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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]

Optical-resolution fluorescence imaging through and within complex samples presents a major challenge due to random light scattering, with substantial implications across multiple fields. While considerable advancements in coherent imaging through severe multiple scattering have been recently introduced by reflection matrix processing, approaches that tackle scattering in incoherent fluorescence imaging have been limited to sparse targets, require high-resolution control of the illumination or detection wavefronts, or require a very large number of measurements. Here, we present an approach that allows the adaptation of well-established reflection matrix techniques to scattering compensation in incoherent fluorescence imaging. We experimentally demonstrate that a small number of conventional wide-field fluorescence microscope images acquired under unknown random illuminations can effectively be used to construct a virtual fluorescence-based reflection matrix. Processing this matrix by an adapted matrix-based scattering compensation algorithm allows reconstructing megapixel-scale images from <150 acquired frames, without any spatial light modulators or computationally intensive processing. [ABSTRACT FROM AUTHOR]

Extensive experimental research on high-pressure spray has been conducted for decades to deepen our understanding and optimize its use in transportation, aviation, and propulsion applications; however, the near-field and in-nozzle flow characteristics are not fully understood. Dense near-field spray is among the most challenging diagnostic tasks since light is severely scattered and diffused by the liquid droplets and columns. In this work, the near-field spray and in-nozzle flow characteristics of an aeration nozzle at elevated pressures were characterized by neutron radiography imaging at the Oak Ridge National Laboratory High Flux Isotope Reactor. Neutron imaging benefits via strong penetration depths for some metals (i.e., aluminum, lead, and steel) and is sufficiently sensitive to detection of light elements, especially for hydrogen-based molecules, due to the large incoherent scattering cross section of neutrons. Both two-dimensional snapshots of the near-field spray and a three-dimensional tomographic scan of the nozzle geometry and in-nozzle water were obtained. This work provides new quantitative characterization of practical metal nozzle geometry for accurate boundary conditions, internal flow patterns inside the nozzle, and high-pressure spray flows. The findings may be used to improve performance and operating conditions of transportation vehicles and propulsion systems. [ABSTRACT FROM AUTHOR]

We present the first simultaneous observations of a traveling ionosphere wave (TID) event, measuring electron concentration ( N e ), vertical plasma drift ( V z ), and ion and electron temperatures ( T i , T e ) using the Arecibo incoherent scatter radar. A TID with a period of 135 min was evident in all four state variables in the thermosphere. The amplitudes of V z and relative T i fluctuations show only small height variations from 200 to 500 km and their vertical wavelengths increase with altitude. The T e fluctuation shows different characteristics from EISCAT in both phase and amplitude. When the geomagnetic dip angle is 45°, half of the driving gravity wave's (GW's) equatorward velocity is mapped to V z . This meridional-to-vertical velocity coupling amplifies GW's effect in N e through vertical transport. The amplifying and anisotropic effects of the geomagnetic field explain the ubiquitous presence of TIDs and their preferred equatorward propagation direction in the geomagnetic mid-latitudes, as well as the midnight collapse phenomenon observed at Arecibo. [ABSTRACT FROM AUTHOR]

An algorithm for obtaining ion vector velocities and neutral winds in the lower thermosphere (100–150 km) was applied to the Sanya incoherent scatter radar (SYISR; located at 18.3°N, 109.6°E) for the first time. The observational experiment transmitted alternating code pulses with a code width of 20 μs. The ion vector velocities and neutral winds were derived from multiple‐beam line‐of‐sight ion velocities. To verify the reliability, we first analyzed the variations and errors of the ion vector velocity and the neutral wind at different time scales. Then, we used an empirical model (HWM) and a theoretical model (NCAR‐TIEGCM) for comparison. Both comparisons exhibited good consistency in terms of neutral wind velocity. Furthermore, we compared the SYISR neutral winds with the meteor radar and ICON/MIGHTI winds. The zonal (meridional) wind speeds of the meteor radar and SYISR are 24.95 m/s (13.95 m/s) and 20.68 m/s (16.85 m/s), respectively, at 6:30 LT at 100 km. The amplitudes and phases of the tides derived from the SYISR data are in accordance with those of the meteor radar. The ICON/MIGHTI and SYISR showed consistencies in terms of the wind velocity when ignoring interannual variation. Plain Language Summary: In the lower thermosphere, atmospheric waves propagating from lower altitudes transfer energy and momentum to the ionosphere, playing a critical role in atmosphere/ionosphere coupling. Atmospheric fluctuations are measured mainly through neutral wind measurements. However, measuring neutral winds, especially height profiles in the lower thermosphere, is very challenging and rare. In this paper, using the newly built Sanya incoherent scatter radar (SYISR), we developed a sophisticated algorithm to derive ion vector velocities and neutral winds from multiple‐beam SYISR measurements between 100 and 150 km. Evaluations show that the data are reliable and useful. Key Points: Neutral winds from 100 to 150 km are obtained using 85 days of continuous SYISR alternating code observationsThe neutral winds derived from the SYISR data are in good agreement with those of the HWM and NCAR‐TIEGCMThe neutral winds derived from the SYISR data are further evaluated with meteor radar and ICON/MIGHTI [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]

Through applying a 4‐MHz linear frequency modulation waveform, which has high range resolution and signal intensity, we successfully detected for the first time the ionospheric 150‐km echo enhancement at 430–450 MHz of the Ultra‐High‐Frequency (UHF) band using the newly built Sanya Incoherent Scatter Radar (SYISR). The obtained low signal enhancement (less than 0.5 dB) explains why previous UHF experiments did not detect them. We also found that our measured fine structure shows a much wider forbidden region than previous results and covers a much larger altitudinal and local time region. In comparison with recent upper‐hybrid instability theory and simulation, our results confirmed the predicted higher altitude occurrence, wider gaps between enhancements, the turn corner feature around sunrise, and perhaps the weak enhancement, which provide an independent evaluation of the newly proposed mechanism in UHF band. Future UHF experiments could further improve the physical understanding of 150‐km echo phenomenon. Plain Language Summary: In the altitude range of 130–170 km above the Earth's surface, the electrons ionized by the solar radiation effecting on the neutrals can generate enhanced radar echo with specific layered structures if the radar beam points perpendicular to or slightly off perpendicular to the geomagnetic field. Up to date, this phenomenon was only detected by lower frequency radars (30–60 MHz). Its physical mechanism has been a puzzle over several decades. Recently, due primarily to the advancement of numerical simulation technology and the improvement of computing power, this puzzle was resolved well by the newly proposed across scales energy transformation physical mechanism in several recent publications. However, these simulations also concluded that it is hard to be detected by higher frequency radars. In this paper, using our newly built Sanya Incoherent Scatter Radar, we applied a novel experimental setup to gain very high range resolution and signal intensity. We finally successfully detected this phenomenon in 430–450 MHz. By comparing our results with previous measurements and theoretical simulation, we can enhance current physical understanding from the perspective of observations. Key Points: Successful first detection of daytime 150‐km echoes in the Ultra‐High‐Frequency band with Sanya Incoherent Scatter RadarWe detected wider forbidden region and larger altitudinal and local time coverage than previous resultsNarrow layering features not following the zenith angle point to field‐aligned irregularitie echoes [ABSTRACT FROM AUTHOR]

Coherent diffractive imaging with X‐ray free‐electron lasers could enable structural studies of macromolecules at room temperature. This type of experiment could provide a means to study structural dynamics on the femtosecond timescale. However, the diffraction from a single protein is weak compared with the incoherent scattering from background sources, which negatively affects the reconstruction analysis. This work evaluates the effects of the presence of background on the analysis pipeline. Background measurements from the European X‐ray Free‐Electron Laser were combined with simulated diffraction patterns and treated by a standard reconstruction procedure, including orientation recovery with the expand, maximize and compress algorithm and 3D phase retrieval. Background scattering did have an adverse effect on the estimated resolution of the reconstructed density maps. Still, the reconstructions generally worked when the signal‐to‐background ratio was 0.6 or better, in the momentum transfer shell of the highest reconstructed resolution. The results also suggest that the signal‐to‐background requirement increases at higher resolution. This study gives an indication of what is possible at current setups at X‐ray free‐electron lasers with regards to expected background strength and establishes a target for experimental optimization of the background. [ABSTRACT FROM AUTHOR]

The G‐condition (NmF2 ≤ NmF1) was observed by ground‐based ionosondes at Mohe and Beijing during the geomagnetic storm occurred on 23 and 24 April 2023. We studied exospheric temperature (Tex) responses during the G‐condition. Tex was derived from electron density (Ne) profiles (∼150–200 km) by the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988). The retrieved Tex showed obvious enhancements, with relative deviation of ∼10%–35% and ∼3%–20% at Mohe and Beijing, respectively. Additionally, chemical reaction rate increased by ∼15%–100% and ∼13%–30%, and O/N2 decreased by ∼17%–35% and ∼23%–30% at Mohe and Beijing, respectively. Under photochemical equilibrium assumption, peak Ne is inversely proportional to chemical reaction rate and proportional to O/N2. Increased chemical reaction rate and decreased O/N2 indicate a decrease in peak Ne. Compared to the increased Tex, the relative enhancement in Tex is more significantly associated with the G‐condition, with relative deviation above ∼10% during the G‐condition. Plain Language Summary: Normally, the peak Ne in the F2 layer (NmF2) is the largest in the full Ne profile. G‐conditions are special events in ionospheric observations, when NmF2 is smaller than or equal to NmF1 (i.e., NmF2 ≤ NmF1). Due to shielding effect of the F1 layer, no information above hmF1 can be obtained from ground‐based ionosondes, and Ne peak height is typically below 200 km. In previous works, incoherent scatter radar (ISR) observations are usually used to extract thermospheric parameters (Tex, neutral composition and thermospheric wind). In this paper, ground‐based ionosonde observations (∼150–200 km) at Mohe and Beijing prior to, during and after the G‐condition (from 22 to 26 April 2023) were selected to derive Tex using the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988). Furthermore, corresponding neutral temperature (Tn) and densities were further calculated by replacing the Tex module in the NRLMSISE‐00 model with the retrieved Tex. In our analysis, the retrieved Tex and chemical reaction rate showed significant enhancements and O/N2 showed obvious decrease compared to quiet reference days. This is consistent with the analysis of the mechanism of G‐conditions formation in previous studies. Key Points: The G‐condition (NmF2 ≤ NmF1) was observed by ground‐based ionosondes at Mohe and Beijing during the G4 storm on 24 April 2023Relative enhancements in Tex during the G‐condition were greater than that in other periods, with relative deviation above ∼10%Enhanced Tex with corresponding increased chemical reaction rate and decreased O/N2 collectively contributed to a decrease in Ne [ABSTRACT FROM AUTHOR]

The relationship between the post‐sunset equatorial Plasma Bubbles (EPBs) and the Rayleigh‐Taylor instability growth rates γRT $\left({\gamma }_{RT}\right)$ was studied by means of the occurrence probabilities. EPB observations were obtained through the Rate of Total Electron Content Index (ROTI) measured by ground‐based Global Navigation Satellite System (GNSS) receivers in the South American equatorial region. Data from the period 2013 to 2022 were utilized. The γRT ${\gamma }_{RT}$ calculations were based on F‐layer vertical drift Vp $\left({V}_{p}\right)$ measurements obtained from the São Luís ionosonde (2.33°S, 44.2°W, dip angle: −0.5°), the International Reference Ionosphere model (IRI‐2016), Horizontal Wind Model (HWM‐14), and Spectrometer Incoherent Scatter Model‐2,000 (NRLMSISE‐00). The EPB occurrence probability increased from 0% to 100% as γRT ${\gamma }_{RT}$ increased from 0 to 2.2×10−3s−1 $2.2\times 1{0}^{-3}\hspace*{.5em}{\mathrm{s}}^{-1}$. Additionally, the occurrence probability exhibited seasonal variations, with a gradual increase during the equinox months, minimal occurrence during the June solstice, and a sudden increase during the December solstice. We found that the occurrence probability varied with day‐to‐day, season and solar activity for the same γRT ${\gamma }_{RT}$. Plain Language Summary: Equatorial plasma bubble (EPB) in the earth's equatorial and low latitude ionosphere is a large‐scale plasma depletion producing a large‐scale irregularity, which causes problems in the space based telecommunications and Global Navigation Satellite System (GNSS) positioning systems. Prediction and forecasting of such ionospheric irregularities are an important issue in the space technology applications. Present work tries to visualize probability of EPB occurrence at a certain location and time of the year. For this purpose, we calculated the occurrence probability using past observed data (temporal variabilities of Total Electron content (TEC) and ionospheric layer height) and ionospheric models. Our results show that the occurrence probability increases gradually when the irregularity index of the ionosphere increases. We also found that the gradual increase has a seasonal dependency, very low probability during June‐July, and high probability during December–January. Key Points: The relationship between the Equatorial Plasma Bubbles and the R‐T instability growth rate was studied by probability of occurrenceProbability gradually rises with growth rate during equinox seasons, but experiences a sudden surge during the December solsticeFor the same growth rate, the probability varies with the seasons and solar cycle [ABSTRACT FROM AUTHOR]

To investigate the impact of non-linear interactions on dynamic coarse graining, we study a simplified model system featuring a tracer particle in a complex environment. Using a projection operator formalism and computer simulations, we systematically derive generalized Langevin equations (GLEs) describing the dynamics of this particle. We compare different kinds of linear and non-linear coarse-graining procedures to understand how non-linearities enter reconstructed GLEs and how they influence the coarse-grained dynamics. For non-linear external potentials, we show analytically and numerically that the non-Gaussian parameter and the incoherent intermediate scattering function will not be correctly reproduced by the GLE if a linear projection is applied. This, however, can be overcome by using non-linear projection operators. We also study anharmonic coupling between the tracer and the environment and demonstrate that the reconstructed memory kernel develops an additional trap-dependent contribution. Our study highlights some open challenges and possible solutions in dynamic coarse graining. [ABSTRACT FROM AUTHOR]

Polyisoprene (PI) melts have been studied, with most reports focusing on systems with high 1,4-cis content. In contrast, 1,4-trans PI homopolymers or random copolymers have seldom been examined, despite a handful of investigations suggesting a distinct dynamic behavior. Herein, we employ all-atom simulations to investigate the effect of chemical architecture on the dynamics of cis and trans-PI homopolymers, as well as copolymers. We examine the thermodynamic, conformational, and structural properties of the polymers and validate the performance of the models. We probe chain dynamics, revealing that cis-PI presents accelerated translation and reorientation modes relative to trans as recorded by the mean square displacement of the chain center-of-mass as well as by the characteristic times of the lower modes in a Rouse analysis. Interestingly, progressing to higher modes, we observe a reversal with trans units exhibiting faster dynamics. This was further confirmed by calculations of local carbon–hydrogen vector reorientation dynamics, which offer a microscopic view of segmental mobility. To obtain insight into the simulation trajectories, we evaluate the intermediate incoherent scattering function that supports a temperature-dependent crossover in relative mobility that extends over separations beyond the Kuhn-length level. Finally, we analyzed the role of non-Gaussian displacements, which demonstrate that cis-PI exhibits increased heterogeneity in dynamics over short-timescales in contrast to trans-PI, where deviations persist over times extending to terminal dynamics. Our all-atom simulations provide a fundamental understanding of PI dynamics and the impact of microstructure while providing important data for the design and optimization of PI-based materials. [ABSTRACT FROM AUTHOR]

In this study, we used the Sanya Incoherent Scatter Radar (SYISR) to observe the altitude profiles of traveling ionospheric disturbances (TIDs) during a moderate magnetic storm from 13 to 15 March 2022. Three TIDs were recorded, including two large-scale TIDs (LSTIDs) and one medium-scale TID (MSTID). These LSTIDs occurred during the storm recovery phase, characterized by periods of ~110–155 min, downward phase velocities of 22–60 m/s, and a relative amplitude of 17–25%. A nearly vertical front was noted at ~350–550 km, differing from AGW theory predictions. This structure is more attributed to the combined effects of sunrise-induced electron density changes and pre-sunrise uplift. Moreover, GNSS observations linked this LSTID to high-latitude origins, indicating a connection to polar magnetic storm excitation. However, the second LSTID was observed at lower altitudes (150–360 km) with a higher elevation angle (~17°). This LSTID, observed by the SYISR, was absent in the GNSS data from mainland China and Japan, suggesting a potential local source. The MSTID exhibited a larger relative amplitude of 29–36% at lower altitudes (130–210 km) with severe upward attenuation. The MSTID may be related to atmospheric gravity waves from the lower atmosphere. AGWs are considered to be the perturbation source for this MSTID event. [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of ∼ ${\sim} $30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration. Plain Language Summary: On 8 April 2024, a total solar eclipse traversed across North America with a dense network of observational equipment in place, providing a great opportunity for analyzing ionospheric effects during the eclipse. This paper presents a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density response to this solar eclipse, utilizing Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm satellite in situ measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS) over continental US and adjacent regions with the SAMI3 as the background model. The observations and SAMI3‐TIDAS data assimilation reveals the time‐evolving 3‐D spatial distribution of the ionospheric electron density during the eclipse, highlighting key features of altitude‐dependent ionospheric variation with significant discrepancies and time delays between the bottomside and topside ionosphere. Key Points: The altitude‐resolved Ne response to the solar eclipse in the 3‐D domain was effectively reconstructed by TIDAS‐SAMI3 data assimilationThe eclipse led to a substantial ionospheric Ne reduction of 20%–50%, with the maximum depletion occurring in the F region of 200–250 kmThe Ne showed a time‐delayed variation with increasing altitude, from 5 to 10 min in the bottomside to 1–2.5 hr in the topside ionosphere [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]

The temperature trend in the mesosphere and lower-thermosphere (MLT) region can be regarded as an indicator of climate change. Using temperature profiles measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during 2002–2023 and binning them based on the yaw cycle, we obtain a continuous dataset with a wide local time coverage at 50° S–80° N or 80° S–50° N. The seasonal change in temperature, caused by the forward drift in the SABER yaw cycle, is removed using the climatological temperature of the Naval Research Laboratory's Mass Spectrometer Incoherent Scatter Radar model (MSIS2.0). The corrected temperature without any waves is regarded as the mean temperature. At 50° S–50° N, the cooling trends in the mean temperature are significant in the MLT region and are in agreement with previous studies. The novel finding is that the cooling trends of ≥ l2 K per decade exhibit seasonal symmetry and reach peaks of ≥ 6 K per decade at high latitudes around the summer solstice. Moreover, there are warming trends of 1–2.5 K per decade at an altitude range of 10 -2 –10 -3 hPa , specifically at latitudes higher than 55° N in October and December and at latitudes higher than 55° S in April and August. Over the past 22 years, the mesopause temperature (altitude) in the northern summer polar region has been ∼ 5–11 K (∼ 1 km) colder (lower) than that in the corresponding southern region. The trends in the mesopause temperature are dependent on latitudes and months, but they are negative at most latitudes and reach larger magnitudes at high latitudes. These results indicate that the temperature in the high-latitude MLT region is more sensitive to dynamic changes. [ABSTRACT FROM AUTHOR]

This work reports the development of the first version of the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model. This is an empirical model of E‐region electron density (Ne) between 90 and 120 km developed using radio occultation measurements from the COSMIC‐1 mission. This first version captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles but it does not account for longitudinal variability at constant local‐time. This work also reports a validation of E‐PROBED simulations through comparisons with ionosondes and incoherent scatter radar (ISR) E‐region Ne profiles. E‐PROBED generally agrees with these ground‐based observations during day‐time. During night‐time, there is a large disparity between E‐PROBED and ISR values. Finally, this work compares E‐PROBED with E‐region Ne simulated by the International Reference Ionosphere (IRI) and the Specified Dynamics—Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD‐WACCM‐X). One of the main differences amongst these models is on the simulation of variabilities that cannot be attributed to photoionization. IRI barely simulates any variability not driven by photoionization. Both E‐PROBED and SD‐WACCM‐X simulates variability not driven by photoionization. Another main difference is in the absolute magnitude of night‐time E‐region Ne values. Both IRI and SD‐WACCM‐X are substantially lower than E‐PROBED. This work first concludes that E‐PROBED can conveniently provide E‐region Ne latitude—local time variabilities and structures that COSMIC‐1 observes. This work also concludes that E‐region Ne have significant non‐photoionization driven variabilities. Plain Language Summary: Navigation and communication systems reliant on sending signals into space require accurate predictions from empirical models of ionospheric electron density (Ne) values because the frequency of the appropriate signals to use are a function of Ne. Empirical models currently struggle the most with the ionospheric E‐region (between 90 and 120 km) because most of these models were constructed using sparsely sampled E‐region Ne data from ground‐based observations. Recently, E‐region Ne profiles were finally retrieved from Global Navigation Satellite Systems Radio Occultation (GNSS RO) measurements. This work reports on a new empirical model of monthly‐mean E‐region Ne that uses global observations of E‐region Ne from GNSS RO. The model is called the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model, and it captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles. It agrees well with both ground‐based observations and first principles Physics‐based model simulations. These findings suggest that E‐PROBED makes a viable alternative E‐region Ne empirical model for those interested in an E‐region Ne model constrained by satellite observations. Key Points: The first version of E‐region Prompt Radio Occultation Based Electron Density Model (E‐PROBED) is developedE‐PROBED agrees with ground‐based observations during day‐time but not during night‐timeE‐PROBED agrees with SD‐WACCM‐X that E‐region Ne have significant non‐photoionization‐driven variabilities [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]

This study considers critical data reduction steps and data analysis approaches required to determine explicitly the atomic arrangements in nanoparticles from time‐of‐flight neutron total scattering. A practical procedure is described for removing parasitic backgrounds caused by the incoherent scattering of hydrogen inevitably present in most nanoparticle samples and normalizing the recovered coherent scattering intensities onto an absolute scale. A model‐free analysis is presented of a pair‐distribution function derived from total scattering that can be used to determine thermal expansion coefficients and particle sizes directly from experimental data without knowledge of the material's structure. Finally, atomistic whole‐nanoparticle refinements of yttrium‐doped ceria nanoparticles from neutron total‐scattering data are demonstrated using the reverse Monte Carlo method implemented in the RMCProfile software. These results reveal a strong dependence of the cation–oxygen and oxygen–oxygen distances on the coordination numbers, which leads to gradients of these distances near the particle surface. The details are dependent on the surface coverage by ligands and adsorbates and on the structure of grain boundaries in nanocrystalline agglomerates. The refined models confirm the expectations of more substantial disorder at particle surfaces, with a distorted surface layer extending over several coordination shells. The results highlight the feasibility of whole‐nanoparticle refinements from neutron data, calling for further development of data reduction and analysis procedures. [ABSTRACT FROM AUTHOR]

Total electron content (TEC) is a key parameter for characterizing the ionosphere. Conventional TEC measurement methods have low spatial resolution, hindering accurate representation of ionospheric spatial characteristics. Synthetic Aperture Radar (SAR) and Interferometric SAR (InSAR) have shown their potential for high-spatial-resolution TEC estimation. However, SAR-based absolute TEC estimation typically relies on full-polarimetric SAR images, while the InSAR-based method can only extract differential TEC. This study proposes a novel TEC mapping method integrating Time Series InSAR (TS-InSAR) with the International Reference Ionospheric (IRI) model and does not leverage full-polarimetric SAR data. The Advanced Land Observing Satellite-1 (ALOS-1) SAR images covering the Chile and Greenland regions were collected to test the proposed method. The TECs derived from the IRI model and International GNSS Service (IGS) and the electron density observed by the Incoherent Scattering Radar (ISR) site of SONDRESTROM were acquired to evaluate the performance and validate the accuracy of the proposed method, respectively. The results show that the proposed method can achieve TEC mapping with resolutions of tens of meters, much higher than the tens or hundreds of kilometers of the IRI and IGS TECs. The spatial features of our method-derived TECs show overall good consistency with those of the IRI and IGS TECs, but the described spatial information is more detailed. Also, compared to the original IRI model, the improvement rates of root-mean-square errors (RMSEs) between the estimated electron density and ISR observations exceeded 21.67% after the update using the TEC obtained from the proposed method. These experiments demonstrate the proposed method's feasibility and reliability for ionospheric TEC mapping. • Using the Rate of TEC (ROT) Index (ROTI) to evaluate ionospheric perturbation and determine the time-series InSAR reference image. • Combining time-series InSAR and IRI model to estimate absolute TEC. • Providing a novel guideline for high-spatial-resolution TEC mapping. [ABSTRACT FROM AUTHOR]

Observations using Arecibo Observatory's highly sensitive Incoherent Scattering Radar (AO‐ISR) show ionospheric descending layers from as high as ∼400 km, much higher than earlier studies, with continuity down to 90 km. The AO‐ISR was operated to observe the ion‐line and plasma‐line with coded‐long‐pulse for high temporal and spatial resolution of 35/10 s and 300 m, respectively, during 01–06 February 2019. We found multiple layering structures descending from 400 to 90 km in all these six days. These layers are traditionally called intermediate descending layers (IDLs) (>130 km and below F‐peak), upper semi‐diurnal daytime and nighttime layers (110–130 km), and lower diurnal layers (<110 km). We have denoted the new daytime descending layers above the hmF2 as top‐side descending layers (TDLs). All these layers are collectively named ionospheric descending layers (IonDLs) since all of them are connected with some discontinuity at the F1‐peak (i.e., 170 km), except for the daytime lower‐diurnal layer. The most pronounced IonDLs occur in the twilight times. IonDLs mainly occur in shear zones of the vertical ion drifts and are favored by downward ion drifts, and their descent speeds increase with increasing altitude. The estimated phase velocities of the waves in the F‐region are comparable with the descending speed of the IonDLs. Furthermore, IonDLs/IDLs occur with and without spread‐F events but intensified spread‐F events raise their beginning altitude. The TDLs and IDLs are driven by gravity waves with periods of 1.5–4 hr. Key Points: We found ionospheric descending layers (IonDLs) that start near ∼400 km and descend down to ∼90 kmWe observe traditional intermediate descending layers (IDLs) starting from ∼300–350 km (below ∼170 km) during the nighttime (daytime)The daytime top‐side descending layers (from 400 km) and IDLs are governed by the waves with periods of ∼1.5–4 hr [ABSTRACT FROM AUTHOR]

We present a new technique for the upcoming tri‐static incoherent scatter radar system EISCAT 3D (E3D) to perform a volumetric reconstruction of the 3D ionospheric electric current density vector field, focusing on the feasibility of the E3D system. The input to our volumetric reconstruction technique are estimates of the 3D current density perpendicular to the main magnetic field, j⊥, and its covariance, to be obtained from E3D observations based on two main assumptions: (a) Ions fully magnetized above the E region, set to 200 km here. (b) Electrons fully magnetized above the base of our domain, set to 90 km. In this way, j⊥ estimates are obtained without assumptions about the neutral wind field, allowing it to be subsequently determined. The volumetric reconstruction of the full 3D current density is implemented as vertically coupled horizontal layers represented by Spherical Elementary Current Systems with a built‐in current continuity constraint. We demonstrate that our technique is able to retrieve the three dimensional nature of the currents in our idealized setup, taken from a simulation of an active auroral ionosphere using the Geospace Environment Model of Ion‐Neutral Interactions (GEMINI). The vertical current is typically less constrained than the horizontal, but we outline strategies for improvement by utilizing additional data sources in the inversion. The ability to reconstruct the neutral wind field perpendicular to the magnetic field in the E region is demonstrated to mostly be within ±50 m/s in a limited region above the radar system in our setup. Plain Language Summary: We introduce a novel method for the upcoming EISCAT 3D (E3D) radar system to reconstruct the 3D electric current density vector in Earth's ionosphere. Here we present the new technique and assess its feasibility for the E3D system. The input to the 3D reconstruction technique relies on estimates of the current density perpendicular to the Earth's magnetic field, obtained from the E3D observations. We include estimates of uncertainties originating from the observations of the 3D ion velocity vectors and electron density in our reconstruction. Comparisons with simulations of an active auroral ionosphere exemplify that our technique provides reasonably accurate estimates of current density, especially in the 90–150 km altitude range. Our results demonstrate success in retrieving the horizontal part of the electric current system in the E region, while the vertical part has more uncertainty. Our method offers insight into how electric currents flow in a specific region of the Earth's atmosphere. The results can be further improved with additional data sources; this flexibility is a significant advantage of our approach. Overall, our study facilitates the advanced knowledge of Earth's upper atmosphere using innovative radar observations in companion with advanced analysis techniques. Key Points: A technique for volumetric reconstruction of 3D electric current density from tri‐static incoherent scatter radar observations is presentedConsidering the anticipated noise levels, the radar system is likely to produce good current density estimates in a limited regionThe reconstruction technique is particularly well suited for inclusion of additional data sources that improve overall performance [ABSTRACT FROM AUTHOR]

Specification and forecast of ionospheric parameters, such as ionospheric electron density (Ne), have been an important topic in space weather and ionospheric research. Neural networks (NNs) emerge as a powerful modeling tool for Ne prediction. However, heavy manual adjustments are time consuming to determine the optimal NN structures. In this work, we propose to use neural architecture search (NAS), an automatic machine learning method, to mitigate this problem. NAS aims to find the optimal network structure through the alternate optimization of the hyperparameters and the corresponding network parameters within a pre‐defined hyperparameter search space. A total of 16‐year data from Millstone Hill incoherent scatter radar (ISR) are used for the NN models. One single‐layer NN (SLNN) model and one deep NN (DNN) model are both trained with NAS, namely SLNN‐NAS and DNN‐NAS, for Ne prediction and compared with their manually tuned counterparts (SLNN and DNN) based on previous studies. Our results show that SLNN‐NAS and DNN‐NAS outperformed SLNN and DNN, respectively. These NN predictions of Ne daily variation patterns reveal a 27‐day mid‐latitude topside Ne variation, which cannot be reasonably represented by traditional empirical models developed using monthly averages. DNN‐NAS yields the best prediction accuracy measured by quantitative metrics and rankings of daily pattern prediction, especially with an improvement in mean absolute error more than 10% compared to the SLNN model. The limited improvement of NAS is likely due to the network complexity and the limitation of fully connected NN without the time histories of input parameters. Plain Language Summary: Neural network (NN) models have garnered significant attention for their application in predicting physical parameters in the ionosphere, notably ionospheric electron density (Ne). In this study, we introduce a novel approach aimed at enhancing the performance of NN models by employing the advanced technique known as neural architecture search (NAS). Leveraging a data set spanning 16 years of Ne measurements obtained from the incoherent scatter radar located at the Millstone Hill observatory, we conduct a comprehensive analysis. This analysis encompasses training both manually calibrated NN models and NN models optimized via NAS. The NN models fine‐tuned through NAS achieve a notable improvement in their ability to predict Ne when compared to their manually adjusted counterparts. This improvement underscores the efficacy of NAS in optimizing neural network hyperparameters for ionospheric modeling. Furthermore, we delve into a thorough exploration of the factors contributing to the somewhat limited improvements observed in the context of our current data set. This investigation yields valuable insights and prompts valuable discussions on the potential avenues for further refinement in ionospheric prediction methodologies. Key Points: Neural architecture search (NAS) can automate hyperparameter optimization for neural networks (NN) to predict Ne at 350 km altitudeA total of 16‐year of data from incoherent scatter radar are used for NN models and their NAS counterparts, which outperformed IRI2016The limited improvement of NAS could be due to the network complexity and the lack of temporal mechanism of fully connected NNs [ABSTRACT FROM AUTHOR]

We discuss robust estimations for the variance of normally distributed random variables in the presence of interference. The robust estimators are based on either ranking or the geometric mean. For the interference models used, estimators based on the geometric mean outperform the rank-based ones in both mitigating the effect of interference and reducing the statistical error when there is no interference. One reason for this is that estimators using the geometric mean do not suffer from the "heavy tail" phenomenon like the rank-based estimators do. The ratio of the standard deviation over the mean of the power random variable is sensitive to interference. It can thus be used as a criterion to combine the sample mean with a robust estimator to form a hybrid estimator. We apply the estimators to the Arecibo incoherent scatter radar signals to determine the total power and Doppler velocities in the ionospheric E-region altitudes. Although all the robust estimators selected deal with light contamination well, the hybrid estimator is most effective in all circumstances. It performs well in suppressing heavy contamination and is as efficient as the sample mean in reducing the statistical error. Accurate incoherent scatter radar measurements, especially at nighttime and at E-region altitudes, can improve studies of ionospheric dynamics and composition. [ABSTRACT FROM AUTHOR]

Pulsating Aurora (PsA) is one of the major classes of diffuse aurora associated with precipitation of a few to a few tens of keV electrons from the magnetosphere. Recent studies suggested that, during PsA, more energetic (i.e., sub‐relativistic/relativistic) electrons precipitate into the ionosphere at the same time. Those electrons are considered to be scattered at the higher latitude part of the magnetosphere by whistler‐mode chorus waves propagating away from the magnetic equator. However, there have been no actual cases of simultaneous observations of precipitating electrons causing PsA (PsA electrons) and chorus waves propagating toward higher latitudes; thus, we still do not quite well understand under what conditions PsA electrons become harder and precipitate to lower altitudes. To address this question, we have investigated an extended interval of PsA on 12 January 2021, during which simultaneous observations with the Arase satellite, ground‐based all‐sky imagers and the European Incoherent SCATter (EISCAT) radar were conducted. We found that, when the PsA shape became patchy, the PsA electron energy increased and Arase detected intense chorus waves at magnetic latitudes above 20°, indicating the propagation of chorus waves up to higher latitudes along the field line. A direct comparison between the irregularities of the magnetospheric electron density and the emission intensity of PsA patches at the footprint of the satellite suggests that the PsA morphology and the energy of PsA electrons are determined by the presence of "magnetospheric density ducts," which allow chorus waves to travel to higher latitudes and thereby precipitate more energetic electrons. Plain Language Summary: Pulsating Aurora (PsA) is a kind of diffuse aurora associated with periodic precipitation of energetic electrons from the near‐Earth space into the atmosphere. Recent research has shown that, during PsA events, energetic particles at the sub‐relativistic energy range precipitate into the atmosphere. We speculate that such particles are scattered by wave‐particle resonance with natural electromagnetic waves, called chorus waves, at higher magnetic latitude regions. However, there has been no experimental case of PsA during which propagation of the chorus waves to higher magnetic latitudes was confirmed; thus, we still do not fully understand when and why PsA electrons become more energetic. Here, we investigate a PsA event on 12 January 2021, simultaneously observed by the Arase satellite, ground‐based all‐sky imagers and the European Incoherent SCATter (EISCAT) radar. We found that, when the PsA shape was patchy, the energy of precipitating electrons increased and chorus waves were observed at high latitudes in the magnetosphere. Comparing the magnetospheric electron density with the PsA brightness seen from the ground, we suggest that both the PsA shape and the energy of precipitating electrons were influenced by the so‐called magnetospheric ducts, which guide chorus waves to high‐latitudes regions where they interact with more energetic electrons. Key Points: Examined simultaneous observations of Pulsating Aurora (PsA) with the Arase satellite, ground‐based all‐sky imagers, and the EISCAT radarFound a relationship among the patchy PsA, the enhanced energy of PsA electrons, and the chorus wave propagation to high‐latitudes (>20°)Arase observations suggest that the observed relationship can be explained by the ducted propagation of chorus waves [ABSTRACT FROM AUTHOR]

The Nitric Oxide (NO) emission at 5.3 μm wavelength is a well‐known coolant above 100 km. It effectively regulates thermospheric temperature during space weather events. We studied NO cooling emission over Tromsø (geographic:69.59°N, 19.22°E; cgm:66.58°,102.94°), Norway by using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) simulation driven by both Heelis and Weimer models as sources of geomagnetic forcing during October–November 2003 storm. The Weimer driven TIEGCM significantly overestimates thermospheric Nitric Oxide and Atomic Oxygen densities and underestimates temperature as compared to the Heelis driven simulation. The density ratio between the Weimer and Heelis driven estimations decreases with increasing altitude for both NO and atomic oxygen densities. The Heelis driven Joule heating rate agrees very well with the European incoherent scatter (EISCAT) radar measurements. It peaks during the main phase of the storm with magnitude about 4–5 times higher than that driven by Weimer model which peaks during the recovery phase. The difference in Joule heating rates between the Heelis and Weimer driven models increases with storm intensity, reaching a peak discrepancy of about an order of magnitude during the October‐November 2003 storm. An early and stronger NO cooling enhancement is predicted by Heelis driven TIEGCM simulation. It overestimates NO cooling emission by about 2–3 times as compared to SABER observations and about 4–5 times the Weimer driven calculation. This strong difference between the two models can be attributed to the model calculations of high latitude electric field and convection patterns. Key Points: The Nitric Oxide cooling emission is studied over Tromsø, Norway, by using the TIEGCM simulations driven by both Weimer and Heelis modelsStrong difference in Nitric Oxide emission is estimated between the Heelis and Weimer models driven TIEGCM simulationsThe strong dissimilarity can be attributed to the model calculation of Joule heating rate [ABSTRACT FROM AUTHOR]

Electron density irregularities in the ionosphere can give rise to scintillations, affecting radio wave phase and amplitude. While scintillations in the cusp and polar cap regions are commonly associated with mesoscale density inhomogeneities and/or shearing, the auroral regions exhibit a strong correlation between scintillation and density structures generated by electron precipitation (arcs). We aim to examine the impact of electron precipitation on the formation of scintillation‐producing density structures using a high‐resolution physics‐based plasma model, the "Geospace Environment Model of Ion‐Neutral Interactions," coupled with a radio propagation model, the "Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere." Specifically, we explore the effects of varying spatial and temporal characteristics of the precipitation, including electron total energy flux and their characteristic energies, obtained from the all‐sky‐imagers and Poker Flat Incoherent Scatter Radar observations, on auroral scintillation. To capture small‐scale structures, we incorporate a power‐law turbulence spectrum that induces short wavelength features sensitive to scintillation. Finally, we compare our simulated scintillation results with satellite‐observed scintillations, along with spectral comparisons. Key Points: A physics‐based plasma model and a radio propagation model are used to examine the effects of precipitation on auroral scintillationPrecipitation fluxes with higher energy, a turbulent noise spectrum, and a faster‐moving arc intensify scintillationThe presence of small‐scale precipitation amplifies both phase and amplitude scintillations [ABSTRACT FROM AUTHOR]

Using the latest coupled geospace model Multiscale Atmosphere‐Geospace Environment (MAGE) and observations from Jicamarca Incoherent scatter radar (ISR) and ICON ion velocity meter (IVM) instrument, we examine the pre‐reversal enhancement (PRE) during geomagnetic quiet time period. The MAGE shows comparable PRE to both the Jicamarca ISR and ICON observations. There appears to be a discrepancy between the Jicamarca ISR and ICON IVM with the later showed PRE about two times larger (∼40 m/s). This is the first time that MAGE is used to simulate the PRE. The results show that the MAGE can simulate the PRE well and are mostly consistent with observations. Key Points: The Multiscale Atmosphere‐Geospace Environment is able to simulate the pre‐reversal enhancement (PRE) very wellThe PRE was a persistent feature on different time scales (day, hour, and minute) during this fall equinox time periodJicamarca Incoherent scatter radar observed PRE is about half of size of that by the ICON ion velocity meter [ABSTRACT FROM AUTHOR]

The paper presents the results of a numerical assessment of the contribution of the ionospheric D region to the total electron content during six powerful X‐ray flares that occurred in September 2017. The calculation of the electron concentration in the lower ionosphere was carried out using a plasma‐chemical model of the ionospheric D region. This model was verified using the data of ground‐based radiophysical measurements in the VLF (very low frequency) range and data of the incoherent scattering radar. To calculate the ionization rate at the D region heights, we used real data on the radiation flux measured by the GOES and SDO satellites during the considered flares. The total electron content was estimated using GNSS data. As a result of the analysis, it was found that the contribution of the lower ionosphere to the TEC change varied from 7% to 23% for flares with different spectra. A functional dependency has been obtained that can be used to estimate the contribution of the D region to the TEC increment depending on the spectrum of the flare. Key Points: A technique for estimating the redistribution of the electron concentration in the ionosphere during solar flares is presentedIt was found that the contribution of the D region to the TEC change varied from 7% to 23% during the flares in September 2017A functional dependency of the D region contribution to the TEC change on the fraction of X‐rays in the flare spectrum is obtained Plain Language Summary: Changes in solar radiation fluxes are observed in a wide range of wavelengths during solar flares. The variations of total electron content in the ionosphere occurs due to an increase in radiation in the UV and X‐ray wavelength ranges. The main increase in ionization caused by flares occurs in the F region (h > 120 km), so the electron concentration increment in the lower part of the ionosphere (h < 90 km) is often neglected. The existing experimental measurement methods do not allow us to estimate the redistribution of the electron concentration in the ionosphere during disturbances; therefore, to estimate the contribution of the lower ionosphere to the total electron content during flares, we used GNSS data and theoretical plasma‐chemical model of the D region. In this work, it is shown that the contribution of the D region to the change of the total electron content can reach 23% during X‐ray flares. [ABSTRACT FROM AUTHOR]

Vertical profiles of nitric oxide (NO) concentration are derived between 120 and 250 km using updated NO emission rates measured by Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. The Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 model is used to provide the required parameters of temperature, atomic oxygen number density, and molecule oxygen number density needed to derive the NO concentrations using a non‐local thermodynamic equilibrium (non‐LTE) model. The SABER NO concentration shows a significant correlation with solar activity with larger peak NO concentrations and higher altitude extent during solar maximum years compared to those during the solar minimum years. The SABER NO agrees well with the MSIS 2.1 NO at altitudes above 120 km for all latitudes, while the pronounced SABER‐MSIS NO discrepancy below 120 km is likely due to the temperature underestimation by MSIS 2.1. A detailed error analysis is presented and considers systematic and random errors in all the terms in the non‐LTE model used to derive the NO concentration. Random error in MSIS 2.1 temperature and atomic oxygen dominates the uncertainty in single NO profiles above 120 km. We estimated a systematic error up to ∼36% between 120 and 250 km during solar maximum years. Plain Language Summary: Nitric oxide (NO) is a minor constituent of the Earth's thermosphere and regulates the energy budget by emitting infrared radiation that cools the region. Understanding the distribution of the NO concentration is important in understanding the energy redistribution in the Earth's upper atmosphere. In this paper, a new NO concentration data set between 120 and 250 km from January 2002 to the present day is derived from the latest Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) NO emission measurements and Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 empirical model of temperature and O, and O2 number density. The derived SABER NO concentration varies with solar activity. The SABER NO agrees well with the empirical MSIS 2.1 NO between 120 and 250 km. Below 120 km, the temperature may be warmer than predicted by MSIS 2.1, leading to a substantial difference between SABER NO and MSIS 2.1 NO. A comprehensive error analysis considering systematic and random errors in all the terms in the model used to derive the NO concentration from SABER data shows that the systematic uncertainty of the SABER NO density is up to 36% above 120 km. Key Points: The nitric oxide (NO) 5.3 μm infrared emission rates derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements have been updatedVertical profiles of NO concentration, spanning 20+ years, are derived from the SABER NO emission rates, and Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 T, n(O), and n(O2)The SABER NO concentrations exhibit good agreement (±30%) with those in the MSIS 2.1 empirical model between 120 and 250 km [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]

Using experiments by Sanya (18.3°N, 109.6°E) Incoherent Scatter Radar (SYISR) during 22 days, we first obtained the line‐of‐sight ion velocity in multiple directions, further calculated three‐dimensional vector velocity through least squares fitting, and subsequently derived northward neutral wind and electric fields from 150 to 500 km based on the momentum equation. In order to verify the reliability of the derived northward neutral wind and electric field, we used NCAR‐TIEGCM and HWM for comparative analysis. They had a good consistency in terms of diurnal variation and direction. The SYISR vertical ExB velocity during the daytime is larger than that of TIEGCM, and the direction reverses occur after 20:00 for both TIEGCM and SYISR. We further compared our wind data with that of ICON satellite. They showed a positive correlation of ∼0.62. We also used the ∆H method to verify our eastward electric field. They showed a roughly linear dependency, although the correlation coefficient is not very high. Plain Language Summary: In the low latitude ionosphere, neutral wind drives ions moving across the geomagnetic field line and generates electric field. Therefore, neutral wind and electric field are two key drivers in low latitude ionospheric dynamics. Monitoring both parameters is important for low latitude ionosphere investigation. However, measuring neutral wind and electric field, especially height profile in the F region, is very challenging and rare. In this paper, using the newly built Sanya Incoherent Scatter Radar, we have developed sophisticated algorithm to derive neutral wind and electric field from multiple beams SYISR measurements. Further evaluations show the data is reliable and therefore useful. Key Points: Northward neutral wind and electric field from 150 to 500 km are obtained using 22 days continuously SYISR observationsThe derived northward neutral wind and electric field are in good agreement with well‐known modelsThe results are further independently evaluated by ICON satellite and geomagnetism data [ABSTRACT FROM AUTHOR]

Since the installation at the Antarctic Jang Bogo Station (JBS) in 2017, Korea Polar Research Institute (KOPRI) has been operating the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) equipped with Dynasonde analysis (JVD). The two‐dimensional ion velocity is one of the key ionospheric parameters obtained from the JVD. The ionospheric ion velocities are compared with simultaneous, but independent, measurements of the Doppler velocity obtained from SuperDARN East radar at Dome C. The JVD ion velocity vector is projected to the line‐of‐sight direction of the SuperDARN observation over the JBS to be directly compared with each other. The result of comparison shows a reasonable agreement with the correlation coefficient of 0.72. The linear regression coefficient of about 0.5 represents that the JVD ion velocity is generally smaller than the SuperDARN observations by the regression coefficient, which may result from the different height ranges of the measurements. It is also found that the correlation coefficient increases with increasing magnetic activity (Kp), which suggests that the small‐scale ionospheric density irregularities tend to move with large‐scale plasma motion that is driven by enhanced plasma convection with increasing Kp. Plain Language Summary: The ion drift velocity in the polar ionosphere is one of the key parameters for understanding not only the dynamics of the ionosphere but also the magnetosphere‐ionosphere coupling processes and the magnetospheric energy transfer to the neutral atmosphere via ion‐neutral interactions. There are several ground‐based observational techniques to monitor the ion velocities in the polar region. For example, the incoherent scatter radars (ISRs) can determine the ion motion in the polar region, but usually require expensive resources for their maintenance and operation, which makes them affordable only to large organizations or international consortiums such as EISCAT. Another widely utilized observational system for the ion velocities in the polar region is the SuperDARN radars but they are relatively scarce in Antarctica. The most affordable technique for monitoring the ionosphere is the ionospheric sounding systems capable of observing not only the ionospheric densities but also the ion velocities in the bottomside ionosphere. An advanced sounding system has been operated at Jang Bogo Station in Antarctica since 2017 to produce ionospheric parameters including ion density and velocities. The observed ion velocities are compared with simultaneously observed SuperDARN ion velocities over the JBS and we discuss similarities and differences between the two measurements. Key Points: Validation of Dynasonde analysis (JVD) ion drift velocities by using simultaneous but independent SuperDARN observationsJVD ion velocities are generally in a good agreement with SuperDARN radar observationsSmall‐scale ionospheric density irregularities tends to move with large‐scale plasma contours as the geomagnetic activity increases [ABSTRACT FROM AUTHOR]

In this study, the atmospheric changes for the 9.0-magnitude Tohoku earthquake, which occurred on March 11, 2011, are analyzed. The March 11, 2011 earthquake was preceded by a large foreshock on March 09, 2011 with magnitude M 7.3 and depth 32 km at 02:45:20 UT near the east coast of Honshu, Japan. The earthquake doesn't limit its effects on the Earth's lithosphere, hydrosphere and biosphere; it also extends its effects to the atmosphere because of the gas emissions, which produce large-scale seismic waves from the ground and release gases into the atmosphere. In this study, the anomalies of the atmospheric parameters are studied by using one of the atmospheric models from the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Extension 2000 (NRLMSISE-00) model data to analyze the atmospheric anomalies of the Tohoku Earthquake on March 11, 2011. The atmospheric parameters of atomic oxygen (O), hydrogen (H), atomic nitrogen (N), helium (He), argon (Ar), molecular oxygen (O2), molecular nitrogen (N2), total mass density (ρ), neutral temperature (Tn), exospheric temperature (Tex) and anomalous oxygen (AO) are used for analysis during the earthquake occurrence. The epicenter of the Tohoku earthquake, with a geographical location of latitude 38.30° N and longitude 142.37° E, is used for the NRLMSISE-00 model as input parameters to analyze the output of atmospheric parameters. To compare the atmospheric changes caused by the earthquake, 5 days before and after the earthquake are considered. To detect where the atmospheric parameters increased or decreased from the earthquake day, the percentage deviation of the NRLMSISE-00 model is applied. The results indicate that there were atmospheric parameter anomalies that occurred a few days before, following and during the earthquake on March 11, 2011. Except for hydrogen (H), all atmospheric parameters average daily percentage deviation values were positive during the 5 days before and after with respect to the main earthquake shock on March 11, 2011. The NRLMSISE-00 model can capture the atmospheric parameter anomalies of the Tohoku earthquake well. [ABSTRACT FROM AUTHOR]

Laser Thomson scattering (LTS) is a minimally invasive measurement technique used for determining electron properties in plasma systems. Sheath model closure validation requires minimally invasive measurements of the electron properties that traverse the boundaries between the bulk plasma, the presheath, and the plasma sheath. Several studies have probed the radial properties along the surface of discharge electrodes with laser-based diagnostics and electrostatic probes. These measurements provide valuable insight into the electron properties in this dynamic region. However, sheath model calibration requires plasma property measurements perpendicular to plasma bounding surfaces, in this case, along the electrode normal vector between discharge electrodes. This work presents the development of a discharge plasma cell and laser Thomson scattering system with a measurement volume step of 1 mm normal to plasma bounding surfaces. The laser Thomson scattering measurements are made between a set of discharge electrodes separated by ∼25 mm that are used to generate a pulsed argon plasma. The spatial distribution of electron temperature and density is measured at several discharge voltages between 8 and 20 kV at a pressure of 8 Torr-Ar. It is determined that the system is statistically stationary and resembles a classic DC discharge plasma. The results are some of the first laser diagnostic-based "between electrode" measurements made along the plasma bounding electrode normal vector. A one-dimensional sheath model is applied to determine the near cathode electron properties, and it is determined that the edge of the presheath is probed in the high-voltage cases. As the lengths of the presheath and sheath decrease with decreasing voltage, the region recedes below the closest probed point to the cathode. To improve the performance of the diagnostic, the step size of the interrogation volume should decrease by an order of magnitude from 1 mm to less than 100 μm, and the data acquisition strategy should be revised to increase the signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The performance of different height models (BSE-1979, AMTB-2013, and SHU-2015) in the International Reference Ionosphere (IRI-2016) is evaluated over the Indian equatorial and low latitude sectors. The impact of deviations in the modelled hmF 2 on the estimation of electron density profiles and the Total Electron Content (TEC) is examined using the three different hmF 2 methods in IRI-2016. For this study digisonde observations over Trivandrum (8.5 °N, 76.9 °E) and Ahmedabad (23.0 °N, 72.5 °E) along with GPS observations over Colombo (6.8 °N, 79.8 °E) and Ahmedabad during a high solar activity year 2014 are considered. A comparison of electron density profiles between digisonde and IRI-2016 at both the equatorial station, Trivandrum, and the low latitude station, Ahmedabad, revealed significant deviations with higher deviations in the post sunset hours at the equator. The three height methods have shown significant differences in their predictions of hmF 2 at both locations. At the equator, the SHU-2015 method shows better estimates closer to the measurement values, than the other two methods except at Pre-Reversal Enhancement (PRE) time, during which, the AMTB-2013 method shows better performance. At the low latitude station, the newly added methods (SHU-2015 and AMTB-2013) do not indicate any improvements in estimating the hmF 2. The availability of three different height methods in IRI-2016 is exploited to understand the modifications if any, in the ionospheric height structure and TEC for different hmF 2 methods and the corresponding results are presented. As the variations in hmF 2 are used to derive vertical drift on many occasions in the absence of incoherent scatter radar, this study is significant in assessing the relative efficacy of the three height models and hence useful to understand various ionospheric processes over the low/equatorial latitudes. [ABSTRACT FROM AUTHOR]

The 233 MHz EISCAT_3D radar system currently under construction in northern Fennoscandia will be able to resolve ionospheric structures smaller than the transmit beam dimensions through the use of interferometric imaging. This capability is made possible by the modular design and digitisation of the 119 91-antenna panels located at the main Skibotn site. The main array consists of a cluster of 109 panels, with 10 outlier panels producing unique interferometry baselines. In the present study synthesized incoherent scatter radar signal measurements are used for interferometric imaging analysis with the EISCAT_3D system. The Geospace Environment Model of Ion-Neutral Interactions (GEMINI) model is used to simulate a Kelvin-Helmholtz instability in the cusp region at 50 m resolution to obtain plasma parameters which are then used to generate the synthetic data. The ionospheric data is forward propagated to the EISCAT_3D array, noise is added to the synthetic data, and then an inversion of the data is performed to reconstruct the incoherent scatter spectra at relatively fine scales. By using Singular Value Decomposition (SVD) with Tikhonov regularization it is possible to pre-calculate the inversion matrix for a given range and look direction, with the regularization value scaled based on the SNR. The pre-calculation of the inversion matrix can reduce computational overhead in the imaging solution. This study provides a framework for data processing of ion-line incoherent scatter radar spectra to be imaged on fine-scales. Furthermore, with more development it can be used to test experimental set-ups and to design experiments for EISCAT_3D by investigating the needed integration time for various signal-to-noise ratios, beam patterns and ionospheric conditions. [ABSTRACT FROM AUTHOR]

Multiple Large‐Scale Traveling Ionospheric Disturbances (LSTIDs) are observed in the European sector in both day‐time and night‐time during the magnetic storm on March 23–24, 2023. The Total Electron Content (TEC) observation from a network of GNSS receivers shows the propagation of LSTIDs with amplitudes between around 0.5 and 1 TECU originating from auroral and polar cusp regions down to southern Europe (35°N) with velocities between around 500 and 1,600 [m/s]. We study the energy deposition to the LSTIDs in the source regions and the resulting horizontal propagation over storm‐time background density by using continuous measurements of EISCAT incoherent scatter radars in northern Norway and Svalbard that allow for estimating the source energy to the thermosphere‐ionosphere system via Joule heating and particle precipitation. Both EISCAT and GNSS TEC data show that the electron density decreased to 50% in the auroral zone after the storm onset. The ionospheric heating caused a nearly 250% increase in the electron temperature above 200 km altitude and the ion temperature above 100 km altitude. We find that Joule Heating acts as a primary energy source for the night‐time LSTIDs triggered in the auroral region, while the day‐time LSTIDs can be also driven by precipitating particles in the polar cusp. We also find that a significant background density decrease over the whole European sector is caused by this storm for the following day, during which almost no clear LSTIDs are observed. Key Points: Multiple day‐time and night‐time LSTIDs are triggered at the high latitudes and propagated over Europe down to 35 N on March 23–24, 2023We evaluate source energy deposition of LSTIDs from Joule heating and particle precipitation using TEC and incoherent scatter radar dataJoule heating has more contribution to LSTID generation than particle precipitation during night‐time compared to day‐time [ABSTRACT FROM AUTHOR]

Ions in the F region ionosphere at 150–400 km altitude consist mainly of molecular NO+ and O2+ ${\mathrm{O}}_{2}^{+}$, and atomic O+. Incoherent scatter (IS) radars are sensitive to the molecular‐to‐atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. In this paper, we introduce a novel combination of Bayesian filtering, smoothness priors, and chemistry modeling to solve for F1 region O+ ion fraction from EISCAT Svalbard IS radar (75.43° corrected geomagnetic latitude) data during the international polar year (IPY) 2007–2008. We find that the fraction of O+ ions in the F1 region ionosphere is controlled by ion temperature and electron production. The median value of the molecular‐to‐atomic ion transition altitude during IPY varies from 187 km at 16–17 MLT to 208 km at 04–05 MLT. The ion temperature has maxima at 05–06 MLT and 15–16 MLT, but the transition altitude does not follow the ion temperature, because photoionization lowers the transition altitude. A daytime transition altitude maximum is observed in winter, when lack of photoionization leads to very low daytime electron densities. Both ion temperature and the molecular‐to‐atomic ion transition altitude correlate with the Polar Cap North geomagnetic index. The annual medians of the fitted transition altitudes are 14–32 km lower than those predicted by the International Reference Ionosphere. Plain Language Summary: Ions in the F region ionosphere at 150–400 km altitudes consist mainly of molecular NO+ and O2+ ${\mathrm{O}}_{2}^{+}$, and atomic O+. Incoherent scatter radars are sensitive to the molecular‐to‐atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. This causes bias to the fitted temperatures and leaves behavior of the F1 region ion composition poorly known. We apply a novel combination of inverse mathematics and chemistry modeling to analysis of EISCAT Svalbard incoherent scatter radar data, and solve for both the ion temperature and the fraction of O+ ions at the same time. We find that the ion composition is considerably different from a standard model, that it undergoes regular diurnal variations, and it is affected by geomagnetic activity. The typical variations can be qualitatively explained by known diurnal variations in ion temperature, solar photoionization, and ion chemistry. When the ionosphere above Svalbard is in almost complete darkness in mid‐winter, ion composition in the daytime ionosphere is considerably different from that observed during the other seasons. Key Points: We use novel data analysis techniques and chemistry modeling to fit atomic oxygen ion fractions to EISCAT Svalbard radar data during IPYWe characterize the F1 region ion composition dependence on local time, solar zenith angle, and geomagnetic activityThe molecular‐to‐atomic ion transition altitudes are 14–32 km lower than those predicted by the International Reference Ionosphere [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]

Auroral electron precipitation during a substorm exhibits complex spatiotemporal variations which are still not fully understood, especially during the very dynamic phase immediately following the onset. Since during disturbed times the auroral oval typically extends across several hundreds of kilometers in the latitudinal direction, one may expect that precipitating electron spectra differ at locations close to the open–closed field line boundary (OCB) compared to the central part of the auroral oval. We carry out a statistical study based on 57 auroral breakups associated with substorm onsets observed above Tromsø (66.7° N geomagnetic latitude, i.e. central oval) and 25 onsets occurring above Svalbard (75.4° N geomagnetic latitude, i.e. poleward boundary) between 2015 and 2022. The events were selected based on the availability of both optical observations and field-aligned incoherent scatter radar measurements. Those are two sets of different substorms; hence we compare solar wind driving conditions and geomagnetic indices for the two event lists in the statistical sense. Using the ELSPEC method (based on the inversion of the electron density profile) on the radar data, we retrieve precipitating electron fluxes within 1–100 keV around each onset time, and we apply the superposed epoch analysis method on the electron spectra at each location. We compare the statistical precipitation characteristics above both sites in terms of peak differential flux, energy of the peak, integrated energy flux, and their time evolution during the minutes following the onset. We find that the integrated energy flux associated with events occurring in the central part of the auroral oval (Tromsø) exhibit a sharp peak up to 25 mW m-2 in the first two minutes following the auroral breakup, before decreasing and reaching stable values around 7 mW m-2 for at least 20 min. In turn, no initial peak is seen near the open–closed field line boundary (Svalbard), and values remain low throughout (1–2 mW m-2). A comparison of the median spectra indicates that the precipitating flux of > 10 keV electrons is lower above Svalbard than above Tromsø by a factor of at least 10, which may partly explain the differences. However, it proves difficult to conclude whether the differences originate from the latitude at which the auroral breakup takes place or from the fact that the breakups seen from Svalbard occur equatorward from the radar beam, which only sees expansion-phase precipitation after a few minutes. [ABSTRACT FROM AUTHOR]

Joule heating is the primary source of the high latitude thermospheric energy dissipation during geomagnetic storm period. The nitric oxide (NO) emission at 5.3 μm accounts for the majority of Joule heating energy due to its radiative nature. It is the dominating cooling agent above 100 km that effectively regulates the thermospheric temperature. We studied the relationship between the response time of NO cooling emission and Joule heating rate during geomagnetic storm periods by using the NO emission observations by Sounding of Atmosphere using Broadband Emission Radiometry (SABER) onboard NASA's Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite and the Joule heating rate measured by the European incoherent scatter (EISCAT) radar over Tromsø (geographic:69.59°N, 19.22°E), Norway, and the thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) simulation. We selected seven geomagnetic storms during which there were continuous measurements of the Pederson conductivity and electric fields when the TIMED/SABER satellite was in the northview mode. The TIEGCM gives a fair description of NO cooling. However, it was found that the Joule heating rates obtained with the TIEGCM often do not show good agreement with those from EISCAT observations. The Joule heating rate peaks with 3–7 hr of storm' onset. The NO cooling flux takes longer time to respond to the energy input during storm period. The fastest response of the NO cooling emission to the Joule heating rate is found during the strongest storm. The weakest storm does not have the longest response time. No correlation between the response time of NO cooling flux with respect to the Joule heating rate and storm's intensity was observed. Key Points: The magnitude of thermospheric cooling emission strongly depends on the storm's intensityTime lag of nitric oxide (NO) cooling emission with respect to Joule heating rate has little dependence on the storm's intensityStrong discrepancy is observed between the modeled and measured Joule heating rates [ABSTRACT FROM AUTHOR]

We present two methods for improved calibration of multi-point electron density measurements from incoherent scatter radars (ISR). They are based on the well-established Flatfield correction method used in imaging and photography, where we exploit the analogy between independent measurements in separate pixels in one image sensor and multi-beam radar measurements. Applying these correction methods adds to the current efforts of estimating the magic constant or system constant made for the calibration of multi-point radars, increasing data quality and usability by correcting for variable, unaccounted, and unpredictable variations in system gain. This second-level calibration is especially valuable for studies of plasma patches, irregularities, turbulence, and other research where inter-beam changes and fluctuations of electron density are of interest. The methods are strictly based on electron density data measured by the individual radar and require no external input. This is of particular interest when independent measurements of electron densities for calibration are available only in one pointing direction or not at all. A correction factor is estimated in both methods, which is subsequently used to scale the electron density measurements of a multi-beam ISR experiment run on a phased array radar such as RISR-N, RISR-C, PFISR, or the future EISCAT3D radar. This procedure could improve overall data quality if used as part of the data-processing chain for multi-beam ISRs, both for existing data and for future experiments on new multi-beam radars. [ABSTRACT FROM AUTHOR]

Meteoric ablation in the Earth's atmosphere produces particles of nanometer size and larger. These particles can become charged and influence the charge balance in the D region (60–90 km) and the incoherent scatter observed with radar from there. Radar studies have shown that, if enough dust particles are charged, they can influence the received radar spectrum below 100 km, provided the electron density is sufficiently high (>109 m 3). Here, we study an observation made with the EISCAT VHF radar on 9 January 2014 during strong particle precipitation so that incoherent scatter was observed down to almost 60 km altitude. We found that the measured spectra were too narrow in comparison to the calculated spectra. Adjusting the collision frequency provided a better fit in the frequency range of ± 10–30 Hz. However, this did not lead to the best fit in all cases, especially not for the central part of the spectra in the narrow frequency range of ±10 Hz. By including a negatively charged dust component, we obtained a better fit for spectra observed at altitudes of 75–85 km, indicating that dust influences the incoherent-scatter spectrum at D-region altitudes. The observations at lower altitudes were limited by the small number of free electrons, and observations at higher altitudes were limited by the height resolution of the observations. Inferred dust number densities range from a few particles up to 104 cm -3 , and average sizes range from approximately 0.6 to 1 nm. We find an acceptable agreement with the dust profiles calculated with the WACCM-CARMA (Whole Atmosphere Community Climate Model-Community Aerosol Radiation Model for Atmospheres) model. However, these do not include charging, which is also based on models. [ABSTRACT FROM AUTHOR]

EISCAT_3D is a project to build a multi-site phased-array incoherent scatter radar system in northern Fenno-Scandinavia. We demonstrate via numerical simulation how useful monochromatic images taken by a multi-point imager network are for auroral research in the EISCAT_3D project. We apply the generalized aurora computed tomography (G-ACT) method to modelled observational data from real instruments, such as the Auroral Large Imaging System (ALIS) and the EISCAT_3D radar. G-ACT is a method for reconstructing the three-dimensional (3D) distribution of auroral emissions and ionospheric electron density (corresponding to the horizontal two-dimensional (2D) distribution of energy spectra of precipitating electrons) from multi-instrument data. It is assumed that the EISCAT_3D radar scans an area of 0.8° in geographic latitude and 3° in longitude at an altitude of 130 km with 10 × 10 beams from the radar core site at Skibotn (69.35° N, 20.37° E). Two neighboring discrete arcs are assumed to appear in the observation region of the EISCAT_3D radar. The reconstruction results from G-ACT are compared with those from the normal ACT as well as the ionospheric electron density from the radar. It is found that G-ACT can interpolate the ionospheric electron density at a much higher spatial resolution than that observed by the EISCAT_3D radar. Furthermore, the multiple arcs reconstructed by G-ACT are more precise than those by ACT. In particular, underestimation of the ionospheric electron density and precipitating electrons' energy fluxes inside the arcs is significantly improved by G-ACT including the EISCAT_3D data. Even when the ACT reconstruction is difficult due to the unsuitable locations of the imager sites relative to the discrete arcs and/or a small number of available images, G-ACT allows us to obtain better reconstruction results. [ABSTRACT FROM AUTHOR]

It is demonstrated that Kikuchi features become clearly visible if reflection high‐energy electron diffraction (RHEED) patterns are filtered using digital image processing software. The results of such pattern transformations are shown for SrTiO3 with mixed surface termination for data collected at different azimuths of the incident electron beam. A simplified analytical approach for the theoretical description of filtered Kikuchi patterns is proposed and discussed. Some examples of raw and filtered patterns for thin films are shown. RHEED patterns may be treated as a result of coherent and incoherent scattering of electron waves. The effects of coherent scattering may be considered as those occurring due to wave diffraction by an idealized crystal and, usually, only effects of this type are analysed to obtain structural information on samples investigated with the use of RHEED. However, some incoherent scattering effects mostly caused by thermal vibrations of atoms, known as Kikuchi effects, may also be a source of valuable information on the arrangements of atoms near the surface. Typically, for the case of RHEED, Kikuchi features are hidden in the intensity background and researchers cannot easily recognize them. In this paper, it is shown that the visibility of features of this type can be substantially enhanced using computer graphics methods. [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]

This paper presents the results of a study of a short event that occurred during the decay phase of a C6.0 circular flare (GOES). The event was simultaneously observed by the Irkutsk Incoherent Scatter Radar at 161 MHz and by the Siberian Radioheliograph (SRH) at 5.6 GHz. The SRH data were obtained at a single frequency, but with a high time resolution (0.2 s). This made it possible to localize the microwave source and carry out its temporal profile using imaging photometry. The similarity of the time profiles at both frequencies and the close values of the periods of the quasi-periodic oscillations seen on both profiles indicate a close relationship between the emeission in the meter and microwave ranges. According to the images at 5.6 GHz, the burst source was located near the circular ribbon of the flare, but was not a part of it. The event was folowed by an eruption that manifested itself in the meter range as the Type II radio burst at frequencies above 200 MHz, while a burst at 161 MHz demonstrated the emission of the Type III radio burst. An analysis of the three-dimensional structure of the magnetic field, reconstructed on the basis of vector magnetograms, showed that the event source was located on relatively open field lines, which were above the dome structure of the circular flare. The active region topology near the microwave source of the event suggests the following scenario based on the expansion of the dome structure of the circular flare due to gas dynamic processes. This caused an impact on relatively open field lines, which resulted to eruption, acceleration of electrons, and their propagation along open field lines. It is shown that the event was triggered by a flare, despite the absence of a direct spatial connection between the burst and flare ribbons. [ABSTRACT FROM AUTHOR]

The plasma–neutral coupling in the mesosphere–lower thermosphere strongly depends on the ion–neutral collision frequency across that region. Most commonly, the collision frequency profile is calculated from the climatologies of atmospheric models. However, previous measurements indicated that the collision frequency can deviate notably from the climatological average. Direct measurement of the ion–neutral collision frequency with multifrequency incoherent scatter radar (ISR) measurements has been discussed before, though actual measurements have been rare. The previously applied multifrequency analysis method requires a special simultaneous fit of the two incoherent scatter spectra, which is not possible with standard ISR analysis software. The difference spectrum method allows us to infer ion–neutral collision frequency profiles from multifrequency ISR measurements based on standard incoherent scatter analysis software, such as the Grand Unified Incoherent Scatter Design and Analysis Package (GUISDAP) software. In this work, we present the first results by applying the difference spectrum method. Ion–neutral collision frequency profiles obtained from several multifrequency EISCAT ISR campaigns are presented. The profiles obtained with the difference spectrum method are compared to previous collision frequency measurements, both from multifrequency ISR and other measurements, as well as results from empirical and comprehensive atmosphere models. Ion–neutral collision frequency measurements can be applied to improve first-principle ionospheric models. [ABSTRACT FROM AUTHOR]