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In this work, we analyse the performance of an Ensemble Kalman filter (EnKF) approach, the simultaneous multiplicative column normalized method SMART+ and the NeQuick model to estimate the topside ionosphere and plasmasphere. The slant total electron content (STEC) measurements of 11 Low Earth Orbit (LEO) satellites are used as input for the EnKF and SMART+ to update the NeQuick model which serves as background model. Our comparative case study is implemented globally for altitudes between 430 and 20 200 km for two periods of the year 2015 covering moderate to perturbed ionospheric conditions. The performance of the methods is investigated regarding their capability to reproduce electron densities. For that purpose, the independent electron density measurements of the Van Allen Probes (VAP) twin satellites, Swarm Langmuir Probes (LP) and FORMOSAT-3/COSMIC ionospheric radio occultation (IRO) profiles serve as reference. The results reveal that in median the NeQuick model underestimates the IRO electron densities in the moderate period and for high latitudes in the perturbed period. For the NeQuick model the median of the absolute relative residuals is about 33 % in the moderate period and around 25 % in the perturbed period. SMART+ reduces these residuals to about 24 % and 23 %, respectively, whereas EnKF doesn't provide an improvement. The validation with VAP electron densities shows that in both periods and for all altitudes, the electron densities provided by the NeQuick model, the EnKF and SMART+ are in general significantly lower than the VAP measurements with a median of the relative residuals equal to about 80 %. The lowest median and RMS values of the VAP residuals are produces by EnKF, reducing the NeQuick statistics by up to 9 %. Further we observe that the electron densities provided by the NeQuick model are in median lower than the calibrated LP in-situ measurements for all three Swarm satellites in the moderate period and higher in the perturbed period. SMART+ shows the lowest median and standard deviation for the absolute relative residuals. These are up to 20 % lower than for the NeQuick model. For the LP in-situ measurements, the EnKF has the worst performance. Overall, the results underpin that even though the data of 11 satellite missions has been assimilated to adjust the a priori information of the NeQuick model, only limited improvements can be achieved especially for the plasmasphere. [ABSTRACT FROM AUTHOR]

The study of the physical properties of the topside ionosphere is fundamental to investigating the energy balance of the ionosphere and developing accurate models to predict relevant phenomena, which are often at the root of Space Weather effects in the near-Earth environment. One of the most important physical parameters characterising the ionospheric medium is electrical conductivity, which is crucial for the onset and amplification of ionospheric currents and for calculating the power density dissipated by such currents. We characterise, for the first time, electrical conductivity in the direction perpendicular to the geomagnetic field, namely Pedersen and Hall conductivities, in the topside ionosphere at an altitude of about 450 km. For this purpose, we use eight years of in situ simultaneous measurements of electron density, electron temperature and geomagnetic field strength acquired by the Swarm A satellite. We present global statistical maps of perpendicular electrical conductivity and study their variations depending on magnetic latitude and local time, seasons, and solar activity. Our findings indicate that the most prominent features of perpendicular electrical conductivity are located at low latitudes and are probably driven by the complex dynamics of the Equatorial Ionisation Anomaly. At higher latitudes, perpendicular conductivity is a few orders of magnitude lower than that at low latitudes. Nevertheless, conductivity features are modulated by solar activity and seasonal variations at all latitudes. [ABSTRACT FROM AUTHOR]

This study investigates the latitudinal characteristics of the nighttime electron temperature, as observed by the Defense Meteorological Satellite Program F16 satellite, and its dependence on solar and geomagnetic activities between 2013 and 2022 in the topside ionosphere, only for the winter hemispheres. The electron temperature in both hemispheres exhibited a low-temperature zone at the equator and a double high-temperature zone at the sub-auroral and auroral latitudes along the magnetic latitude. In addition, we further studied the temperature crest/trough positions in the temperature zone at different latitudes. As the solar activity intensity decreased (increased), the temperature trough position at the equator shifted from the Southern (Northern) to the Northern (Southern) Hemisphere, and the temperature double-crest positions at the sub-auroral and auroral latitudes gradually approached (moved away from) each other. Furthermore, during the geomagnetic disturbance time, the temperature double-crest positions both moved toward lower latitudes, but the temperature trough position was not sensitive to geomagnetic activity. Our analysis demonstrates that the values and correlations of the electron temperature and density varied in different temperature characteristic zones (the temperature crest/trough positions ±2°), possibly due to the different energy control factors of the electrons at different latitudes. This may also indirectly indicate the energy coupling process between the topside ionosphere and different regions at different latitudes. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the measurements of Langmuir Probes (LPs) onboard ESA's Swarm mission overestimate ion densities on the nightside by up to 50%. The overestimation is due to the assumption of oxygen‐only plasma for ion density calculations, which is often violated at mid‐latitudes on the nightside. In this study, we present a calibration model that resolves the nighttime overestimation by Swarm LPs. Using observations by Swarm FacePlate (FP) as a reference, we develop a neural network (NN) model that adjusts LP data to the FP measurements. The model incorporates dependence on solar and geomagnetic conditions, parameterized by the P10.7 and Hp30 indices, location, day of the year and local time. Our model reveals a distinct double‐crest pattern in nighttime density overestimation by LPs, centered at ∼30° quasi‐dipole latitude in both hemispheres. This overestimation intensifies during low solar activity and shows strong seasonal dependence. During solstices, the crests are more pronounced in the local winter hemispheres, while during equinoxes the crests are weaker and exhibit hemispheric symmetry. This morphology aligns with the presence of light ions diffusing downward from the plasmasphere. Validating the LP data in conjunctions with Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) observations showed a much stronger agreement after applying the developed correction: for Swarm B, nighttime correlation with COSMIC increased from 0.74 to 0.93. The NN‐calibrated LP data set has numerous applications in ionospheric research, and the developed model can provide useful insights into the ion composition in the topside ionosphere. Plain Language Summary: Operating since late 2013, ESA's Swarm mission has provided an extensive data set of plasma densities in the topside ionosphere, frequently used in space weather research. Recent studies have shown that the Langmuir Probes (LPs) onboard the Swarm mission overestimate ion densities during nighttime by up to 50%. This overestimation is due to the presence of light ions not accounted for in the ion density calculations. In this study, we develop a new NN‐based calibration model for Swarm observations, which resolves the nighttime overestimation by the LPs for the first time. Our results reveal a distinct double‐crest pattern on the nightside, centered at ∼30° quasi‐dipole latitude in both hemispheres, which corresponds to decreased effective ion masses and thus causes the overestimation of ion densities. This pattern closely mimics the behavior of light ions diffusing downwards from the plasmasphere. The developed NN‐based calibration improves the quality of the LP observations and allows using them for improving the existing models of the topside ionosphere, among other applications. Furthermore, the developed model provides insights into the ion composition and light ion dynamics in the topside ionosphere. Key Points: We present a neural network‐based calibration model that resolves the nighttime overestimation of ion densities by Swarm Langmuir Probes (LPs)This overestimation, peaking at 30°QDLat in both hemispheres, mimics the morphology of light ions diffusing downward from the plasmasphereThe corrected LP ion densities are in excellent agreement with COSMIC radio occultation data [ABSTRACT FROM AUTHOR]

Previous studies paid little attention to the ionospheric storm‐time altitudinal differences due to insufficiency of ionospheric measurements. In this work, multiple instrumental observations were used to investigate the ionospheric storm‐time response at low latitudes in the American and Asian‐Australian sectors during the May 2021 geomagnetic storm. The ground‐based Global Navigation Satellite Systems (GNSS) total electron content (TEC) and Low Earth Orbit (LEO) satellite topside TEC presented opposite (positive/negative) variations in the low‐latitude and equatorial region of both sectors during this storm. The electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) and the Sanya Incoherent Scatter Radar showed a good agreement and well explained the opposite variations between GNSS TEC and LEO satellite topside TEC. The F2‐layer peak height (hmF2) and peak density (NmF2) displayed inverse variations, and the feature was present mostly in the regions between equatorial ionization anomaly crests. The combined modulation effects of the storm‐time zonal electric fields and the field‐aligned transports possibly resulted in the contrary variations of hmF2 and NmF2 in the low‐latitude and equatorial region, leading to the storm‐time altitudinal differences during this storm. Relatively, the storm‐time thermospheric composition disturbances might be a minor factor responsible for these differences. Plain Language Summary: The ionospheric storm‐time responses have been the research hotspot for nearly 80 years. When geomagnetic storms occur, the temperature, electron density, and electric field of the ionosphere are violently disturbed due to the enhanced energy coupled into the ionosphere, which are called ionospheric storms. The increases (decreases) of ionospheric electron density and total electron content are called positive (negative) storm during ionospheric storm. The ionospheric storm‐time responses present complex variations such as sectoral differences, latitudinal differences, and altitudinal differences. The altitudinal differences are mainly reflected in that the ionospheric storm‐time responses are inconsistent at different altitudes. This study focuses on the ionospheric storm‐time altitudinal differences at low latitudes during the May 2021 geomagnetic storm. In the low‐latitude and equatorial region, the ionospheric storm‐time electron densities at different altitudes show inconsistent variations. The storm‐time zonal electric fields and field‐aligned transports may play significant roles in the ionospheric storm‐time altitudinal differences in the low‐latitude and equatorial region during this storm. Key Points: The ionospheric storm‐time altitudinal differences were studied by comparing Global Navigation Satellite Systems (GNSS) total electron content (TEC), Swarm, COSMIC‐2, Sanya Incoherent Scatter Radar, Ionospheric Connection Explorer, and Global Scale Observations of the Limb and DiskThe ground‐based GNSS TEC and topside Low Earth Orbit TEC revealed opposite variations in both the Asian‐Australian and American sectorsStorm‐time zonal electric fields and field‐aligned transports possibly played key roles [ABSTRACT FROM AUTHOR]

This study focuses on the open question of the electron temperature (Te) variation with solar activity in the topside ionosphere at mid- and high latitudes. It takes advantage of in situ observations taken over a decade (2014–2023) from Langmuir probes on board the low-Earth-orbit Swarm B satellite and spanning an altitude range of 500–530 km. The study also includes a comparison with Te values modeled using the International Reference Ionosphere (IRI) model and with Millstone Hill (42.6° N. 71.5° W) incoherent scatter radar observations. The largest Te variation with solar activity was found at high latitudes in the winter season, where Te shows a marked decreasing trend with solar activity in the polar cusp and auroral regions and, more importantly, at sub-auroral latitudes in the nightside sector. Differently, in the summer season, Te increases with solar activity in the polar cusp and auroral regions, while for equinoxes, variations are smaller and less clear. Mid-latitudes generally show negligible Te variations with solar activity, which are mostly within the natural dispersion of Te observations. The comparison between measured and modeled values highlighted that future implementations of the IRI model would benefit from an improved description of the Te dependence on solar activity, especially at high latitudes. [ABSTRACT FROM AUTHOR]

Electron density (Ne) and electron temperature (Te) observations collected by Langmuir Probes on board the European Space Agency (ESA) Swarm B satellite are used to characterize their correlation in the topside ionosphere at an altitude of about 500 km. Spearman correlation coefficient values (RSpearman) are calculated on joint probability distributions between Ne and Te for selected conditions. The large data set of Swarm B observations at 2‐Hz rate, covering the years 2014–2022, allowed investigating the correlation properties of the topside ionospheric plasma on a global scale, for different diurnal and seasonal conditions, with both a coverage and a detail never reached before. Results are given as maps of RSpearman as a function of the Quasi‐Dipole (QD) magnetic latitude and magnetic local time (MLT) coordinates. The characterization of the correlation at high latitudes, along with the description of the diurnal trend at all latitudes, are the new findings of this study. The main correlation features point out a negative correlation at the morning overshoot, during daytime at mid latitudes, and during nighttime at the ionospheric trough and subauroral latitudes. Conversely, a positive correlation dominates the nighttime hours at mid and low latitudes and, to a minor extent, the low latitudes from 09 MLT onwards. A seasonal dependence of the correlation is noticeable only at very high latitudes where the general pattern of the negative correlation does not hold around ±75° QD latitude in the summer season. Results from Swarm B have been statistically compared and discussed with observations from the Arecibo, Jicamarca, and Millstone Hill incoherent scatter radars. Key Points: The correlation between the electron density and temperature in the topside ionosphere has been investigated through 9 years of Swarm B in‐situ observationsThe latitudinal, diurnal, and seasonal variations of the correlation have been studied on a global scale at an unprecedented spatial and temporal resolutionFirst ever study of the correlation at high latitudes, which highlighted a general pattern of negative correlation except for the summer season at the polar cusp [ABSTRACT FROM AUTHOR]

This study investigates the diurnal variation of topside ionospheric plasma density using the Ionospheric Connection Explorer (ICON) observations from May to July in 2020 and 2021. The total ion density exhibits daytime double‐maxima (DDM) patterns, also known as "bite‐out" at magnetic latitudes from 10°S to 20°N and longitudes of 180°–276°E, but a single peak in other longitudes. The total ion density between 180° and 276°E reaches its first peak around 12 LT (Local Time) in both hemispheres and the second peak at 14–15 LT in the Northern Hemisphere, gradually shifting to around 16–17 LT in the Southern Hemisphere. The formation of the first peak is mainly influenced by vertical plasma drift, while the second peak is associated with both neutral wind and vertical plasma drift. Furthermore, this study provides direct observational evidence for the influence of neutral winds on the longitude and latitude differences of the topside DDM. Stronger southward magnetic meridional winds and vertical plasma drifts are observed at approximately 11–15 LT in the longitude sector with DDM compared to other longitudes, leading to a valley of ion density in the Southern Hemisphere around 15 LT. In the Northern Hemisphere, ion density continues to accumulate to form a second peak at 15 LT. Key Points: Diurnal double maxima (DDM) occur in the daytime topside ionosphere around 180°–276°E in June SolsticeThis study presents probably evidence that neutral trans‐equatorial winds cause the formation of the topside ionospheric DDMSimulations analyzed the respective contributions of neutral winds and E × B drift to the topside ionospheric DDM [ABSTRACT FROM AUTHOR]

The post-midnight irregularities in equator and its adjcent regions, which have remained least explored so far, are investigated using the in situ electron density (Ne) measurements from 2019 to 2021 obtained by the China Seismo-Electromagnetic Satellite (CSES) in the topside ionosphere. The followings are salient points of our findings: 1) Equatorial and low latitude post-midnight irregularities are distributed along the dip equator, exhibiting a wavenumber 4 longitude variation pattern and a seasonal variation pattern characterized by peaks in summer and winter and valleys in spring and autumn during low solar activity (LSA) period in the topside ionosphere, 2) The longitude variation pattern of post-midnight irregularities aligns with that of the background Ne, with irregularities concentrated at the peaks of background Ne, 3) The occurrence rate of post-midnight irregularities increases significantly with higher solar flux, which can be attributed to the rapid increase in the background Ne under LSA conditions. Additionally, the growth rate of occurrence rate varies across different longitudes, with the Pacific and the Atlantic longitudes standing out as prominent regions, and 4) Post-midnight irregularities in the topside ionosphere may have two possible origins. One is irregularities generated at the lower altitudes that drift upward into the topside ionosphere, while the other is the irregularities generated directly in the topside ionosphere. The latter is supported by the presence of conditions conducive to irregularity generation in the topside ionosphere during midnight hours in LSA period. Given the scarcity of research on post-midnight irregularity compared to post-sunset irregularity, further investigations are essential to fully comprehend their generation mechanisms. The abundance of post-midnight Ne measurements from the CSES satellite offers a valuable opportunity for this research, which can enhance our understanding of post-midnight ionospheric dynamics, and their variations with solar activity. [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]

The study of the physical properties of the topside ionosphere is fundamental to investigating the energy balance of the ionosphere and developing accurate models to predict relevant phenomena, which are often at the root of Space Weather effects in the near-Earth environment. One of the most important physical parameters characterising the ionospheric medium is electrical conductivity, which is crucial for the onset and amplification of ionospheric currents and for calculating the power density dissipated by such currents. We characterise, for the first time, electrical conductivity in the direction perpendicular to the geomagnetic field, namely Pedersen and Hall conductivities, in the topside ionosphere at an altitude of about 450 km. For this purpose, we use eight years of in situ simultaneous measurements of electron density, electron temperature and geomagnetic field strength acquired by the Swarm A satellite. We present global statistical maps of perpendicular electrical conductivity and study their variations depending on magnetic latitude and local time, seasons, and solar activity. Our findings indicate that the most prominent features of perpendicular electrical conductivity are located at low latitudes and are probably driven by the complex dynamics of the Equatorial Ionisation Anomaly. At higher latitudes, perpendicular conductivity is a few orders of magnitude lower than that at low latitudes. Nevertheless, conductivity features are modulated by solar activity and seasonal variations at all latitudes.

This study investigates the latitudinal characteristics of the nighttime electron temperature, as observed by the Defense Meteorological Satellite Program F16 satellite, and its dependence on solar and geomagnetic activities between 2013 and 2022 in the topside ionosphere, only for the winter hemispheres. The electron temperature in both hemispheres exhibited a low-temperature zone at the equator and a double high-temperature zone at the sub-auroral and auroral latitudes along the magnetic latitude. In addition, we further studied the temperature crest/trough positions in the temperature zone at different latitudes. As the solar activity intensity decreased (increased), the temperature trough position at the equator shifted from the Southern (Northern) to the Northern (Southern) Hemisphere, and the temperature double-crest positions at the sub-auroral and auroral latitudes gradually approached (moved away from) each other. Furthermore, during the geomagnetic disturbance time, the temperature double-crest positions both moved toward lower latitudes, but the temperature trough position was not sensitive to geomagnetic activity. Our analysis demonstrates that the values and correlations of the electron temperature and density varied in different temperature characteristic zones (the temperature crest/trough positions ±2°), possibly due to the different energy control factors of the electrons at different latitudes. This may also indirectly indicate the energy coupling process between the topside ionosphere and different regions at different latitudes.

The Swarm satellite mission has been used for numerous studies of the ionosphere. Here we use a global product, based on electron density measurements from Swarm that characterises ionospheric variability. The IPIR (Ionospheric Plasma IRregularities product) provides characteristics of plasma irregularities in terms of their amplitudes, gradients and spatial scales and assigns them to geomagnetic regions. Ionospheric irregularities and fluctuations are often the cause of errors in position, navigation, and timing (PNT) based on the Global Navigation Satellite Systems (GNSS), in which signals pass through the ionosphere. The IPIR dataset also provides an indication, in the form of a numerical value index (IPIR index), of the severity of irregularities affecting the integrity of trans -ionospheric radio signals and hence, the accuracy of GNSS positioning. We analysed datasets from Swarm A and ground-based scintillation receivers. Time intervals (when Swarm A passes over the field of view of the ground-based GPS receiver) are compared to ground-based scintillation data, collecting an azimuthal selection of the GNSS data relevant to the Swarm satellite overpass. We provide validations of the IPIR product against the ground-based measurements from 23 ground-based receivers, focusing on GPS TEC and scintillation data in low-latitude, auroral and polar regions, and in different longitudinal sectors. We have determined the median, mean, maximum and standard deviation of the parameter values for both datasets and each conjunction point. We found a weak correlation of the intensity of both phase and amplitude scintillation with the IPIR index. [ABSTRACT FROM AUTHOR]

We investigate the response of the topside ionosphere, auroral and polar sectors, to the forcing of the geospace during September 2017. Specifically, we aim at characterizing such a response in terms of the involved spatial scales and of their intensification during the different auroral and polar cap activity conditions experienced in the selected month, that is characterized by severe geomagnetic storm conditions. For our purposes, we leverage on and compare various in situ plasma density data products provided by the Swarm constellation of the European Space Agency (ESA). The spatio-temporal variability of the involved scales in the plasma density observation is featured through the application of the Fast Iterative Filtering (FIF) signal decomposition technique and, for the first time in the ionospheric field, of a FIF-derived dynamical spectrum called "IMFogram". The instantaneous time-frequency representation provided through the IMFogram illustrates the time development of the multi-scale processes with spatial and temporal resolutions higher than those obtained with traditional signal processing techniques. To demonstrate this, the IMFogram is tested against Fast Fourier and Continuous Wavelet Transforms. With our fine characterization, we highlight how scale cascading and intensification processes in the plasma density observations follow the ionospheric currents activity, as depicted through the auroral activity and polar cap indices, and through the field-aligned currents data product provided by Swarm. [ABSTRACT FROM AUTHOR]

We model the electron density in the topside of the ionosphere with an improved machine learning (ML) model and compare it to existing empirical models, specifically the International Reference Ionosphere (IRI) and the Empirical‐Canadian High Arctic Ionospheric Model (E‐CHAIM). In prior work, an artificial neural network (NN) was developed and trained on two solar cycles worth of Defense Meteorological Satellite Program data (113 satellite‐years), along with global drivers and indices to predict topside electron density. In this paper, we highlight improvements made to this NN, and present a detailed comparison of the new model to E‐CHAIM and IRI as a function of location, geomagnetic condition, time of year, and solar local time. We discuss precision and accuracy metrics to better understand model strengths and weaknesses. The updated neural network shows improved mid‐latitude performance with absolute errors lower than the IRI by 2.5 × 109 to 2.5 × 1010 e−/m3, modestly improved performance in disturbed geomagnetic conditions with absolute errors reduced by about 2.5 × 109 e−/m3 at high Kp compared to the IRI, and high Kp percentage errors reduced by >50% when compared to E‐CHAIM. Plain Language Summary: The sun interacts with the outer layers of the Earth's atmosphere via the solar wind. Coronal mass ejections and solar flares travel from the sun along the solar wind creating space weather on Earth. A severe storm will cause widespread power outages and disrupt communication services. Unfortunately, we cannot predict space weather as well as we can predict normal weather. In this work, we explain a machine learning model that can predict how the sun will impact one outer layer of the Earth's atmosphere, called the ionosphere. The model combines data that is easier to measure directly and uses it to create these predictions, and compares them to existing physical/mathematical models of the ionosphere. Key Points: We present an upgraded machine learning (ML) model for topside electron density predictions at any locationWe analyze precision/accuracy of the ML model, Empirical‐Canadian High Arctic Ionospheric Model, and International Reference Ionosphere on Defense Meteorological Satellite Program/DEMETER data across solar local times, months, Kp, and MLatThe neural network produces lower mean absolute error by >50% than E‐CHAIM in highly disturbed geomagnetic conditions [ABSTRACT FROM AUTHOR]

Based on the plasma parameters measured by the Ion Velocity Meter (IVM) instrument on the Ionospheric Connection Explorer (ICON) satellite from 2020 to 2021, we present an analysis of seasonal variations in ion density, ion temperature, and migrating tides in the low-latitude topside ionosphere. The interannual variations in total ion density and O+ density are directly impacted by solar radiation. However, the concentration of H+ is not highly related to solar activity. The measurements show that the hemispheric dividing latitude for the seasonal variation in Ti is at about 9°N. We suggest that the reason for the hemispheric dividing latitude being 9°N is because measurements at this geographical latitude represent the closest match to the geomagnetic equator. An anticorrelation in the seasonal variations between the total ion density (as well as the O+ density) and the ion temperature is observed at all observed latitudes while the correlations between H+ density and the ion temperature are positive in most of the latitudes except for serval degrees around 9°N. The latitudinal variations in the correlation coefficients lead us to suggest that thermal conduction is likely more important than ion-neutral collision in the ion energy budget at 600 km. Additionally, semiannual oscillations with peak amplitudes in winters and summers at the extra-equatorial latitudes are revealed in the observations of diurnal migrating tides in the topside ionosphere, which are different from the latitudinal and seasonal distributions of diurnal migrating tides captured in the lower thermospheric temperature and total electron content. [ABSTRACT FROM AUTHOR]

This study focuses on the open question of the electron temperature (Te) variation with solar activity in the topside ionosphere at mid- and high latitudes. It takes advantage of in situ observations taken over a decade (2014–2023) from Langmuir probes on board the low-Earth-orbit Swarm B satellite and spanning an altitude range of 500–530 km. The study also includes a comparison with Te values modeled using the International Reference Ionosphere (IRI) model and with Millstone Hill (42.6° N. 71.5° W) incoherent scatter radar observations. The largest Te variation with solar activity was found at high latitudes in the winter season, where Te shows a marked decreasing trend with solar activity in the polar cusp and auroral regions and, more importantly, at sub-auroral latitudes in the nightside sector. Differently, in the summer season, Te increases with solar activity in the polar cusp and auroral regions, while for equinoxes, variations are smaller and less clear. Mid-latitudes generally show negligible Te variations with solar activity, which are mostly within the natural dispersion of Te observations. The comparison between measured and modeled values highlighted that future implementations of the IRI model would benefit from an improved description of the Te dependence on solar activity, especially at high latitudes.

The post-midnight irregularities in equator and its adjcent regions, which have remained least explored so far, are investigated using the in situ electron density (Ne) measurements from 2019 to 2021 obtained by the China Seismo-Electromagnetic Satellite (CSES) in the topside ionosphere. The followings are salient points of our findings: 1) Equatorial and low latitude post-midnight irregularities are distributed along the dip equator, exhibiting a wavenumber 4 longitude variation pattern and a seasonal variation pattern characterized by peaks in summer and winter and valleys in spring and autumn during low solar activity (LSA) period in the topside ionosphere, 2) The longitude variation pattern of post-midnight irregularities aligns with that of the background Ne, with irregularities concentrated at the peaks of background Ne, 3) The occurrence rate of post-midnight irregularities increases significantly with higher solar flux, which can be attributed to the rapid increase in the background Ne under LSA conditions. Additionally, the growth rate of occurrence rate varies across different longitudes, with the Pacific and the Atlantic longitudes standing out as prominent regions, and 4) Post-midnight irregularities in the topside ionosphere may have two possible origins. One is irregularities generated at the lower altitudes that drift upward into the topside ionosphere, while the other is the irregularities generated directly in the topside ionosphere. The latter is supported by the presence of conditions conducive to irregularity generation in the topside ionosphere during midnight hours in LSA period. Given the scarcity of research on post-midnight irregularity compared to post-sunset irregularity, further investigations are essential to fully comprehend their generation mechanisms. The abundance of post-midnight Ne measurements from the CSES satellite offers a valuable opportunity for this research, which can enhance our understanding of post-midnight ionospheric dynamics, and their variations with solar activity.

Constructing realistic vertical electron density profiles is a crucial step in accurately estimating the ionospheric total electron content (TEC) in empirical models. Inadequate representation of electron density profiles leads to under‐ or over‐estimation of TEC in model outputs. One of the widely used ionospheric empirical model, the NeQuick2 exhibits large uncertainties over equatorial and low latitudes, particularly in the topside ionosphere. The NeQuick2 model employs semi Epstein type of polynomials to characterize the topside ionospheric structure, wherein, the scale height (H) is the key parameter. NeQuick2 model estimates H value using empirical formulations, which contain three major parameters namely, Ho—the scale height at the F‐layer peak, g—height gradient in H, and r—which controls the increase in H at higher altitudes. In this study, a systematic analysis has been carried out with particular focus on the equatorial and low latitudes to explore the variability of topside scale height and electron density profiles on the above three empirical parameters using COSMIC observations over Trivandrum and Ahmedabad, in India. Model uncertainties in estimating Ho and their impacts on the estimation of TEC are investigated. Data assimilation analysis has been carried out to obtain a quantitative understanding on the effect of deviations in topside scale height on the uncertainties in accurate estimation of TEC over equatorial and low latitudes. This study reveals the impact of scale height uncertainties on errors in modeled TEC and demonstrates the need for improvement in empirical formulations for improving accuracies in ionospheric modeling over equatorial and low latitude sectors. Key Points: Effect of empirical parameters on the variability of topside scale height and density in NeQuick2 is investigated over the low latitudesObserved diurnal and latitudinal variations of Ho parameter are not well produced in the NeQuick2 modeled outputsModeled TEC shows significant improvements after ingesting the observed Ho values in the NeQuick2 over the equatorial and low latitudes [ABSTRACT FROM AUTHOR]

• Low-latitude topside ionosphere mismodelling made by NeQuick. • Improving of the NeQuick topside representation through COSMIC-1 data. • Different topside tests with different values of the NeQuick topside parameters. This paper deals with the well-known mismodelling characterizing the NeQuick topside ionosphere at low latitudes, i.e., the fact that the model keeps the two electron density humps typical of the equatorial ionization anomaly as the altitude increases from the height of the F2-layer electron density peak, without merging them in a single peak above the geomagnetic equator as expected. This is because the NeQuick topside ionosphere modelling strongly depends on several bottomside ionosphere parameters, which causes an essential coupling between the topside and the bottomside that in many cases behave differently. This means that this kind of topside ionosphere modelling may lead to inaccurate results, as it is the maintenance at low latitudes of the electron density double hump structure in the topside as the altitude increases. On the base of some recently published results, this paper analyzes the role played by the three NeQuick scale height parameters H 0 , g and r in the description of the electron density above the F2 layer peak height. The results of this work pave the way for a possible solution of this low-latitude NeQuick topside ionosphere mismodelling. [ABSTRACT FROM AUTHOR]

FORMOSAT‐7/COSMIC‐2 is the largest equatorial multi‐satellite constellation of six full‐size satellites to study the equatorial ionosphere. Each satellite is equipped by an ion velocity meter (IVM) instrument to provide high rate in situ plasma density observations along the low‐inclined satellite orbits at ∼530–550 km altitude. Six satellites provide an unprecedented dense coverage of the entire equatorial region around the globe and allow reliable detection of equatorial plasma bubbles (EPBs) and plasma density irregularities at different local times/longitudinal sectors simultaneously. We present a method for detection of EPBs in FORMOSAT‐7/COSMIC‐2 in situ plasma density data and construction of the global maps of EPB geolocations. The results in the form of time series and IVM‐based global Bubble Maps have a great potential for both near real‐time monitoring of space weather conditions and long‐term statistical analysis of EPB occurrence in regional or global scales. We present first FORMOSAT‐7/COSMIC‐2 derived climatological characteristics of the post‐sunset and post‐midnight EPBs occurrence probability and their apex altitudes during a period of low solar activity. Also, we demonstrate the good performance of the FORMOSAT‐7/COSMIC‐2 IVM‐based Bubble Maps when compared to optical images and ground‐based ionosonde observations. Plain Language Summary: Recently lunched satellite mission FORMOSAT‐7/COSMIC‐2 represents the largest equatorial mission of six spacecrafts that fly near the equator at 550 km altitude and measures various characteristics of the Earth's ionospheric plasma. Each satellite is equipped with an instrument for contact measurements of ionospheric plasma density and plasma motions—an ion velocity meter. Six satellites operate together around the globe, and they can probe the ionosphere at different longitudes and local times simultaneously. We use FORMOSAT‐7/COSMIC‐2 observations for detection of equatorial plasma bubbles (EPBs)‐huge plasma depletions developed after sunset over the equator. Detection of such structures is very important because of their impact on radio signals propagation. In this paper, we describe methodology on how we can detect EPBs and how to create global maps of their geolocation. The obtained results with time series of identified bubbles and global bubble maps can be used for near real‐time monitoring of ionospheric depletions and for statistical analysis of their occurrence around the globe. Key Points: Six satellites with in situ probe provide unprecedented coverage of equatorial region for reliable detection of equatorial plasma bubbles (EPBs)We developed new approaches for construction the global maps of EPBs geolocationsResults agreed well with independent observations and climatological characteristics of plasma bubbles occurrence [ABSTRACT FROM AUTHOR]

Electron density measured at high latitudes by the Swarm satellites was compared with measurements by the incoherent scatter radars at Resolute Bay and Poker Flat. Overall, the ratio of Swarm-based electron density to that measured by the radars was about 0.5–0.6. Smaller ratios were observed at larger electron densities, usually during the daytime. At low electron densities less than 3 × 1010 m−3, the ratios were typically above 1, indicating an overestimation effect. The overestimation effect was stronger at night and for Swarm B. It was more evident at lower solar activity when the electron densities in the topside ionosphere were lower. [ABSTRACT FROM AUTHOR]

This work presents statistical models of the variability of plasma in the topside ionosphere based on observations made by the European Space Agency’s (ESA) Swarm satellites. The models were developed in the “Swarm Variability of Ionospheric Plasma” (Swarm-VIP) project within the European Space Agency’s Swarm+4D-Ionosphere framework. The configuration of the Swarm satellites, their near-polar orbits and the data products developed, enable studies of the spatial variability of the ionosphere at multiple scale sizes. The statistical modelling technique of Generalised Linear Modelling (GLM) was used to create models of both the electron density and measures of the variability of the plasma structures at horizontal spatial scales between 20 km and 100 km. Despite being developed using the Swarm data, the models provide predictions that are independent of these data. Separate models were created for low, middle, auroral and polar latitudes. The models make predictions based on heliogeophysical variables, which act as proxies for the solar and geomagnetic processes. The first and most significant term in the majority of the models was a proxy for solar activity. The most common second term varied with the latitudinal region. This was the Solar Zenith Angle (SZA) in the polar region, a measure of latitude in the auroral region, solar time in the mid-latitude region and a measure of latitude in the equatorial region. Other, less significant terms in the models covered a range of proxies for the solar wind, geomagnetic activity and location. In this paper, the formulation, optimisation and evaluation of these models are discussed. The models show very little bias, with a mean error of zero to two decimal places in 14 out of 20 cases. The models capture some, but not all, of the trends present in the data, with Pearson correlation coefficients of up to 0.75 between the observations and the model predictions. The models also capture some, but not all, of the variability of the ionospheric plasma, as indicated by the precision, which ranged between 0.20 and 0.83. The addition of the thermospheric density as an explanatory variable in the models improved the precision in the polar and auroral regions. It is suggested that, if the thermosphere could be observed at a higher spatial resolution, then even more of the variability of the plasma structures could be captured by statistical models. The formulation and optimisation of the models are presented in this paper. The capability of the model in reproducing the expected climatological features of the topside ionosphere, in supporting GNSS-based ionospheric observations and the performance of the model against the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM), are provided in a companion paper (Spogli L et al. 2024. J Space Weather Space Clim https://doi.org/10.1051/swsc/2024003).

Statistical models of the variability of plasma in the topside ionosphere based on the Swarm data have been developed in the “Swarm Variability of Ionospheric Plasma” (Swarm-VIP) project within the European Space Agency’s Swarm+4D-Ionosphere framework. The models can predict the electron density, its gradients for three horizontal spatial scales – 20, 50 and 100 km – along the North-South direction and the level of the density fluctuations. Despite being developed by leveraging on Swarm data, the models provide predictions that are independent of these data, having a global coverage, fed by various parameters and proxies of the helio-geophysical conditions. Those features make the Swarm-VIP models useful for various purposes, which include the possible support for already available ionospheric models and proxy of the effect of ionospheric irregularities of the medium scales that affect the signals emitted by Global Navigation Satellite Systems (GNSS). The formulation, optimisation and validation of the Swarm-VIP models are reported in Paper 1 (Wood et al. 2024. J Space Weather Space Clim. in press). This paper describes the performance assessment of the models, by addressing their capability to reproduce the known climatological variability of the modelled quantities, and the ionospheric weather as depicted by ground-based GNSS, as a proxy for the ionospheric effect on GNSS signals. Additionally, we demonstrate that, under certain conditions, the model can better reproduce the ionospheric variability than a physics-based model, namely the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM).

Using the topside electron density (Ne) measurements recorded over Cyprus and Russia, we investigate the latitudinal variation in the topside electron density during the interval 2014–2020, encompassing a period of high-to-low solar activity. The selected topside electron density dataset employed in this study is based on the in situ Langmuir probe data on board the European Space Agency (ESA) Swarm satellites, in the vicinity of the three Digisonde stations in Nicosia (35.14°N, 33.2°E), Moscow (55.5°N, 37.3°E) and Saint Petersburg (60.0°N, 30.7°E). Our investigation demonstrates that the ratio Ne_Swarm/NmF2 between the coincident Ne_Swarm and the Digisonde NmF2 observations is higher than one on various occasions over Nicosia during the nighttime, which is not the case over Moscow and Saint Petersburg, signifying a discrepancy feature of the electron density at Swarm altitudes which depends not only on the solar activity and time of day but also on the latitude. [ABSTRACT FROM AUTHOR]

Satellites in Low Earth Orbit (LEO) are essential for sounding the topside ionosphere. In this work, we present and validate a data set of Total Electron Content (TEC) and in situ electron density observations from the Gravity Recovery And Climate Experiment (GRACE) and GRACE‐Follow‐On missions as well as a TEC data set from the CHAllenging Minisatellite Payload mission. Concerning TEC, special emphasis is put to ensure optimal consistency to the already existing Swarm and Gravity field and steady‐state ocean circulation explorer (GOCE) TEC data sets. The newly processed satellite missions allow covering two full solar cycles with LEO slant TEC. Furthermore, the twin satellite missions GRACE and GRACE‐FO equipped with inter‐satellite K‐band ranging allows to derive the horizontal TEC and, due to the small inter‐satellite distance of the satellite pairs, an approximation for local electron density. However, the derived value of electron density is relative and requires calibration using external information. In this work, the calibration is performed using the IRI‐2016 model. Radar observations, as well as in situ electron density observations available from Swarm B Langmuir probes, are used for validation. Conjunctions between satellites are used to validate the TEC time series. The newly derived data set is shown to be highly consistent with the already existing data sets with standard deviations below 3 TECU for TEC (even 1 TECU was reached for low solar flux) and an offset below 7 × 1010 m−3 with a standard deviation near 1 × 1011 m−3 for the electron density. Key Points: Two full solar cycles are covered by Low Earth Orbit (LEO) Global Positioning System (GPS) Total Electron Content (TEC) observations by adding CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery And Climate Experiment (GRACE) and GRACE‐Follow‐On (FO) to support SwarmLocal electron density is approximated using inter‐satellite K‐band ranging of the GRACE satellitesLEO GPS TEC from the missions considered and GRACE(‐FO) electron density observations are highly consistent to Swarm observations [ABSTRACT FROM AUTHOR]

Based on the plasma parameters measured by the Ion Velocity Meter (IVM) instrument on the Ionospheric Connection Explorer (ICON) satellite from 2020 to 2021, we present an analysis of seasonal variations in ion density, ion temperature, and migrating tides in the low-latitude topside ionosphere. The interannual variations in total ion density and O+ density are directly impacted by solar radiation. However, the concentration of H+ is not highly related to solar activity. The measurements show that the hemispheric dividing latitude for the seasonal variation in Ti is at about 9°N. We suggest that the reason for the hemispheric dividing latitude being 9°N is because measurements at this geographical latitude represent the closest match to the geomagnetic equator. An anticorrelation in the seasonal variations between the total ion density (as well as the O+ density) and the ion temperature is observed at all observed latitudes while the correlations between H+ density and the ion temperature are positive in most of the latitudes except for serval degrees around 9°N. The latitudinal variations in the correlation coefficients lead us to suggest that thermal conduction is likely more important than ion-neutral collision in the ion energy budget at 600 km. Additionally, semiannual oscillations with peak amplitudes in winters and summers at the extra-equatorial latitudes are revealed in the observations of diurnal migrating tides in the topside ionosphere, which are different from the latitudinal and seasonal distributions of diurnal migrating tides captured in the lower thermospheric temperature and total electron content.

Electron density (Ne) and temperature (Te) values recorded by the Langmuir probe onboard the first satellite of the China Seismo-Electromagnetic Satellite (CSES-01) mission allow calculating quantities such as the rate of change of electron density index (RODI) and the rate of change of electron temperature index (ROTEI), which are essential to describe the ionospheric irregularities and their dynamics. These two indices depend significantly on two parameters, i.e., the measurement sampling time and the width of the sliding windows used for their computation. Ne and Te measurements from CSES-01 present two different sampling times, i.e., 3 s in the survey mode and 1.5 s in the burst mode. The purpose of this article is to understand what are the best values of these two parameters to be used when computing RODI and ROTEI based on CSES-01 data. The main results of the study show the following: The shorter the data sampling time, the higher the values of the calculated ionospheric indices, which means that it is not possible to merge values of either RODI or ROTEI calculated with a different sampling time; the wider the sliding window used to calculate the indices, the higher the indices. A reasonable compromise between the data sampling time and the satellite orbital velocity suggests that the optimal way to calculate RODI and ROTEI from CSES-01 should be done by considering data with a 3-s sampling time (i.e., in the survey mode or in the downsampled burst mode), and a 24-s wide sliding window.

By analyzing the latitudinal distributions of ionospheric ions observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions satellite at 670 km altitude in different seasons, we found that the large‐scale depletion of nighttime oxygen ions is prevalent at the low and middle latitudes of winter hemisphere (center ∼ 30°) under different geomagnetic conditions. The latitudinal width of the winter oxygen ion depletion (WOD) region is about 20°–60° at different longitudes, and its upper boundary latitudes are mostly lower than the midlatitude trough (|λ| ≥ 50°) of nighttime hydrogen ions (H+) near the plasmapause. In the WOD region, the density ratio of oxygen and hydrogen ions and that of their neutral densities (O and H) approximately agree with the chemical equilibrium relationship of charge exchange reactions (H + O+ ↔ H+ + O) predicted by previous theory, indicating that the charge exchange reactions are mainly responsible for the large‐scale oxygen ion depletion at the low and middle latitudes in winter hemisphere. Plain Language Summary: Previous studies focused on the nighttime midlatitude trough of ionospheric electrons (e−) and light ions (H+ and He+). Although the percentage of nighttime oxygen ions was found to decrease in winter hemisphere at 400 km altitude, it is not clear whether the winter oxygen ion depletion (WOD) is a common phenomenon in the topside ionosphere. Moreover, the mechanism of the WOD has not been understood clearly. Here, our statistical results indicate that the large‐scale depletion of nighttime oxygen ions is prevalent at the low and middle latitudes of winter hemisphere under different geomagnetic conditions. By analyzing the relationship among the latitudinal changes in the ion bulk velocity, densities and their neutral densities, we found that the charge exchange reactions mainly cause the nighttime oxygen ion depletion at the low and middle latitudes in winter hemisphere in the topside ionosphere. Key Points: The nighttime oxygen ions decrease remarkably at the low and middle latitudes of winter hemisphere in the topside ionosphereThe latitudes of large‐scale oxygen ion depletion (center ∼ 30°) are mostly lower than the midlatitude trough of nighttime hydrogen ionsThe winter depletion of nighttime oxygen ions is largely due to the charge exchange reactions between oxygen ions and atomic hydrogen [ABSTRACT FROM AUTHOR]

The study of electrical currents in the topside ionosphere is of great importance, as it may allow a better understanding of the processes involved in the Sun–Earth interaction and magnetosphere–ionosphere–thermosphere coupling, two crucial aspects debated by the Space Weather scientific community. In this context, investigating the electrical conductivity parallel to the geomagnetic field in the topside ionosphere is of primary importance because: (1) it provides information on the capability of the ionosphere to conduct currents; (2) it relates current density and electric field through Ohm's law; (3) it can help to quantify the dissipation of currents; (4) it is generally modeled and not locally measured by in situ missions. In this work, we used in situ measurements of electron density and temperature recorded between 2019 and 2021 by the China Seismo-Electromagnetic Satellite (CSES-01) flying with an orbital inclination of 97.4° and at an altitude of about 500 km to compute the parallel electrical conductivity in the topside ionosphere at low and middle latitudes at the two fixed local times (LT) characterizing the CSES-01 mission: around 02 and 14 LT. The results, which are discussed in light of previous literature, highlight the dependence of conductivity on latitude and longitude and are compared with those obtained using values both measured by the Swarm B satellite (flying at a similar altitude) and modeled by the International Reference Ionosphere in the same time period. In particular, we found a diurnal variation in parallel electrical conductivity, with a slight hemispheric asymmetry. Daytime features are compatible with Sq and equatorial electrojet current systems, containing "anomalous" low values of conductivity in correspondence with the South Atlantic region that could be physical in nature. [ABSTRACT FROM AUTHOR]

The China Seismo-Electromagnetic Satellite (CSES-01) provides in situ electron density (Ne) observations through Langmuir probes (LPs) in the topside ionosphere since February 2018. CSES-01 is a sun-synchronous satellite probing the ionosphere around two fixed local times (LTs), 14 LT in the daytime sector and 02 LT in the night-time sector, at an altitude of about 500 km. Previous studies evidenced that CSES-01 seems to underestimate Ne measurements with respect to those acquired by similar satellites or obtained from different instruments. To overcome this issue, we calibrated CSES-01 LP Ne observations through Swarm B satellite data, which flies approximately at CSES-01 altitude. As a first step, Swarm B LP Ne observations were calibrated through Faceplate (FP) Ne observations from the same satellite. Such calibration allowed solving the Ne overestimation made by Swarm LP during nighttime for low solar activity. Then, the calibrated Swarm B LP Ne observations were used to calibrate CSES-01 Ne observations on a statistical basis. Finally, the goodness of the proposed calibration procedure was statistically assessed through a comparison with Ne observations by incoherent scatter radars (ISRs) located at Jicamarca, Arecibo, and Millstone Hill. The proposed calibration procedure allowed solving the CSES-01 Ne underestimation issue for both daytime and nighttime sectors and brought CSES-01 Ne observations in agreement with corresponding ones measured by Swarm B, ISRs, and with those modelled by the International Reference Ionosphere (IRI). This is a first fundamental step towards a possible future inclusion of CSES-01 Ne observations in the dataset underlying IRI for the purpose of improving the description of the topside ionosphere made by IRI. [ABSTRACT FROM AUTHOR]

Modeling the Earth's ionosphere is a critical component of forecasting space weather, which in turn impacts radio wave propagation, navigation and communication. This research focuses on predicting the electron density in the topside of the ionosphere using satellite data, in particular from the Defense Meteorological Satellite Program, a collection of 19 satellites that have been polar orbiting the Earth for various lengths of times, fully covering 1982 to the present. An artificial neural network was developed and trained on two solar cycles worth of data (113 satellite‐years), along with global drivers and indices such as F10.7, interplanetary magnetic field, and Kp to generate an electron density prediction. We tested the model on six years of subsequent data (26 satellite‐years), and found a correlation coefficient of 0.87. Once trained, the model can predict topside electron density at any location specified by latitude and longitude given current/recent geomagnetic conditions. We validated the model via comparison with data from the DEMETER satellite which orbited at a similar altitude, and taking that as an independent source of true electron density values. Comparing our results to the International Reference Ionosphere, we find that our model works better at low to mid‐latitudes, and for quiet and moderately disturbed geomagnetic conditions, but not for highly disturbed conditions. Plain Language Summary: The sun interacts with the outer layers of the Earth's atmosphere. These interactions cause space weather, which like normal weather can be calm or stormy. A severe storm could cause widespread power outages and disrupt communication services. Unfortunately, we cannot predict space weather as well as we can predict normal weather. This work begins to solve this problem by developing a tool that can predict how the sun will impact one outer layer of the Earth's atmosphere, called the ionosphere. The tool combines data that is easier to measure directly and uses it to create these predictions, which we hope to eventually use as a part of an overall space weather forecasting system. Key Points: We present a machine learning model using global geomagnetic index/solar wind parameters to make topside electron density predictions at any locationThe wealth of available satellite data makes this well suited to machine learning, whereas current models of topside ionosphere are lackingThis model has a correlation coefficient of 0.87 on testing data, and can outperform the International Reference Ionosphere at the high altitudes of the topside [ABSTRACT FROM AUTHOR]

During the geomagnetic storm on 3–6 November 2021, the in‐situ plasma density and the gridded total electron content (TEC) maps registered a persistent presence of the strip‐like bulges (electron density enhancement, also known as the shoulders in previous literatures) at lower‐middle latitudes. The observed strip‐like bulge resided in the Pacific‐America‐Atlantic sector and lasted from 07:00 UT on 4 November to 20:00 UT on 6 November. For the first time, the temporal evolution of the 2‐D strip‐like bulge was recorded by gridded TEC maps, though observations were limited over the North American continents. The TEC maps showed a continuous shrinking of the nightside midlatitude plasma band structure right before the presence of the narrow strip‐like bulge. Simultaneous measurements from the Ionospheric Connection Explorer satellite (ICON) revealed an equatorward turning of the field‐aligned ion transportation driven by the disturbance equatorward neutral winds. However, the enhanced fountain effect at low latitudes was not observed during the entire formation phase of the strip‐like bulge. We propose that the storm‐induced enhanced equatorward thermospheric neutral winds push the plasma equatorward along the field lines, and the plasma band structure was developed into the strip‐like bulge. In addition, the ion composition of both the plasma band structure and the strip‐like bulges are dominated by H+, and the maintenance of the strip‐like bulge could be due to the compensation of the downward plasmaspheric content flux. Key Points: The temporal evolution of the 2‐D strip‐like bulge is recorded on gridded total electron content (TEC) mapsThe strip‐like bulge is a consequence of the narrowing midlatitude band structure driven by the disturbance neutral windThe enhanced equatorial fountain effect is not involved during the formation of the strip‐like bulge [ABSTRACT FROM AUTHOR]

The ionosphere experiences strong diurnal and seasonal changes. The longitudinal variations of electron density (Ne) in the ionosphere at the middle latitudes also show strong diurnal and seasonal changes. In this paper, we use in situ Ne measurements from the DEMETER satellite and electron density profiles retrieved from the COSMIC data to study the local time (LT) and seasonal dependence of the longitudinal variations of topside Ne at middle latitudes during 2007–2009. With regard to the diurnal trend, the reversal phase of longitudinal peaks/valleys of topside Ne with a 12 hr interval occurred in less than half of the cases, and there were less cases with eastward phase shift of the longitudinal variations of topside Ne with LT in winter than those in other seasons. The seasonal trends of transition longitudes of topside Ne might be westward from winter to summer and eastward from summer to winter in the daytime and in the opposite direction at night in both hemispheres in some cases and sometimes they were located within 20° of longitude at 52°N in other cases. The longitudinal peaks/valleys of hmF2 and/or NmF2 and the longitudinal peaks/valleys of topside Ne were within 30° of longitude in most cases at all local times, in all seasons, and in both hemispheres. Exceptions to this were independent of season or LT. Key Points: Reversal phase of longitudinal peaks/valleys of topside Ne with a 12 hr interval occurred in less than half of the casesLess cases with eastern phase shift of longitudinal variations of topside Ne during a day in winter than those in other seasonsSeasonal trends of the transition longitudes of topside Ne in the daytime and at night were in the opposite direction in both hemispheres [ABSTRACT FROM AUTHOR]

In‐situ plasma density (Ne) and Over‐Satellite Electron Content (OSEC) measured at Low‐Earth‐Orbit (LEO) are basic parameters for analyzing ionospheric plasma. In this study, we calculate the ratio of OSEC to Ne (OSEC‐to‐Ne ratio; α) measured by Swarm in the daytime topside, which is analogous to the traditional slab thickness of the ionosphere. Dependence of the α on latitude, local time, season, and solar activity generally agrees with the known climatology of vertical scale height (VSH) of the topside ionosphere. Hence, the α can be deemed a proxy for the VSH. Calibrated Swarm‐A and Swarm‐B data at two different altitudes further demonstrate that lower‐altitude Ne and α can reasonably estimate higher‐altitude Ne. In addition, a new method is proposed to estimate effective ion mass from the α and electron/ion temperature measured by Swarm. The estimates are broadly consistent with recent reports on topside ion composition. All these results demonstrate the usefulness of the α in constraining vertical profiles and composition of the topside ionosphere. Caveats for using the α are given, and possible directions of future improvement are discussed. Plain Language Summary: For precise orbit determination and navigation, modern Low‐Earth‐Orbit (LEO) satellites usually carry multiband Global Navigation Satellite System (GNSS) signal receivers. The received signals can be used for deriving Over‐Satellite Electron Content (OSEC) between LEO spacecraft and GNSS satellites, which complements in‐situ plasma density (Ne) measured by dedicated scientific payloads. In this study, we extract additional information from the combination of OSEC and Ne measured by Swarm: the OSEC‐to‐Ne ratio (α). It has a dimension of length and is analogous to the conventional slab thickness, which represents how thick the ionospheric layer is. Climatology of the α generally agrees with previous studies on the vertical scale height, which is another measure of ionospheric thickness. From in‐situ Ne and the α, we can estimate plasma density above the spacecraft. Performance of the estimation is demonstrated by two Swarm satellites at different altitudes. Moving one step further, we propose a method to derive effective ion mass from the α and plasma temperature measurements onboard the same spacecraft. The effective ion mass (or equivalently, ion composition) is broadly consistent with recent reports on direct ion measurements. We also discuss the limitations of our approaches and suggest directions for future improvement. Key Points: Ratio of over‐satellite electron content (OSEC) to in‐situ plasma density (Ne) measured by Swarm, which has a dimension of length, is statistically investigatedClimatology of the OSEC‐to‐Ne ratio generally agrees with previous studies on topsideThe OSEC‐to‐Ne ratio combined with plasma temperature data can give constraints to effective ion mass [ABSTRACT FROM AUTHOR]

The ionospheric scale height is a key parameter that defines the shape of the electron density profile. Given that its direct calculation requires knowledge of the physical and chemical state of the plasma, the concept of the effective scale height was proposed to facilitate its application. It is defined as the scale height that fits the electron density profiles with the α‐Chapman function. Its main characteristics, including the diurnal and altitudinal variation are still under investigation. This paper focuses on the specification of its key characteristics at middle latitudes based on coincident Digisonde and Swarm A satellite data. Based on the assumption that the topside electron density profile is approximated with the α‐Chapman model of variable scale height, the effective scale height at the Swarm A altitude is calculated and compared with the effective scale height at hmF2 $hmF2$. A general conclusion is that the topside effective scale height at ∼450 km exhibits a diurnal variation with highest values in the early morning and evening sectors, while at hmF2 $hmF2$ it gets its maximum values at noon. At night, the effective scale height tends to increase with altitude, however in the daytime, its behavior is more complex. On a local scale, evidence for connection between spread F irregularities and high variability in the topside effective scale height is reported. These preliminary results indicate the potential to further exploit Swarm and Digisonde data for the development of a model for the effective scale height, able to support realistic electron density reconstruction models. Key Points: A clear diurnal pattern is identified in the topside effective scale height with peak values in the early morning and evening sectorsAt night, the topside effective scale height tends to increase with altitude, while in the daytime its altitudinal dependence is complexEvidence for connection between spread F irregularities and high variability in the topside effective scale height is reported [ABSTRACT FROM AUTHOR]

Can geomagnetic storms during low solar activity trigger formation of extreme equatorial plasma bubbles (EPBs) that affect low and midlatitudes? We analyzed the ionospheric response to the 25–26 August 2018 geomagnetic storm and revealed formation of intense ionospheric plasma irregularities over broad latitudinal ranges from equatorial toward middle latitudes in the American and Pacific sectors. Storm‐induced penetration electric fields created favorable conditions for strong fountain effect uplifted the equatorial ionosphere, enhancement of the equatorial ionization anomaly (EIA), and postsunset EPBs generation. We found two patterns of equatorial ionospheric irregularities expansion toward low and middle latitudes: (a) storm‐induced EPBs developed over a large latitudinal extent between widely spread EIA crests, (b) narrow channel of the ionospheric irregularities stretched away from the EPB location toward the auroral zone in the northwestward direction. The EPBs latitudinal extent largely exceeded climatological‐based expectations for solar minimum conditions; EPBs reached atypically high latitudes (20°–25° magnetic latitude [MLAT]) in the Pacific Ocean sector. The poleward‐streaming plasma density depletions were registered along the western coast of North America. The ionospheric irregularities transported in the northwestward direction toward midlatitudes reaching as high as 40°–45° MLAT. The passage of these ionospheric irregularities coincided with the spread‐F conditions recorded at a midlatitude ionosonde (42° MLAT)—rather atypical phenomenon for midlatitudes. It is suggested that enhanced westward drifts associated with prompt penetration and Sub Auroral Polarization Stream electric fields can support the northwestward plasma transportation. Plain Language Summary: The 25–26 August 2018 geomagnetic storm occurred at the background of very low solar activity. For this storm, we registered occurrence of very intense ionospheric plasma irregularities developed in the equatorial region and expanded toward much higher latitudes. We revealed that the storm‐induced equatorial ionospheric irregularities expended toward middle latitudes in two ways: (a) storm‐induced plasma bubbles initially developed with a large latitudinal extent in the uplifted equatorial ionosphere, and (b) poleward part of the largely expanded can be involved into transportation in the northwestward across midlatitudes. The ionospheric irregularities were transported in the northwestward direction toward middle latitudes reaching as high as 40°–45° magnetic latitude (MLAT). The passage of these ionospheric irregularities coincided with the spread‐F conditions recorded at a midlatitude ionosonde (42° MLAT)—it is rather atypical phenomenon for midlatitudes, such scattered echoes are typically observed at nighttime at ionosondes located in the equatorial region. It is suggested that enhanced westward drifts associated with strong storm‐time electric fields can support the northwestward plasma transportation. Key Points: Geomagnetic storm in solar minimum initiated development of equatorial ionospheric irregularities of large latitudinal extent upto 25° magnetic latitude (MLAT)Plasma depletions of equatorial origin transported in northwestward direction across midlatitudes toward auroral zoneAtypical ionospheric irregularities moving toward higher latitudes cause rare spread‐F echoes at a midlatitude ionosonde (42° MLAT) [ABSTRACT FROM AUTHOR]

We investigate an equatorial plasma depletion (EPD) event in Japan near the early morning sector (04:30-7:30 LT) during the recovery phase of a geomagnetic storm occurred on Memorial weekend (May 28) 2017 by using multi-instrumental measurements, including ground-based Global Navigation Satellite Systems (GNSS) network, ionosonde stations, and space-based Swarm and Defense Meteorological Satellite Program (DMSP) missions. The detailed 2-D evolution of the early morning EPD is well observed using GNSS ROTI maps with high temporal-spatial resolution. Moreover, ROTI data from BeiDou Geostationary Earth Orbit satellites are used to detect the EPD signature for the first time. The dawn EPD signature was also observed at the topside ionosphere by in situ electron density (Ne) measurements and ROTI results by space-borne Swarm Alpha (A), Charlie (C), and DMSP F17 satellites, indicating that the equatorial depletions arose to higher altitudes. We suggest that both the eastward disturbance dynamo electric field and overshielding electric field near the early morning sector created a favorable condition for the growth of the Rayleigh-Taylor (R-T) instability, and thus result in the generation of the dawn EPD.

Electron density measured at high latitudes by the Swarm satellites was compared with measurements by the incoherent scatter radars at Resolute Bay and Poker Flat. Overall, the ratio of Swarm-based electron density to that measured by the radars was about 0.5–0.6. Smaller ratios were observed at larger electron densities, usually during the daytime. At low electron densities less than 3 × 1010 m−3, the ratios were typically above 1, indicating an overestimation effect. The overestimation effect was stronger at night and for Swarm B. It was more evident at lower solar activity when the electron densities in the topside ionosphere were lower.

Using the topside electron density (Ne) measurements recorded over Cyprus and Russia, we investigate the latitudinal variation in the topside electron density during the interval 2014–2020, encompassing a period of high-to-low solar activity. The selected topside electron density dataset employed in this study is based on the in situ Langmuir probe data on board the European Space Agency (ESA) Swarm satellites, in the vicinity of the three Digisonde stations in Nicosia (35.14°N, 33.2°E), Moscow (55.5°N, 37.3°E) and Saint Petersburg (60.0°N, 30.7°E). Our investigation demonstrates that the ratio Ne_Swarm/NmF2 between the coincident Ne_Swarm and the Digisonde NmF2 observations is higher than one on various occasions over Nicosia during the nighttime, which is not the case over Moscow and Saint Petersburg, signifying a discrepancy feature of the electron density at Swarm altitudes which depends not only on the solar activity and time of day but also on the latitude.

• First time study on the NWA using in situ measurements. • Discovery of the larger longitude extension of the NWA in mid-latitudes. • Discovery of the NWA in the equatorial regions. The nighttime winter anomaly (NWA) phenomenon is studied using the in situ electron density measurements from the CSES (China Seismo-Electronmagnetic Satellite) satellite orbiting above the F2 layer peak height (hmF2) region. Observations in the present study indicates that: (1) The NWA phenomenon can appear at mid-latitudes in large longitude ranges above the hmF2 region during low solar activity period, with the most obvious part being located at the American longitude sector during northern winter time and at the Asian longitude sector during the southern winter time as mention in previous studies. (2) The NWA phenomenon can also appear in the equatorial region. The equatorial NWA is located at the American to Eurasian longitude sectors during northern winter time and at the eastern Pacific longitude sector during southern winter time, and its occurrence and distribution vary with solar activity, which is similar to that of the mid-latitude NWA. (3) The formation mechanism for the mid-latitude NWA can be explained by the ionosphere-plasmasphere coupling process, and the formation mechanism for the equatorial NWA can be explained by the wind-driven plasma transporting process. These results are helpful for the in-depth understanding of the topside ionosphere, and the in situ electron density measurements from the CSES satellite provide us good opportunities to study the ionospheric climatological phenomena due to its special and unified observation local times. [ABSTRACT FROM AUTHOR]

The ions composition and their densities have been studied for different solar activity periods - along with their diurnal, seasonal and annual variations - for half of the 23rd solar cycle, covering solar minima (1995) to solar maxima (2000) over Indian sector (65-95°E and 5-35°N) at an average altitude of ~500 km. The study has been done by processing the data obtained from in situ measurement made by separate Retarding Potential Analyser (RPA) for electrons and ions, aboard Indian satellite SROSS C2. The plasma density has been found to be rich in O+ ion for all instances of time and showed a direct increase with solar activity. H+ has been observed to be in plenty during night time, especially from moderate to high solar activity period. The difference between H+ and O+ densities widens with increasing value of F10.7. He+ always constitutes a small part of plasma but its density exceeds H+ - during moderate to high solar activity period. O2+ has been found to be a minor constituent, even 3-4 folds lesser than He+ density. A positive correlation with solar activity has been found for O2+. [ABSTRACT FROM AUTHOR]

This paper reports the characteristics of the topside ionospheric O+ diffusive flux (ΦO+) during both geomagnetically quiet (0 ≤ Kp ≤ 2) and moderate (2 < Kp ≤ 4) times using incoherent scatter radar observations at Millstone Hill (42.6°N, 288.5°E) for solar minimum from 1970 to 2018. ΦO+ partially characterizes plasma mass exchange between the upper and lower part of the topside ionosphere through diffusion and sometimes serves as upper boundary conditions for ionosphere‐thermosphere models. The altitude where the flux sign changes (mainly during daytime) is termed the transition height and the time when the flux sign changes (mainly at dawn and dusk) is termed the transition time. At quiet times, the daytime transition height is ∼100 km above the F2 peak height (hmF2) in summer, and it is about 50 km above hmF2 in other seasons; the transition time is before 18 solar local time (SLT) in spring and winter, but after 18 SLT in summer and autumn. The daytime average upward ΦO+ above the transition height shows a significant seasonal variation with a minimum of 2.2×108cm−2s−1 in summer and a maximum of 3.7×108cm−2s−1 in autumn. Under geomagnetically moderate conditions, the transition height increases by ∼20 km in spring, winter, and autumn, but moves up by about 20–50 km in summer. The transition time occurs later by ∼1 hr in summer but ∼1 hr earlier in other seasons. The mean upward ΦO+ peaks in summer and minimizes in spring. Plain Language Summary: The topside ionospheric O+ diffusive flux (ΦO+) is a part of the total plasma flux that is a critical parameter to characterize the plasma exchange between the upper and lower part of the topside ionosphere, but has been poorly known due to lack of direct measurements. Using long‐term incoherent scatter radar (ISR) data observed at Millstone Hill for nearly five decades, this study investigates ΦO+ variations as a function of the altitude above the F2 peak height (hmF2), local time, and season in both geomagnetically quiet and moderate times, and further explores the physical mechanism causing these variations. During quiet times, the summer daytime transition height where ΦO+ changes from downward to upward along magnetic field lines occurs at ∼hmF2+100 km. However, it is usually near hmF2+50 km in other seasons. In addition, the transition time when ΦO+ changes from upward to downward at a fixed height is earlier than 18 SLT in spring and winter, but later than 18 SLT in summer and autumn. We also found that an increase in geomagnetic activity decreases the vertical gradient of plasma density during the daytime in all seasons, resulting in a ∼20 km increase in the transition height and an earlier transition time near dusk in spring, autumn, and winter, but a larger transition height change of about 20–50 km and a later transition time in summer. The change in the seasonal variations of the upward ΦO+ results from the competition between the variations of plasma diffusive velocity and density when the geomagnetic activity increases, as well as the changes in plasma vertical density profiles. Key Points: Topside ionospheric O+ diffusive flux (ΦO+) climatology at solar minimum is derived from incoherent scatter radar data at Millstone HillO+ diffusive flux has seasonal, local time, and altitude dependenceGeomagnetic activity changes the spatiotemporal variations of Φ(O+) by modulating the vertical gradient of the plasma density [ABSTRACT FROM AUTHOR]

The study of electrical currents in the topside ionosphere is of great importance, as it may allow a better understanding of the processes involved in the Sun–Earth interaction and magnetosphere–ionosphere–thermosphere coupling, two crucial aspects debated by the Space Weather scientific community. In this context, investigating the electrical conductivity parallel to the geomagnetic field in the topside ionosphere is of primary importance because: (1) it provides information on the capability of the ionosphere to conduct currents; (2) it relates current density and electric field through Ohm’s law; (3) it can help to quantify the dissipation of currents; (4) it is generally modeled and not locally measured by in situ missions. In this work, we used in situ measurements of electron density and temperature recorded between 2019 and 2021 by the China Seismo-Electromagnetic Satellite (CSES-01) flying with an orbital inclination of 97.4° and at an altitude of about 500 km to compute the parallel electrical conductivity in the topside ionosphere at low and middle latitudes at the two fixed local times (LT) characterizing the CSES-01 mission: around 02 and 14 LT. The results, which are discussed in light of previous literature, highlight the dependence of conductivity on latitude and longitude and are compared with those obtained using values both measured by the Swarm B satellite (flying at a similar altitude) and modeled by the International Reference Ionosphere in the same time period. In particular, we found a diurnal variation in parallel electrical conductivity, with a slight hemispheric asymmetry. Daytime features are compatible with Sq and equatorial electrojet current systems, containing “anomalous” low values of conductivity in correspondence with the South Atlantic region that could be physical in nature.

One approach to determining the height structure of the mid-latitude medium-scale travelling ionospheric disturbances (MSTIDs) above the F layer peak is to involve simultaneous satellite measurements in the ground-based ionosonde measurements near Kazan. The electron concentration data obtained from the Swarm satellite constellation were the most suitable for our purposes. For accurate spatial attachment to satellite data, a strict selection of daytime satellite flythroughs data was used: their trajectory should not be located in latitude further than 100 km from the Kazan's longitude, and the spatial extent of irregularities in electron plasma density measurements should be at least more 100 km without smaller (high-frequency) irregularities. Of all the satellites in the pass es over ~ 2 years (2016–2018), we managed to select only seven such cases. For the cases found, mutual correlation functions of sequences of values of two series of the critical frequency and electron concentration were constructed. The correlation function has a bright negative peak, with a spread within 100 km, which, taking into account the typical horizontal wavelength of the MSTIDs (~ 200 km), means the antiphase behavior of electronic concentrations within the MSTID below and above the peak of the F layer of the daytime mid-latitude ionosphere.

Abstract We use Mars Atmosphere and Volatile EvolutioN observations of the upstream solar wind, and Mars Express observations of ionospheric electron densities and magnetic fields, to study how the topside ionosphere (>320 km) of Mars is affected by variations in solar wind dynamic pressure. We find that high solar wind dynamic pressures result in the topside ionosphere being depleted of plasma at all solar zenith angles, coincident with increased induced magnetic field strengths. The depletion of topside plasma in response to high solar wind dynamic pressures is observed in both weak and strong crustal magnetic field regions. Taken together, our results suggest that high solar wind dynamic pressures lead to ionospheric compression, increased ion escape, and reduced day‐to‐night plasma transport in the high‐altitude nightside ionosphere.

The China Seismo-Electromagnetic Satellite (CSES-01) provides in situ electron density (Ne) observations through Langmuir probes (LPs) in the topside ionosphere since February 2018. CSES-01 is a sun-synchronous satellite probing the ionosphere around two fixed local times (LTs), 14 LT in the daytime sector and 02 LT in the night-time sector, at an altitude of about 500 km. Previous studies evidenced that CSES-01 seems to underestimate Ne measurements with respect to those acquired by similar satellites or obtained from different instruments. To overcome this issue, we calibrated CSES-01 LP Ne observations through Swarm B satellite data, which flies approximately at CSES-01 altitude. As a first step, Swarm B LP Ne observations were calibrated through Faceplate (FP) Ne observations from the same satellite. Such calibration allowed solving the Ne overestimation made by Swarm LP during nighttime for low solar activity. Then, the calibrated Swarm B LP Ne observations were used to calibrate CSES-01 Ne observations on a statistical basis. Finally, the goodness of the proposed calibration procedure was statistically assessed through a comparison with Ne observations by incoherent scatter radars (ISRs) located at Jicamarca, Arecibo, and Millstone Hill. The proposed calibration procedure allowed solving the CSES-01 Ne underestimation issue for both daytime and nighttime sectors and brought CSES-01 Ne observations in agreement with corresponding ones measured by Swarm B, ISRs, and with those modelled by the International Reference Ionosphere (IRI). This is a first fundamental step towards a possible future inclusion of CSES-01 Ne observations in the dataset underlying IRI for the purpose of improving the description of the topside ionosphere made by IRI.

The topside ionosphere accounts for a dominant part of the ionospheric total electron content, whereas accurate global modeling of topside ionospheric electron density (Ne) profile has not been fully achieved. In this study, a high precision Ne profile model, named α‐Chapman Based Electron Density Profile Model (α‐Chapman‐Based‐EDP), was built by using ∼4.5 million Ne profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC‐1) radio occultations. We first describe each of the profiles using five parameters of the α‐Chapman function, that is, peak density (NmF2) and height (hmF2) of F2 layer, scale height (Hm) as well as its altitude change rates, and then built a model for each of the parameters as a function of latitude, longitude, month, local time, and solar activity, through Empirical orthogonal function (EOF) analysis and Fourier expansion. Combining all the five models, we construct the α‐Chapman‐Based‐EDP. Compared with observations from COSMIC‐1 and ‐2, the model captures the ionospheric climatology well, such as solar activity dependence, seasonal variation, and spatial pattern, including the equatorial ionization anomaly and midlatitude trough as well as their variabilities. Our model can describe nearly 80% variability of Ne in F region. In contrast, the IRI2016 cannot well reproduce these characteristics, with errors higher than our model. The potential applications of our model were also discussed. A dense matrix data calculated by the model will be released in https://www.researchgate.net/profile/Qiaoling%5fLi5 with the permissions of COSMIC organizations. Key Points: A global model of Ne profile in F region ionosphere was constructed based on the α‐Chapman functionThe model gives three‐dimensional Ne as well as five key parameters of Ne profile, including NmF2, hmF2, Hm, and its change rates with heightThe model reasonably reproduces the equatorial ionization anomaly and midlatitude trough [ABSTRACT FROM AUTHOR]

• Comparison of Swarm observations with Digisonde and IRI estimations. • Comparison of COSMIC RO observations, Digisonde and IRI estimations. • Comparison of COSMIC, Digisonde and IRI Ne profile for the matched peak. We examine the systematic differences between topside electron density measurements recorded by different techniques over the low-middle latitude operating European station in Nicosia, Cyprus (geographical coordinates: 35.14oN, 33.2oE), (magnetic coordinates 31.86oN, 111.83 oE). These techniques include space-based in-situ data by Langmuir probes on board. European Space Agency (ESA) Swarm satellites, radio occultation measurements on board low Earth orbit (LEO) satellites from the COSMIC/FORMOSAT-3 mission and ground-based extrapolated topside electron density profiles from manually scaled ionograms. The measurements are also compared with International Reference Ionosphere Model (IRI-2016) topside estimations and IRI-corrected NeQuick topside formulation (method proposed by Pezzopane and Pignalberi (2019)). The comparison of Swarm and COSMIC observations with digisonde and IRI estimations verifies that in the majority of cases digisonde underestimates while IRI overestimates Swarm observations but in general, IRI provides a better topside representation than the digisonde. For COSMIC and digisonde profiles matched at the F layer peak the digisonde systematically underestimates topside COSMIC electron density values and the relative difference between COSMIC and digisonde increases with altitude (above hm F2), while IRI overestimates the topside COSMIC electron density but after a certain altitude (~150 km above hm F2) this overestimation starts to decrease with altitude. The IRI-corrected NeQuick underestimates the majority of topside COSMIC electron density profiles and relative difference is lower up to approximately 100 km (above the hm F2) and then it increases. The overall performance of IRI-corrected NeQuick improves with respect to IRI and digisonde. [ABSTRACT FROM AUTHOR]

The modeling of topside ionospheric electron density using height‐varying scale height methods have received increasing attention. The modified Chapman function with a variable scale height, known as Vary‐Chap function, can well represent the topside electron density profiles, and is considered as a promising direction to improve the International Reference Ionosphere model. In order to facilitate modeling, a new formula with four unknown coefficients is proposed to fit the Vary‐Chap scale height, and these unknowns can be determined by the least squares technique. The approximated electron density profile is reconstructed by using the fitted scale height, and its average percentage error relative to the measured electron density profile is quantitatively analyzed by using the Alouette/ISIS topside sounder data sets and COSMIC radio occultation databases, and the accuracies of five other approaches (i.e., Nava et al., 2008, https://doi.org/10.1016/j.jastp.2008.01.015; Nusmei et al., 2012, https://doi.org/10.1029/2012RS004989; Porl et al., 2019, https://doi.org/10.1007/s10712-019-09521-3; Reinisch et al., 2007, https://doi.org/10.1016/j.asr.2006.05.032; Wu et al., 2020, https://doi.org/10.1029/2019JA027637) to approximate the topside electron density profiles are also compared. From the perspective of average percentage error, our formula performs well to reconstruct the topside electron density profiles, especially at the altitudes between hmF2 and hmF2 + 1,200 km.Key Points: A new formula with four unknowns is proposed to fit the Vary‐Chap scale height profileThe accuracies of five other methods to approximate the topside ionospheric electron density profile are evaluatedOur formula has a high accuracy to reconstruct the topside electron density profiles, especially below the altitude of hmF2 + 1,200 km [ABSTRACT FROM AUTHOR]