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This study investigated the latitudinal variations of post‐sunset enhancements in the ionospheric electron density during the geomagnetic quiet period in May 2021 with a combination of high‐precision ionospheric parameters obtained from four ionosondes, Beidou geostationary satellite (BD‐GEO) receiver network and Sanya incoherent scatter radar (SYISR). We identified four categories of post‐sunset enhancement phenomena (Types 1–4), each with unique spatial and temporal evolutions, yet uniformly accompanied by a decrease in hmF2. Measurements of plasma drift vector velocities from SYISR and hmF2 gradients across various latitudes provided pivotal insights, confirming that the ionospheric post‐sunset enhancements can result from downward plasma motion due to westward electric field, downward field‐aligned drift, or a combination of both. For Type 1, dominated by field‐aligned drift, plasma density enhancements not only intensify at low latitudes but may also extend to mid‐latitudes, exhibiting a distinct temporal delay with increasing latitude. In contrast, Type 4, primarily driven by the westward electric field, is characterized by modest increases in plasma density confined to localized low‐latitude regions, with no observable latitudinal time delay in the peak of enhancements. Types 2 and 3, which are subject to the combined influence of the westward electric field and field‐aligned drift, exhibit plasma density increases at certain low‐latitude areas, with Type 2 presenting a delayed pattern and Type 3 showing none with rising latitude. Meanwhile, neutral winds can partially account for the observed post‐sunset enhancement from low to middle latitudes. These findings offer new insights into the factors influencing ionospheric behavior after sunset. Key Points: Four types of distinctive latitudinal variations of post‐sunset enhancements were identified with multiple data sets during the quiet periodAccompanying the increase in the electron density at sunset, the decrease in hmF2 is favorable for the formation of post‐sunset enhancementDownward field‐aligned drift and westward electric field govern the spatial scale of post‐sunset enhancements at low and middle latitudes [ABSTRACT FROM AUTHOR]

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

In this study, we offer an extensive examination of the F‐region ionospheric disturbances during the May 2024 superstorm, focusing primarily on the middle‐low latitude regions of East Asia. Our analysis is grounded in a wealth of data sources including Total Electron Content (TEC), ionospheric parameters NmF2 and hmF2, Electron Density Profile (EDP) retrieved from Radio Occultation (RO) data, and ∑[O]/[N2] from the Global Ultraviolet Imager (GUVI), among others, complemented by model simulations. The observed negative ionospheric storm effect, characterized by a significant and long‐lasting reduction in electron density across the entire China, commenced immediately following the sudden storm commencement (SSC) on 10 May and continued through the main and early recovery phase of the storm on 11 May. On 11–12 May, positive ionospheric storm impacts were initially observed in a restricted geographical area from the post‐midnight to sunrise, first manifesting over the eastern regions of China and then shifting to the central regions. Subsequently, a pronounced negative storm effect persisted throughout the later stages of recovery phase. In contrast, the western regions of China experienced a positive storm effect on 12 May followed by a comparatively mild negative storm phase. This persistent extensive zonal gradient in electron density across the East Asian region resembles the scenarios depicted in prior superstorms attributed to the thermospheric circulation patterns. The disparity in the ionospheric response from east to west in this area is probably a common feature during superstorms, potentially resulting from an arch‐shaped structure of elevated ∑[O]/[N2]. Key Points: The superstorm in May 2024 led to a significant and persistent decrease in electron density at middle‐low latitude over Eastern ChinaPositive ionospheric effects spread from post‐midnight eastern China to central China, with western China seeing a weaker positive stormThe disparity in ionospheric response from east to west mainly arose from an arch‐shaped structure of elevated ∑[O]/[N2] [ABSTRACT FROM AUTHOR]

An unseasonal equatorial plasma bubble (EPB) event occurred in the East/Southeast Asian sector during the geomagnetic storm on 1 December 2023, causing strong amplitude scintillations from equatorial to middle latitudes. Based on the observations from multiple instruments over a large latitudinal and longitudinal region, the spatial features of the super EPB were investigated. The EPB developed vertically at a fast rising speed ∼470 m/s over the magnetic equator and extended to a very high middle latitude more than 40°N, despite that the storm intensity was not very strong with the minimum SYM‐H index −132 nT. In the zonal direction, the super EPB covered over a specific region ∼95–140°E, where the local sunset roughly coincided with southward turning of interplanetary magnetic field (IMF) Bz component. Before the onset of the super EPB, significant upward plasma drift up to ∼110 m/s was observed over the magnetic equator, which could amplify the growth rate of Rayleigh‐Taylor instability and lead to the generation of the super EPB. The significant drift was likely caused by eastward penetration electric field (PEF) due to sharp southward turning of IMF Bz. The local time of storm onset and duration of IMF Bz southward turning during the storm main phase may partly determine the onset region and zonal coverage of the EPB. Plain Language Summary: Previous studies have reported unexpected generation and development of equatorial plasma bubbles (EPBs) during geomagnetic storms, with different EPB morphology varied from case to case. It is important to figure out potential factors controlling the occurrence or absence of EPBs over a specific region during geomagnetic storms, their latitudinal and longitudinal development. In this study, super EPB extending to middle latitudes more than 40°N was observed over a limited zonal region ∼95–140°E during the main phase of the 1 December 2023 geomagnetic storm. Significant upward plasma drift was observed before the onset of the EPB, which coincided with sharp southward turning of interplanetary magnetic field (IMF) Bz that could induce remarkable penetration electric field. Based on the combination of multiple kinds of observations over a large region, the potential factors controlling the generation and development of the EPB over latitude and longitude were investigated. It was suggested that the onset time, duration and intensity of IMF Bz southward turning during the main phase of geomagnetic storm play an important role in controlling the EPB spatial features, including the onset region, zonal coverage and latitudinal extension. Key Points: Ionospheric super bubble with fast rising speed occurred during storm time, extended to more than 40°N and caused strong midlatitude scintillationThe super bubble occurred over a confined longitudinal sector ∼95–140°E where the local sunset roughly coincided with interplanetary magnetic field (IMF) Bz southward turningSignificant upward plasma drift due to penetration electric field (PEF) could be responsible for the generation of the super bubbles [ABSTRACT FROM AUTHOR]

In this study, multiple instrumental observations including Global Navigation Satellite System total electron content (TEC), plasma drift velocity measured by Sanya (18.3°N, 109.6°E, dip latitude 12.6°N) Incoherent Scatter Radar (SYISR) and F2‐layer peak electron density (NmF2) and peak height (hmF2) from ionosonde and SYISR have been used to investigate ionospheric responses during a minor yet highly geo‐effective geomagnetic storm on 26–27 May 2021. Our findings revealed a significant time delay in the post‐sunset plasma density enhancement peak across different latitudes over East Asia, that is, the lower the geographic latitude, the earlier the peak appeared. The plasma density enhancement was accompanied by a decrease in hmF2 prior to NmF2 peak around sunset. The newly built SYISR measurements around sunset verified that the field‐aligned drift decreased the ionosphere with a notable time delay at latitude, beneficial to electron density enhancements at lower altitudes within the 16–30°N latitudinal band but a small TEC change. While at 30–50°N, it is possible that the competition between storm‐induced equatorward winds and downward field‐aligned drift depressed hmF2 decline and the buildup increased both NmF2 and TEC. The ICON observations suggested that the meridional wind during this minor storm event modulated the direction of plasma transport near sunset, playing a dominant role in post‐sunset plasma density enhancement from low to middle latitudes. These results provide fresh insight into the electrodynamic mechanisms of post‐sunset enhancements at middle and low latitudes over East Asia, and also enhance our understanding of the intricate behaviors within the ionosphere‐thermosphere system in response to a minor storm. Key Points: The local time of post‐sunset plasma density enhancement peak had a notable time delay with increasing latitude during a minor stormThe plasma drift during postsunset hours observed by Sanya incoherent scatter radar exhibited strong response to the minor geomagnetic stormThe meridional winds changing from equatorward to poleward explained the post‐sunset enhancement from low to middle latitudes [ABSTRACT FROM AUTHOR]

Ionospheric equatorial plasma bubbles (EPBs) are usually generated around sunset over equatorial to low latitudes. In this study, super EPBs extending to middle latitudes were observed, which were freshly generated at both post‐sunset and near‐sunrise periods over East/Southeast Asia during the geomagnetic storm on 26–28 February 2023. The post‐sunset (near‐sunrise) EPB persisted ∼4 (8) hours, drifting eastward (westward) and extending up to ∼35°N. Strong L‐band scintillations, deep total electron content (TEC) depletions and significant positioning errors were caused by the post‐sunset EPB. However, the scintillations and TEC depletions linked with the near‐sunrise EPB were relatively weaker, and no apparent positioning error was caused. Before the onsets of the post‐sunset EPBs, rapid upward vertical plasma drifts of ∼60 m/s, which could be linked with storm‐time prompt penetration electric fields, were observed at magnetic equator. The upward vertical drifts could amplify the F‐region bottomside perturbation via increasing the growth rate of Rayleigh‐Taylor (R‐T) instability and lead to the generation of post‐sunset EPBs. On the other hand, before the onset of near‐sunrise EPBs, the uplift of F layer was more significant at higher latitudes than that at magnetic equator, probably indicating the presence of equatorward neutral wind. Both the equatorial F layer uplift and equatorward neutral wind could contribute to the growth of R‐T instability and favor the generation of near‐sunrise EPBs. Plain Language Summary: Equatorial plasma bubble (EPB) is a kind of ionospheric irregularity which could cover a large area up to thousands of kilometers and cause strong radio signal scintillation and positioning error. It is usually generated at post‐sunset hours when the pre‐reversal enhancement of the eastward electric field could increase the growth rate of the Rayleigh‐Taylor instability, and observed at equatorial and low latitudes. Here we report unusual cases of super EPBs extending to middle latitudes, which were freshly generated at both post‐sunset and near‐sunrise periods during the geomagnetic storm on 26–28 February 2023. Based on the observations by multiple instruments from equatorial to middle latitudes, a comparison study is performed to analyze the characteristics of the EPBs during the two periods, that is, sunset and sunrise hours, including their evolution processes, scintillation characteristics and effects to GNSS positioning, and possible driving mechanisms. The results showed that relatively weaker scintillation, total electron content depletion and positioning error were associated with the near‐sunrise EPB than the post‐sunset EPB. Storm‐time electric field and/or equatorward neutral wind, together with the bottomside perturbations could contribute to the generation of the EPBs at different periods. Key Points: Ionospheric super bubbles extending to middle latitudes were freshly generated at post‐sunset and near‐sunrise periods during storm timeThe sunrise bubbles caused relatively weaker scintillation, total electron content depletion and positioning error than the post‐sunset bubblesRapid uplift of F layer driven by storm‐time electric fields, and equatorward neutral wind could contribute to the generation of the bubbles [ABSTRACT FROM AUTHOR]

Measuring the plasma motion vector field accurately is beneficial to understanding the physical processes in the ionosphere. As a newly built advanced modular phase array Incoherent Scatter Radar in Sanya (SYISR), SYISR can measure the line‐of‐sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift at low latitudes in East Asia. Compared to the traditional ISR, it can operate continuously for a long‐time and perform with fast and flexible scanning. This paper evaluates different beam configurations and inversion methods for vector ion velocity detection by comparing the vector ion velocities input in the forward modeling and derived by inversion. For the mono‐static SYISR, the all‐sky scan mode is suitable for observing the distribution of vector ion velocity with height, and the fine scan mode is suitable for observing the spatial distribution of two components of vector ion drift perpendicular to the magnetic field in the northern region of SYISR's non‐grating lobe. For the future tri‐static SYISR, it could simultaneously observe the spatial distribution of all three components of the vector ion velocity. Besides, the inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocity, especially when the LOS ion velocity has a large error. These results will benefit the design and optimization of the actual experiments for the vector ion velocity measurement and inversion using the SYISR system. Key Points: A procedure is constructed to guide and evaluate the detection of the vector ion velocity by the Incoherent Scatter Radar in Sanya (SYISR) systemThe ion velocity inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocityMono‐static and tri‐static SYISR can be used to detect the height profile of vector ion velocity and spatial distribution, respectively [ABSTRACT FROM AUTHOR]

In this study, multiple data sets from Beidou geosynchronous Earth orbit (GEO) satellite, ionosonde, magnetometer, Swarm satellite, and meteor radar were used to investigate an anomalous disturbance of the ionosphere in East Asia on 1 November 2016. During 03:00–08:00 universal time (UT), the total electron content (TEC) observed by the Beidou GEO satellite decreased by up to 8.17% at low latitudes and increased by up to 140.19% at middle to low latitudes in East Asia compared to the average TEC on the reference days. The F2 peak electron density (NmF2) measured by ionosondes decreased by 16.00% at Sanya (109.4°E, 18.3°N) and increased by 51.96% and 53.97% at Wuhan (114.4°E, 30.5°N) and Shaoyang (111.3°E, 27.1°N), respectively. The equatorial electric field (EEF) observed by the Swarm satellite at 06:59 UT on 1 November was approximately 77.57% higher than on 4 November. The variations of equatorial electrojet near 100°E and 120°E on 1 November presented an east‐west asymmetry in which there was a significant enhancement in the east while there was no obvious increase in the west. The TEC enhancement derived by the Center for Orbit Determination in Europe also showed the same east‐west asymmetry in the two longitudes. Based on the comprehensive analysis of multiple observational data, the increase in EEF caused the ionospheric TEC and NmF2 to decrease at low latitudes and increase at middle to low latitudes in East Asia during 03:00–08:00 UT on 1 November 2016. The lower atmospheric tidal forcing could contribute to the enhancement of the EEF. Key Points: An anomalous disturbance of ionospheric total electron content (TEC) and NmF2 occurred in East Asia during the geomagnetic quiet day on 1 November 2016The enhancements in the equatorial electrojet and CODE‐TEC near 100°E and 120°E present an east‐west asymmetryThe equatorial electric field enhancement causes the ionospheric TEC to decrease at low latitudes and increase at middle to low latitudes [ABSTRACT FROM AUTHOR]

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

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

The ionospheric electron density is expected to enhance after sunrise during geomagnetic quiet days. In this paper, we report an anomalous large‐scale ionospheric TEC depletion structure occurring in morning hours over low latitudes in China under relatively quiet geomagnetic conditions. The depletion structure, which occurred over a wide longitude range of >35°, developed toward higher latitudes at the speed of ∼250 m/s. A statistical analysis using the data in 2018–2020 showed that such large‐scale daytime TEC depletion mainly occurred in summer (mostly with Kp < 3), with large intensities at ∼16–30°N. The geomagnetic ΔH measurements at the stations near and away from the magnetic dip equator showed positive values, which rules out the possible contribution from a westward electric field. Observations from multiple ionosondes revealed a net downward plasma transport during the TEC depletion. An enhanced poleward neutral wind during the early morning hours, which may induce downward plasma transport, was surmised to play a leading role in driving the large‐scale morning TEC depletion. Plain Language Summary: Under geomagnetic storms, the daytime ionosphere may undergo significant plasma density depletion over wide longitude and latitude regions. Possible driving mechanisms of large‐scale plasma density depletion include the suppression of the fountain effect by westward equatorial electrojets, the upward plasma diffusion caused by increasing electron temperature, and the decreased O/N2 ratio transported by perturbed neutral circulation. In this paper, we report cases of anomalous large‐scale plasma density depletion structures in morning hours of geomagnetic quiet days over low latitudes in China. The large‐scale plasma density depletion structure, which produced TEC reduction of ∼4 TECu, covered >3,500 km in the east‐west direction and developed poleward. Based on the observations from multiple ionosondes and magnetometers at equatorial and low latitudes, it is unlikely that the possible driving sources are from geomagnetic activities. We surmise that there could be an enhanced poleward neutral wind over a wide longitude region which induced downward plasma transport and produced the depletion structure. Key Points: A daytime large‐scale TEC depletion structure covering ∼35° in longitude under relatively quiet geomagnetic conditions was reportedThe daytime large‐scale depletion structure with poleward development mainly occurred in summer with large intensities at 16–30°NIt is surmised that enhanced poleward neutral wind could be a possible driver for the daytime large‐scale depletion structure [ABSTRACT FROM AUTHOR]

A special F2 layer stratification structure named F3 layer occurs frequently in equatorial and low latitude ionosphere during summer daytime. In this study, a new phenomenon of decreasing occurrence of the F3 layer, and narrowing differences of virtual heights between the F3 and F2 layers in the recovery phase of geomagnetic storms is reported. We named this phenomenon as the inhibition of F3 layer event (IFLE). Using the ionosonde observations during summer of 2012–2015 at Sanya (18.3°N, 109.6°E, dip latitude 12.6°N), we found that IFLE occurred during 14 geomagnetic storms (−127 nT ≤ Dstmin ≤ −22 nT), which was accompanied by the thinning and lowering bottom ionosphere, and decreasing the crest‐to‐trough ratio of total electron content (TEC). Together with the ion drift data measured by Defense Meteorological Satellite Program F18, we suggest that the IFLE is mainly caused by the westward disturbance dynamo electric field (DDEF; downward drift velocity), taking disadvantage of the formation of the F3 layer. The observed decrease in the crest‐to‐trough ratio of TEC also indicates that the westward DDEF should prompt IFLE by providing less plasma from the equatorial region to the low latitude. Hence, IFLE then can be a good indicator to show how the magnetosphere‐ionospheric coupling process affects the low and equatorial ionosphere. Notably, the results also indicate that even a very weak geomagnetic storm can generate significant changes in ionospheric state at low latitude. Key Points: A new phenomenon of the inhibition of F3 layer over Sanya during recovery phase of geomagnetic storms in summer is reportedEven weak geomagnetic storms can produce significant changes in the low latitude ionosphere over Sanya in the recovery phaseInhibition of F3 layer event is mainly caused by the westward disturbance dynamo electric field [ABSTRACT FROM AUTHOR]

The effects of the 13–14 March 1989 great magnetic storm on the East Asian ionosphere have been re‐investigated using ground‐based and satellite measurements as well as theoretical simulation. Within introduction of new data like the Chinese ionosondes and DMSP F8 and F9 and low‐equatorial magnetometer data, we are able to track the ionospheric response at both the bottomside and topside ionosphere from middle to low latitude to obtain an overall understanding of storm‐time ionospheric change. Through the comparative study of different longitude bands, we found that the East‐Asian ionosphere was characterized by a strong westward electron density gradient persisting over a day at both the bottomside and topside ionosphere at mid‐low latitudes during the main and recovery phases of the storm. This feature was not studied in the previous literature for this event at this area. We then examine the effect through a numerical simulation work from the Thermosphere Ionosphere Electrodynamics General Circulation Model under Apex and Dipole geomagnetic fields. It is seen that the model well reproduces the zonal gradients during this event under realistic geomagnetic field model. The conditions favor for this structure requires a hemispheric asymmetry response of storm thermosphere as well as background condition and also the storm development which requires further investigation. This study shows that even very close stations would manifest totally different storm behaviors during the superstorm event suggesting a great challenge in the space weather prediction. Key Points: Use dense ionosonde network to study the east‐west differences of the ionospheric response to the March 1989 stormReveal a long‐duration strong zonal gradient in ionospheric electron density during superstorm event in the East Asian regionPropose that north‐south asymmetry of the geomagnetic configuration may cause the zonal gradient in the East Asian region in equinox [ABSTRACT FROM AUTHOR]

A new method for estimating the equatorial plasma bubbles (EPBs) motions from airglow emission all‐sky images is presented in this paper. This method, which is called cloud motion wind (CMW) and widely used in satellite observation of wind, could reasonably derive zonal and meridional velocity vectors of EPBs drift by tracking a series of successive airglow images. Airglow emission images data are available from all‐sky airglow camera in Hainan Fuke (19.5°N, 109.2°E) supported by China Meridional Project, which can receive the 630.0 nm emission from ionosphere F region at low latitudes to observe plasma bubbles. A series of pretreatment technology is utilized to preprocess the raw observation. Then the regions of plasma bubble extracted from the images are divided into several small tracing windows, and each tracing window can find a target window in the searching area in following image, which is considered as the position tracing window moved to. According to this, velocity of each tracing window was calculated by CMW. The maximum correlation coefficient is adopted to analyze the velocity of plasma bubbles due to its better performance than histogram of oriented gradient. All‐sky images from Hainan Fuke, an example on 17 September 2014, are analyzed to investigate the plasma bubble drift velocities using CMW. For comparison and validation, EPBs motions obtained from three traditional methods are also investigated. The advantages and disadvantages of using CMW are discussed. The results of CMW are compared with slant total electron contents data and show a good consistency, but some errors are also discussed. Plain Language Summary: The advantages of our method are as follows. First, CMW can obtain the velocity of different parts in EPB. Second, the result of CMW has a relatively good stability. The disadvantage of CMW is that the computational complexity is sometimes large, because self‐adaptive method is used and this process involves looping operations. The result of CMW shows a good consistency with GPS STEC data, which means that the results of CMW are generally reliable. Key Points: Equatorial plasma bubbles velocity is derived by cloud motion wind (CMW) technology by tracking a series of successive airglow imagesCMW can obtain the velocity of different parts in EPBResults of CMW are compared with STEC data and show a good consistency, but some errors are also discussed [ABSTRACT FROM AUTHOR]

Abstract: Using C/NOFS satellite observations, this paper studies the disturbance field‐aligned plasma drifts in the equatorial topside ionosphere during eight geomagnetic storms in 2011–2015. During all six storms occurred in the solstices, the disturbance field‐aligned plasma drift is from winter to summer hemisphere especially in the morning‐midnight local time sector and the disturbance is stronger in June solstice. The two storms occurred at equinoxes have very little effect on the field‐aligned plasma drift. Using the plasma temperature data from DMSP satellites and Global Positioning System‐total electron content, it is suggested that the plasma density gradient seems likely to cause the disturbance winter‐to‐summer plasma drift while the role of plasma temperature gradient is opposite to the observed plasma drift. [ABSTRACT FROM AUTHOR]

Abstract: The ionospheric sporadic E (Es) layer has significant impact on radio wave propagation. The traditional techniques employed for Es layer observation, for example, ionosondes, are not dense enough to resolve the morphology and dynamics of Es layer in spatial distribution. The ground‐based Global Navigation Satellite Systems (GNSS) technique is expected to shed light on the understanding of regional strong Es occurrence, owing to the facts that the critical frequency (foEs) of strong Es structure is usually high enough to cause pulse‐like disturbances in GNSS total electron content (TEC), and a large number of GNSS receivers have been deployed all over the world. Based on the Chinese ground‐based GNSS networks, including the Crustal Movement Observation Network of China and the Beidou Ionospheric Observation Network, a large‐scale strong Es event was observed in the middle latitude of China. The strong Es shown as a band‐like structure in the southwest‐northeast direction extended more than 1,000 km. By making a comparative analysis of Es occurrences identified from the simultaneous observations by ionosondes and GNSS TEC receivers over China middle latitude statistically, we found that GNSS TEC can be well employed to observe strong Es occurrence with a threshold value of foEs, 14 MHz. [ABSTRACT FROM AUTHOR]