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In this paper, we consider pseudoharmonic heat flow with small initial horizontal energy and give the existence of pseudoharmonic maps from closed pseudo-Hermitian manifolds into closed Riemannian manifolds.

Plasma blob is generally a low‐latitude phenomenon occurring at the poleward edge of equatorial plasma bubble (EPB) during post‐sunset periods. Here we report a case of midlatitude ionospheric plasma blob‐like structures occurring along with super EPBs over East Asia around sunrise during the May 2024 great geomagnetic storm. Interestingly, the blob‐like structures appeared at both the poleward and westward edges of EPBs, reached up to 40°N magnetic latitudes, and migrated westward several thousand kilometers together with the bubble. The total electron content (TEC) inside the blob‐like structures was enhanced by ∼50 TEC units relative to the ambient ionosphere. The blob‐like structure at the EPB poleward edge could be partly linked with field‐aligned plasma accumulation due to poleward development of bubble. For the blob‐like structure at the EPB west side, one possible mechanism is that it was formed and enhanced accompanying the bubble evolution and westward drift. Accompanying the generation of equatorial plasma bubble (EPB), an extra structure with plasma density enhancement may occur. The density‐enhanced structure is known as plasma blob. Generally, plasma blobs mainly appear at the poleward edge of EPB, being low‐latitude phenomena occurring at 10–20° magnetic latitudes. Whereas previous simulations showed that plasma blobs could appear at the east/west side of EPBs, there were few observational evidences and the driving mechanism is unclear. In this study, midlatitude plasma blob‐like structures occurring up to 40°N magnetic latitude was observed along with super EPBs during magnetic storm. Different from most blobs appearing at the EPB poleward edge, the blob‐like structures in the present study appeared at both the EPB poleward and westward edges. By using ground‐based observations from GNSS receiver networks and in‐situ measurements onboard spacecraft, the morphology and evolution of the super plasma blob‐like structures are visualized. Potential mechanisms responsible for their generation are investigated. The results highlight the coexistence of large‐scale plasma depletion and blob structures and their complex evolution with longitude and latitude, and have implications for better understanding the sudden changes of plasma density in time and altitude observed by radar. Super plasma blob‐like structures occurred up to 40°N MLat along with equatorial plasma bubbles near sunrise during storm timeThe blob‐like structures mainly occurred at the bubble poleward and westward edges and migrated westward along with the bubbleThe poleward development and evolution of the bubble could contribute to the formation of super blob‐like structures Super plasma blob‐like structures occurred up to 40°N MLat along with equatorial plasma bubbles near sunrise during storm time The blob‐like structures mainly occurred at the bubble poleward and westward edges and migrated westward along with the bubble The poleward development and evolution of the bubble could contribute to the formation of super blob‐like structures

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/N2decreased 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/N2indicate 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. Normally, the peak Ne in the F2layer (NmF2) is the largest in the full Ne profile. G‐conditions are special events in ionospheric observations, when NmF2is smaller than or equal to NmF1(i.e., NmF2≤ NmF1). Due to shielding effect of the F1layer, no information above hmF1can 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/N2showed obvious decrease compared to quiet reference days. This is consistent with the analysis of the mechanism of G‐conditions formation in previous studies. 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/N2collectively contributed to a decrease in Ne The G‐condition (NmF2≤ NmF1) was observed by ground‐based ionosondes at Mohe and Beijing during the G4 storm on 24 April 2023 Relative 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/N2collectively contributed to a decrease in Ne

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. 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 Four types of distinctive latitudinal variations of post‐sunset enhancements were identified with multiple data sets during the quiet period Accompanying the increase in the electron density at sunset, the decrease in hmF2 is favorable for the formation of post‐sunset enhancement Downward field‐aligned drift and westward electric field govern the spatial scale of post‐sunset enhancements at low and middle latitudes

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]. 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] The superstorm in May 2024 led to a significant and persistent decrease in electron density at middle‐low latitude over Eastern China Positive ionospheric effects spread from post‐midnight eastern China to central China, with western China seeing a weaker positive storm The disparity in ionospheric response from east to west mainly arose from an arch‐shaped structure of elevated ∑[O]/[N2]

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. 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. 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 Ionospheric super bubble with fast rising speed occurred during storm time, extended to more than 40°N and caused strong midlatitude scintillation The super bubble occurred over a confined longitudinal sector ∼95–140°E where the local sunset roughly coincided with interplanetary magnetic field (IMF) Bz southward turning Significant upward plasma drift due to penetration electric field (PEF) could be responsible for the generation of the super bubbles

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. 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 The local time of post‐sunset plasma density enhancement peak had a notable time delay with increasing latitude during a minor storm The plasma drift during postsunset hours observed by Sanya incoherent scatter radar exhibited strong response to the minor geomagnetic storm The meridional winds changing from equatorward to poleward explained the post‐sunset enhancement from low to middle latitudes

In the past decades, the Incoherent Scatter Radar (ISR) has been demonstrated to be one of the most powerful instruments for ionosphere monitoring. The Institute of Geology and Geophysics at the Chinese Academy of Sciences was founded to build a state‐of‐the‐art phased‐array ISR at Sanya (18.3°N, 109.6°E), a low‐latitude station on Hainan Island, named the Sanya ISR (SYISR). As a first step, a prototype radar system consisting of eight subarrays (SYISR‐8) was built to reduce the technical risk of producing the entire large array. In this work, we have summarized the preliminary experimental results based on the SYISR‐8. The amplitude and phase among 256 channels were first calibrated through an embedded internal monitoring network. The mean oscillation of the amplitude and phase after calibration were about 1 dB and 5°, respectively, which met the basic requirements. The beam directivity was confirmed by crossing screen of the International Space Station. The SYISR‐8 was further used to detect the tropospheric wind profile and meteors. The derived winds were evaluated by comparison with independent radiosonde and balloon‐based GPS measurements. The SYISR‐8 was able to observe several typical meteor echoes, such as the meteor head echo, range‐spread trail echo, and specular trail echo. These results confirmed the validity and reliability of the SYISR‐8 system, thereby reducing the technical risk of producing the entire large array of the SYISR to some extent.

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. 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

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. 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. 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 Ionospheric super bubbles extending to middle latitudes were freshly generated at post‐sunset and near‐sunrise periods during storm time The sunrise bubbles caused relatively weaker scintillation, total electron content depletion and positioning error than the post‐sunset bubbles Rapid uplift of F layer driven by storm‐time electric fields, and equatorward neutral wind could contribute to the generation of the bubbles

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 Fregion 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. 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. 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

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. Seasonal dependence of disturbance field‐aligned plasma drift in the equatorial topside ionosphere is presentedDisturbance field‐aligned plasma drift is from winter to summer hemisphereConjugate asymmetry of plasma density gradient seems likely to cause the disturbance winter‐to‐summer plasma drift

The ionospheric sporadic E(Es) layer has significant impact on radio wave propagation. The traditional techniques employed for Eslayer observation, for example, ionosondes, are not dense enough to resolve the morphology and dynamics of Eslayer in spatial distribution. The ground‐based Global Navigation Satellite Systems (GNSS) technique is expected to shed light on the understanding of regional strong Esoccurrence, owing to the facts that the critical frequency (foEs) of strong Esstructure 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 Esevent was observed in the middle latitude of China. The strong Esshown as a band‐like structure in the southwest‐northeast direction extended more than 1,000 km. By making a comparative analysis of Esoccurrences 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 Esoccurrence with a threshold value of foEs, 14 MHz. Large‐scale band‐like strong Esstructures elongated more than 1,000 km were detected by ground‐based GNSSGNSS TEC can be well employed to observe strong Esoccurrence with a threshold value of foEs, 14 MHzTracking of BDS GEO signal can observe Esstructures of fixed points continuously

Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF/UHF coherent scatter radars in the past, no simultaneous radar and optical instruments were employed to investigate the characteristics of meteor trail irregularity and its corresponding meteoroid. By installing multiple video cameras near the Sanya VHF radar site, an observational campaign was conducted during the period from November 2016 to February 2017. A total of 242 optical meteors with simultaneous non‐specular echoes backscattered from the plasma irregularities generated in the corresponding meteor trails were identified. A good agreement between the angular positions of non‐specular echoes derived from the Sanya radar interferometer and those of optical meteors was found, validating that the radar system phase offsets have been properly calibrated. The results also verify the interferometry capability of Sanya radar for meteor trail irregularity observation. The non‐specular echoes with simultaneous optical meteors were detected at magnetic aspect angles greater than ~78°. Based on the meteor visual magnitude estimated from the optical data, it was found that the radar non‐specular echoes corresponding to brighter meteors survived for longer duration. This could provide observational evidence for the significance of meteoroid mass on the duration of meteor trail irregularity. On the other hand, the simultaneous radar and video common‐volume observations showed that there were some cases with optical meteors but without radar non‐specular echoes. One possibility could be that some of the optical meteors appeared at extremely low altitudes where meteor trail irregularities rarely occur.

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. 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 A procedure is constructed to guide and evaluate the detection of the vector ion velocity by the Incoherent Scatter Radar in Sanya (SYISR) system The ion velocity inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocity Mono‐static and tri‐static SYISR can be used to detect the height profile of vector ion velocity and spatial distribution, respectively

A detailed observation of the coseismic ionospheric disturbances (CIDs) after the Mw7.8 Gorkha earthquake has been carried out using total electron content (TEC) data from 333 GPS stations. This inland earthquake occurred on 25 April 2015, at a depth of ~15 km. Following the quake at 6:11 UT, groups of CIDs were observed from 06:18 to 07:00 UT. The CIDs we observed could be classified into three groups: CIDs related to gravity waves (GWs), CIDs related to acoustic waves (AWs), and CIDs related to seismic Rayleigh surface waves. The GW‐related CIDs traveled at a velocity of ~500 m/s. They were mainly observed to the south and north of the epicenter, within 600 km of the event, with a maximum amplitude of 0.18 total electron content unit (TECU; 1 TECU = 1016el m−2). The AW‐related CIDs traveled at a velocity of ~700–1400 m/s. They could be observed for 1000 km in all directions from the epicenter, with an amplitude of ~0.3 TECU. The CIDs related to Rayleigh waves could be observed only to the east of the epicenter, at a maximum distance of 1800 km. They had an apparent velocity of ~2.0–2.3 km/s. Through comparing the seismogram data from Incorporated Research Institutions for Seismology to the TEC data, we found that the azimuthal characteristics of the ground vibrations agree with the directivity of the CIDs related to Rayleigh surface waves. Investigation of ionosphere response to inland quake by a large GPS‐TEC data setDetailed characteristics of CIDs following the Gorkha earthquakeFar‐field CIDs and seismic Rayleigh surface waves show similar directivity

The July 2012 geomagnetic storm is an extreme space weather event in solar cycle 24, which is characterized by a southward interplanetary geomagnetic field lasting for about 30 h below −10 nT. In this work, multiple instrumental observations, including electron density from ionosondes, total electron content (TEC) from Global Positioning System, Jason‐2, and Gravity Recovery and Climate Experiment, and the topside ion concentration observed by the Defense Meteorological Satellite Program spacecraft are used to comprehensively present the regional differences of the ionospheric response to this event. In the Asian‐Australian sector, an intensive negative storm is detected near longitude ~120°E on 16 July, and in the topside ionosphere the negative phase is mainly existed in the equatorial region. The topside and bottomside TEC contribute equally to the depletion in TEC, and the disturbed electric fields make a reasonable contribution. On 15 July, the positive storm effects are stronger in the Eastside than in the Westside. The topside TEC make a major contribution to the enhancement in TEC for the positive phases, showing the important role of the equatorward neutral winds. For the American sector, the equatorial ionization anomaly intensification is stronger in the Westside than in the Eastside and shows the strongest feature in the longitude ~110°W. The combined effects of the disturbed electric fields, composition disturbances, and neutral winds cause the complex storm time features. Both the topside ion concentrations and TEC reveal the remarkable hemispheric asymmetry, which is mainly resulted from the asymmetry in neutral winds and composition disturbances. Use multi‐instrument observations to study the regional differences of the ionospheric response to the July 2012 stormThe positive and negative storms have salient regional and altitudinal differencesThe hemispheric asymmetry in the ionospheric response over the American sector

A new method for estimating daily averaged peak height of the OH airglow layer from a ground‐based meteor radar (MR) and a Fabry‐Perot interferometer (FPI) is presented. The first results are derived from 4 year simultaneous measurements of winds by a MR and a FPI at two adjacent stations over center China and are compared with observations from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. The OH airglow peak heights, which are derived by using correlation analysis between winds of the FPI and MR, are found to generally peak at an altitude of 87 km and frequently varied between 80 km and 90 km day to day. In comparison with SABER OH 1.6 μm observations, reasonable similarity of airglow peak heights is found, and rapid day‐to‐day variations are also pronounced. Lomb‐Scargle analysis is used to determine cycles of temporal variations of airglow peak heights, and there are obvious periodic variations both in our airglow peak heights and in the satellite observations. In addition to the annual, semiannual, monthly, and three monthly variations, the shorter time variations, e.g., day‐to‐day and several days' variations, are also conspicuous. The day‐to‐day variations of airglow height obviously could reduce observation accuracy and lead to some deviations in FPI measurements. These FPI wind deviations arising from airglow height variations are also estimated to be about 3–5 m/s from 2011 to 2015, with strong positive correlation with airglow peak height variation. More attention should be paid to the wind deviations associated with airglow height variation when using and interpreting winds measured by FPI. Airglow heights are derived by simultaneous observation of MR and FPI. It provides a chance for continuous studying their temporal variationIn addition to the long‐term variations, the day‐to‐day variations are also conspicuousFPI wind difference arising from height variations is about 3–5 m/s. One should pay attention to this difference when using FPI winds

The storm‐enhanced density (SED) is an electron density enhancement phenomenon extending from the later afternoon at middle latitude to the noontime in high latitude within a narrow band during early stage of geomagnetic storm. Previous investigations were mostly focused on the northern America region due to sparse data coverage in other regions. Recent observational analysis and simulation studies have shown that the SED might be quite complicated both physically and spatial/temporal evolution. In this paper, we obtained the global ionospheric electron density with the spatial/temporal resolution of 5° in latitude, 10° in longitude, ~30 km around F2peak, and 0.5 h in time, during 17 March 2013 geomagnetic storm through assimilating ground and LEO‐based total electron content (TEC) data into the model. A total of ~450 ground‐based Global Navigation Satellite Systems stations' and ~10 LEO satellites' observations were applied in the assimilation. Of all the data, six satellites with ionospheric radio occultation profiling capability provided the key altitudinal variation information. The SED associated with the tongue of ionization (TOI) and boundary blob can be well identified from the data assimilation results, although their amplitude of enhancement was only up to ~6 TECU (TEC unit, 1 TECU = 1016el m−2). All structures show very dynamic and complicated time evolution features. During this storm time, we identified two separate SED/TOI/blob structures corotating from Europe to American with conjugate occurrence. This partly supports the mechanism of convection expansion. Given a significant amount of radio occultation will be available in the near future, this method will help up us to resolve global large‐scale ionospheric disturbance down to very small spatial and temporal scale in storm time. Global optimized ionospheric electron density was obtained through assimilating 10 LEO satellites and ~450 ground GNSS observationsGlobal storm‐time‐enhanced structures in middle and high latitudes such as SED, TOI, and boundary blob were identifiedBoth the SED and TOI show corotating and magnetic conjugacy occurrence

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 F2peak 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. 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 An anomalous disturbance of ionospheric total electron content (TEC) and NmF2 occurred in East Asia during the geomagnetic quiet day on 1 November 2016 The enhancements in the equatorial electrojet and CODE‐TEC near 100°E and 120°E present an east‐west asymmetry The equatorial electric field enhancement causes the ionospheric TEC to decrease at low latitudes and increase at middle to low latitudes

The 2015 St. Patrick's Day geomagnetic storm with SYM‐Hvalue of −233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the Asian‐Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in low latitude and equatorial regions. PEFs occur on 17 March in both the American sector and the Asian‐Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the low‐latitude ionospheric response during this storm. The negative storm effects caused by the long‐duration DDEFs are intense over the Asian‐Australian sector, while the repeated elevations of hmF2and the equatorial ionization anomaly intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3layer features are caught on 17 March in the American equatorial region, proving the effects of the multiple strong eastward PEFs. The disturbed electric fields play a decisive role in the ionospheric responses in low latitude and equatorial regions during the stormPenetration electric fields and disturbance dynamo electric fields occur in both the Asian‐Australian sector and the American sectorThe salient storm time F3 layer is observed in the American equatorial region but not in the East Asian sector

In this paper, an ionospheric electron density reanalysis algorithm was used to generate global optimized electron density during the 17–18 March 2013 geomagnetic storm by assimilating ~10 low Earth orbit satellites based and ~450 ground global navigation satellite system receiver‐based total electron content into a background ionospheric model. The reanalyzed electron density could identify the large‐scale ionospheric features quite well during storm time, including the storm‐enhanced density, the positive ionospheric storm effect during the initial and main phases, and the negative ionospheric storm effect during the recovery phase. The simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model can reproduce similar large‐scale ionospheric disturbances as seen in the reanalysis results. Both the reanalysis and simulations show long‐lasting (>17 h) daytime negative storm effect over the Asia sector as well as hemispheric asymmetry during the recovery phase. Detailed analysis of the Global Ultraviolet Imager‐derived O/N2ratio and model simulations indicate that the polar ward meridional wind disturbance, the downward E × B drift disturbance and O/N2depletion might be responsible for the negative storm effect. The hemispheric asymmetry is mainly caused by the geomagnetic field line configuration, which could cause hemispheric asymmetry in the O/N2depletion. Global ionospheric storm effects during 17 March 2013 geomagnetic storm were studied based on the data assimilation and TIEGCM simulationBoth the reanalysis and simulations show long‐lasting and hemispheric asymmetry negative storm effect during the recovery phaseThe wind, E x B drift disturbance, and hemispheric asymmetry O/N2 depletion might be responsible for the negative storm effect

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_Tiand the overestimated IRI_Te. The comparison between the daytime SYISR_Tiand ICON_Tishows 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. 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. 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 Sanya incoherent scatter radar (SYISR) is a new phased array system dedicated to the ionospheric observations at low latitudes in Asia sector SYISR is operational, providing high‐quality ionospheric measurements in multiple radar scanning modes and radio pulse schemes modes Data processing procedures and initial ion‐line results using typical experiment modes are reported

Ionospheric observations from the ground‐based GPS receiver network, CHAMP and GRACE satellites and ionosondes were used to examine topside and bottomside ionospheric variations at low and middle latitudes over the Pacific and American sectors during the October 2003 superstorms. The latitudinal variation and the storm time response of the ground‐based GPS total electron content (TEC) were generally consistent with those of the CHAMP and GRACE up‐looking TEC. The TECs at heights below the satellite altitudes during the main phases were comparable to, or even less than, the quiet time values. However, the storm time CHAMP and GRACE up‐looking TECs showed profound increases at low and middle latitudes. The ground‐based TEC and ionosonde data were also combined to study the TEC variations below and above the F2peak height (hmF2). The topside TECs above hmF2at low and middle latitudes showed significant increases during storm time; however, the bottomside TEC below hmF2did not show so obvious changes. Consequently, the bottomside ionosphere made only a minor contribution to the ionospheric positive phase seen in the total TEC at low and middle latitudes. Moreover, at middle latitudes F2peak electron densities during storm time did not have the obvious enhancements that were seen in both the ground‐based and topside TECs, although they were accompanied by increases of hmF2. Therefore, storm time TEC changes are not necessarily related to changes in ionospheric peak densities. Our results suggest that TEC increases at low and middle latitudes are also associated with effective plasma scale height variations during storms. Bottomside ionosphere made a minor contribution to ionospheric positive stormTEC changes are not necessarily related to ionospheric peak densitiesTEC increases are also associated with effective plasma scale height variations

Previous works have shown that coseismic ionospheric disturbances (CIDs) after the tsunamigenic 2011 Tohoku earthquake (Tohoku EQ, Mw9.1) covered a vast area and were observed thousands of kilometers away from the epicenter. For the purpose of making a comprehensive comparison between powerful oceanic and inland EQs, we conduct a retrospective investigation of CIDs and geomagnetic responses to the 2008 Wenchuan EQ (Mw7.9) using a combination of techniques, total electron content, HF Doppler, and ground magnetometer. It is the very first study to present CIDs recorded by different techniques at co‐located sites and profiled with regard to changes of both ionospheric plasma and current (geomagnetic field) simultaneously. The integrated observation also shows that (1) in the Wenchuan case, most of the ionospheric and geomagnetic disturbances were observed within 1000 km distance which is far less than the Tohoku case; (2) two groups of CIDs were found with maximum amplitudes in the direction of azimuth 150° and 135°, respectively; and (3) the geomagnetic changes were only registered by three magnetometers located to the east and southeast of the epicenter. All the facts indicate that the main directional lobe of Wenchuan EQ energy propagation is to southeast and perpendicular to the direction of the fault rupture, but this kind of directivity is not that distinct in the Tohoku case. We suggest that the different fault slip (inland or submarine) affecting the way of couplings of lithosphere with atmosphere may contribute to the discrepancies between the two events. Ionospheric and geomagnetic changes caused by a major inland earthquakeStrong directivity of CID distribution related to fault rupture directionDifferent features in CIDs of inland case compared with oceanic case

Here we report two cases of coseismic ionospheric disturbances observed through a GPS network in China after the great Wenchuan earthquake at 06:28 UT on 12 May, 2008. One is detected 7.9 min after the earthquake and had an intensive “N” shape oscillated waveform with a pronounced amplitude of about 1 TECU, which propagates approximately southward to the distance about 1000 km with the horizontal phase velocity of 600±84 m/s and the period of 9.5±1.3 min. The other is detected 8.5 min after the earthquake and has an oscillated waveform more like a positive pulse with an amplitude of about 0.5 TECU, which propagates eastward to the distance about 800 km with the horizontal phase velocity of 720±59 m/s and the period of 7.4±0.8 min. These two coseismic ionospheric disturbances are caused by the acoustic gravity waves excited by partial transformation of the acoustic waves originated from the energy release of the earthquake, somewhere near the epicenter. The directional preferences of these two coseismic ionospheric disturbances may be associated with the oblique geomagnetic field lines and the background winds filtering effect.

About 8 year electron density profile (EDP) data from the COSMIC/FORMOSAT‐3 satellites radio occultation technique were used to investigate the additional stratification of the F2 (the so‐called F3 layer) layer over the equatorial and low‐latitude ionosphere on a global scale for both the bottomside and topside ionosphere. The F3 layer was recognized through the altitude differential profile featured by two maxima existing from the selected EDP profile. There were ~37,000 (bottomside) and 25,000 (topside) cases of F3 layer selected out of ~1.27 million occultation events at equatorial and low‐latitude areas during the period of April 2006 to August 2014. The statistical results for the bottomside ionosphere resemble that reported in Zhao et al. (2011a), while in the topside the highest occurrence of F3 layer shows a 3–4 h delay depending on the altitude range of the stratification. The magnetic latitude distribution shows different dependence with a tendency to form a single crest toward high altitude. Also, the seasonal variation is weaker in the topside ionosphere compared to the bottomside one, especially in the high altitude. Then we build up an empirical model of the F3 layer occurrence using the bottomside statistics based on empirical orthogonal function (EOF) decomposition as it gets the inherent characters inside the data set and converges quickly. The model well grasps the main features of the F3 occurrence, e.g., the F3 occurrence's sensitivity on the magnetic latitude. Further, in order to accommodate the ground observation a corrected factor was introduced. As F3 layer is an important phenomenon in the low‐latitude ionosphere, we have made an attempt to describe its feature with a consecutive function although future work needs to be done for an overall expression of this structure. The first attempt to parameterize the F3 occurrence using occultation techniquePresented the features of F3 layer for both the bottom and topside ionosphereThe model well grasps the main features of the F3 layer occurrence as observed

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. 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. 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 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 receiving This paper details the hardware, key parameters, data processing and preliminary experimental results of SYISR The key parameters meet or are superior to the original design, and the preliminary observations are encouraging

This paper presents a statistical result of large‐scale traveling ionospheric disturbances (LSTIDs) associated with moving solar terminator in China during 12 months from February 2011 to January 2012. The LSTIDs are identified by the two‐dimensional total electron content (TEC) perturbation maps, which are built based on the observations of GPS network data from China. The GPS observations are combined with the observations from an ionosonde chain established by the Institute of Geology and Geophysics, Chinese Academy of Sciences. A total of 135 LSTID events are identified at dawn, while there is indiscernible LSTID at dusk. Meanwhile, these LSTIDs are captured by the ionosonde chain which shows that there are perturbations in the virtual heights during the passage of LSTIDs at the height between 200 and 700 km. The occurrence rate of LSTIDs shows a maximum in winter and a minimum in summer. The LSTIDs propagate across China with phase front widths larger than 1500 km. The propagation direction of LSTIDs is northwestward in winter, southwestward in summer, and quasi‐westward in equinoxes, respectively. The average period, horizontal phase velocity, and horizontal wavelength of LSTIDs are 79 ± 12 min, 288 ± 43 m/s, and 1503 ± 205 km, respectively. The relative TEC perturbations of the LSTIDs attenuate as the LSTIDs travel across China. 135 LSTIDs are observed in China after sunrise during Feb 2011 to Jan 2012The propagation directions of LSTIDs have seasonal dependencyThe solar termiantor‐related LSTIDs are generated under the height of 200 km

Larger-scale traveling ionospheric disturbances (LSTIDs) are studied using the total electron content (TEC) data observed from 246 GPS receivers in and around China during the medium storm on 28 May 2011. It is the first attempt to get the two-dimensional TECperturbation maps in China. Two LSTID events are detected: one is in southwestern China before midnight propagating from low to middle latitude to the distance of about 1200 km with the phase front extending to about 500 km, and the other is in northeastern China after midnight propagating from middle to low latitudes to the distance of about 1200 km with the phase front extending to nearly 1400 km. By using the multichannel maximum-entropy method, we get the propagation parameters of these two LSTIDs. The LSTID that occurs before midnight has a higher horizontal phase velocity and a larger damping rate corresponding to the after midnight LSTID, and this may be caused by the relatively large vertical background TEC0and high atmospheric temperature in the southwest of China before midnight. According to the variations of magnetic H component observed in high latitudes, the source region for the after midnight LSTID is likely to be located 1400–2600 km east of 140°E and north of 42°N; the before midnight LSTID is propably excited by the atmospheric gravity waves (AGWs) generated by the Joule heating of the equatorial electrojet.

Abstract: The hourly values of the ionospheric F2 layer critical frequency, foF2, recorded at Wakkanai ionosonde station (45.4°N, 141.7°E) have been collected to construct a middle-latitude single-station model for forecasting foF2 under geomagnetic quiet and disturbed conditions. The module for the geomagnetic quiet conditions incorporates local time, seasonal, and solar variability of climatological foF2 and its upper and lower quartiles. It is the first attempt to predict the upper and lower quartiles of foF2 to account for the notable day-to-day variability in ionospheric foF2. The validation statistically verifies that the model captures the climatological variations of foF2 with higher accuracy than IRI does. The storm-time module is built to capture the geomagnetic storm induced relative deviations of foF2 from the quiet time references. In the geomagnetically disturbed module, the storm-induced deviations are described by diurnal and semidiurnal waves, which are modulated by a modified magnetic activity index, the Kf index, reflecting the delayed responses of foF2 to geomagnetic activity forcing. The coefficients of the model in each month are determined by fitting the model formula to the observation in a least-squares way. We provide two options for the geomagnetic disturbed module, including or not including Kalman filter algorithm. The Kalman filter algorithm is introduced to optimize these coefficients in real time. Our results demonstrate that the introduction of the Kalman filter algorithm in the storm time module is promising for improving the accuracy of predication. In addition, comparisons indicate that the IRI model prediction of the F2 layer can be improved to provide better performances over this region.

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 ΔHmeasurements 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. 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/N2ratio 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. 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 A daytime large‐scale TEC depletion structure covering ∼35° in longitude under relatively quiet geomagnetic conditions was reported The daytime large‐scale depletion structure with poleward development mainly occurred in summer with large intensities at 16–30°N It is surmised that enhanced poleward neutral wind could be a possible driver for the daytime large‐scale depletion structure

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. 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 A new phenomenon of the inhibition of F3 layer over Sanya during recovery phase of geomagnetic storms in summer is reported Even weak geomagnetic storms can produce significant changes in the low latitude ionosphere over Sanya in the recovery phase Inhibition of F3 layer event is mainly caused by the westward disturbance dynamo electric field

The global traveling ionospheric disturbances (TIDs) during the drastic magnetic storms of October 29–31, 2003 were analyzed using the Global Position System (GPS) total electron content (TEC) data observed in the Asian-Australian, European and North American sectors. We collected the most comprehensive set of the TEC data from more than 900 GPS stations on the International GNSS Services (IGS) website and introduce here a strategy that combines polynomial fitting and multi-channel maximum entropy spectral analysis to obtain TID parameters. The results of our study are summarized as follows: (1) large-scale TIDs (LSTIDs) and medium-scale TIDs (MSTIDs) were detected in all three sectors after the sudden commencement (SC) of the magnetic storm, and their features showed longitudinal and latitudinal dependences. The duration of TIDs was longer at higher latitudes than at middle latitudes, with a maximum of about 16 h. The TEC variation amplitude of LSTIDs was larger in the North American sector than in the two other sectors. At the lower latitudes, the ionospheric perturbations were more complicated, and their duration and amplitude were relatively longer and larger. (2) The periods and phase speeds of TIDs were different in these three sectors. In Europe, the TIDs propagated southward; in North America and Asia, the TIDs propagated southwestward; in the near-equator region, the disturbances propagated with the azimuth (the angle of the propagation direction of the LSTIDs measured clockwise from due north with 0°) of 210° showing the influence of Coriolis force; in the Southern Hemisphere, the LSTIDs propagated conjugatedly northwestward. Both the southwestward and northeastward propagating LSTIDs are found in the equator region. These results mean that the Coriolis effect cannot be ignored for the wave propagation of LSTIDs and that the propagation direction is correlated with polar magnetic activity.

Incoherent scatter radars (ISR) are among the most powerful ground‐based instruments for solar‐terrestrial physics, measuring multiple plasma parameters over almost the entire vertical extent of the ionosphere. In Sanya, on Hainan Island, China, an advanced high‐power phased array ISR, known as the Sanya incoherent scatter radar (SYISR), is under construction. A tristatic system is planned, with a transmitter at Sanya (SY) Station (109.6°E, 18.3°N), together with receivers at SY, Fuke (FK) (109.1°E, 19.5°N), and Qiongshan (QS) (110.2°E, 19.7°N) on Hainan Island. With the pulse width increasing, the SNR increases for a phased array monostatic ISR, while the SNR initially increases and then remains unchanged for a phased array bistatic ISR. SNR first increases and then decreases as the detection height increases from 100 km to 1,000 km for both the monostatic and bistatic ISR for typical ionospheric conditions. The relative error decreases fast with increasing pulse width when the pulse width is lower than 130 μs for both monostatic and bistatic ISR. When the pulse width is over 130 μs, the relative error decreases very slowly for a monostatic ISR, while it decreases very slowly and then stays unchanged with increasing pulse width for a bistatic ISR. Relative error increases slowly below 500 km, but increases fast for heights over 500 km for both the monostatic ISR and bistatic ISR. It is shown that a tilt angle of 20°–30° and a clockwise‐rotation angle of 157° are reasonable choices for the FK receiving array in the SYISR tristatic system. Multistatic phased array Incoherent scatter radars (ISR) equation for soft target is derivedThe detection performances of phased array monostatic and bistatic ISR are analyzed for pulse width, height, beam width and scattering volumeOptimal tilt and rotation angles of receiving array are suggested for Sanya incoherent scatter radar (SYISR) tristatic system Multistatic phased array Incoherent scatter radars (ISR) equation for soft target is derived The detection performances of phased array monostatic and bistatic ISR are analyzed for pulse width, height, beam width and scattering volume Optimal tilt and rotation angles of receiving array are suggested for Sanya incoherent scatter radar (SYISR) tristatic system

This work statistically analyzed the characteristics of the summer descending sporadic E layers (Es) recorded at four Chinese ionospheric sounding stations of Sanya (109.4°E, 18.3°N, and I: 24.1°), Wuhan (114.4°E, 30.5°N, and I: 45.5°), Beijing (116.2°E, 40.3°N, and I: 58.7°), Mohe (122.5°E, 52.0°N, and I: 69.6°), and simulated the vertical ion convergence derived from the Horizontal Wind Model (HWM14) for the period from June 1, 2014 to August 31, 2017. Both observations and simulations show that a daytime Es layer starting at ∼120 km in the morning descends to ∼100 km in the afternoon. In addition, the descending rate of the Es layer has obvious altitude and latitude dependence. The Es layer descends faster (∼2.1 km/h) at higher altitudes (above 110 km), and meanwhile, it descends relatively slowly (∼1.5 km/h) at low altitudes (below 110 km). The mean descending rate (∼1.5–1.8 km/h) of the Es layers gradually increases from Sanya to Mohe. It is undoubtedly that the wind shear convergence nodes controlled by the atmospheric tides can drive the Es layer to move downwards. However, the simulation results in this paper show that the wind shear convergence nodes and the Es layer traces do not overlap completely, especially in the vertical direction, which suggests that wind shear cannot fully explain the details of Es layer evolution. We should pay attention to the combined effect of wind shear and metallic ion density in future research, especially in the details of the evolution of Es layer in the vertical direction. Both observations and simulations show that the Es layer traces and wind shear convergence nodes have a downward movement trend, which is mainly controlled by the atmosphere tidesThe Es layer’s descending rates increase with altitude and latitude from Sanya to MoheThe Es layer traces observed by the ionosondes and the wind shear convergence nodes derived from the HWM14 model do not completely overlap, especially in the vertical direction Both observations and simulations show that the Es layer traces and wind shear convergence nodes have a downward movement trend, which is mainly controlled by the atmosphere tides The Es layer’s descending rates increase with altitude and latitude from Sanya to Mohe The Es layer traces observed by the ionosondes and the wind shear convergence nodes derived from the HWM14 model do not completely overlap, especially in the vertical direction

Using TIME3D-IGGCAS model, we simulated the east–west differences in F-region peak electron density (NmF2) in the Far East mid-latitude region near the longitudinal sectors with very clear zonal variations of geomagnetic declination, and mainly analyzed the influence of the geomagnetic field configuration on the east–west differences. We found that, after removing the longitudinal variations of neutral parameters, TIME3D-IGGCAS can better represent the observed relative east–west difference (Rew) features. Rewis mainly negative (West NmF2 > East NmF2) at noon and positive (East NmF2 > West NmF2) at evening–night. The magnitude of daytime negative Rewis weaker in winter and stronger in summer, and the daytime Rewshows two negative peaks around two equinoxes. With the increasing solar flux level, the magnitude of Rewmainly becomes larger, and the two daytime negative peaks slightly shift to June Solstice. With the decreasing geographical latitude, Rewmainly becomes positive, and the two daytime negative peaks slightly shift to June Solstice. Our simulation also suggested that the thermospheric zonal wind plays an important role in the formation of the ionospheric east–west differences in the Far East mid-latitude region. The observed solar activity dependency of the ionospheric EW differences may be driven primarily by corresponding zonal wind changes with solar activity, whereas the observed latitudinal dependency of the differences is associated with primarily zonal wind and secondarily meridional wind latitudinal variations.

Sudden stratospheric warming (SSW) events in Northern Hemisphere winter atmosphere play a significant effect on large ionospheric disturbances. Nevertheless, the SSW effects on small‐scale structure in ionosphere, that is, the ionospheric Flayer irregularities, are generally lack of understanding. This work carefully studied the equatorial Flayer scintillation during the SSW event extending from 4 January to 5 February in 2013 by using the data of COSMIC/FORMOSAT‐3 (a Constellation Observing System for Meteorology, Ionosphere, and Climate mission). The occurrence rate of ionospheric Flayer scintillation was found to be apparently decreased by about 30% during the SSW period. This phenomenon was obviously seen in American sector (90°W–0°) and African sector (0°–90°E). Another prominent feature was the ~0.5 local time hour delay of maximum scintillation occurrence rate associated with the suppression of equatorial Flayer irregularities in the first 2 hr after the sunset during the SSW event in January 2013. It is the first time to study the response of small scale structure in ionosphere during SSW by COSMIC data setsThe occurrence rate of ionospheric F‐layer scintillation weakened by about 30% during the January 2013 SSW eventThe weakening is obvious in American and African sectors, not obvious in other sectors, which are attributed to low scintillation in winter

For the first time, we develop a local time and day of year dependent climatological model in different geomagnetic activity based on 630.0 nm Fabry‐Perot Interferometer (FPI) measurements over Kelan (38.7°N, 111.6°E) during 2011–2019. With this model, we analyze the regional climatological characteristics of nighttime thermospheric neutral winds, and a comparison is made with two recent versions of the Horizontal Wind Model series (HWMs), HWM07 and HWM14. Due to the location of Kelan far from the northern magnetic pole longitude at middle latitude, the statistical characterizations of meridional and zonal winds show some unique local time, seasonal, solar cycle and geomagnetic activity dependence. There are predominantly annual variations of horizontal winds characterized by rapid transitions from eastward December winds to westward or less eastward June winds and largest equatorward meridional winds during summer months. To some degree, the wind patterns show casual solar activity dependence especially for the zonal component. The maximum southward winds show negative relationship with increasing solar activity in summer months, while the maximum eastward winds present negative trend in winter months. The Kp dependencies are characterized by larger westward perturbation winds for all night and enhanced northward winds in the premidnight sector and equatorward surges in the postmidnight sector during strongly active conditions. With increasing solar flux, the westward perturbation winds become weaker at nights. Results also reveal that HWM14 predictions are in better agreement with Kelan FPI winds in the shape and magnitude than HWM07, and the consistency improves with increasing Kp. We construct climatological models of thermospheric winds with Kelan FPI during 2011–2019 under quiet and disturbed geomagnetical conditionsWe evaluate the applicability of HWM07 and HWM14 over Kelan and find better agreement for HWM14 predictions especially during geomagnetically active periodsWe make a comprehensive analysis of the statistical characteristics of seasonal, local time, solar activity, and geomagnetic activity dependence of thermospheric winds over Kelan

Using total electron content (TEC) data from 902 global navigation satellite system stations over China, we analyzed the ionospheric responses following two similar launches of the Long March 4B from Taiyuan, China, that is, China‐Brazil Earth Resources Satellite 3 launched on 9 December 2013 and China‐Brazil Earth Resources Satellite 4 launched on 7 December 2014. By adopting a method that can detect TEC depletion effectively, filtered TEC series, and built two‐dimensional disturbances map, we found that the ionospheric disturbances of these two launches were almost the same. We identified three types of disturbances following both launches: depletions, shock wave‐related traveling ionospheric disturbances (TIDs), and acoustic wave‐related circle TIDs. The buildup time of depletion was influenced by the amount of exhaust expelled into the ionosphere with an amplitude of ∼12 TEC unit (TECU; 1 TECU = 1016el/m2). After the electron depletion was formed, it drifted westward for approximately 300 km. The shock wave‐related TIDs can only be observed near the trajectory and reflect the acceleration of the rockets with an amplitude of ∼0.6 TECU. The acoustic wave‐related circle TIDs were observed at the southeast of the launch site. From our observations, this type of TID includes three groups of wave trains, and the speed and period are ∼900 m/s and ∼3.8 min, respectively. The amplitudes of the three groups of TIDs are ∼0.065, ∼0.045, and ∼0.02 TECU. We observed traveling ionospheric disturbances caused by shock waves and circle waves during two rocket launches; TEC depletion was also addressedThe ionospheric depletion was influenced by the amount of exhaust expelled and probably by the E× Bdrift and/or background windShock wave‐related TIDs can be found near the trajectory, and acoustic wave‐related circle TIDs can be found southeast of the launch site

Based on the coordinated observations by the incoherent scatter radar (ISR), ionosonde, magnetometers, and GPS receivers, the electrodynamic effects on the equatorial and low‐latitude ionosphere have been investigated during the intense solar flare (X1.5/2B) on 13 September 2005. In the initial stage of the flare, the ISR and ionosonde measurements at Jicamarca show the decreases of 10.14 m/s and 20 km in the upward vertical E × B drift velocity and the F2region peak height, respectively, while equatorial electrojet (EEJ) strength over American sector indicates a sudden increase of 53.7 nT. The decrease of the upward vertical E × B drift velocity reveals the weakening of eastward electric field during the flare, which is firstly and directly observed by instrument. It is well known that the variation of equatorial electric field is mainly attributed to the ionospheric dynamo electric field and partially affected by the penetration of interplanetary electric field. The observations during this flare suggest that the flare‐induced increase of Cowling conductivity changes the ionospheric dynamo electric field and further results in the weakening of eastward electric field and the decrease of the upward vertical E × B drift velocity. Meanwhile, the upward vertical E × B drift velocity and the EEJ strength during the flare are negatively correlated, which is contrary to the knowledge established by Anderson et al. ([Anderson, D., 2002]) based on 10 days of observations in the Peruvian longitude sector. The difference may be caused by the flare‐induced enhancement of Cowling conductivity. In addition, GPS total electron content (TEC) observations from six stations in the American equator and low latitudes show an enhancement of 1.47–3.09 TEC units. The measurements of GPS and ISR indicate that the contribution of the enhanced photoionization to the increase of TEC is more than that of electrodynamic effect during the initial stage of the intense flare. Measurements show a decrease in upward drift and increase in EEJThe decrease of upward drift is firstly observed by instrument during the flareThere is a negative correlation between upward drift and EEJ during the flare

The longest total solar eclipse of this century occurred in East and South Asia on 22 July 2009. The eclipse was accompanied with a medium magnetic storm, whose main phase onset occurred ∼27 min after the passage of the Moon's umbral shadow. Using TEC data from 60 GPS stations, we construct differential TEC maps to investigate the ionosphere response to the solar eclipse in central China in the range of 26°N–36°N, 108°E–118°E (i.e., the magnetic latitude 15°N–25°N). During the eclipse's totality, a “shadow” in the ionosphere shown as TEC depletion area was formed ∼100 km south of the Moon's umbral path with a maximum decrease of 5 TECU. The TEC depletion area moved eastward, following the movement of the totality area with a time lag of ∼10 min. Enhancements of TEC due to the storm are observed after the main phase onset. The relative drop of TEC due to the solar eclipse is evidently larger at lower latitudes than that at higher ones and around noontime than that in the morning. By modeling work, we find that the latitudinal dependence of the TEC response may result from latitudinal variation of magnetic inclination, which influences the diffusion of ionization among different layers. Besides, the local time dependence of TEC response is closely related to the local time variation of background atmosphere density, which affects the electron loss efficiency in the ionosphere.