atmospheric region in which a large number of charged particles interact. Therefore, the production and loss mechanisms in this region play an important role in the modeling and interpretation of the ionosphere. In this study, the cross-section (CS) values, calculated by kinetic theory, were used to validate the International Reference Ionosphere (IRI) model at four different heights (210 km, 240 km, 340 km, and 410 km) on the equinox (March and September) and solstice (June and December) days in 2008 and 2015. The CS values were obtained by using both the temperatures of ions and electrons predicted in the IRI 2007, 2012, and 2016 versions, and the temperatures measured by the Kharkiv incoherent scatter radar (ISR). As a result of the calculation, the CS values in the solar minimum year (in 2008) were greater than those with moderate solar activity in the solar declining phase year (in 2015). The three versions of IRI show almost the same value in 2008. While these values are consistent with the values obtained from the ISR during the day, they are quite low at night. Validation of the model was carried out using Root Mean Square Error (RMSE) and Mean Absolute Percentage Error (MAPE) metrics. According to RMSE and MAPE values, it can be said that the predictions of all three versions of the IRI model are quite good. It can also be stated that the estimations of IRI-2016 are generally better than those of other versions of the IRI model. However, it is important to use ISR data to improve estimations of models at lower altitudes. [ABSTRACT FROM AUTHOR]

As the first advanced modular phase array incoherent scatter radar (ISR) established in the Eastern Hemisphere at low latitudes, Sanya ISR (SYISR) can measure the line-of-sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift. Three beam-scanning modes are designed for plasma drift detection: meridian, zonal and cross-shaped (both meridian and zonal) plane, which will provide the distribution of plasma drift in latitude/longitude as well as altitude. The altitude profile of plasma drift and the first presented distribution of low latitude plasma drift in the meridian plane for March to May 2021 are inversed through LOS velocity using cross-shaped and meridian beam-scanning modes, respectively. A statistical correlation coefficient between the v p n and crest-to-trough ratio (CTR) of equatorial ionization anomaly (EIA) TEC and a case study of magnetic storm response in plasma drift show that the inversed plasma drift can be a good indicator in response to the changes in atmospheric tide and solar wind at different time scales and explain the corresponding ionospheric electron density variations at low and equatorial latitudes. This study proves that the SYISR-measured plasma drift is reliable and will play an important role in the atmosphere-ionosphere-magnetospheric coupling study in the East Asian region. [ABSTRACT FROM AUTHOR]

A new incoherent scattering radar established at Sanya (18.3°N, 109.6°E; SYISR) has made it possible to directly measure the fine structure of the ionospheric E‐F valley. An intriguing phenomenon termed "ionospheric drizzle" was revealed by the month‐long continuous observation of complete ionospheric profiles with a temporal and vertical resolution of 0.8 s and 4.5 km. It manifests as weak plasma "streaks" in the strength of ∼103–104 cm−3 that detach from the F‐layer base and extends downward to the sporadic E (Es) layer during pre‐dawn hours, with an apparent descent speed of ∼10–20 m/s. The frequently observed "ionospheric drizzle" provides additional plasma to enhance the Es layer. Subsequent testing particle simulations were performed, revealing the critical role of the neutral winds in the plasma descent. This downward coupling process enhancing the Es layer is thus supposed to be important in communication and navigation. Key Points: Using the Sanya ISR, "ionospheric drizzle", a phenomenon linking the F‐layer and sporadic E layer at pre‐dawn hours, was revealedStreak‐like plasma detaches from the F‐layer base with a descent speed of ∼10–20 m/s and significantly enhances the Es layer at ∼120 kmTest particle simulations are performed to explain the critical role of neutral winds in the "ionospheric drizzle" formation [ABSTRACT FROM AUTHOR]

The plasma lines observed by Sanya incoherent scatter radar (SYISR) are dependent on the enhancement of Langmuir waves due to superthermal photoelectrons generated by solar EUV radiation. The plasma line power spectrum can be obtained using long-pulse and alternating-code transmission signals during the period from sunrise to noon almost every day. For the power spectrum of the long pulse, the CLEAN algorithm that has been applied in this field is used to verify the feasibility of this method for SYISR in only a few cases. However, it is difficult to deal with alternating code with such a low SNR using the general deconvolution method. The irreversible-migration filtering (IMF) method has been developed to separate signal noise from the measurements of the alternating code. Some experimental results from the SYISR measurements validate the excellent performance of the IMF method for alternating code. Additionally, an example observation of the electron density with a high time and range resolution is derived. The results show that plasma line detection can be a powerful new observational capability for SYISR as an ionospheric experimental mode for ionospheric calibration, when possible, which can be simultaneously measured with the ion line for constant radar calibration in the standard fitting of the ion line. [ABSTRACT FROM AUTHOR]

Ducts (field-aligned plasma density enhancements) provide a link into the magnetosphere and can guide whistler waves. Inside ducts, wave-particle interactions occur efficiently; therefore, their presence contributes to the removal of energetic particles from the magnetosphere. We present experimental results concerning the characteristics, behavior, and excitation thresholds of ducts induced by extraordinary (X-mode) polarized high-power HF radio waves emitted towards the magnetic zenith (MZ) into the upper ionosphere at EISCAT (European Incoherent SCATter). The features and behavior of ducts were diagnosed by the EISCAT incoherent scatter radar (ISR) at Tromsø and the CUTLASS (SuperDARN) Finland radar at Hankasalmi. The state of the ionosphere was monitored by the Dynasonde in Tromsø. It was found that the electron density Ne enhancements inside ducts were of 50–80% above the background Ne values and their transverse size (normal to the magnetic flux tube) corresponded to about 3–4° in the north–south direction. They were generated during magnetically quiet periods and extended from ~300 to 320 km up to the upper altitude limit of the EISCAT radar measurements (600–700 km), when heater frequencies were both below and above the critical frequency of the F2 layer (fH ≤ foF2 and fH > foF2), regardless of whether HF-induced plasma and ion lines were generated or not. Comparing the O-/X-mode effects from the EISCAT radar observations, it was shown that the creation of the strong Ne ducts is a typical characteristic of the X-mode pulses. As a rule, electron density enhancements were not observed during O-mode pulses. A plausible mechanism for the creation of X-mode artificial ducts is discussed. [ABSTRACT FROM AUTHOR]

The radar constant calibration in incoherent scatter radar ion line processing is essential for the data quality and was not paid enough attention in previous studies. In this investigation, based on several experiments made by the newly built Sanya incoherent scatter radar (SYISR), we made and evaluated the ion line calibration by plasma line both in case study and statistically. The calibration factor had local time and altitude variations, due to the corresponding variations of the transmitted power, the radar gain, and the noise temperature. We obtained a mean calibration factor of 1.35 by the simultaneous measured plasma line and ion line electron density and applied it to a one-month ion line observation calibration. Through a co-located ionosonde measured foF2 evaluation, the calibration decreased the mean deviation from −1.92 MHz (−18%) to −0.33 MHz (−3%), which resulted in much better agreement between the ion line foF2 after calibration and the ionosonde results. The existed deviations between after calibration and ionosonde results were due to the uncertainties either in the used calibration factor or the ionosonde measurements. An empirical Te/Ti usage in raw electron density estimation and ignoring the seasonal and short-term variations of the effecting factors might influence the calibration performance. Using the to-be-completed SYISR Tristatic System, the performance of plasma line calibration technique is expected to be improved in the future. [ABSTRACT FROM AUTHOR]

Ionospheric heating is known to induce density and temperature irregularities in the ionospheric plasma, which have been observed by various in‐situ and radar techniques. Analysis of incoherent scatter radar (ISR) plasma line spectra offers high resolution reconstruction of ionospheric features, but the plasma line profile is frequently difficult to reconstruct at altitudes where the heater is generating Langmuir turbulence and thus creating additional signals in the ISR spectrum. This work presents a novel, automated method based on Gabor filter image analysis to reconstruct plasma line spectra recorded with the coded long‐pulse technique. The method permits the unsupervised reconstruction of the Langmuir frequency with resolution in frequency (sub‐kHz) and altitude (∼300 m) typical of the coded long‐pulse technique even in regions directly coupling to the heater. The technique is applied to ISR data from the June 2019 Arecibo Observatory heating experiment, demonstrating the formation of field‐aligned irregularities of magnitude ∼0.5% in the Langmuir frequency during daytime conditions and spatial scale perpendicular to the magnetic field (fluctuation periodicity of several kilometers, corresponding to k ≈ 1 km−1) consistent with the patches of depletions of free electrons previously observed by in‐situ methods. Additionally, the method allows observation of the formation and evolution of the bunches over the course of several hours. Additional F‐layer irregularities were found in the ambient ionosphere in regions away from the heater coupling altitude. The automated nature of this technique offers the potential for precision analysis of Arecibo's ISR data archives for a variety of ionospheric physics applications. Key Points: A novel technique for extracting incoherent scatter radar plasma line profiles is presentedThe technique illuminates magnetic field aligned plasma irregularities and their temporal evolutionIonospheric heating‐induced plasma irregularities were observed to induce scintillation in high‐frequency oblique sounding signals [ABSTRACT FROM AUTHOR]

Space objects around the Earth are a potential pollution source for ground-based radio observations. The Sanya incoherent scatter radar (SYISR) is a newly built active digital phased array, all solid-state transmitting and digital receiving incoherent scatter radar in Sanya (18.3°N, 109.6°E), with the main purpose of ionospheric monitoring and investigations. In this study, we presented the effect of the greatly increased number of space objects on ionospheric observations through SYISR. Firstly, we showed the space object pollution on the range-time-intensity (RTI), autocorrelation function (ACF)/power spectra, and ionosphere parameter of SYISR measurements. An altitude of around 600 km is the region where space objects occur most frequently. Then, we eliminated the space object pollution using the traditional smallest of constant-false-alarm-rate (SO-CFAR) algorithm. However, pollution from smaller space objects remains, whose reflected echo is comparable to or lower than the background ionosphere, which results in unrealistic retrieved ionospheric electron density. Furthermore, we quantitatively assessed the space object effect based on the current space object orbit database and simulation. The pollution should linearly increase with the increase in the number of space objects in the future. Among the space objects, whose radar cross section (RCS) and orbit information are now published, there still exist ~9000 (~37% of the total number) space objects, whose effect is difficult to eliminate. This study is beneficial to the data process of SYISR and has implications for similar types of ionospheric observations by radar. [ABSTRACT FROM AUTHOR]

As the observation accuracy of parameters in the ionosphere cannot be directly checked, the comparison with other observations is the main way to evaluate the data quality of satellite measurements. Through the comparative analysis between the in situ electron density (Ne) observed by the China Seismo-Electromagnetic Satellite (CSES) and Ne at about 500 km altitude detected by Qujing Incoherent Scatter Radar (ISR), it was found that the pattern of CSES Ne is consistent with that of ISR Ne, and the correlation coefficient between the two sets of data is above 0.88 for different groups according to the magnitude. The value of CSES Ne is lower than that of ISR Ne, and the median value of the ratio for the difference between the conjugate data is 84.04%. Based on the comparison in the daytime between CSES Ne and ionosonde observations in China, it was found that the trend of the two datasets is mostly similar, and the correlation coefficient in some locations can reach up to 0.7. The distribution of CSES Ne and correlation coefficients at different latitudes show that the relationship is relatively better around the peak of the equatorial ionization anomaly (EIA). The differences in the value between CSES Ne and ionosonde data also exist, the relative change of which is about 80–95% in the daytime. [ABSTRACT FROM AUTHOR]

A new incoherent scatter radar (ISR) has recently been dedicated at Sanya (SYISR), China (geographic 18.34°N and 109.62°E) to detect the low‐latitude ionospheric plasma by conducting continuous operation and electronic beam steering on a pulse‐to‐pulse basis. This paper provides an overview of the processing procedure of SYISR ion line data and presents some preliminary observational results under various transmitting signal schemes and different working modes, such as the zenith‐directed mode and meridional and zonal scanning modes, with comparison to the international reference ionosphere model (IRI‐2016), ionosonde and Ionospheric Connections Explorer (ICON) satellite measurements. The diurnal and altitude versus latitude variation characteristics of electron density (Ne), electron temperature (Te), and ion temperature (Ti) are in accordance with those of other ISR measurements in mid‐low latitudes, and IRI shows some discrepancies with SYISR observations, including the sunrise and sunset enhancement in SYISR_Ti and the overestimated IRI_Te. The comparison between the daytime SYISR_Ti and ICON_Ti shows good consistency based on 31 observational samples. The measurement of line‐of‐sight velocity (Vi) for long pulse can reveal the properties of the medium scale traveling ionospheric disturbances manifesting the radar's potential to study the mesoscale ionospheric variation in the Sanya area. The derived vector velocities with a better measurement accuracy during the day are generally in line with ionospheric plasma physics. The results show that SYISR can obtain continuous ionospheric parameters through multi‐beam scanning measurements at ∼700 km horizontal scale, which provides important information for studying the atmosphere‐ionosphere‐magnetosphere coupling and ionospheric scintillation at low latitudes in East Asia. Plain Language Summary: China's first phased‐array incoherent scatter radar was completed in 2021 in Sanya, which is called SYISR. SYISR aims to investigate fundamental scientific problems in the ionosphere, including atmosphere‐ionosphere‐magnetosphere coupling, regional ionospheric properties and ionospheric irregularities in southern China. SYISR can directly measure electron density, electron and ion temperature, and drift velocity of the ionospheric plasma and can be used to indirectly obtain the temperature of the neutral atmosphere, wind field and irregularity information. This paper describes in detail the data processing from the IQ signal to the derived ionospheric parameters for SYISR. Ionospheric experiments in various scanning modes have been performed, mainly including a single beam in zenith‐directed mode and multiple beams in meridian and zonal scanning modes, and the obtained results are evaluated. These experimental results are evaluated through comparison with the IRI‐2016 model, digital ionosonde and Ionospheric Connections Explorer satellite. The comparison results indicate that SYISR is able to provide reliable, unique, and important ionospheric information for the study of ionospheric science at low latitudes in East Asia. Key Points: Sanya incoherent scatter radar (SYISR) is a new phased array system dedicated to the ionospheric observations at low latitudes in Asia sectorSYISR is operational, providing high‐quality ionospheric measurements in multiple radar scanning modes and radio pulse schemes modesData processing procedures and initial ion‐line results using typical experiment modes are reported [ABSTRACT FROM AUTHOR]

To provide new insights into the relationship between geomagnetic conditions and plasma irregularity scale‐sizes, high‐latitude irregularity spectra are computed using a novel Incoherent Scatter Radar (ISR) technique. This new technique leverages: (a) the ability of phased array Advanced Modular ISR (AMISR) technology to collect volumetric measurements of plasma density, (b) the slow F‐region cross‐field plasma diffusion at scales greater than 10 km, and (c) the high dip angle of geomagnetic field lines at high‐latitudes. The resulting irregularity spectra are of a higher spatiotemporal resolution than what has been previously possible with ISRs. Spatial structures as small as 20 km are resolved in less than 2 minutes (depending on the radar mode). In this work, we focus on Resolute Bay ISR‐North (RISR‐N) observations operating in the "imaginglptight" mode. In addition to having an unprecedented view of the size and occurrence of irregularities as they traverse the polar cap, we find that as the F‐region plasma density decreases below approximately 2.5 × 1010 m−3 at 350 km altitude the spectral power shifts to scale‐sizes lower than 50 km. Additionally, near magnetic local noon, the spectral power shifts to scales greater than 50 km, and from 15 to 6 magnetic local time, the spectral power shifts to scales lower than 50 km. This reflects the role of photoionization dominating high‐latitude ionospheric structuring in the polar cap. Key Points: A novel mapping technique is developed to compute high latitude ionospheric irregularity spectra at a high spatiotemporal resolutionThe spectral power shifts to structures smaller than 50 km at low plasma densities and from dusk to dawnPhotoionization dominates structuring in the polar cap and creates structures 100 km and larger [ABSTRACT FROM AUTHOR]

Led by the Institute of Geology and Geophysics, Chinese Academy of Sciences, we have built a brand‐new modular active digital phased array, with all solid‐state transmission and digital receiving incoherent scatter radar (ISR) in Sanya (18.3°N, 109.6°E), a station in low latitude China called Sanya ISR (SYISR) since 2015. The development of SYISR involved the indoor design and development of key components, an outdoor prototype test, and the production and debugging of the entire array. The unique features of SYISR include a single‐channel directly connected T/R unit and antenna, a radar array monitoring and calibration network, environmental adaptability design and open architecture. The entire radar has 4,096 channels and 5,930 modules in total. All the technical indices, mainly including a >2 MW peak power, a 43 dBi antenna gain and a <120 K noise temperature, either meet or are superior to the designed value through an independent evaluation. Waveforms of single pulse, linear frequency modulation, Barker code, long pulse and alternating code have been implemented to meet multiple purposes. Four observational modes for ionospheric experiments, including zenith stare, perpendicular to geomagnetic field, meridian scan, and all sky scan, have been developed. We have implemented time domain decoding, frequency domain decoding, and statistical inversion methods in calculating the autocorrelation function and power spectra in signal processing. The preliminary experimental results on ionospheric parameters, plasma lines, irregularities and hard targets are reasonable and encouraging, which greatly enhances our confidence in achieving our scientific goals in the future. Plain Language Summary: The Earth's ionosphere (∼60–1,000 km above the sea level) is composed of dense plasma, which shows complicated variations versus altitude, geographic location, and solar activity level. When a ground‐based radio wave is transmitted onto these plasmas, it will generate stimulated radiation, with a small portion of them radiating back to the ground. If this backscattered radiation is collected by a ground‐based radar, a variety of ionospheric parameters, including plasma density, temperature, and movement speed, can be derived through a complicated signal processing algorithm. An ionospheric monitoring method called incoherent scatter radar (ISR) was therefore developed. However, since this backscattered radiation is very weak, the ISR should be built with sufficiently large power (usually megawatts) and apertures (usually hundreds of square meters). This leads to the high cost of building an ISR, with only a few having been built in the world thus far. In this paper, we describe a newly built ISR, called Sanya ISR (SYISR), in low latitude China by applying modern radar technologies, such as modular active digital phased arrays, all solid‐state transmitting and digital receiving. The development of SYISR followed the principle of a gradual and orderly progress, including indoor design, outdoor prototype testing and the entire radar construction and debugging. The radar technical indices have been evaluated, and all meet the designed value. We also developed a signal processing algorithm and a variety of observational modes. Finally, preliminary experimental results are shown, which are reasonable and encouraging. In the future, we will gradually achieve our scientific goals via effective data accumulation and analysis. Key Points: Sanya incoherent scatter radar (SYISR) is a newly built incoherent scatter radar in low latitude China with modular active electronic scanning, digital phased array, solid‐state transmitting, and digital receivingThis paper details the hardware, key parameters, data processing and preliminary experimental results of SYISRThe key parameters meet or are superior to the original design, and the preliminary observations are encouraging [ABSTRACT FROM AUTHOR]

We have investigated the difference between two standard techniques for deriving the ionospheric ion velocity using data taken with the EISCAT incoherent scatter radar between 1987 and 2007. For large‐scale convection flows, there is little difference between the tristatic and monostatic techniques, though the biggest relative difference occurs during periods when the interplanetary magnetic field (IMF) is strongly northward. At small scales the difference between the two techniques is correlated with a measure of the variability of the tristatic measurement. This suggests that small‐scale flow bursts, such as those associated with enhanced auroral arcs, could explain the local time variation in the velocity difference distributions. The difference in velocities obtained from the monostatic and tristatic techniques can make a significant difference in the estimate of the magnitude of Joule heating in the thermosphere. Considering only the electric field dominated component of Joule heating, Q, the difference in the two techniques can be as much as 52% of the tristatic measurement (Qm = 0.48Qt) in the morning sector (0–6 MLT), during a moderate to large geomagnetic storm. This reduces to a difference of 36% at non‐storm times in the same MLT period. Careful averaging of the velocity field with the future EISCAT_3D radar system will allow us to establish the impact of both spatial and temporal scales on the magnitude of the observations. Plain Language Summary: We compare two different methods of using an ionospheric radar to measure the speed of ions moving through the high‐altitude ionosphere. One method measures over a shorter time and smaller space than the other. We find that although both methods show similar large‐scale patterns (100–1,000s of km) in the velocity at different times of day and for different levels of space weather driving, there can be large differences at small scales (10–100s of km). This can affect estimates of the heating of the atmosphere that are important for understanding satellite drag, for example. The difference in heating can be very large at certain times of day (nearly double). Key Points: Two techniques using incoherent scatter radars to estimate the ion velocity and hence the ionospheric electric field have been comparedTristatic and monostatic radar methods capture the bulk motion of the ionosphere but small scale differences can reach hundreds of ms‐1Calculations of Joule heating from the tristatic method are >40% greater than from the monostatic depending on magnetic local time and geomagnetic activity [ABSTRACT FROM AUTHOR]

Different types of incoherent scatter radar (ISR) echoes are observed associated with aurora, including some which have been interpreted as signatures of cavitating Langmuir turbulence (CLT). Akbari et al. (2013) https://doi.org/10.1002/jgra.50314 discussed two instances of correlation between CLT and naturally occurring radio emissions called medium frequency burst (MFB) which occur at substorm onsets. Based on that observation, radio detections of MFB from Toolik Lake Observatory have been applied to investigate occurrence of CLT in ISR data from Poker Flat Incoherent Scatter Radar and their possible correlation with MFB. Of 131 MFB events, 25 occurred within 15 min of an ISR echo detection, compared to 6 of 116 intervals of a control set with similar local time and seasonal distribution. The difference is significant at the 10−4 level, suggesting that ISR echoes are more probable during substorm onset times identified using MFB as a proxy. However, only four observed ISR echoes coincident with one MFB event showed both specific characteristics consistent with CLT. Furthermore, investigation of the angle of arrival of MFB suggests that the electromagnetic emissions do not originate from the plasma volume where the ISR detects the echoes. The small number of coincident ISR echoes and MFB is expected due to the different volumes in which the emissions and the echoes are detected. 50% of the MFB events occurred within 20 min of a substorm onset independently identified versus 8% of the control set intervals, confirming the correlation of MFB with substorm onsets. Key Points: Langmuir turbulence events appear rare, estimated to occur in a few percent of examined intervalsA higher prevalence of echoes was found associated with medium frequency bursts (MFBs) than for a control set of similar dates and timesA connection between Langmuir turbulence and MFBs was neither proved nor disproved, however this study contains future search space [ABSTRACT FROM AUTHOR]

This study presents an improved method to estimate differential energy flux, auroral power and field‐aligned current of electron precipitation from incoherent scatter radar data. The method is based on a newly developed data analysis technique that uses Bayesian filtering to fit altitude profiles of electron density, electron temperature, and ion temperature to observed incoherent scatter spectra with high time and range resolutions. The electron energy spectra are inverted from the electron density profiles. Previous high‐time resolution fits have relied on the raw electron density, which is calculated from the backscattered power assuming that the ion and electron temperatures are equal. The improved technique is applied to one auroral event measured by the EISCAT UHF radar and it is demonstrated that the effect of electron heating on electron energy spectra, auroral power, and upward field‐aligned current can be significant at times. Using the fitted electron densities instead of the raw ones may lead to wider electron energy spectra and auroral power up to 75% larger. The largest differences take place for precipitation that produces enhanced electron heating in the upper E region, and in this study correspond to fluxes of electrons with peak energies from 3 to 5 keV. Finally, the auroral power estimates are verified by comparison to the 427.8 nm auroral emission intensity, which shows good correlation. The improved method makes it possible to calculate unbiased estimates of electron energy spectra with high time resolution and thereby to study rapidly varying aurora. Key Points: We use the BAFIM‐ELSPEC analysis combination to calculate the energy spectra of precipitating electrons with high time resolutionUsing the fitted electron density data in precipitation events leads to wider energy spectra, and larger auroral power and field‐aligned current estimatesAuroral power calculated using the BAFIM‐ELSPEC analysis correlates well with the 427.8 nm emission line intensity [ABSTRACT FROM AUTHOR]

We present an analysis of additional ionospheric F‐region layers, the F3 layers at Arecibo (18.3°N, 32°N geomagnetic latitude). The F3 layer is often observed in low geomagnetic latitudes observed above the F2 layer. This is the first time that the F3 layer is reported at Arecibo, which has a dip angle of 43.6°. Three F3 layer events were captured by the Arecibo incoherent scatter radar at around 400 km in the period of 03:00–07:00 LT, 04:00–06:00 LT, and 02:00–06:30 LT on 30 November, 1 December, and 2 December 2016, respectively. The formation of the observed layers is largely due to the change of the height gradient of the vertical ion drift, which accumulates the electrons at around 400 km. This generation mechanism is further verified by the simulation. The altitudinal variation of the vertical ion drift is mainly due to the effect of the westward electric field. Plain Language Summary: In the low geomagnetic latitudes of +/−15°, an additional ionospheric layer above the ionospheric F2 peak is often observed. The additional layer, often called F3 layer, is suggested to be generated by the upward ion drift over the F2 peak. Using incoherent scatter radar (ISR) measurements at Arecibo (18.3°N, 32°N geomagnetic latitude), Puerto Rico, we provide the first observations of F3 layer events at geomagnetic mid‐latitudes. Three events of the F3 layer were captured by the Arecibo ISR at around 400 km in the period of 03:00–07:00 LT, 04:00–06:00 LT, and 02:00–06:30 LT on 30 November, 1 December, and 2 December 2016, respectively. Our analysis shows that the observed F3 layers were formed due to the change in the height gradient of the downward vertical ion drifts that result in a large plasma influx at about 400 km. The vertical variation of the downward vertical ion drift is largely due to the effect of the westward electric field. Our results provide a good example that the F3 layer could be formed largely due to the vertical transport at the geomagnetic mid‐latitude, which improves our understanding of the F3 layer. Key Points: Events of an additional ionospheric (F3) layer are first reported at AreciboThe formation of the F3 layers is mainly due to the gradient of the vertical ion driftThe height variation of the vertical ion drift is largely due to the zonal electric field [ABSTRACT FROM AUTHOR]

The temporal and spatial variation of the upper transition height ( H T ) is studied using the measurements of the Incoherent Scatter Radar (ISR) at Arecibo Observatory (18.35 ° N , 66.75 ° W) , Puerto Rico (magnetic latitude 30 ° N). Observed ion compositions, electron densities, electron and ion temperatures are analysed to understand the diurnal and seasonal variations of H T at low and high solar activity covering solar cycles 23 and 24. The results show that during the low solar activity conditions, the mean H T is very low (around 750 km during the daytime and 500 km at night), while during high solar activity conditions, it reaches about 1600 km. Under very quiet geomagnetic conditions, the mean H T is about 700 km during day time and 500 km during night time. The obtained results are compared with the International Reference Ionosphere (IRI) model predictions using the TBT15 model ion composition option. During low and moderate solar activity conditions, the TBT15 model overestimates about 5–10% at night and 15–20% in the afternoon. Furthermore, from the current investigation using the IRI's TBT15 and DY85 model ion composition options, it is found that the newer TBT15 model option is more appropriate to investigate H T variations as compared to the DY85 model option. [ABSTRACT FROM AUTHOR]

In the present study, the results obtained by incoherent scatter radar (ISR) and empirical models (NeQuick2 and IRI-2016) of the variations in mid-latitude ionospheric electron density during partial solar eclipses on March 29, 2006 and March 20, 2015 over Kharkiv (49.60° N, 36.30° E, Ukraine) are presented and the prediction performances of the models during these partial solar eclipses are compared. The electron density (Ne) values are obtained for seven (190, 210, 240, 290, 340, 410, and 490 km) different altitudes. The percent deviation of Ne values, statistical correlation coefficient and root mean square error (RMSE) during solar eclipses are applied to make this investigation. According to the percent deviation of Ne values measured by ISR, a decrease in electron density was observed at all heights during the eclipse of March 29, 2006, whereas there was a decrease in Ne values at only 190, 210, and 240 km altitudes during the eclipse of March 20, 2015. However, at other altitudes (290, 340, 410, and 490 km), the increase has occurred in the electron density during the eclipse on March 20, 2015. The IRI-2016 model predicted that Ne values increase at all altitudes in the solar eclipse on March 20, 2015, and a decrease in Ne at all altitudes occurs on March 29, 2006. It can be said that the NeQuick 2 model predicts a decrease in the first three heights and an increase in other heights. When the predictions of the empirical models were compared, according to the root mean square error (RMSE) calculations, the predictions of the IRI-2016 model can be said to be better than that of NeQuick 2 at all heights during eclipse of March 29, 2006. Besides, the IRI-2016 model's prediction can be stated to be better than NeQuick 2 model at the first three altitudes (190, 210 and 240 km), while the NeQuick 2 model's prediction can be said to be better than the IRI-2016 model at the last four (290, 340, 410, and 490 km) altitudes during March 20, 2015 eclipse. It can be stated that ionospheric models do not accurately reflect the ionospheric response to partial solar eclipses. The inclusion of ISR data in empirical models will help models accurately predict the ionospheric response under all conditions. [ABSTRACT FROM AUTHOR]

Some improvements introduced in the Autoscala program are presented. They include improvements in E valley modeling of the electron density profile N e (h), and in the link between the E valley and bottom-side F regions. An abrupt variation in N e (h) generated by the previous version of Autoscala under night conditions has been eliminated. A series of ionograms recorded by the Millstone Hill digisonde (42.6°, 288.5°) were automatically interpreted by the previous version of Autoscala and by the new one. Data from Incoherent Scatter Radar (ISR) were used to comparatively assess the performance of the two versions. For this purpose, the root mean square errors (RMSE s) of the N e (h) provided by Autoscala were calculated relative to the corresponding values provided by ISR. A more accurate overall modeling of N e (h) was achieved by the new Autoscala version (RMSE = 0.51 MHz for the new version against RMSE = 0.67 MHz for the previous one). [ABSTRACT FROM AUTHOR]

Observation of the 3-dimensional (3-D) electron density of the ionosphere is useful to study large-scale physical processes in space weather events. Ionospheric data assimilation and ionospheric tomography are methods that can create an image of the 3-D electron density distribution. While multiple techniques have been developed over the past 30 years, there are relatively few studies that show the accuracy of the algorithms. This paper outlines a novel simulation approach to test the quality of an ionospheric tomographic inversion. The approach uses observations from incoherent scatter radar (ISR) scans and extrapolates them spatially to create a realistic ionospheric representation. A set of total electron content (TEC) measurements can then be simulated using real geometries from satellites and ground receivers. This data set, for which the 'truth' ionosphere is known, is used as input for a tomographic inversion algorithm to estimate the spatial distribution of electron density. The reconstructed ionospheric maps are compared with the truth ionosphere to calculate the difference between the images and the truth. To demonstrate the effectiveness of this simulation framework, an inversion algorithm called MIDAS (Multi-Instrument Data Analysis Software) is evaluated for three geographic regions with differing receiver networks. The results show the importance of the distribution and density of GPS receivers and the use of a realistic prior conditioning of the vertical electron density profile. This paper demonstrates that when these requirements are met, MIDAS can reliably estimate the ionospheric electron density. When the region under study is well covered by GPS receivers, as in mainland Europe or North America, the errors in vertical total electron content (vTEC) are smaller than 1 TECu (2–4%). In regions with fewer and more sparsely distributed receivers, the errors can be as high as 20–40%. This is caused by poor data coverage and poor spatial resolution of the reconstruction, which has an important effect on the calibration process of the algorithm. [ABSTRACT FROM AUTHOR]

Abstract: The investigation concerns the size and motion of spatial fine structure in auroral radio absorption as observed by imaging riometer. Electron density profiles in the D and E regions determined by simultaneous incoherent-scatter radar were inverted to provide an estimate of the energy spectrum of the incoming electrons, and it is shown that the gradient-curvature drift at these energies is not consistent with the observed motion of the absorption fine structure, being too large by a factor of 5 or 6. However, a comparison with F-region drifts determined by the SuperDARN radar network supports a hypothesis that the cause lies in moving structures within the magnetosphere. [Copyright &y& Elsevier]

Abstract: We describe a sounding technique that allows us to improve spatial resolution at the Irkutsk Incoherent Scatter Radar (IISR) without losing spectral resolution. The technique also allows us to decrease temperature estimation errors caused by the Faraday effect. The technique is based on transmitting various duration pulses without any modulation and on subtracting correlation matrices of the received signal grouped by sounding pulse duration. We show theoretically and experimentally that the technique allows us to solve the problem of improving spatial resolution. Accumulation time for the technique is approximately four times longer than that for the alternating codes technique with the same spatial resolution. The number of lags in the correlation function with high spatial resolution does not depend on necessary spatial resolution. In the proposed technique, all the lags are obtained with the same spatial resolution and with the same signal-to-noise ratio. The technique is valid within the quasi-static ionospheric parameter approximation. [Copyright &y& Elsevier]

The present paper is devoted to the description of an improved method for determination of the ionospheric electron density values by Kharkov Incoherent Scatter Radar. This method allows to improve the resolution of the incoherent signals parameters up to ∼20 km in the range of 100-400 km and ∼100 km in the range of 200-1100 km. This approach was used to investigate variability of the ionospheric electron density over East European region in the heights interval of 100-1000 km during the period from 2003 to 2008, including case-studies of solar eclipses and ionospheric storms. [ABSTRACT FROM AUTHOR]

Many modern ionospheric studies rely on incoherent scatter radars (ISR) since this kind of radar is able to detect various ionospheric parameters over very long ranges. The performance of ISR significantly depends on its coding system. In recent decades, a new type of coding system, which is the so-called composite coding, was presented. It used to be constructed by using a certain code to modulate alternating code to achieve better detection resolution and anti-noise performance for ISRs. In the present study, a new composite coding system was presented, which is constructed based on complementary codes and alternating codes. In this paper, the performance of the new composite code will be compared with that of several traditional codes to show that the new composite code can help to improve the detection performance of the ISR. According to the analysis based on the ambiguity function, the present composite coding system helps to improve the range resolution and detection range for ISR detections. In addition, numerical tests on anti-noise performance show that the complementary composite coding system has a good anti-noise performance and helps to reduce the necessary times of incoherent integration. As a result, the composite coding system can improve the time resolution. [ABSTRACT FROM AUTHOR]

A long-term statistical analysis of thermospheric tides in an altitude range from 150 to 400 km is presented. The analysis is based on a dataset with 31 multi-day experiments conducted from an incoherent scatter radar at Arecibo Observatory between 1984 and 2015. This is the first time that the climatological mean and seasonal variations of the thermospheric tides and their response to solar activities are reported using an extensive dataset. The climatological mean amplitude of diurnal tide (DT) is dominant while the amplitudes of the semidiurnal tide (SDT) and terdiurnal tide (TDT) are comparable. Below 250 km, the SDT and TDT phases show downward propagation with vertical wavelengths of 300 and 240 km, respectively. Above 250 km, the DT is the most prominent component except in autumn while the SDT dominates below 250 km except in winter. Above 250 km, the DT is the strongest in winter and its amplitude varies around 28 m/s. Below 250 km, the SDT is prominent in autumn and fluctuates around 35 m/s. The TDT is the most important in winter compared to the other three seasons and its amplitude is slightly less than 20 m/s. The DT and SDT amplitudes show the opposite response to solar activity. Above 250 km, the former increases with increasing solar activity while the latter is the opposite. The enhanced DT amplitude and its phase structure under the high solar activity indicates that the in-situ EUV radiation plays a major role in generating the thermospheric DT over Arecibo. • A long-term observational study on the thermospheric tides is presented over Arecibo for the first time. • Climatology and seasonal variation of the thermospheric tides are revealed. • The thermospheric diurnal tide responds most strongly to solar activity among the three tidal components. [ABSTRACT FROM AUTHOR]

Although the horizontal density structures of the polar ionosphere have been extensively studied mostly using the F-region peak density or total electron content, there are relatively few studies on the vertical density structures. In this review, we present the climatology of the polar ionospheric density not only in the F-region but also in the E-region and topside ionosphere, in comparison with the mid-latitude ionosphere, using long-term incoherent scatter radar (ISR) observations at Millstone Hill, Tromsø, and Svalbard. The ISR data during the period of 1995–2015 are analyzed to study on the variations with local time, season, and solar/geomagnetic activity. The diurnal variations of the F-region density are much smaller in the polar region than in the mid-latitude, particularly in summer. At Svalbard, there is a characteristic double-peak structure in the diurnal variation of the polar ionosphere in winter only for high solar activity. The diurnal variation of hmF2 decreases with increasing latitude and eventually disappears at Svalbard for low solar activity but the hmF2 and its diurnal variations in the polar ionosphere are remarkably enhanced for high solar activity. The distinctive irregularity in the mid-latitude F1-layer nearly disappears in the polar region, especially at Svalbard. The anomalous seasonal variations of the F-region density are less evident in the polar ionosphere especially for low solar activity and for high magnetic activity conditions. The polar E-region density shows characteristic nighttime peaks induced by auroral precipitation but it does not necessarily increase with solar activity. The topside ionospheric density variations are much stronger in the polar region for high solar activity. Finally, it is found that the polar ionospheric density profiles more strongly respond to increasing solar activity as well as the magnetic activity compared with the mid-latitude ionosphere. • Diurnal variation of the polar ionosphere is largely much smaller than the mid-latitude ionosphere. • But hmF2 shows remarkable diurnal variations for high solar activity in the polar region. • Seasonal variations of the ionosphere are less evident and only occur during low magnetic activity in the polar region. • Auroral contribution to the E-region density is largest in winter for low solar activity. • Ionospheric responses to solar and magnetospheric energy inputs are much more complex in the polar region. [ABSTRACT FROM AUTHOR]

Simultaneously estimating plasma parameters of the ionosphere presents a problem for the incoherent scatter radar (ISR) technique at altitudes between ~ 130 and ~ 300 km. Different mixtures of ion concentrations and temperatures generate almost identical backscattered signals, hindering the discrimination between plasma parameters. This temperature–ion composition ambiguity problem is commonly solved either by using models of ionospheric parameters or by the addition of parameters determined from the plasma line of the radar. Some studies demonstrated that it is also possible to unambiguously estimate ISR signals with very low signal fluctuation using the most frequently used non-linear least squares (NLLS) fitting algorithm. In a previous study, the unambiguous estimation performance of the particle swarm optimization (PSO) algorithm was evaluated, outperforming the standard NLLS algorithm fitting signals with very small fluctuations. Nevertheless, this study considered a confined search range of plasma parameters assuming a priori knowledge of the behavior of the ion composition and signals with very large SNR obtained at the Arecibo Observatory, which are not commonly feasible at other ISR facilities worldwide. In the present study, we conduct Monte Carlo simulations of PSO fittings to evaluate the performance of this algorithm at different signal fluctuation levels. We also determine the effect of adding different combinations of parameters known from the plasma line, different search ranges, and internal configurations of PSO parameters. Results suggest that similar performances are obtained by PSO and NLLS algorithms, but PSO has much larger computational requirements. The PSO algorithm obtains much lower convergences when no a priori information is provided. The a priori knowledge of Ne and T e / T i parameters shows better convergences and "correct" estimations. Also, results demonstrate that the addition of N e and T e parameters provides the most information to solve the ambiguity problem using both optimization algorithms. [ABSTRACT FROM AUTHOR]

A total of 4991 ionograms recorded from April 1997 to December 2017 by the Millstone Hill Digisonde (42.6°N, 288.5°E) were considered, with simultaneous Ne(h)[ISR] profiles recorded by the co-located Incoherent Scatter Radar (ISR). The entire ionogram dataset was scaled with both the Autoscala and ARTIST programs. The reliability of the hmF2 values obtained by ARTIST and Autoscala was assessed using the corresponding ISR values as a reference. Average errors Δ and the root mean square errors RMSE were computed for the whole dataset. Data analysis shows that both the Autoscala and ARTIST systems tend to underestimate hmF2 values with |Δ| in all cases less than 10 km. For high magnetic activity ARTIST offers better accuracy than Autoscala, as evidenced by RMSE[ARTIST] < RMSE[Autoscala], under both daytime and nighttime conditions, and considering all hours of the day. Conversely, under low and medium magnetic activity Autoscala tends to estimate hmF2 more accurately than the ARTIST system for both daytime and nighttime conditions, when RMSE[Autoscala] < RMSE[ARTIST]. However, RMSE[Autoscala] slightly exceeds RMSE[ARTIST] for the day as a whole. RMSE values are generally substantial (RMSE > 16 km in all cases), which places a limit on the results obtainable with real-time models that ingest ionosonde data. [ABSTRACT FROM AUTHOR]