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Sporadic E (Es) layers could be composed of metallic ions and formed, modified, or transported by the action of convective electric fields in the high latitude ionosphere. In this paper, by utilizing simultaneous observations from Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radars at Zhongshan Station (ZHS, 69.4°S, 76.4°E), Antarctica, a thin Es layer, which initially formed in the lower F region and descended into the lower E region, with wavelike structures, was recorded by Digisonde on 14 November 2019. The Es layer‐related concurrent ionospheric irregularities were also detected by the SuperDARN ZHS HF radar. By using a global‐scale 2‐D convection map, combined with images from the Special Sensor Ultraviolet Spectrographic Imager instruments onboard Defense Meteorological Satellite Program (DMSP) spacecraft, it is proposed that the flow shears associated with the duskside convective circulation are responsible for the evolution of the Es layer. Moreover, using the HF radar elevation angle data to measure the scatter height, it is strongly suggested that the Es layer was elongated with convection circulation. The electrodynamic processes responsible for the formation and evolution of the Es layer are discussed. Key Points: An Es layer formed in the lower F region and descended to the E region, observed simultaneously by Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radarsThe formation and evolution of the Es layer related to the afternoon convection reversalSuperDARN HF radar measurements suggest the Es layer is elongated with convection circulation [ABSTRACT FROM AUTHOR]

A longitudinal chain and a latitudinal chain of ionosondes, located at Yakutsk (129.60°E, 62.00°N), Mohe (122.37°E, 53.50°N), Beijing (116.25°E, 40.25°N), Wuhan (114.61°E, 30.53°N), Sanya (109.42°E, 18.34°N), I‐Cheon (127.54°E, 37.14°N), San Vito (17.80°E, 40.60°N), Roquest (0.50°E, 40.80°N), Millstone Hill (71.50°W, 42.60°N), and Alpena (83.56°W, 45.07°N) ionosondes, are utilized to statistically investigate the sporadic E layer (Es) occurrence rate and variation. The Es occurrence rates at all stations show a similar dependence on local time and season. However, the seasonal dependence of Es occurrence is more distinct at middle latitudes than that at low latitudes along the 120°E longitude. Results at different stations along ∼40°N latitude show much lower Es occurrence at the American sector than that at European and East Asian sectors, which is most likely because the horizontal geomagnetic field in north America is smaller than in other regions. The statistical results demonstrate that the seasonal variations of Es occurrence and strength are related to the annual variation of meteor counts. Simulation results show that the summer maximum of the ion vertical drift velocity driven by the variations of neutral densities and winds could also contribute to the seasonal variations of Es occurrence and intensity. The seasonal variations of the quasi‐6‐day and quasi‐16‐day oscillations in Es are also investigated. The seasonal variations of periodic disturbances of the Es differ between each station. Planetary waves in the MLT region cannot fully explain the seasonal variations of the periodic disturbance of Es. Solar and geomagnetic activities can also result in periodic variations of the Es. Plain Language Summary: Sporadic E layer is an ionized cloud composed of metallic ions. Meteor flux and atmospheric waves, such as planetary waves and tides, play an important role in the formation of Es. In this work, we investigate the latitudinal and longitudinal distribution of Es occurrence and variations based on ionosonde data of 10 stations around the world. The Es occurrence rates at all stations show a similar dependence on season, which are mostly related to the variation of meteor input. In addition, simulation results show that the summer maximum of the ion vertical drift velocity driven by the variations of neutral densities and winds could also contribute to the seasonal variation of Es occurrence and intensity. The seasonal variations of quasi‐6‐day and quasi‐16‐day oscillation in sporadic E layer are also studied. The results demonstrate that the seasonal variations of periodic disturbances in Es differ between each station and enhanced quasi periodic oscillation can be observed in all seasons. Besides, in addition to the PWs in the MLT region, geomagnetic and solar activities can also lead to the periodic disturbance in Es layers. Key Points: The occurrence rate of Es in north America is lower than that at 120{degree sign}E sector due to horizontal geomagnetic field differenceEffects of meteor counts, neutral density, and winds contribute to the seasonal dependence of Es layer occurrenceSolar and geomagnetic activities can also contribute to the periodic oscillations in the Es layer variations [ABSTRACT FROM AUTHOR]

In this paper, we examine the mechanism of middle‐ and low‐latitude sporadic‐E (Es) layer formation. Observations of ionosondes and meteor radars at stations of different latitudes show that Es layer also formed at low‐altitude nonwindshear points, which is distinct from prototypical windshear theory. The theoretical modeling results show that metallic ions can effectively accumulate by the combined effect of the zonal and meridional winds in the mesosphere and the lower thermosphere region. Numerical simulation results also demonstrate that metallic ions can accumulate without wind shear at lower height (90–105 km) as a consequence of the MLT winds leading to the ion drifting downward. We suggest that the turbopause at 90–105 km plays an important role in forming low‐altitude Es layer in the middle‐ and low‐latitude regions. Plain Language Summary: Sporadic‐E layer is an enhanced ionization patch, which can reflect radio wave signals, and plays a significant role in the telecommunication and navigation system. The occurrence of Es layer is inconsecutive and influenced by many factors, such as wind, electric field, and meteor input, which makes the prediction of Es layer very difficult. In this study, we try to investigate the process of the formation of Es layer from electrodynamic theory of ionosphere. Our simulation results demonstrate that the wind shear, a classical theory for the Es layer formation at midlatitude, is not the only mechanism leading to the generation of Es layer, and turbopause at 90–105 km height can provide the concentration location for metallic ions. Key Points: Windshear Theory of sporadic‐E (Es) formation is reexaminedMeteor radar and ionosonde observations show that Es can form at nonwindshear pointsSimulation results show that Es formation can be affected by the turbopause [ABSTRACT FROM AUTHOR]

A sodium Doppler lidar system with three‐directional measurements of sodium density, atmospheric wind field, and temperature was established at Zhongshan (69.4°S, 76.4°E), Antarctica. On November 14, 2019, a sporadic sodium layer (SSL) was observed at an altitude range of 93–103 km. The temporal/spatial sodium density variations of this SSL are associated with a strong sporadic E (Es) layer at nearly the same height, which is modulated by the convective electric field. By considering the structures and the time lags of the SSL's growth at three positions, the SSL appears to have a horizontal advection in an approximately westward direction with a velocity of the order of 80 m/s. This is consistent with the zonal wind velocity derived from the lidar system itself. The temporal/spatial sodium density variations strongly indicate that the formation and perturbation of SSLs are related to the evolution of ES layers due to varied electric fields and atmospheric gravity waves, while it is advected by the horizontal wind. Plain Language Summary: A sporadic E layer (Es) could be formed by metallic ions and then modified or transported by the action of magnetospheric electric fields in the high latitude ionosphere. It has been widely proposed that Es layers play an important role in the formation of sporadic sodium layers (SSL), but detailed studies of their dynamic process and evolution are still lacking. A three‐frequency Sodium (Na) resonance fluorescence Doppler lidar has been recently deployed by the Polar Research Institute of China, which could measure the sodium density, temperature, and wind profiles simultaneously in three directions. To clarify the dynamic properties of Es/SSL, we have performed observations of an event at Zhongshan Station (69.4°S, 76.4°E), Antarctica, which includes sodium density profiles and wind velocity measured by the multidirectional lidar system, detection of the Es layer by a collocated Digisonde radar, F region ion velocity, that is, electric field, derived by the SuperDARN HF radar network, as well as gravity wave perturbations determined from the Davis medium frequency radar. Key Points: Separation of temporal/spatial variations in sporadic sodium layer (SSLs) is observed by a three‐directional lidar system at Zhongshan, AntarcticaThe formation and perturbance of SSLs are associated with a sporadic E layer at the same heightThe SSLs are advected by the background wind [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers are known for their layered structures with enhanced electron density. Recently, some scientists have studied the relationship between the occurrence rate and intensity of the Es layer in the global distribution. However, the relationship between the Es layer parameters (Es layer occurrence rate, Es layer intensity, Es layer thickness) in the vertical direction seems to be ignored to date. This paper highlights the vertical profiles of the occurrence rate, intensity, and thickness of the Es layers by using scintillation index (S4) from FORMOSAT‐3/COSMIC satellites from July 2007 to March 2015. The results show that there are some persistent differences between the occurrence rate, intensity, and thickness of the Es layers in the vertical profiles, which is different from their general similarity in the latitude distribution. It is found that the Es layer occurrence rate has a maximum value at ∼102 km altitude, but the Es layer intensity and Es layer thickness reach their maximum at an altitude of ∼110 km. Besides, the intensity and thickness of the Es layer exhibit a high linear relationship, which suggests that the larger electron density of the Es layer, the thicker its thickness. The Horizontal Wind Model (HWM14) indicates that the latitudinal variations in Es layers parameters are highly coincident with the zonal wind shear. However, the altitudinal variations in Es layers parameters do not agree well with the zonal wind shear. Key Points: The vertical variations of the occurrence rate, intensity, and thickness of the Sporadic E (Es) layers are presented in detailTheir vertical variations exist obvious discrepancies, which is different from their high similarity in latitudeCompared with the Es layer occurrence rate, the Es layer intensity can better reflect the evolution of the Es layer in vertical direction [ABSTRACT FROM AUTHOR]

The COSMIC radio occultation observations are used to investigate the wavenumber‐4 (WN4) structure of the Sporadic E (Es) layer during 2007–2018. The WN4 pattern is observed in the Es occurrence rate over the middle‐ and low‐latitudes. This pattern is stronger at the low‐latitudes than at the middle‐latitudes. It shows the peak intensity in latitude bands between 5° and 15° dip latitudes in both hemispheres. The WN4 structure occurs predominantly during daytime. It appears above 110 km in the morning, then descends slowly down to about 105 km, and finally disappears after sunset. The WN4 structure moves eastward with a speed of approximately 90°/day. It is prominent in boreal autumn and summer, and disappears in boreal winter. The dominant role of the DE3 tide for the formation of the WN4 structure is supported by similar features between the Es occurrence rate and the DE3 tide of the zonal wind shear, such as the phase, eastward speed, seasonal variation, symmetrical distribution, and strong activity at the low‐latitudes. Key Points: The wavenumber‐4 (WN4) structure is observed in the sporadic E layer over the middle‐ and low‐latitudesThe diurnal, seasonal, and altitudinal variations of the WN4 pattern are analyzedThe diurnal eastward non‐migrating tide with zonal wavenumber‐3 plays the dominant role in the formation of the WN4 structure [ABSTRACT FROM AUTHOR]

Ionospheric sporadic E layers are irregularities with enhanced plasma density which occur in the ionosphere E region. In the present work, we have focused on an interesting wavenumber 4(WN4) pattern of sporadic E layer based on the Constellation Observing System for Meteorology, Ionosphere and Climate radio occultation S4 indices observation. It is found that amplitude of WN4 pattern has a significant seasonal dependency with the maximum appearing in summer/autumn and minimum in winter. The seasonal variation of the WN4 pattern is basically consistent with the eastward propagating diurnal tide with zonal wavenumber 3 (DE3) tide features. Inconsistencies, mainly an anomalous weak amplitude during the summer in southern hemisphere and propagation direction (speed), are also found. By removing the zonal ion drift factor effect, WN4 pattern of sporadic E layers exhibits a closer relation with the DE3 tide with larger amplitude in the northern hemisphere and ~90° shift in longitude on average during autumn in the southern hemisphere. In general, it is reasonable to conclude that DE3 tide greatly contributes to this WN4 pattern of sporadic E layers. [ABSTRACT FROM AUTHOR]

We investigate the correlation of sporadic E (Es) with the occurrence of medium‐scale traveling ionospheric disturbances (MSTIDs) at night in middle latitudes (25°–40°N and 25°–40°S magnetic latitudes) by examining their occurrence climatology. The occurrence climatology of Es and MSTIDs is derived using the Challenging Minisatellite Payload satellite data acquired in 2001–2008 and 2001–2009, respectively. Electron density irregularities and radio scintillations are used as the detection proxies of MSTIDs and Es, respectively. The occurrence rate of MSTIDs shows a semi‐annual variation with the primary peak during June solstices and the secondary peak during December solstices in both hemispheres. However, the occurrence rate of Es shows a seasonal variation with a pronounced peak in summer in both hemispheres. The occurrence of MSTIDs during local summer and equinoxes is correlated with the occurrence of local Es, but the high occurrence rate of MSTIDs in local winter is not correlated with local winter hemisphere Es. MSTIDs in the winter hemisphere are correlated with magnetically conjugate MSTIDs in the summer hemisphere; these summer hemisphere MSTIDs are correlated with the occurrence of Es in the summer hemisphere. The occurrence rate of MSTIDs clearly shows an increase with decreasing solar activity, but the solar cycle dependence of Es is not obvious from the data. This observation suggests that the generation of MSTIDs is significantly affected by factors other than Es such as the growth rate of the Perkins instability, atmospheric gravity waves, and the F region conductance. Key Points: Nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) in summer and equinoxes are correlated with sporadic E in the local hemisphereWinter hemisphere MSTIDs can be explained by summer hemisphere sporadic EThe role of sporadic E in winter is contingent upon the conjugacy of MSTIDs [ABSTRACT FROM AUTHOR]

Observations of backscatter from field‐aligned plasma density irregularities in sporadic E (Es) layers made with a 30‐MHz coherent scatter radar imager in Ithaca, New York are presented and analyzed. The volume probed by the radar lies at approximately 54° geomagnetic latitude, under the midlatitude trough and at the extreme northern edge of the zone where Es layers are prevalent. Nonetheless, the irregularities exhibit many of the characteristics of quasiperiodic echoes observed commonly at lower middle latitudes. These include a tendency to occur in elongated bands stretching from the northwest to southeast in the Northern hemisphere separated by tens of kilometers and propagating to the southwest. In addition, the irregularities were found to exhibit finer‐scale structures with secondary bands oriented nearly normally to the primary bands. We investigate the proposition that the primary bands are telltale of Es‐layer structuring caused by neutral Kelvin Helmholtz (KH) instability in the lower thermosphere and that the secondary bands signify secondary KH instability. Results from a 3D numerical simulation of KH support this proposition. Key Points: Quasiperiodic (QP) sporadic E layers observed under the midlatitude trough using coherent scatter radarRadar imagery shows secondary bands of irregularities normal to the primary QP bandsThe secondary bands are attributed to secondary Kelvin Helmholtz instability in the lower thermosphere [ABSTRACT FROM AUTHOR]

Sporadic potassium (Ks) and sodium (Nas) layers were investigated using more than 1,500 h of simultaneous Na/K LIDAR data at São José dos Campos, Brazil (23.1°S, 45.9°W). These events were observed on more than 88% of the analyzed nights. Although Na and K are both alkali metals, their sporadic layers show different behaviors. The Nas layers were in general 0.7 km wider and occurred ∼2 km higher than Ks layers. Moreover, the Nas layers progression is faster than that of Ks layers not only in the process of reaching their maximum amplitude but also in terms of dissipation. Our study showed that most Ns events (80% of Ks layers and 90% of Nas layers) have a strength factor of upto 5. Moreover, on average, Ks presented a strength factor ∼1.3 times stronger than that of Nas. The relative frequency of occurrence for both Ks and Nas layers exhibits an almost semiannual variation, with maxima in January/October and February/September, respectively. On the other hand, no similar variation was observed in the peak density or in the peak height. The peak density shows an annual variation, with a maximum occurring during winter for Ks events and a semiannual variation with a maxima occurring at the equinoxes for Nas. Different seasonal behaviors were also seen in the peak height of the identified layers, with Ks presenting a semiannual variation with maxima around January/October, while an annual variation occurred in Nas with lower layers occurring around winter. Key Points: This study presents the first simultaneous analysis of averaged K and Na sporadic layer parameters in the Southern HemisphereSeasonal and nocturnal variations in sporadic events were observed in K and Na layersThe differing behaviors of neutral sporadic layers were described as seen in K and Na mesospheric layers [ABSTRACT FROM AUTHOR]

The great solar eclipse traveled eastward through the American Continent from coast to coast on August 21, 2017 and provided a rare opportunity to investigate the ionospheric responses to the rapid solar radiation variations. We have analyzed the peak electron density and density gradient variations in Sporadic E (Es) and applied the data of four Digisondes close to the eclipse path. All the four Digisondes have recorded the same interesting phenomena. One is the depletion on the fbEs (blanketing frequency of Es layer) curve during the solar eclipse and the other is the enhancements on the semi‐transparency parameter of Es layer Δf after the solar eclipse, where Δf = ftEs−fbEs, and ftEs is the top frequency of Es layer. The wind disturbances on the tidal wind and the photoionization decrease during the eclipse are put forward as a possible reason for the peak plasma density reduce. The periodic gravity waves have been observed in both ionospheric E and F region. However, the Δf enhancements occurred after the gravity waves had passed through the Es layer. Thus, we suggest that the solar induced waves did not raise the plasma density gradient directly. Gravity waves breaking may occur in the Mesosphere/Lower Thermosphere region to deposit momentum and produce turbulence, and so the accompanying instabilities may be generated to enhance the electron density gradient in E‐region. Key Points: Four Digisondes have recorded the same interesting phenomena in Sporadic E during and after the solar eclipse of August 21, 2017Depletion on fbEs curves were recorded during the solar eclipse and the minimum fbEs appeared around the eclipse maximumEnhancement on ftEs‐fbEs curves were recorded after the solar eclipse and may be related to the gravity wave breaking [ABSTRACT FROM AUTHOR]

In this study, the difference in the sporadic E layer occurrence ratio between two low‐magnetic‐latitude (±5° to ±15°) regions is analyzed by using COSMIC scintillation observations covering the period from 2007.001 to 2017.365. Obvious longitudinal, seasonal, and LT dependencies of this phenomenon are found. Most features of the differences between two hemispheres could be explained by the difference in zonal vertical wind shear between the two hemispheres induced by the difference between the magnetic and geographic latitudes of the studied regions. This result also indicates that the wind shear mechanism still dominates sporadic E layer formation in low‐magnetic‐latitude regions. Key Points: The difference in the sporadic E layer occurrence ratio between two low‐magnetic‐latitude regions is analyzedThe difference in the sporadic E layer occurrence ratio between two hemispheres shows obvious longitudinal, seasonal, and LT dependenciesThe longitudinal, seasonal, and LT dependencies of this difference could be partly explained by the difference in vertical zonal wind shear [ABSTRACT FROM AUTHOR]

An unseasonal equatorial plasma bubble (EPB) event over South-East Asia was observed on July 22, 2014 that has not been studied before. An investigation into this event is presented with the 26th July, 2014 as a comparison, non-bubble day. The 22nd July EPB event occurred in the late post-sunset sector and was associated with a small upward plasma drift. This event was highlighted using a new filter on the SCINDA S4 data. Ionosonde data show that sporadic E was present during the growth period for the EPB event. Modeling results from Thermosphere-Ionosphere Electrodynamics (TEC) Global Circulation Model were used to conduct a numerical experiment investigating the direct effect of sporadic E on the linear R-T growth rate. It was shown that sporadic E located in the correct latitude and local time can increase the linear growth rate. The seeding conditions were investigated using TEC data from Patumwan, Thailand. Wave-like structures were observed for both days of interest, with larger amplitudes on 22nd July compared with the 26th July. Finally, simulations using the high-resolution model PBMOD showed that for forcing from above conditions similar to the days of interest, EPBs would form in the presence of large seed perturbations. Therefore, it is likely that this unseasonal event was caused by large seed perturbations in TEC. [ABSTRACT FROM AUTHOR]

Sporadic E (Es) structures have been observed occasionally covering a large horizontal scale of more than 1,000 km over China. Their onset locations and propagation features, and related generation mechanisms still remain unknown. In this study, a statistical analysis of large-scale Es structures is performed based on the ionospheric total electron content obtained from ground-based receiver networks, in combination with data from multiple ionosondes in China. The large-scale strong Es structures mainly occur during summer months, with dominant horizontal azimuth in the east-west and northwestsoutheast directions and dimensions of 1,000--3,000 km along the elongation. They predominantly drift southwestward at the speed of 30--210 m/s. The main onset region for the large-scale Es structures over China is identified for the first time, which is around 20°--45°N and 100°--125°E. Based on the morphological features of large-scale Es structures, and the observation of concurrent cases of traveling ionospheric disturbances, we surmise that gravity waves could play an important role in the generation of large-scale Es structures. [ABSTRACT FROM AUTHOR]

During disturbed periods, E region electric fields can cause anomalous Es layer behavior, which is observed in the digital ionosonde data. To investigate the influence of these electric fields in the Es layer development, we analyzed a set of 20 magnetic storms from 2015 to 2018 over Boa Vista (BV, 2.8°N, 60.7°W, dip ~18°), São Luís (SLZ, 2.3°S, 44.2°W, dip ~8°), and Cachoeira Paulista (CXP, 22.41°S, 45°W, dip ~35°). The electric field zonal components during the main and recovery phases of each magnetic storm are computed to study the corresponding characteristics of these Es seen in ionograms. Additionally, a numerical model (MIRE, Portuguese acronym for E Region Ionospheric Model) is used to analyze the Es layer dynamics modification around disturbed times. Using observation data and simulations, we were able to establish a threshold value for the electric field intensity for each region that can affect the Es layer formation. The results sustain that the strong Es layer in BV can be an indicator of the disturbed dynamo event. At SLZ, on the other hand, the Es layers are affected by the competition mechanisms of their formation, as equatorial electrojet irregularities and winds, during the main phase of the magnetic storm. Over CXP, the Es layer dynamics are dominated by the wind shear mechanism. Finally, this study provides new insights into the real impact of the electric field in the Es layer development over the Brazilian sector. Thus, our results lead to a better understanding of the underlying mechanisms related to the Es layer formation and dynamics. [ABSTRACT FROM AUTHOR]

We present passive observations of midlatitude sporadic E (Es) structures in the 30–50 MHz range using the long wavelength array telescopes in New Mexico. Spectral and temporal analysis of the structures reveals that some of the emissions can be characterized by broadband, spark‐like radiation occurring at a repetition frequency of 60 Hz. The azimuthal distribution indicates that the brightest emissions come from the directions of several large metropolitan centers with distances ranging from 700 to 1,250 km from the telescopes. This implies that the source is unintended human‐made noise, originating from devices connected to 60 Hz alternating current power. The geometry of our observations rule out coherent scatter from field‐aligned irregularities, and indicate that the unintended noise must undergo a specular reflection off of over‐dense plasma distributed approximately parallel to Earth's surface. On certain occasions the reflections are seen at such high frequencies and high elevation angles that the local plasma frequency must be above 30 MHz. During both day and night Es conditions, the dense structures are observed to be arranged in propagating fronts, and travel at speeds of 70–100 m/s. We also present observations that reveal quasi‐periodic amplitude oscillations with frequencies consistent with atmospheric gravity waves and acoustic waves. Plain Language Summary: Dense, patchy layers of plasma that form in the E layer of the ionosphere (sporadic E) often reflect high frequency radio waves from distant, over‐the‐horizon sources. While detectable with current radar techniques, sporadic E is difficult to image over large regions due to the limited field of view of a typical radar. We show that unintentional, human‐made radio emissions, originating from the electrical grid, act as a widespread set of radio sources, with hotspots centered on large metropolitan areas. The radio emissions reflect off of sporadic E plasma and are readily detectable and imaged by radio telescopes such as the Long Wavelength Array (LWA) in New Mexico. We present LWA observations that prove this capability, which enables the tracking and mapping of sporadic E over large regions. Key Points: Radio emissions from the electrical grid act as a widespread illumination source for imaging sporadic EThe technique described in this paper enables tracking and mapping of sporadic E structuresSince the source of RF is broadband, this method may also enable tracking of sporadic E electron density [ABSTRACT FROM AUTHOR]

Nearly 200 hr of observing with the Murchison Widefield Array (MWA) radio telescope during the Galactic and Extragalactic All‐sky MWA (GLEAM) survey was used to assemble images of ionospheric structure. These images cover a nearly 50° wide area on the sky at a cadence of 10 min over many 5–7 hr long observing runs. They are generated by tracking the apparent motions of ∼200–800 cosmic radio sources caused by changes in the transverse gradient of the ionospheric total electron content. Spectral analysis of these images revealed that the data set was dominated by three distinct signatures. The first is consistent with field‐aligned structures within the topside ionosphere/lower plasmasphere previously imaged with the MWA. The second are structures that are relatively large and aligned nearly east/west. Regional weather data imply that these are preferentially detected when there is a noticeable shear within the subtropical jet stream, which passes near the MWA. This suggests that this signature may be related to gravity waves launched by jet stream shear. The final signature is consistent with the properties of so‐called electrobuoyancy waves that are known to occur at midlatitudes at night. Detections of these were more common when regional sporadic E (ES) was present, supporting a proposed connection between these waves and polarization electric fields that may arise within ES. We discuss the implications for future observations with the Square Kilometer Array. Key Points: One hundred eighty hours of data from the GLEAM survey at frequencies below 100 MHz were used to image ionospheric structures over a ∼50° field of viewMost wavefronts were found to have one of three orientations: (1) field aligned, (2) nearly east‐west, and (3) northeast to southwestCategory (2) wavefronts appear associated with shear within the jet stream; category (3) waves appear associated with sporadic E [ABSTRACT FROM AUTHOR]

A complex daytime sporadic E (Es) case with extremely high critical frequency (foEs) was observed over the low latitude of China on 19 May 2018. Simultaneous observational results from two very high frequency (VHF) radars, two ionosondes, and multiple Global Navigation Satellite System total electron content and scintillation receivers are analyzed to investigate the evolution of the complex Es occurrence, which consisted of a relatively weak ambient Es layer (foEs < 8 MHz) and band‐like strong Es structures (foEs > 17 MHz) drifting from higher latitude. The strong Es structures elongated more than 500 km in the northwest‐southeast direction, drifted southwestward at a speed of ~65 m/s. VHF radar backscatter echoes were generated when the strong Es structures passed the radar field of view, with different echo patterns due to different radar and antenna configurations. No VHF radar backscatter echo was associated with the ambient Es layer. The mechanisms responsible for the formations of the ambient Es layer and band‐like strong Es structures are addressed and discussed. Key Points: Daytime complex Es composed of a relatively weak ambient Es layer and drifting strong Es was observed over low‐latitude ChinaThe strong Es structures showed band‐like elongation aligned NW‐SE, drifting southwestward with velocity of ~65 m/sVHF radar backscatter echoes were generated with different echo patterns due to different radar and antenna configurations [ABSTRACT FROM AUTHOR]

This study analyzes strong sporadic E layer (Es) formation in Boa Vista (BV, 2.8°N, 60.7°W, dip: 18°), a low‐latitude region in the Brazilian sector, which occurred far after the onset of a magnetic storm recovery phase. Such occurrences were observed during seven magnetic storms with available data for BV. Thus, the ionospheric behavior on days around the magnetic storm that occurred on 20 January 2016 was investigated to search for possible explanations. This analysis indicated that the probable mechanism acting during the Es layer strengthening is the zonal westward electric field caused by a disturbance dynamo. The same evidence was also observed in two other magnetic storms at the same location. Hence, a numerical model of the E region dynamics, called MIRE (Portuguese acronym for E Region Ionospheric Model), was used to confirm whether the disturbance dynamo could cause the Es layer intensification. The inputs for the model were the electric field deduced from the vertical drift and the wind components provided by GSWM‐00 model. The simulations indicate that the Es layer density is significantly enhanced when the zonal electric field is present compared to the reference scenario with only the winds. Therefore, it is concluded that the disturbance dynamo electric field is the likely cause of the strong Es layers in the analyzed cases. Finally, the combined results from the model and observational data seem to contribute significantly to advance our understanding of the role of the electric fields in the Es layer formation at low latitudes. Key Points: Data analysis and modeling taking into account wind and E field are employed to study anomalous Es layers in Boa VistaThe results show that the most probable mechanism responsible for the Es layer strengthening in Boa Vista is the zonal westward electric field caused by the disturbance dynamoSimulations support our assumption that the disturbance dynamo electric field leads the observed Es layer intensification in Boa Vista [ABSTRACT FROM AUTHOR]

Ionospheric anomalies before earthquakes termed as seismo‐ionospheric precursors have been extensively discussed due to the importance of earthquake prediction. Electric field driven mechanism is commonly used to explain seismo‐ionospheric precursors. Kim et al. (1993) first theoretically demonstrated that seismogenic electric fields could form intensified sporadic E layers in higher altitudes, which is so influential that motivates considerable statistical work on variations of foEs and h'Es before earthquakes. We attempt to establish a physical model of sporadic E layers at middle latitudes based on wind shear theory with the objective to reexamine the influence of seismogenic electric fields on sporadic E layers. The results show that zonal electric fields of several mV/m can influence the formation of higher sporadic E layers: For existing higher sporadic E layers, eastward/westward electric fields are capable of inhibiting/facilitating the existing layers accompanied by ascent/descent in altitude. For normal E region without sporadic E layers, the effect of eastward electric fields is not notable, while westward electric fields are able to form wider sporadic E layers. Taking into account their complicated response and inherently high variability, a suggestion is made about the necessity to be cautious when attributing variations of sporadic E layers to forthcoming earthquakes. Key Points: A sporadic E layer model considering ionospheric and metallic ions at middle latitudes is established based on shear wind theoryResponse of higher sporadic E layers depends on the directions of presumably seismogenic electric fieldsComplicated response and inherently variability of sporadic E layers make it hard to attribute sporadic E layer variations to earthquakes [ABSTRACT FROM AUTHOR]

This paper describes insights into the nature of corotating plasmaspheric irregularities (CPIs) enabled by a newly developed method for range‐ and time‐resolved tomographic images of these structures. The method is based on high‐precision measurements of total electron content gradients measure toward cosmic radio sources using an interferometric radio telescope, the Very Large Array (VLA). Exploiting hundreds of hours of data from the VLA Low‐band Ionosphere and Transient Experiment, we confirm that CPIs are predominantly detected at night and at L shells ∼2–4. Combining this large data set with other ionospheric, atmospheric, and space weather databases, we have established that CPI detection rates are much higher when levels of solar and geomagnetic activity are low. We also demonstrate that higher detection rates coincide with drivers of perturbations in the ionospheric electric field, including sporadic E and gravity waves. The gravity wave sources most commonly associated with CPIs are jet stream bend/shear and substorm‐triggered atmospheric disturbances. We also show that simulations of the plasmaspheric impact of both gravity wave‐driven electric field perturbations and polarization electric fields associated with electrobuoyancy waves/sporadic E produce results that are very similar to what is observed with the VLA. Key Points: A new method has been developed to generate range‐ and time‐resolved images of corotating plasmaspheric irregularities (CPIs)More than 300 hr of range/time imaging data have been generated with the Very Large Array (VLA) radio telescope using this methodThese data imply that CPIs are generally caused by perturbations in the ionospheric electric field due to gravity/electrobuoyancy waves [ABSTRACT FROM AUTHOR]

This paper investigates the variations of middle‐ and low‐latitude sporadic E (Es) layers during 2011–2012 and 2012–2013 sudden stratospheric warming (SSW) events by using observations from six ionosondes in East Asia over China and Japan region. Observations of Es critical frequency (foEs) at all these ionosondes present obvious enhancements around the days of new and full moon during SSW periods. Wavelet spectra of foEs during two SSW events exhibit noticeable enhanced 14.5‐day modulation, which resembles the lunar semimonthly period. In addition, simultaneous wind measurements by meteor radar also show enhancement of 14.5‐day periodic oscillation after SSW onset. Tidal components extracted from meteor radar wind measurements in mesosphere and lower thermosphere region and from ionospheric F region critical frequency (foF2) are noticeably modulated by the lunar phase. These observational evidences reveal that the semimonthly lunar period of 14.5 days can be a contributing factor to variations of Es layer during SSW event. Key Points: The foEs observed at middle‐ and low‐latitude show evident enhancement around the day of new and full moon during 2011–2012 and 2012–2013 SSWfoEs presents enhancement of 14.5‐day wave‐type oscillationsThe variations of sporadic E layers in middle and low latitudes during SSW periods can be closely linked to the lunar cycle [ABSTRACT FROM AUTHOR]

Ionospheric sporadic E layer is thin irregularity with much higher plasma density in the ionospheric E region (~90‐120 km). Morphology of sporadic E intensity is investigated by using the COSMIC radio occultation data during 2007‐2012. Maximum gradient value of total electron content fluctuation is defined as Smax index, and it shows good linear relationship with the sporadic E intensity parameter (ordinary critical frequency, foEs) in the low‐latitude and midlatitude regions. By using this Smax index as the sporadic E layer intensity, distribution of sporadic E intensity is obtained. Some known features are confirmed such as the hemisphere summer enhancement, diurnal and semidiurnal behavior, and intensity depression anomaly regions. New findings are also obtained, especially the different behavior pattern in the low‐latitude region. (1) Both positive and negative correlations exist between sporadic E intensity and solar activity, and this correlation also shows complex magnetic latitude dependency. (2) Weaker seasonal variation is found in the low‐latitude region. Besides, hemisphere asymmetries of sporadic E intensity/location are also found during the equinox. (3) Longitudinal wave number‐4 feature of sporadic E intensity is prominent in the lower latitude region, which may be related to the diurnal eastward wave number‐3 tide. In a word, it is feasible to investigate the sporadic E layer intensity distribution based on radio occultation data, which also allows for a global morphology analysis. Key Points: Correlation between sporadic E intensity and solar activity shows complex magnetic latitude dependencyWeaker seasonal variation is identified in low‐latitude region, as well as hemisphere asymmetries of sporadic E intensity during equinoxLongitudinal WN4 feature is prominent in the lower latitude region, which may be related to the diurnal eastward WN3 tide [ABSTRACT FROM AUTHOR]

In the long era of sporadic E layer (Es) ionosonde research, the ordinary wave critical frequency foEs is used as a measure of Es layer occurrence and intensity. This practice, however, undermines that sporadic E layers are in fact metal ion layers, whereas foEs relates inherently to the sum of both the layer metal and the regular E region plasma densities. This means that foEs acts as an overestimate of Es intensity, a fact that somehow has been overlooked and escaped attention. To deal with this deficiency, an algorithm is introduced which combines the ionosonde‐measured foEs and virtual height h'Es parameters with International Reference Ionosphere model predictions of E region electron densities, to arrive at a new critical frequency which depends on sporadic E metal plasma density only; therefore, it represents an unbiased estimate of Es layer intensity. The extensive use of foEs in numerous sporadic E studies must have introduced errors in various results in relation with the Es variability in the time, frequency, and space domains. It is therefore important that this new parameter, notated as foμEs, is now adopted and used instead of foEs in the evaluation of sporadic E. The same methodology applies also for the sporadic E blanketing critical frequency fbEs, which at times is taken as an alternative estimate of Es intensity; similarly, a new parameter, notated as fbμEs, should be used instead of fbEs. Key Points: The ionosonde foEs critical frequency is used as a measure of sporadic E layer intensity; this undermines that Es are metal ion layersfoEs overestimates sporadic E intensity because it relates to the sum of the layer metal atomic ion and E region molecular ion densitiesA method is developed combining ionosonde Es parameters and IRI model electron densities to derive an unbiased estimate of Es intensity [ABSTRACT FROM AUTHOR]

SUMMARY This paper presents a new sporadic control approach to the tracking problem for MIMO closed-loop systems. An LTI sampled data plant with unmeasurable state affected by external unknown disturbances is considered. The plant is interconnected to an event-based digital dynamic output-feedback controller via a network. Both the external reference and the unknown disturbance are assumed to be generated as the free output response of unstable LTI systems. The main feature of the new event-driven communication logic (CL) is that it works without the strict requirement of a state vector available for measurement. The purpose of the CL is to reduce as much as possible the number of triggered messages along the feedback and feedforward paths with respect to periodic sampling, still preserving internal stability and without appreciably degrading the control system tracking capability. The proposed event-driven CL is composed of a sensor CL (SCL) and of a controller CL (CCL). The SCL is based on the computation of a quadratic functional of the tracking error and of a corresponding suitably computed time-varying threshold: a network message from the sensor to the controller is triggered only if the functional equals or exceeds the current value of the threshold. The CCL is directly driven by the SCL: the dynamic output controller sends a feedforward message to the plant only if it has received a message from the sensor at the previous sampled instant. Formulation of the controller in discrete-time form facilitates its implementation and provides a minimum inter-event time given by the sampling period. An example taken from the related literature shows the effectiveness of the new approach. The focus of this paper is on the stability and performance loss problems relative to the sporadic nature of the control law. Other topics such as network delay or packets dropout are not considered. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Meteorological processes (cold fronts, mesoscale convective complexes, thunderstorms) in the troposphere can generate upward propagating waves in the neutral atmosphere affecting the behaviour of the ionosphere. One type of these waves are the internal atmospheric gravity waves (AGWs) which are often generated by thunderstorms. Davis & Johnson (2005) found in low pressure systems that a localized intensification of the sporadic E layer (Es) can be attributed to lightnings. To confirm this result, we have performed two different statistical analysis using the time series of the critical frequency (foEs), the virtual height of the sporadic E layer (h'Es), and meteorological observations (lightnings, Infrared maps) over the ionospheric station of Rome (41.9° N, 12.5° E). In the first statistical analysis, we separated the days of 2009 into two groups: stormy days and fair-weather days, then we studied the occurrence and the properties of the Es separately for the two different groups. No significant differences have been found. In the second case, a superposed epoch analysis (SEA) was used to study the behaviour of the critical frequency and virtual height 100 hours before and after the lightnings. The SEA shows a statistically significant decrease in the critical frequency after the time of the lightnings, which indicates a sudden decrease in the electron density of the sporadic E layer associated with lightnings. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Strain-induced crystallization (SIC) of natural rubber (NR) samples with different strain rates at a fixed strain was investigated by synchrotron radiation X-ray diffraction measurements, which provided the evolution trends of crystal sizes and crystallinity during the SIC process. It was found that the Avrami index was about 1 during the crystallization of NR after the cessation of stretch, which demonstrated that sporadic nucleation occurred during SIC process. The increase of the crystallinity was attributed to the increase of the number of new crystallites rather than the growth of the crystal size. An unexpected relationship between the final crystallinity and the strain rates was observed. The increase of physical crosslink points originated from either entanglement or crystallite was considered as the reason that leads to the nonmonotonic variation of the final crystallinity with strain rates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

Background: Although the histological features of the amyloid plaques in variant Creutzfeldt–Jakob disease (vCJD) are distinct from those in other forms of prion disease [kuru, sporadic Creutzfeldt–Jakob disease (sCJD) and Gerstmann–Sträussler–Scheinker disease (GSS)], their ultrastructural features have only been described in a single case report. Aims: To study vCJD plaques systematically and compare them with plaques in kuru, sCJD, GSS and Alzheimer disease (AD). Methods: Amyloid plaques were studied by transmission electron microscopy and image analysis in five cases of vCJD, three cases of GSS, two cases of sCJD, one case of kuru and five cases of AD. Immunohistochemistry was performed on paraffin sections from one case of vCJD, two cases of GSS, one case of kuru and two cases of sCJD. Results: The florid plaques in vCJD were either compact or more diffuse; in both forms, the radiating fibrils were organized into thick ‘tongues’, in contrast to kuru plaques. Dystrophic neurites (DNs) containing lysosomal electron-dense bodies or vesicles surrounded florid plaques. Microglial cells were found within florid plaques; occasional amyloid fibrils were identified in membrane-bound pockets of microglial cells. In vCJD, there was significant tau immunoreactivity in DNs around florid plaques while, in sCJD, GSS and kuru, minimal tau immunoreactivity was observed around plaques. Conclusions: The ultrastructure of the florid plaques and DNs in vCJD is more reminiscent of neuritic plaques in AD than kuru or multicentric plaques. These findings may reflect differences both in the strains of the transmissible agents responsible for these disorders and in host factors. [ABSTRACT FROM AUTHOR]

Mutations in the TBX22 gene underlie an X-linked malformation syndrome with cleft palate (CP) and ankyloglossia. Its mutations also result in non-syndromic CP in some populations. To investigate whether mutations in TBX22 play a part in the formation of non-syndromic CP in the Thai population, we performed mutation analysis covering all the coding regions of the TBX22 gene in 53 unrelated Thai patients with non-syndromic CP. We identified four potentially pathogenic mutations, 359G→A (R120Q), 452G→T (R151L), 1166C→A (P389Q), and 1252delG in four different patients. All mutations were not detected in at least 112 unaffected ethnic-matched control chromosomes and had never been previously reported. R120Q and R151L, found in two sporadic cases, were located in the DNA binding T-box domain. P389Q and 1252delG, found in two familial cases, were at the carboxy-terminal region, which has never been described. Our study indicates that TBX22 mutations are responsible for a significant proportion of Thai non-syndromic CP cases confirming its importance as a frequent cause of non-syndromic CP across different populations. [ABSTRACT FROM AUTHOR]

Calcineurin (CaN) is a Ser/Thr protein phosphatase involved in a wide range of cellular responses to calcium mobilizing signals. Previous evidence supports the notion that calcineurin is oxidatively inhibited by mutant Cu, Zn superoxide dismutase (SOD1) typical of familial ALS patients in vitro and in transgenic mice. We report that calcineurin activity is markedly inhibited in lymphocytes from 37 sporadic, eight familial ALS patients and an asymptomatic subject carrying an SOD1 mutation as compared to 28 healthy controls. Two other healthy subjects, heterozygous for the D90A mutation from a recessive pedigree, have normal calcineurin activity. Immunoreactive CaN protein, age, sex and riluzole treatment are not related to calcineurin activity in samples from patients. However, we have observed a marked increase in total protein oxidation in extracts from ALS lymphocytes, as compared to extracts from control subjects. These data confirm that modification of calcineurin activity and possibly of calcineurin-mediated pathways of signal transduction (including modulation of apoptotic neuronal death) may contribute to the pathogenesis of ALS. [ABSTRACT FROM AUTHOR]

Oculo-facio-cardio-dental syndrome (OFCD) is an uncommon multiple congenital anomaly syndrome that is characterized by congenital cataracts, multiple minor facial dysmorphic features, congenital heart defects, and dental anomalies including canine radiculomegaly and oligodontia. Although most cases of OFCD are sporadic, since all reported OFCD individuals have been female, it has been suggested that OFCD is an X-linked dominant trait. Here we report two affected female patients with OFCD, a mother and daughter, who both had congenital cataracts, microphthalmia, characteristic dental anomalies, and typical facial dysmorphisms. These features were diagnostic for OFCD; thus, these cases represent the second documented instance of mother-to-daughter OFCD transmission. In addition to the clinical features typically seen in OFCD individuals, the affected daughter exhibited several additional congenital anomalies including intestinal malrotation and hypoplastic thumbs. Thus, these cases further define and expand the OFCD clinical phenotype. These two individuals also displayed a skewed pattern of X chromosome inactivation. Together, these data strongly support the hypothesis that OFCD is inherited as an X-linked dominant condition. © 2003 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]