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Using two separate databases of in situ ionospheric observations, we present case studies and perform a statistical investigation of the link between energetic precipitating particles during the polar night and high-latitude F-region steepening density spectra. Our study covers approximately 3 years of data obtained near the peak of the 24th solar cycle from four Defense Meteorological Satellite Program satellites and from the European Space Agency's Swarm satellites. Focusing on the midnight sector of the auroral oval, we found that there is a near-perfect co-location between high-energy precipitating particles and occurrence of dissipating F-region plasma density spectra. This is because the precipitating energy flux strongly enhances the E-region Pedersen conductivity, allowing fast and efficient dissipation of kilometer-scale F-region irregularities. Spectra that are possibly non-dissipating are in turn co-located with the distribution of soft electron precipitation. Together, dissipating and non-dissipating density spectra constitute two distinct irregularity regimes. Surprisingly, we also found that efficient dissipation notwithstanding, high-energy precipitating particles cause a net increase in the F-region irregularity power, suggesting that growth and dissipation are interlinked and that some of the observed F-region irregularities may conceivably be generated in the E region. This work is expected to be beneficial for the classification of F-region in situ density spectra and suggests that such density spectra can be used to infer the presence of high-energy or low-energy precipitations based on spectral properties. [ABSTRACT FROM AUTHOR]

Using a large data set of ground‐based GNSS scintillation observations coupled with in situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed occurrence of density irregularities in the northern hemisphere cusp. By examining trends in the two data sets relating to geomagnetic activity, we conclude that observations of irregularities in the cusp grows increasingly likely during storm‐time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a previously documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread, as we demonstrate. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric "irregularity landscape." As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way. Plain Language Summary: The cusp ionosphere is a particularly turbulent part of the interaction between Earth and the solar wind. Here, magnetic field lines connect the ionosphere directly to the solar wind, and a range of plasma physical mechanisms can studied. "Soft" (low‐energy) electrons raining into Earth's atmosphere inside the cusp has been suggested several times in the past as an important contributor to the various energetic phenomena found in the cusp, but little evidence exists that can tell us how important they are. We present a large statistical study of the cusp's input energy (the soft electrons) and how it affects the cusp's turbulent output (widespread plasma irregularities). We find a discrepancy in that soft electrons largely exhibit opposite long‐term trends compared to the cusp plasma turbulence. Put simplistically, the energy budget provided by soft electrons might not be sufficient to explain the enormous energy that are at times pent up in the ionospheric cusp plasma turbulence. These results point to the fact that the community needs to address the role of particle precipitation (electrons, but also ions) in producing the observed ionosphere "irregularity landscape." Key Points: Cusp scintillation occurrence increases significantly with rising geomagnetic activityThe energy flux of soft electron precipitation in the cusp has a statistical tendency to decrease as geomagnetic activity increasesThe increase in cusp scintillation with geomagnetic activity must be caused by drivers other that soft electron precipitation [ABSTRACT FROM AUTHOR]

It has been a concern of space scientists and engineers for many decades to have a well-defined proxy to characterize the fluctuations experienced by radio-based signals propagating through the ionosphere. Scintillation indices were deduced using the signal intensity to show the level of fluctuations in phase and amplitude of radio signal passing through the ionosphere. Subsequently, more than two decades now, the rate of change of Total Electron Content (TEC) index (ROTI) was as well used to characterize the level of fluctuations associated to ionospheric irregularities on radio signals. This paper examines the level of relationship between the rate of change of TEC index (ROTI) and the ionospheric amplitude scintillation (S4) within the belt of equatorial ionization anomaly of West African sector. The study was carried out using the Scintillation Network Decision Aid (SCINDA) Global Positioning System (GPS) data obtained from ground-based stations located at Dakar (Senegal) and Cape Verde. The comparative analysis of ROTI and S4 was done with normalized ROTI. The results obtained have shown that the magnitude of normalized ROTI and S4 corresponds very well, higher in Cape Verde compared to Dakar. It was observed that the correlation coefficient could be ∼0.6 when all the satellites are used. However, the relationship increases with individual satellite in which the coefficient of correlation could be up to ∼0.97 and R2 ∼ 0.94. It can be concluded that the relationship between the S4 and ROTI depends on the location and the level of ionospheric disturbance condition as observed in this work. [ABSTRACT FROM AUTHOR]

We develop a climatological model for the Arctic ionosphere based on more than one solar cycle of data (2010–2021) of the rate of change of the total electron content index (ROTI) maps from the International GNSS Service (IGS). The IGS ROTI maps are daily averaged in magnetic latitude and local time coordinates. To develop a climatological model, the ROTI maps are decomposed into base functions and coefficients using the empirical orthogonal function (EOF) method. The EOF method converges very quickly, and the first four EOFs reflect the majority (96%) of the total data variability. Furthermore, different EOF components can reflect different drivers of ionospheric irregularities. The first EOF reflects the averaged ROTI activity and the impact of the solar radiation and geomagnetic activity; the second EOF reflects the impact of the interplanetary magnetic field (IMF) Bz and electric field; the third and fourth EOFs reflect the dawn‐dusk asymmetry around the auroral oval and polar cap, and they can be related to the IMF By. To build an empirical model, we fit the EOF coefficients using helio‐geophysical indices from four different categories (solar activity; geomagnetic indices; IMF; the solar wind coupling function). The final EOF model is dependent on seven selected indices (F10.7P, Kp, Dst, Bt, By, Bz, and EKL). The statistical data‐model comparisons show satisfactory results with a good correlation coefficient. However, the model cannot capture the significant expansion of the dayside ROTI activity during strong geomagnetic storms. Future effort is needed to provide corrections to the model during severe storms. Plain Language Summary: The ionosphere is often highly structured and contains significant irregularities of plasma density, which can impact the Global Navigation Satellite System (GNSS) services that rely on trans‐ionospheric radio waves. Due to the practical use of GNSS services, there is a high demand for modeling and forecasting of ionospheric irregularities. In this study, we develop a climatological model for the Arctic ionosphere based on a long‐term data set from the International GNSS Service (IGS). The final model is dependent on a few helio‐geophysical indices that represent the main drivers/sources of ionospheric irregularities. The statistical data‐model comparisons show that the model is able to reproduce all the climatological features of ionospheric irregularities. This model can be useful in forecasting the space weather impact for GNSS users using real‐time data as input. Key Points: An empirical model of rate of change of the total electron content index (ROTI) maps in the Arctic is developed based on 12 years of ROTI data using the empirical orthogonal function methodThe statistical data‐model comparisons show satisfactory resultsThe model fails to capture the significant expansion of the dayside ROTI activity during severe geomagnetic storms [ABSTRACT FROM AUTHOR]

The oblique radio wave incidence on a turbulent equatorial conductive collision plasma layer is considered. The "Compensation Effect" has been discovered by us. A complex refractive index of the equatorial terrestrial ionosphere has been derived for the first time. Second-order statistical moments of the spatial power spectrum (SPS) of scattered radio waves are obtained for the first time using the WKB method, taking into account the asymmetry of the problem: the inclined incidence of the wave on a plasma boundary and the asymmetry of the magneto-ionic parameters. It was established for the first time that a certain direction exists along which the inclined incidence radio wave on a plasma layer and the anisotropy parameters of a magnetoplasma compensate each other. This result will have great practical application in communication. In this case, the SPS of scattered radio waves neither widens nor is its maximum displaced. The behavior of this spectrum versus distance propagated by radio waves in the conductive equatorial ionosphere is analyzed numerically for different penetration angles and anisotropy factors of asymmetric anisotropy electron density irregularities. It was shown that the anisotropy factor of elongated plasmonic structures has a substantial influence on the "Compensation Effect" of scattered ordinary and extraordinary waves penetrating in the conductive collision ionospheric plasma the slab. Numerical calculations are carried out for the anisotropic Gaussian correlation function applying IRI experimental data. [ABSTRACT FROM AUTHOR]

This article is an editorial that discusses the vertical coupling in the atmosphere-ionosphere-magnetosphere system. The Earth's ionosphere, which is a plasma layer in the upper atmosphere, plays a role in various phenomena and also affects technological systems. The ionosphere is influenced by the solar wind and the Earth's magnetosphere, as well as the neutral atmosphere below it. The article presents various studies that explore the interactions and dynamics within this system, including the impact of solar wind on atmospheric gravity waves, the generation of auroral spirals, and the effects of geomagnetic activity on the ionosphere. The authors emphasize the need for further research to understand the complex interactions and variability within the atmosphere-ionosphere-magnetosphere system. [Extracted from the article]

In the article action of the anisotropic irregularities in an ionosphere on a satellite channel is investi- gated. Relations between the exponent, characterising depth of fadings in a communication channel, with sta- tistical performances of the aniisotropic irregularities in an ionosphere are spotted.

The response of the ionosphere in the Indian region to the extreme space weather events of 17–19 March 2015 is studied using Global Positioning System (GPS) Total electron Content (TEC) measurements, Thermosphere Ionosphere Electrodynamics-General Circulation Model (TIE-GCM) simulations and High Frequency (HF) Radar observations. The data from Thiruvananthapuram (8.5° N, 77° E geographic, 0.3° N magnetic latitude) and northern hemispheric midlatitude station Hanle (32.80° N, 79° E geographic, 23.9° N magnetic latitude) have been analyzed for the above event. The TEC over Thiruvananthapuram, a dip equatorial location exhibits a decrease during the post sunset hours on 17 March 2015 and is followed by the generation of F region irregularities at an altitude of 600 km, as observed by HF radar as well as TEC scintillations. The TEC over the mid latitude station of Hanle decreased during the storm recovery phase on 18March 2015 in response to negative storm effects. The extent of decrease in TEC as observed during the recovery phase could not be simulated by TIE-GCM over Hanle. This difference is attributed to the enhanced Joule heating taking place in the polar region during St. Patrick's Day storm and the associated composition changes. An anomalous heating scenario was introduced and the simulated TIE-GCM results were found to match the observations. The present study clearly brings out the impact of 17 March 2015, St. Patrick's Day geomagnetic storm on the ionization distribution over equatorial as well as mid latitude stations over the Indian region using observations as well as TIE-GCM simulations and the evolutionary features of ionospheric irregularities over equatorial location using TEC and HF radar observations. The role played by thermosphere ionosphere coupling in modulating the ionization variability over equatorial as well as mid latitude region during periods of extreme geomagnetic forcing is highlighted. [ABSTRACT FROM AUTHOR]

This paper uses a regional simulation of plasma convective instability in the postsunset equatorial ionosphere together with a global atmosphere/ionosphere/plasmasphere GCM (WAM‐IPE) to forecast irregularities associated with equatorial spread F (ESF) for 1–2 hr after sunset. First, the regional simulation is initialized and forced using ionosphere state parameters derived from campaign data from the Jicamarca Radio Observatory and from empirical models. The irregularities produced by these simulations are found to be quantitatively similar to those observed. Next, the aforementioned state parameters are replaced with parameters from WAM‐IPE, and the resulting departures between the simulated and observed irregularities are noted. In one of five cases, the forecast failed to accurately predict ESF irregularities due to the late reversal of the zonal thermospheric winds. In four of five cases, significant differences between the observed and predicted prereversal enhancement (PRE) of the background vertical drifts resulted in degraded forecast accuracy. This highlights the need for improved PRE forecasting in the global‐scale model. Key Points: Equatorial ionospheric F‐region stability can be reproduced quantitatively using a regional data‐driven direct numerical simulation (DNS)DNS model runs become forecasts when initialized and driven with parameters from a comprehensive GCM such as Whole Atmosphere Model with Ionosphere, Plasmasphere, ElectrodynamicThe post‐sunset background vertical plasma drift and zonal thermospheric wind profile are the most critical forecast parameters [ABSTRACT FROM AUTHOR]

Coherent backscatter radar observations made at the Jicamarca Radio Observatory (JRO) have contributed significantly to our understanding of equatorial F-region irregularities. Radar observations, however, have been made predominantly at the Very-High Frequency (VHF) band (50 MHz), which corresponds to measurements of 3-m field-aligned irregularities. The deployment of the 14-panel version of the Advanced Modular Incoherent Scatter Radar (AMISR-14) at Jicamarca provided an opportunity for observations of Ultra-High Frequency (UHF - 445 MHz) echoes which correspond to measurements of irregularities with 0.34 m scale sizes. Here, we present what we believe to be the first report describing the quiet-time climatology of sub-meter equatorial F-region irregularities derived from UHF radar measurements. The measurements were made between August 2021 and February 2023 using a 10-beam AMISR-14 mode that scanned the F-region in the magnetic equatorial plane. The results show how F-region sub-meter irregularities respond to variations in season and solar flux conditions. The results also confirm, experimentally, that the occurrence of UHF F-region echoes is controlled by the occurrence of equatorial spread F (ESF). Higher occurrence rates were observed during pre-midnight hours and during Equinox and December solstice. Reduced occurrence rates were observed during June solstice. The results show that an increase in solar flux was followed by an increase in the altitude where noticeable occurrence rates (≳ 10%) start and in the maximum altitude of these occurrence rates. The observations also show that occurrence rates lasted longer (in local time) during low solar flux conditions. Comparisons with collocated VHF radar observations showed that, despite differences in radar parameters, observation days, and the scale size (one order of magnitude) of the scattering irregularities, the two systems show similar climatological variations with only minor differences in the absolute occurrence rates. Finally, the analysis of the occurrence rates for different beams did not show substantial climatological variations over local (within a few 100s of km) zonal distances around JRO. We point out, however, that observations on a single day can show strong local variations in echo detection and intensity within the AMISR-14 field of view due to the intrinsic development and decay of ESF structures. • We report for the first time the climatology of nighttime F-region echoes observed by a UHF radar at the magnetic equator. • We present the response of the occurrence rate of UHF echoes to variations in season and solar flux that are related to ESF. • We show that the climatology of irregularities from AMISR-14 is similar to that observed by the collocated VHF JULIA radar. • Analysis of the 10-beam observations confirms that the ESF echo climatology does not change within local zonal distances. [ABSTRACT FROM AUTHOR]

Six hundred hours of data from a receiver located at the Cape Verde Atmospheric Observatory at 15°N (dip latitude), has been used to explore the fading correlation of 300–360 MHz trans‐ionospheric signals from the MUOS satellite. Using these data, we have highlighted that the inter‐frequency correlation varies with the fading frequency; components at frequencies close to the Fresnel frequency tend to be well correlated over bandwidths between 15 MHz and greater than 20 MHz, but those at higher fading frequencies are only well correlated over bandwidths between 0.1 and 5 MHz at a correlation threshold of 0.7. When considered over all fading frequencies, flat fading is far more common than frequency selective fading, such that when the frequency separation is 5 MHz and when S4 lies between 0.7 and 0.8, the ratio is ∼16:1, when the separation is 10 MHz the ratio is ∼9:1 and when the separation is 15 MHz it is ∼7:1. Together, the results in this paper suggest that flat fading is the dominant fading mechanism for satellite communication systems, with bandwidths up to 15 MHz, operating in the high VHF and low UHF bands in the equatorial region. At still higher operating bandwidths we expect frequency selective fading to become dominant as the differentially delayed multipath components, occurring via Fresnel scale irregularities, cause destructive and constructive interference. Key Points: Fading close to the Fresnel frequency is well correlated over greater bandwidths than higher frequency fading componentsOver all fading frequencies, flat fading is far more common than frequency selective fading, for signal bandwidths less than 15 MHzFlat fading is the dominant fading mechanism for high VHF and low UHF equatorial satellite communication systems [ABSTRACT FROM AUTHOR]

Spread‐F (SF) is a feature that can be visually observed on ionograms when the ionosonde signals are significantly impacted by plasma irregularities in the ionosphere. Depending on the scale of the plasma irregularities, radio waves of different frequencies are impacted differently when the signals pass through the ionosphere. An automated method for detecting SF in ionograms is presented in this study. Through detecting the existence of SF in ionograms, we can help identify instances of plasma irregularities that are potentially affecting the high‐frequency radio‐wave systems. The ionogram images from Jicamarca observatory in Peru, during the years 2008–2019, are used in this study. Three machine learning approaches have been carried out: supervised learning using Support Vector Machines, and two neural network‐based learning methods: autoencoder and transfer learning. Of these three methods, the transfer learning approach, which uses convolutional neural network architectures, demonstrates the best performance. The best existing architecture that is suitable for this problem appears to be the ResNet50. With respect to the training epoch number, the ResNet50 showed the greatest change in the metric values for the key metrics that we were tracking. Furthermore, on a test set of 2050 ionograms, the model based on the ResNet50 architecture provides an accuracy of 89%, recall of 87%, precision of 95%, as well as Area Under the Curve of 96%. The work also provides a labeled data set of around 28,000 ionograms, which is extremely useful for the community for future machine learning studies. Key Points: Adopt machine learning techniques to detect spread‐F in ionogramsMake an annotated data set of around 28,000 ionograms publicly available [ABSTRACT FROM AUTHOR]

Ionospheric plasma irregularities can be successfully studied with the Swarm satellites. Parameters derived from the in‐situ plasma measurements and from the topside ionosphere total electron content provide a comprehensive dataset for characterizing plasma structuring along the orbits of the Swarm satellites. The Ionospheric Plasma IRregularities (IPIR) data product summarizes these parameters and allows for systematic studies of ionospheric irregularities. IPIR has already been used in investigations of structuring and variability of ionospheric plasma. This report provides a detailed description of algorithms behind the IPIR data product and demonstrates its use for ionospheric studies. Key Points: Ionospheric Plasma IRregularities (IPIR) is a new Swarm data product that characterizes ionospheric plasma irregularitiesPlasma irregularity parameters are assigned to different geomagnetic regionsIPIR allows for both detailed case studies and global statistical studies of ionospheric plasma variability [ABSTRACT FROM AUTHOR]

The presence of large and small-scale irregularities TID of the ionosphere in the path of radio wave propagation from the navigation satellite to the GPS receiver is the main challenge for the application of ionospheric research to improve positioning accuracy using single-frequency receivers. In this paper, in contrast to the traditional methods of compensating for the error associated with the ionospheric delay (the Klobuchar method and the use of TEC values), we propose the calculation of the ionospheric delay by the ray tracing method in a model that is as close as possible to the real conditions. To achieve this proximity, a model is proposed that includes various types of TID irregularities. The corresponding algorithms are presented and the numerical results are given. It is shown that single large-scale TIDs can lead to errors of 10–30 m, errors due to small-scale TIDs do not exceed 2 m. © 2001 Elsevier Science. All rights reserved. • Positioning systems are highly dependent on the state of the ionosphere. • Ionospheric delay is a major source of positioning errors. • The greatest influence is exerted by irregularities. • A method is proposed for direct calculation of delays in a model with irregularities. • The core effect is influence of single large-scale irregularities. [ABSTRACT FROM AUTHOR]

Abstract We report the proposal and results of a low‐cost, easy‐to‐build GPS‐based sensor for detection and monitoring ionospheric irregularities through the detection of amplitude scintillation. The system is based on the Raspberry Pi single‐board computer combined with an Adafruit Ultimate GPS peripheral, which is capable of measuring (at 10‐Hz rate) the intensity of the L1 signals transmitted by GPS satellites. We introduce and discuss results of short‐ and long‐term observations obtained with a prototype of this system deployed in Presidente Prudente, a low magnetic latitude site in Brazil. The deployment and observations were carried out to test the ability of the system to detect ionospheric scintillations and, therefore, monitor the occurrence of ionospheric irregularities associated with equatorial spread F. Our results show that this low‐cost sensor is indeed capable of detecting scintillation events associated with equatorial spread F. Comparison with simultaneous, collocated measurements made by a commercial scintillation monitor are also presented. The joint observations allowed us to quantify the performance of the low‐cost monitor and to identify sources of potential limitations. While the sensor cannot (and it was not intended to) substitute commercial scintillation monitors, the low cost allows its use in studies of ionospheric irregularities (space weather) that require observations made by distributed arrays of small instruments (DASI). Furthermore, the simplicity of the sensor design stimulates its use in educational and citizen science initiatives.

Abstract While low and high‐latitude ionospheric scintillation have been extensively reported, significantly less information is available about the properties of and conditions leading to mid‐latitude scintillations. Here, we report and discuss scintillation observations made in the Southern United States (UT Dallas, 32.99°N, 96.76°W, 43.2°N dip latitude) on June 1st, 2013. The measurements were made by a specialized dual‐frequency GPS‐based scintillation monitor which allowed us to determine main properties of this mid‐latitude scintillation event. Additionally, simultaneous airglow observations and ionospheric total electron content (TEC) maps provided insight on the conditions leading to observed scintillations. Moderate amplitude scintillations (S4>∼0.4) occurred in both L1 and L2C signals, and severe (S4 > ∼0.8) events occurred in L2C signals at low (

The midlatitude ionospheric trough (MIT) is a well‐known feature in the topside ionosphere. Previous MIT related studies mainly focused on the MIT structure itself, while few studies focused on irregularities inside the MIT region. In this study, we provided for the first time the magnetic latitude and magnetic local time distribution of irregularities within the MIT, by using the Swarm in situ plasma measurements. Orbital cases show that both the MIT equatorward and poleward walls are embedded with small‐scale irregularities. Further statistical analysis reveals that the particle precipitation is the dominant mechanism for the dayside MIT region irregularities, while nightside MIT region irregularities are strongly related with the temperature gradient drift instability. The Swarm observations show that the temperature gradient drift instability occur not only in MIT equatorward wall but also in MIT polarward wall, which is different to the theoretical prediction of Hudson and Kelley (1976, https://doi.org/10.1029/JA081i022p03913). Plain Language Summary: Early theoretical studies have pointed out that there are several instabilities for generating plasma density irregularities on both the equatorward and poleward sides of the midlatitude trough. However, few observational studies were performed to focus on the irregularities in the trough region, possibly due to the relatively low‐resolution in situ plasma density measurements of previous missions. The high‐resolution plasma and magnetic measurements from the Swarm satellites provide an excellent data set for investigating the irregularities in the midlatitude trough region. Our results confirm the existence of plasma irregularities frequently observed in the walls of the MIT. Statistical analysis indicate that the dayside MIT region irregularities occur much more frequently than that on the nightside, which is mainly due to the dayside MIT overlapping with the auroral active area, while the nightside MIT is located outside the auroral oval. Study on the diurnal variation of the MIT region irregularities suggests that the dayside irregularities are mainly caused by the auroral particle precipitations, while the nightside irregularities are related to the temperature gradient drift instability proposed by Hudson and Kelley (1976, https://doi.org/10.1029/JA081i022p03913). Key Points: The Swarm in situ Ne and Te measurements are used for the first time to investigate the plasma irregularities inside the MITThe auroral particle precipitations dominate the dayside MIT region irregularitiesThe temperature gradient drift instability contributes mostly to the nightside MIT region irregularities [ABSTRACT FROM AUTHOR]

While low and high‐latitude ionospheric scintillation have been extensively reported, significantly less information is available about the properties of and conditions leading to mid‐latitude scintillations. Here, we report and discuss scintillation observations made in the Southern United States (UT Dallas, 32.99°N, 96.76°W, 43.2°N dip latitude) on June 1st, 2013. The measurements were made by a specialized dual‐frequency GPS‐based scintillation monitor which allowed us to determine main properties of this mid‐latitude scintillation event. Additionally, simultaneous airglow observations and ionospheric total electron content (TEC) maps provided insight on the conditions leading to observed scintillations. Moderate amplitude scintillations (S4>∼0.4) occurred in both L1 and L2C signals, and severe (S4 > ∼0.8) events occurred in L2C signals at low (<30°) elevation angles. Phase scintillation accompanied amplitude fadings, with maximum σϕ values exceeding 0.5 radians in L2C. We also show that the observed phase scintillation magnitudes increased with amplitude scintillation severity. Decorrelation times were mostly between 0.25 and 1.25 s, with mean value around 0.65 s for both L1 and L2C. Frequency scaling of S4 matched fairly well the predictions of weak scattering theory but held for observations of moderate and strong amplitude scintillation as well. Scintillation occurred during the main phase of a modest magnetic storm that, nevertheless, prompted an extreme equatorward movement of the mid‐latitude trough and large background TEC enhancements over the US. Scintillations, however, occurred within TEC and airglow depletions observed over Texas. Finally, scintillation properties including severity and rapidity, and associated TEC signatures are comparable to those associated with equatorial spread F. Plain Language Summary: The ionosphere is a region of the upper atmosphere characterized by a relatively large density of free ions and electrons created, in most part, by solar photoionization. Spatial and temporal variations in the ionospheric electron density cause the diffraction of radio waves such as those used by GPS. The diffraction of the radio waves, as a result, cause fluctuations in the amplitude and/or phase of signal measured by a ground‐based receiver, for instance. These fluctuations are referred to as ionospheric scintillations. Ionospheric scintillations are commonly observed at low and high latitudes, where ionospheric irregularities are known to develop very frequently. Here, we report detailed observations of an ionospheric scintillation event that occurred on June 1st, 2013 over mid‐latitudes (Southern United States) where scintillations are thought to occur very rarely. We were able to capture the event with a specialized scintillation monitor during an educational project. This fortunate observation provided details about the properties (severy, rapidity, frequency scaling, etc.) of the observed mid‐latitude scintillations. The report provides new information about mid‐latitude scintillation that is of interest to a better fundamental understanding of ionospheric irregularities and their impact on signals in the frequency band used by global navigation satellite systems (GPS, Galileo, BeiDou, etc.). Key Points: We present and discuss the detection of L‐Band ionospheric scintillations that occurred over southern United StatesThe severity and temporal properties of the observed scintillations are similar to those of events associated with equatorial plasma bubblesA modest geomagnetic storm prompted extreme background ionospheric conditions and perturbations leading to the observed scintillations [ABSTRACT FROM AUTHOR]

Abstract Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E‐layer during the daytime which counteracts the development of F‐layer plasma irregularities. This study presents two rare daytime ESF‐like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso‐heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillations lasted nearly 3 h. Close inspection of the ionograms from ILR during this interval (1500–1800 UT) showed the occurrence of small‐scale spreading in the F‐layer trace which varied in altitude as the disturbance progressed. Computation of the linear growth rate of the collisional Rayleigh‐Taylor instability showed that the plasma instability was seeded by a traveling ionospheric disturbance (TID). The characteristics of the second event suggest that horizontal stratifications in plasma density distribution at the reflecting ionospheric layer were responsible for the spread F traces in the ionograms. Analysis of GPS TEC data from Nigeria during these events revealed the presence of wave structures consistent with TIDs.

Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E‐layer during the daytime which counteracts the development of F‐layer plasma irregularities. This study presents two rare daytime ESF‐like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso‐heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillations lasted nearly 3 h. Close inspection of the ionograms from ILR during this interval (1500–1800 UT) showed the occurrence of small‐scale spreading in the F‐layer trace which varied in altitude as the disturbance progressed. Computation of the linear growth rate of the collisional Rayleigh‐Taylor instability showed that the plasma instability was seeded by a traveling ionospheric disturbance (TID). The characteristics of the second event suggest that horizontal stratifications in plasma density distribution at the reflecting ionospheric layer were responsible for the spread F traces in the ionograms. Analysis of GPS TEC data from Nigeria during these events revealed the presence of wave structures consistent with TIDs. Key Points: Analysis of Digisonde, High Frequency Doppler receiver, and Global Navigation Satellite System data was undertakenUse of low‐cost High Frequency Doppler equipment for reliable diagnostic of the equatorial ionosphere is demonstratedDaytime equatorial spread F is associated with traveling ionospheric disturbances [ABSTRACT FROM AUTHOR]

We report on simultaneous observation of artificial plasma density irregularities near the magnetic zenith (MZ) by incoherent scatter radar and GNSS satellite in the high latitude. During an EISCAT (European Incoherent Scatter Scientific Association) HF heating experiment, a GLONASS satellite signal intersected the disturbed ionospheric volume along the local magnetic field lines. The satellite signal amplitude and phase were simultaneously perturbed when the electron temperature increased in the F region through O‐mode HF waves. The field‐aligned irregularities (FAIs) and associated density perturbations are most significantly found in the MZ direction. The growth of FAI reached the saturation level in 30 s while large‐scale electron density perturbation on the order of 0.1 TECU developed in a few minutes. The observed density perturbations agree well with recent numerical studies of FAI generation due to the thermal self‐focusing process. Plain Language Summary: Powerful high‐frequency radio waves are used to study electron heating process in the high latitude ionosphere. We observed the development of plasma density structures by analyzing small changes of amplitude and phase of ground‐based satellite signals and incoherent scatter radar measurements. It is concluded that the electron heating causes irregular electron density structures along local magnetic field lines. This experiment shows that coordinated ground‐based satellite and incoherent scatter radar measurements essentially help to better understand the physics of ionospheric plasma dynamics and radio wave propagation. Key Points: HF‐induced field‐aligned plasma irregularities are simultaneously observed by GLONASS satellite and incoherent scatter radarThe satellite signal amplitude fluctuation and TEC deviation are generally correlatedObserved positive density deviations suggest that the density perturbations are likely caused by the thermal self‐focusing process [ABSTRACT FROM AUTHOR]

We present an investigation of polar cap plasma structure lifetimes. We analyze both simulated data from ionospheric models (International Reference Ionosphere model and Mass Spectrometer Incoherent Scatter model) and in situ data from the Swarm satellite mission (the 16 Hz Advanced Plasma Density dataset). We find that the theoretical prediction that E‐region conductance is a predictor of F‐region polar cap plasma structure lifetimes is indeed supported by both in situ‐based observations and by ionospheric models. In situ plasma structure lifetimes correlate well with the ratio of F‐ to E‐region conductance. We present explicit predictions of small scale (∼1 km) structure lifetimes, which range from less than 1 h during local summer to around 3 h during local winter. We highlight a large discrepancy between the observational and theoretical scale‐dependency of decay due to diffusion.Key Points: The ratio of E‐region to F‐region conductance is an accurate predictor of F‐region polar cap plasma diffusionThe observed scale‐dependency of the structure lifetimes deviates strongly from the theoretical predictionsF‐region small‐scale plasma structure lifetimes in the central polar caps range from 1 to 3 h [ABSTRACT FROM AUTHOR]

The formation of polar cap density enhancements, such as tongues‐of‐ionization (TOIs), are often attributed to enhanced dayside reconnection and convection due to solar wind changes. However, ionospheric poleward moving density enhancements can also form in the absence of changes in the solar wind. This study examines how TOI and patch events that are not triggered by solar wind changes relate to magnetospheric processes, specifically substorms. Based on total electron content and Super Dual Auroral Radar Network (SuperDARN) observations, we find substorms that occur at the same time as TOIs are associated with sudden enhancements in dayside poleward flows during the substorm expansion phase. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations also show enhanced field‐aligned currents (FACs) that extend into the dayside ionosphere during this period. We suggest that the global enhancement of FACs and convection during these substorms are the drivers of these TOIs by enhancing dayside convection and transporting high‐density lower‐latitude plasma into the polar cap. However, we also find that not all substorms are coincident with polar cap density enhancements. A superposed epoch study showed that the AL index for TOIs during substorms is not particularly stronger than substorms without TOIs, but epoch studies of AMPERE observations do show events with TOIs to have a higher total FAC on both the dayside and nightside. Our results show the importance of TOI formation during substorms when solar wind drivers are absent, and the importance of considering substorms in the global current system. This work also shows the need to incorporate substorms into models of high‐latitude global convection and currents. Key Points: Enhanced dayside flows without interplanetary magnetic field changes can occur during substormsDayside plasma flows during substorms can transport photoionized plasma into the polar capSubstorms impact dayside currents and patches despite being typically associated with the nightside [ABSTRACT FROM AUTHOR]

This study investigates the quiet-time and storm-time variations of the African equatorial and low latitude ionosphere during 2009–2015. We obtained TEC observation data from four Global Navigation Satellite System (GNSS) stations over the African equatorial and low latitude, namely: Asab (ASAB, geographic coordinates 13.0°N, 42.65°E; geomagnetic coordinates 4.85°N, 114.34°E), Arba Minch University, (ARMI, 6.03°N, 37.56°E; 3.06°S, 109.29°E), Yamoussoukro (YKRO, 6.87°N, 5.24oW; 2.84°S, 67.41°E) and Mbarara (MBAR, 0.60°S, 30.74°E; 10.27°S, 102.36°E). Seven selected major Geomagnetic Storms (GSs) over solar cycle 24 were investigated and the responses of TEC and Rate of change of TEC Index (ROTI) as proxy for ionospheric irregularities to the storms were also studied. We recorded weaker ionospheric plasma density distributions around the magnetic equator (trough) than at the inner flank of the Equatorial Ionization Anomaly (EIA) crests: A clear evidence of EIA formation. Furthermore, hemispherical asymmetry was observed in the latitudinal distribution of TEC, with consistently higher distribution in the Northern Hemisphere (NH) than Southern Hemisphere (SH). TEC and ionospheric irregularities showed clear dependence on solar activity. Negative responses dominated GSs that occurred at night-time and positive effects for those that occurred during the daytime. The enhancement or inhibition of irregularities during GS is strongly dependent on the time of GS unfolding. As previously established, the daily maximum distribution of TEC at 1400–1600 LT and minimum at 0400–0600 LT were validated by this study, using long time (7 years) data. Seasonally, TEC showed semi-annual pattern of variations, with highest values in equinoxes, and lowest in solstices. [ABSTRACT FROM AUTHOR]

The high‐latitude ionosphere is highly dynamical with significant irregularities and density gradients. However, the spatial and temporal distributions of density gradients and irregularities are very different between the Arctic and Antarctic. In this report, we study the interhemispheric asymmetry of the large‐scale (100 km) density gradients in both polar caps. Our results show that density gradients in the Arctic are enhanced during local winter (December solstice) with a peak around 19 UT. The UT and spatial distributions in the Antarctic local winter (June solstice) are similar to the Arctic except that they are reversed by 12 hr, which indicates a mirror symmetry between hemispheres. The 12‐hr difference in the peak density gradients can be explained by the displacements between the geographic and geomagnetic poles. The only asymmetry (anomaly) is the persistence of strong density gradients in the southern polar cap during local summer (December solstice). Plain Language Summary: The high‐latitude ionosphere is highly variable with significant fluctuations in plasma density. So far, the long‐term studies have been conducted in the Arctic due to better ground‐based data coverage, while the knowledge of the Antarctic is limited. We take advantage of the Swarm mission to study the long‐term behaviors in both the Arctic and Antarctic. New statistics reveal phenomena that we understand and an "anomaly" that do not understand. The results during local winter (December in the Arctic and June in the Antarctic) are similar and can be explained by the previous knowledge. However, the anomaly is the persistence of strong density fluctuations in the Antarctic during local summer (December solstice). Key Points: Density gradients (100 km) in the Arctic are enhanced from autumn to spring equinoxes with a significant "hole" during 3–12 UT near December solsticeThe variations of density gradients are mirror symmetric for two hemispheres during local winterDensity gradients in the Antarctic are strong and persistent during local summer (December solstice) [ABSTRACT FROM AUTHOR]

Most of the low‐latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP‐Clemson‐INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°W). However, despite being operational for several years (2001–2012), FCI only made observations during daytime and pre‐midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full‐night F‐region observations. This radar is referred to as Measurements of Equatorial and Low‐latitude Ionospheric irregularities over São Luís, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre‐ and post‐midnight F‐region echoes with focus on the spectral features of post‐midnight echoes and how they compare to spectra of echoes observed in the post‐sunset sector. The radar observations indicate that post‐midnight F‐region irregularities were generated locally and were not a result of "fossil" structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field‐of‐view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F‐region conditions leading to ESF development. Key Points: The MELISSA 30 MHz radar system is described and examples of F‐region observations are introducedResults of the spectral analysis of pre‐ and post‐midnight F‐region echoes are presented and comparedResults indicate the local development of F‐region irregularities in the post‐midnight sector [ABSTRACT FROM AUTHOR]

The uplooking total electron contents (TECs) from the GRACE, SWARM‐A, TerraSAR‐X, and MetOp‐A satellites and in situ electron density (Ne) from SWARM‐A were utilized to investigate the topside ionospheric conditions during the 7–8 September 2017 geomagnetic storm. The rate of TEC index (ROTI) and rate of density index (RODI), which are derivative indices of TEC and Ne, respectively, were also used to characterize the topside ionospheric irregularities. The main results of this study are as follows: (1) There were significant enhancements seen in the uplooking TEC during the first main phase of the storm. (2) The uplooking TEC did not show unusual enhancement at the morning and evening local times in the Asian‐Australian sector during the recovery phase of the storm. (3) Prominent TEC hemispheric asymmetry at the middle and high latitudes was observed at both day and night sectors. (4) Long‐duration recovery of topside TEC with respect to the prestorm condition was also detected in this event. (5) Nighttime ROTI enhancements were presented in a wide latitudinal range from the equator to the poles during the main phases of the storm. (6) The ionospheric electric field disturbances associated with IMF‐Bz fluctuations probably played a very important role in triggering ionospheric irregularities during the relatively weak geomagnetic activity on 7 September, which implies that ionospheric irregularities do not necessarily occur under the severe geomagnetic conditions only. Key Points: Long‐duration depletion of topside TEC recovery was observed during the morning and evening local times in this eventEnhancements of ionospheric irregularities were presented at night with wide latitudinal range during the two main phases of the stormIonospheric electric field disturbances due to Bz fluctuations probably triggered topside ionospheric irregularities before the main phases [ABSTRACT FROM AUTHOR]

We present an overview of the ionospheric conditions during the launch of the Investigation of Cusp Irregularities 3 (ICI‐3) sounding rocket. ICI‐3 was launched from Ny‐Ålesund, Svalbard, at 7:21.31 UT on 3 December 2011. The objective of ICI‐3 was to intersect the reversed flow event (RFE), which is thought to be an important source for the rapid development of ionospheric irregularities in the cusp ionosphere. The interplanetary magnetic field was characterized by strongly negative Bz and weakly negative By. The EISCAT Svalbard radar (ESR) 32‐m beam was operating in a fast azimuth sweep mode between 180° (south) and 300° (northwest) at an elevation angle of 30°. The ESR observed a series of RFEs as westward flow channels that were opposed to the large‐scale eastward plasma flow in the prenoon sector. ICI‐3 intersected the first RFE in the ESR field of view and observed flow structures that were consistent with the ESR observations. Furthermore, ICI‐3 revealed finer‐scale flow structures inside the RFE. The high‐resolution electron density data show intense fluctuations at all scales throughout the RFE. The ionospheric pierce point of the GPS satellite PRN30, which was tracked at Hornsund, intersected the RFE at the same time. The GPS scintillation data show moderate phase scintillations and weak amplitude scintillations. A comparison of the power spectra reveals a good match between the ground‐based GPS carrier phase measurements and the spectral slope of the in situ electron density data in the lower frequency range. It demonstrates the possibility of modelling GPS scintillations from high‐resolution in situ electron density data. Key Points: RFE is associated with sharp flow shear and particle precipitation which result in GPS scintillationsThe spectral slopes between scintillation and in situ measurements are consistent with the phase screen theoryThe study suggests that it is possible to model GPS scintillations from high‐resolution in situ electron density data [ABSTRACT FROM AUTHOR]

We report the proposal and results of a low‐cost, easy‐to‐build GPS‐based sensor for detection and monitoring ionospheric irregularities through the detection of amplitude scintillation. The system is based on the Raspberry Pi single‐board computer combined with an Adafruit Ultimate GPS peripheral, which is capable of measuring (at 10‐Hz rate) the intensity of the L1 signals transmitted by GPS satellites. We introduce and discuss results of short‐ and long‐term observations obtained with a prototype of this system deployed in Presidente Prudente, a low magnetic latitude site in Brazil. The deployment and observations were carried out to test the ability of the system to detect ionospheric scintillations and, therefore, monitor the occurrence of ionospheric irregularities associated with equatorial spread F. Our results show that this low‐cost sensor is indeed capable of detecting scintillation events associated with equatorial spread F. Comparison with simultaneous, collocated measurements made by a commercial scintillation monitor are also presented. The joint observations allowed us to quantify the performance of the low‐cost monitor and to identify sources of potential limitations. While the sensor cannot (and it was not intended to) substitute commercial scintillation monitors, the low cost allows its use in studies of ionospheric irregularities (space weather) that require observations made by distributed arrays of small instruments (DASI). Furthermore, the simplicity of the sensor design stimulates its use in educational and citizen science initiatives. Plain Language Summary: It is well recognized that progress in space sciences can benefit from observations made by distributed arrays of instruments. Additionally, the value of integrated research and education as well as citizen science initiatives became more evident in the past few years. To support such efforts, we propose and investigate the use of off‐the‐shelf, inexpensive electronic devices to create a sensor capable of providing useful information about the space environment near Earth as well as being used in educational activities. We present results of a low‐cost, easy‐to‐build instrument (ScintPi) capable of providing information about irregularities in the density of the Earth's ionosphere. Ionospheric irregularities are known to affect the performance of radio systems used for navigation, communication, and remote sensing. The components of the instrument are readily available and commonly used by hobbyists. Despite its simplicity, we show that this instrument is capable of providing useful information about the state of the ionosphere. More importantly, the low cost of this instrument allows its use in geospace studies that require a dense regional network of measurements, educational programs, and citizen science initiatives. Key Points: We propose a low‐cost, easy‐to‐build GPS‐based sensor (ScintPi) for ionospheric studies and educational and citizen science initiativesComparisons with measurements provided by a collocated, commercial scintillation receiver are presentedResults from short‐ and long‐term observations confirm the ability of ScintPi to detect and monitor ionospheric irregularities [ABSTRACT FROM AUTHOR]

Electromagnetic signals including Global Navigation Satellite Systems (GNSS) signals often experience fluctuations due to ionospheric density structures termed as scintillation. Physical processes contributing to these structures in polar cap regions likely include gradient‐drift instability. GNSS scintillations are frequently observed at high latitudes and are potentially useful diagnostics of how energy from the transient forcing in the cusp or polar cap region cascades, via instabilities, to small scales. This work interfaces a physics‐based plasma model (Geospace Environment Model of Ion‐Neutral Interactions) to a radio wave propagation model (Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere) to explore this cascade. In this work, Geospace Environment Model of Ion‐Neutral Interactions‐Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere is used to simulate signal propagation through gradient‐drift instability over Resolute Bay for comparison with GNSS observations during a geomagnetic storm presented earlier in this series of papers. Our spectral and multiscale analyses show close agreement for simulated and observed data, suggesting the use of our physics‐based platform for understanding and predicting the effects of ionospheric instabilities on navigation and communication. Plain Language Summary: Radio signals such as those from Global Positioning System experience fluctuations due to plasma structures or instabilities in the Earth's upper atmosphere (ionosphere) that cause the signal to bend and scatter, called scintillation. Navigation and communication on the ground can be severely affected during geomagnetically active conditions. It is difficult to understand the ionospheric turbulence and how it is generated by simply looking at the Global Positioning System measurements. For the first time, a physics‐based ionosphere model and 3‐D Electromagnetic (EM) wave propagation model are combined to extract quantitative details about the irregularities. We present our results of comparing observations during a geomagnetic storm to a simulation from a combination of two models Geospace Environment Model of Ion‐Neutral Interactions and Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere. Our simulation results provide basic agreement with observations in terms of the scale sizes and strength of the ionospheric structures, thus providing a promising avenue for a further study of causes of scintillation. Such a state‐of‐the‐art tool provides a way to potentially characterize scintillation coming from different types of instabilities and evolving in time under different types of forcing mechanisms in the ionosphere. Key Points: We have interfaced a high‐latitude plasma instability model with a model of scintillationWe can simulate GNSS scintillation observed on the edges of polar cap patches generated by gradient‐drift instabilitiesSpectral and multiscale analysis results show that the simulated phase has periods and scales similar to the observations [ABSTRACT FROM AUTHOR]

We present a new quantitative theory for the rate of change of total electron content index (ROTI) by noting its straightforward relationship to the phase structure function of ionospheric turbulence. The theory provides the dependence of ROTI on the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of the irregularities. We also present useful approximations to the full theory that elucidate the principal dependencies. We show, for example, that ROTI varies with the effective scan velocity (Veff) to the power ν − 1/2, where 2ν + 1 is the spectral index of the 3‐D irregularities. This dependence on Veff persists in the ratio of ROTI to the intensity scintillation index (S4), which thus depends on the particular viewing geometry for each satellite. While irregularity strength cancels when forming this ratio, the dependence on Fresnel scale remains. We validate the theory by comparing predictions of irregularity strength (CkL) and S4 from 1‐Hz ROTI measurements with calculations derived from 20‐Hz intensity samples collected at Ascension Island. The theory explains the observations well, except when the scan is directed nearly along the field‐aligned irregularities (within 20° of meridional). For cross‐field scans the standard deviations of the error in predicting S4 from 1‐Hz ROTI were 0.05, 0.11, and 0.14 for weak, moderate, and strong scintillation, respectively. These encouraging results suggest the possibility of using dense networks of inexpensive total electron content monitors to provide quantitative diagnostics of ionospheric scintillation and the irregularities that cause them. Plain Language Summary: We present a new theoretical model to relate a widely used statistical measure of fluctuations in total electron content to statistical measures of ionospheric irregularities and the scintillations they produce. This model includes several effects not previously accounted for quantitatively, including the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of magnetic field‐aligned ionospheric irregularities. We validate the theory using measurements collected at Ascension Island, an equatorial station, during solar maximum conditions. Key Points: We present a new theory for the rate of change of total electron content index (ROTI)ROTI varies with sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of field‐aligned irregularitiesThe new theory enables quantitative prediction of intensity scintillation (S4) and irregularity strength (CkL) from measurements of ROTI [ABSTRACT FROM AUTHOR]

The polar ionosphere is often characterized by irregularities and fluctuations in the plasma density. We present a statistical study of ionospheric plasma irregularities based on the observations from the European Space Agency's Swarm mission. The in situ electron density obtained with the Langmuir probe and the total electron content from the onboard global positioning system receiver are used to detect ionospheric plasma irregularities. We derive the irregularity parameters from the electron density in terms of the rate of change of density index and electron density gradients. We also use the rate of change of total electron content index as the irregularity parameter based on the global positioning system data. The background electron density and plasma irregularities are closely controlled by the Earth's magnetic field, with averaged enhancements close to the magnetic poles. The climatological maps in magnetic latitude/magnetic local time coordinates show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. These irregularities may be associated with large‐scale plasma structures such as polar cap patches, auroral blobs, auroral particle precipitation, and the equatorward wall of the ionospheric trough. The spatial distributions of irregularities depend on the interplanetary magnetic field (IMF). By filtering the irregularity parameters according to IMF By, we find a clear asymmetry of the spatial distribution in the cusp and polar cap between the Northern (NH) and Southern Hemispheres (SH). For negative IMF By, irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is in agreement with the high‐latitude ionospheric convection pattern that is regulated by the IMF By component. The plasma irregularities are also controlled by the solar activity within the current declining solar cycle. The irregularities in the SH polar cap show a seasonal variation with higher values from September to April, while the seasonal variation in the NH is only obvious around solar maximum during 2014–2015. Key Points: The high‐latitude ionospheric irregularities are enhanced in the dayside cusp, polar cap, and nightside auroral ovalThe spatial distribution of irregularities in plasma density depends on the IMF ByThe seasonal variations of plasma irregularities in the Northern and Southern Hemispheres are different with respect to the local seasons [ABSTRACT FROM AUTHOR]

Abstract: The Communication/Navigation Outage Forecasting System satellite was operational from 2008, a period of deep solar minimum, to 2015, a period of moderate solar conditions. The behavior of the vertical plasma drift and the distribution of plasma depletions during the deep solar minimum of 2009 deviated substantially from the behavior that was observed during the solar moderate conditions encountered by the Communication/Navigation Outage Forecasting System satellite in 2014, which are typical of previous observations. Presented here are observations of the vertical drift of plasma depletions and the background plasma in which they are embedded. We find that depletions detected at local times after 2100 hr during solar minimum are typically found in background drifts that are weakly downward compared to the strongly downward background drifts observed during moderate solar activity levels. Additionally, at solar minimum, the drift within the depletions is upward with respect to the background as compared with observations at the same local times during solar moderate conditions for which the depleted plasma more nearly drifts with the background. We note that weak background plasma drifts observed throughout the night during solar minimum promote the continued growth of depletions that may evolve more slowly or be continuously generated to appear in the topside in the postmidnight hours. [ABSTRACT FROM AUTHOR]

Knowledge of Mars's ionosphere has been significantly advanced in recent years by observations from Mars Express and lately Mars Atmosphere and Volatile EvolutioN. A topic of particular interest are the interactions between the planet's ionospheric plasma and its highly structured crustal magnetic fields and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument on Mars Express. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp‐like) magnetic fields and spread echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination, and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to interhemispheric ducting in the Earth's ionosphere. Key Points: MARSIS on MEX observed both echoes from distant ionospheric irregularities as well as locally ducted echoes in close successionBoth effects are consistent with the presence of extended, small‐scale field‐aligned density irregularitiesWe suggest that these may be the direct result of strong electric fields present at ionospheric altitudes [ABSTRACT FROM AUTHOR]

We discuss theoretically the excitation of artificial plasma irregularities in the auroral ionosphere by high‐frequency X‐mode radio wave. This is done via a two‐step process. As a first step we adopt the thermal self‐focusing instability excited in the F region of the ionosphere under the action of a strong high‐frequency (HF) radio wave. This instability causes the formation of perturbations of the electron temperature and plasma concentration across the magnetic field. In addition, the plasma becomes depleted in the regions of the electron temperature enhancements and vice versa, since the gradients of plasma concentration and the electron temperature have opposite signs. In such conditions the temperature gradient instability comes into play. As a second step we consider plasma and electron temperature inhomogeneities that appear due to this instability to be responsible for the generation of irregularities with transverse sizes smaller than the typical scales of the self‐focusing instability. Alternative mechanisms such as excitation of the gradient‐drift and the current‐convective instabilities, which are often attributed to the generation of plasma irregularities in the F region and can contribute to the formation of artificial irregularities in the case of X‐mode heating, are also discussed. Key Points: We have investigated F region irregularities produced by high power HF X‐mode radio wavesPlasma instabilities responsible for decametric irregularities in the F region are described via a two‐step processOur theoretical characteristic time for development of decametric irregularities is of the order of 1 min [ABSTRACT FROM AUTHOR]

Abstract: We examine the density structure of equatorial depletions referred to here as equatorial plasma bubbles (EPBs). Data recorded by the Ion Velocity Meter as part of the Coupled Ion Neutral Dynamics Investigation (CINDI) aboard the Communication/Navigation Outage Forecasting System (C/NOFS) satellite are used to study EPBs from 1600 to 0600 h local time at altitudes from 350 to 850 km. The data are taken during the 7 years from 2008 to 2014, more than one half of a magnetic solar cycle, that include solar minimum and a moderate solar maximum. Using a rolling ball algorithm, EPBs are identified by profiles in the plasma density, each having a depth measured as the percent change between the background and minimum density (ΔN/N). During solar moderate activity bubbles observed in the topside postsunset sector are more likely to have large depths compared to those observed in the topside postmidnight sector. Large bubble depths can be observed near 350 km in the bottomside F region in the postsunset period. Conversely at solar minimum the distribution of depths is similar in the postsunset and postmidnight sectors in all longitude sectors. Deep bubbles are rarely observed in the topside postsunset sector and never in the bottomside above 400 km in altitude. We suggest that these features result from the vertical drift of the plasma for these two solar activity levels. These drift conditions affect both the background density in which bubbles are embedded and the growth rate of perturbations in the bottomside where bubbles originate. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies have suggested that weakening downward plasma drifts can produce favorable conditions for the ionospheric Generalized Rayleigh‐Taylor (GRT) instability and explain the occurrence of postmidnight equatorial spread F (ESF). We evaluated this hypothesis using numerical simulations aided by measurements and attempted to explain ESF events observed in the American sector during June solstice, low solar flux conditions. We analyzed plasma drifts and ESF measurements made by the incoherent scatter radar of the Jicamarca Radio Observatory (11.95° S, 76.87° W, ∼1° dip). We found adequate measurements during a prototypical, quiet time event on 4–5 June 2008 when the downward drifts weakened and a fully developed ESF appeared. The measured drifts were used as input for the SAMI2 model. SAMI2 reproduced an “apparent” uplift of the ionosphere based on h ′F measurements that was consistent with expectations and observations. SAMI2 also provided parameters for estimation of the flux tube linear growth rates of GRT instability associated with the weakening drift event. We found that the weakening drifts did produce unstable conditions with positive growth rates. The growth rates, however, were slower than those obtained for typical, premidnight ESF events and those obtained for similar drift conditions in other longitude sectors. We show, however, that departures in the wind pattern, from climatological model predictions, can produce favorable conditions for instability development. Following the hypothesis of Huba and Krall (2013) and using SAMI2 simulations, we show that equatorward winds, when combined with weakening drifts, could have contributed to the unstable conditions responsible for the postmidnight ESF events. [ABSTRACT FROM AUTHOR]

A total solar eclipse occurred on 20 March 2015, with a totality path passing mostly above the North Atlantic Ocean, which resulted in a partial solar eclipse over Belgium and large parts of Europe. In anticipation of this event, a dedicated observational campaign was set up at the Belgian Solar-Terrestrial Centre of Excellence (STCE). The objective was to perform high-quality observations of the eclipse and the associated effects on the geospace environment by utilising the advanced space- and ground-based instrumentation available to the STCE in order to further our understanding of these effects, particularly on the ionosphere. The study highlights the crucial importance of taking into account the eclipse geometry when analysing the ionospheric behaviour during eclipses and interpreting the eclipse effects. A detailed review of the eclipse geometry proves that considering the actual obscuration level and solar zenith angle at ionospheric heights is much more important for the analysis than at the commonly referenced Earth’s surface or at the plasmaspheric heights. The eclipse occurred during the recovery phase of a strong geomagnetic storm which certainly had an impact on (some of) the ionospheric characteristics and perhaps caused the omission of some “low-profile” effects. However, the analysis of the ionosonde measurements, carried out at unprecedented high rates during the eclipse, suggests the occurrence of travelling ionospheric disturbances (TIDs). Also, the high temporal and spatial resolution measurements proved very important in revealing and estimating some finer details of the delay in the ionospheric reaction and the ionospheric disturbances.

This paper presents research on polar cap ionosphere space weather phenomena conducted during the European Cooperation in Science and Technology (COST) action ES0803 from 2008 to 2012. The main part of the work has been directed toward the study of plasma instabilities and scintillations in association with cusp flow channels and polar cap electron density structures/patches, which is considered as critical knowledge in order to develop forecast models for scintillations in the polar cap. We have approached this problem by multi-instrument techniques that comprise the EISCAT Svalbard Radar, SuperDARN radars, in-situ rocket, and GPS scintillation measurements. The Discussion section aims to unify the bits and pieces of highly specialized information from several papers into a generalized picture. The cusp ionosphere appears as a hot region in GPS scintillation climatology maps. Our results are consistent with the existing view that scintillations in the cusp and the polar cap ionosphere are mainly due to multi-scale structures generated by instability processes associated with the cross-polar transport of polar cap patches. We have demonstrated that the SuperDARN convection model can be used to track these patches backward and forward in time. Hence, once a patch has been detected in the cusp inflow region, SuperDARN can be used to forecast its destination in the future. However, the high-density gradient of polar cap patches is not the only prerequisite for high-latitude scintillations. Unprecedented high-resolution rocket measurements reveal that the cusp ionosphere is associated with filamentary precipitation giving rise to kilometer scale gradients onto which the gradient drift instability can operate very efficiently. Cusp ionosphere scintillations also occur during IMF BZ north conditions, which further substantiates that particle precipitation can play a key role to initialize plasma structuring. Furthermore, the cusp is associated with flow channels and strong flow shears, and we have demonstrated that the Kelvin-Helmholtz instability process may be efficiently driven by reversed flow events.

Abstract: The plasma densities detected by the Planar Langmuir Probe (PLP) on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite have been analyzed in detail from launch in April 2008 through December 2009 period. The initial period just after launch which corresponds to one of the quietest periods in the space era, exhibited broad plasma decreases (BPDs) which we have reported previously. In order to treat the data quantitatively, we have detrended the PLP data by using the International Reference Ionosphere (IRI) model to remove variations in density due to changes in spacecraft altitude and latitude along the orbit. In this paper we present results of a statistical analysis of C/NOFS detrended plasma densities as well as neutral densities from the Gravity Recovery and Climate Experiment (GRACE) measured during the same period. The results show periodic structures in both charged and neutral species, most evident in limited local times. These structures resemble wave-4 nonmigrating tides. BPDs persist in the detrended data and appear as one of the minima in the longitudinal signatures, most strikingly on the nightside. Statistical averages of meridional velocities from the Ion Velocity Meter (IVM), also on C/NOFS, during the same period shows that the locations of BPDs coincide with measurements of large downward-directed velocities. [Copyright &y& Elsevier]

In high-frequency (HF) ionospheric communications, the dynamic nature of the high-latitude ionosphere creates small-scale ionospheric irregularities capable of scattering HF radio waves. In addition, there are large-scale irregularities which may be identified using some specific methods. However, small-scale irregularities cannot be easily identified and characterized. The aim of this paper is to introduce a new wavelet-based multifractal dimension mapping approach for the identification and the quantification of the small-scale irregularities. Numerical results show the improvement of the proposed methods compared to those obtained by the classical Legendre multifractal analysis. In our analysis, we have used the scattering function as a tool for the HF channel characterization. Our analysis deals with experimental scattering functions measured during the solar eclipse on August 11, 1999. [ABSTRACT FROM AUTHOR]

Abstract: The probability of occurrence of spread-F can be modeled and predicted using neural networks (NNs). This paper presents a feasibility study into the development of a NN based model for the prediction of the probability of occurrence of spread-F over selected equatorial stations within the Brazilian sector. The input space included the day number (seasonal variation), hour (diurnal variation), sunspot number (measure of the solar activity), magnetic index (measure of the magnetic activity) and magnetic position. Twelve years of spread-F data from Brazil (covering the period 1978–1989) measured at the equatorial site Fortaleza (3.9°S, 38.45°W) and low latitude site Cachoeira Paulista (22.6°S, 45.0°W) are used in the development of an input space and NN architecture for the model. Spread-F data that is believed to be related to plasma bubble developments (range spread-F) was used in the development of the model. The model results show the probability of spread-F occurrence as a function of local time, season and latitude. Results from the Brazilian Sector NN (BSNN) based model are presented in this paper, as well as a comparative analysis with a Brazilian model developed for the same purpose. [Copyright &y& Elsevier]

Abstract: We present a statistical study of the diurnal variations of the E-region backscatter occurrence observed by 15 SuperDARN radars over 3 years. The diurnal variation of echo occurrence observed by each radar is examined for three levels of geomagnetic activity. It is found that the E-region echo occurrence for individual radars exhibits very similar diurnal variations for low geomagnetic activity, with the differences between radars increasing with activity. This indicates that local processes become more important in the generation of the E-region echoes during disturbed periods. It is also shown that the majority of E-region backscatter structures take the form of diffuse echo bands rather than discrete echo patches, with the dominance of diffuse echoes increasing with activity. Finally, a hypothesis that the echo occurrence is controlled by the radar''s coverage of the auroral oval is tested using the electron and ion oval models derived from the DMSP satellite observations. [Copyright &y& Elsevier]

Scintillations are caused by ionospheric irregularities and can affect the propagation of trans-ionospheric radio signals. One way to understand and predict the impact of such irregularities on Global Navigation Satellite System (GNSS) signals is through the spatial/temporal characterization of the scintillation's climatology during different phases of a solar cycle covering different latitudes and longitudes. This characterization is performed using amplitude scintillation index S 4 , during the full solar cycle 24, in the South American (SA) sector. The investigation considers the diurnal, daily, and seasonal variation of S 4 index for climatological purpose, and the goal of this study is to investigate the scintillations covering a large spatial scale during the full solar cycle 24. The characterization shows a latitudinal asymmetry, whereas at the south, the scintillations were more frequent and their peak was more distant from the magnetic equator, which can be attributed by the South Atlantic Magnetic Anomaly (SAMA), and/or by the transequatorial meridional neutral winds. It also shows a longitudinal asymmetry, where the scintillations at the eastern sector occurred between November and February, while at the western sector, they occurred during the months of October, November, February and March, which can be attributed to the difference between the magnetic and geographic equators. The occurrence of scintillations during two distinct geomagnetic storms with similar storm time in the SA sector is also presented. • During solar minimum, weak scintillations occur in a region of ∼10°, centered at 15°-20° N/S, mainly during the equinoxes. • During solar maximum, intense scintillations have a coverage of ∼30° centered at 20°-30° N/S, during equinoxes and summer. • Ionospheric scintillations are more frequent at the southern crest of Equatorial Ionization Anomaly. • Ionospheric scintillations at the easter sector of South America (SA) occurred between November and February. • Ionospheric scintillations at the western sector of SA occurred during the months of October, November, February and March. [ABSTRACT FROM AUTHOR]

The Tiny Ionospheric Photometer (TIP) on the Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) characterizes the nighttime ionosphere using 135.6-nm radiative recombination emission. TIP measures horizontal structure of the ionosphere with high precision and high spatial resolution. Latitudinal, longitudinal, and temporal distribution of the nighttime ionosphere is specified. We present a review of ionospheric observations made with TIP during the first five months of operation. Comparisons are made with other ionospheric sensors in order to validate the TIP observations and to demonstrate TIP resolution and sensitivity performance. Equatorial anomalies observed by TIP are compared with estimates of the E x B vertical drift during the post-sunset pre-reversal enhancement in the Peruvian sector. Low latitude irregularity structures observed by TIP are compared with measurements from ground-based sensors including: imaging photometers, ionosonde, and UHF scintillation receivers. Detailed measurements of low latitude density depletion depth and width are provided. Global ionospheric morphology observed by TIP is compared with similar observations by COSMIC radio occultation, and the GAIM model. The complexity of the underlying neutral winds is revealed by the TIP ionospheric morphology. [ABSTRACT FROM AUTHOR]