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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]

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]

The possible relation between pre‐earthquake processes and ionospheric fluctuations is widely investigated and debated. Here, we conduct a numerical experiment to examine two studies that reported preseismic anomalies in the ionospheric total electron content (TEC) and argue for the significance of their respective analyses based on statistical evaluations. The first study is that by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01), who statistically investigated the relationship between 62 M ≥ 6 earthquakes occurred in China over an 18‐year period and the TEC, which was deduced from the Global Ionospheric Map. According to this study, the TEC showed anomalies with specific polarities at certain days and times that preceded the earthquakes. Earthquake alarms they defined based on these anomalies showed a good performance. We conduct this same analysis using random synthetic earthquakes and obtain results that closely match those by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01), thus indicating that the good performance of the Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) alarm is an artifact. The second study is that by Le et al. (2011, https://doi.org/10.1029/2010ja015781), who classified the TEC time series into anomalous and non‐anomalous days based on the TEC perturbation. They found that the anomalous day rate increased as the nucleation time of the earthquakes was approached, especially for larger and shallower earthquakes. We conduct the same analysis using random synthetic earthquakes. The anomalous day rate that is comparable to their result occurs in ∼40% of the 1,000 random trials, thereby suggesting that their result may also be an artifact. Plain Language Summary: Fluctuations in the ionospheric electron have been considered as a possible precursor of earthquakes. Papers by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) and Le et al. (2011, https://doi.org/10.1029/2010ja015781) are important as positive reports evaluating the precursory behavior from a statistical point of view. Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) reported that the electrons increases/decreases at specific leading times before 62 large earthquakes occurred in China. Earthquake alarms based on these anomalies showed a good performance. Le et al. (2011, https://doi.org/10.1029/2010ja015781) reported that rate of days with large fluctuation of electron contents increases before shallower and larger earthquakes, based on global 736 large earthquakes over 8 years. We test their methods by numerical experiments generating random earthquakes. Even for the random earthquakes never related to ionospheric processes, the method by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) achieves a high alarm performance comparable to their original paper. Regarding Le et al. (2011, https://doi.org/10.1029/2010ja015781), the rate of days with large electron fluctuation calculated by their equation increases before shallower and larger earthquakes even though the given rate is constant for all the earthquakes. These results show that the relation between ionospheric processes and earthquakes reported by the both studies should be artifact derived from the methodology. Key Points: Pre‐seismic ionospheric electron anomalies detected based on global navigation satellite system and Global Ionospheric Map are tested from a statistical point of viewNumerical experiments based on randomly generated earthquakes reproduced the statistical features of the "significant precursor"The statistical significance inferred by previous studies may have been caused by false predictions due to arbitrarily defined "precursor" conditions [ABSTRACT FROM AUTHOR]

In this paper, we have modified the International Reference Ionosphere model 2016 (IRI2016) with the Global Navigation Satellite System total electron content data observation in China by adjusting the ionospheric index (IG12) for given location and time. The capability of the modified IRI2016 to reproduce the F2 layer critical frequency foF2 is validated for high/low solar activity conditions and disturbed geomagnetic conditions for the time period 2014–2017 over low‐latitude ionosonde station located at Sanya in China. The analysis shows that the modified IRI2016 better reproduced the variability of the observed foF2 versus time and seasons. Also, the modified IRI2016 foF2 prediction exhibits lower root mean square error and higher correlation coefficients (R) over different solar activity periods, and significantly reduces the average percentage deviations (△foF2(%)) under enhanced geomagnetic activity periods. Subsequently, the 24 hr ahead forecast foF2 map over China constructed by the combination of the modified IRI2016 and the eXtreme Gradient Boosting algorithm is presented for the first time. The promising performance of the modified IRI2016 indicates that this combination method may be an effective way to predict the ionospheric parameters, though further validations are needed in the future work. Key Points: The International Reference Ionosphere model 2016 (IRI2016) is modified with the Global Navigation Satellite System data by adjusting the ionospheric index for given location and time in ChinaThe modified IRI2016 improves the foF2 prediction for both high/low solar activity conditions and disturbed geomagnetic conditionsThe 24 hr ahead forecast foF2 map over China is constructed for the first time to our knowledge [ABSTRACT FROM AUTHOR]

We present a low‐altitude satellite survey of power line harmonic radiation (PLHR) at 50 Hz over Mainland China. We analyzed the month‐to‐month variation pattern in PLHR occurrence rate and further analyzed its correlation with some influencing factors (i.e., solar radiation, lightning flashes, and electricity consumption) using CSES satellite electric field data from 2019 to 2021. We also investigate the response of PLHR occurrence rate to COVID‐19. The statistical results show the dayside PLHR occurrence rate decreasing from winter to summer solstice and increasing from summer to winter solstice, which indicates it is controlled by the solar radiation. The nightside variation is more complex, which may be due to many sources that could influence the nightside lower ionosphere. The PLHR occurrence rate significantly decreased over Mainland China in February 2020, which is because of the significant decrease in electricity consumption due to the suspension of industrial production caused by COVID‐19. Plain Language Summary: Power line harmonic radiation (PLHR) is the electromagnetic waves radiated by electric power systems at harmonic frequencies of 50 or 60 Hz, depending on the frequency of the system on the ground. Previous research mainly focuses on identification of individual PLHR events and their subsequent analysis. However, the number of base‐frequency PLHR signal events is the most abundant, which is suitable for the statistical study of PLHR occurrence rate and its variation pattern, and further study of the factors affecting its variation pattern. In this paper, we use 3 years of electric field data from the China Seismo‐Electromagnetic Satellite (CSES) which is an LEO satellite launched into orbit in February 2018 to investigate the month‐to‐month variation pattern of PLHR occurrence rate over Mainland China and its correlation with the influencing factors. The response of PLHR occurrence rate to COVID‐19 are also investigated. Key Points: The month‐to‐month variation in power line harmonic radiation (PLHR) incidence is controlled by solar radiation and influenced by changes in electricity consumptionThe variation in nightside PLHR occurrence rate is complex, due to the nightside lower ionosphere being influenced by multiple sourcesThe PLHR occurrence rate over Mainland China decrease in February 2020 due to the decrease in electricity consumption caused by COVID‐19 [ABSTRACT FROM AUTHOR]

In this paper, we first investigate the effects of a postevening Weddell Sea Anomaly (WSA)‐like plasma patch on a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID) over Xinglong, China (40.4°N, 117.6°E; Mlat. ~30.4°N). We found that some of this kind of WSA‐like plasma patches moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they traveled westward into China. During 2012–2016, these plasma patches frequently occurred during the period of May–August with a monthly occurrence rate of over 20%, causing nighttime plasma density enhancements (NPDEs) in midlatitudes of China. Over 50% of these structures were accompanied by concurrent MSTID and Es. We propose that an intense polarization electric field (PEF) associated with an MSTID/Es from the more northern regions of EIA could frequently drive these plasma patches poleward. On the night of 27 June 2014 (Kp < 2−), either an MSTID and an intense Es layer (the maximum foEs >12 MHz) occurred at the onset of the poleward motion of a WSA‐like plasma patch, lending credence to this possibility. The observed plasma patch was pushed poleward to interact with the observed MSTID, causing some poleward extending C‐shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one. We attributed those poleward extending airglow depletions to the interaction between the MSTID and the plasma patch because of the secondary gradient drift instability and those of airglow enhancements to the redistribution of plasma. Plain Language Summary: This study found that postevening Weddell Sea Anomaly (WSA)‐like patchy plasma structures frequently occurred during May–August of 2012–2016 in the East Asian regions. Among these plasma patches, a small portion moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they moved westward into China. On the magnetically quiet night of 27 June 2014 (Kp < 2−), such a plasma patch already moved westward and poleward to the midlatitude region of China, affecting the evolution/propagation of a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID). This paper, therefore, investigates the interaction between a midlatitude MSTID and a poleward moving WSA‐like plasma patch. We mainly investigate the possible physical mechanisms behind this kind of plasma patches and how they could affect the propagation/evolution of MSTIDs over midlatitudes of China. Key Points: Poleward extending C‐shaped airglow depletions/enhancements of MSTID were caused by interaction between an MSTID and a postevening WSA‐like plasma patchWestward and northward moving WSA‐like plasma patches frequently caused nighttime midlatitude plasma enhancements over Xinglong, ChinaAn intense PEF associated with an MSTID/Es from the more northern regions of EIA could frequently cause poleward motion of the WSA‐like plasma patch [ABSTRACT FROM AUTHOR]

In this paper, the interaction between equatorial to low‐latitude postmidnight ionospheric irregularities and large‐scale traveling ionospheric disturbances (LSTIDs) on 14 December 2015 during strong geomagnetic disturbances in China is studied by using multiple ground‐based observations. Postmidnight irregularities initiated near the magnetic equator and extended northward to higher latitudes (∼28°N) until the local early morning period (∼05 LT), which is unusual in the Northern Hemisphere winter. Multibeam observations by the very high frequency coherent scatter radar at Fuke (19.5°N, 109.1°E; dip latitude 14.4°N), Hainan Island, China, showed that the meter‐scale field‐aligned irregularities were still in the evolutionary phase during the postmidnight period (∼04 LT) instead of the fossil structures that must already be in their decay phase. Meanwhile, the emergence of the postmidnight ionospheric irregularities was accompanied by the passage of LSTIDs, as revealed by total electron content measurements. Southward and northward LSTIDs, with velocities of ∼550–570 m/s and periods of ∼80–100 min, that originated from opposite hemispheres met at ∼27°–28°N, and the LSTIDs might produce localized perturbation during their passage to initiate the instability for the postmidnight irregularity development. The height of the ionospheric F‐layer measured by the collocated Digisonde at Fuke was suggested to be modulated by the overshielding penetration electric field and storm‐related atmospheric gravity waves (AGWs). This interesting case gives insight into the important role of storm‐related AGWs in providing seeding sources for plasma bubble development and reveals unique characteristics of coupling between LSTIDs and plasma bubbles during the postmidnight sector. Key Points: Low‐latitude postmidnight ionospheric irregularities and opposite‐propagating large‐scale traveling ionospheric disturbances (LSTIDs) were observed simultaneouslyThe quasiperiodic F‐layer height uplifting might be caused by the storm‐related atmospheric gravity waves and overshielding penetration electric fieldThe LSTIDs produced localized perturbations, initiating the development of instabilities for seeding postmidnight equatorial plasma bubbles [ABSTRACT FROM AUTHOR]

The Meteor and ionospheric Irregularity Observation System (MIOS), which consists of multi‐station optical subsystem at Ledong (18.4°N, 109°E) and Sanya (18.3°N, 109.6°E), and radar subsystem including a 38.9 MHz all‐sky interferometric radar and a 47.5 MHz coherent phased array radar at Ledong, has been in full operational since December 2021. This paper describes the system design and first results of meteor plasma density irregularities and corresponding meteoroids. The MIOS optical subsystem consists of a few tens of video cameras for observing optical meteor trail and spectrum. The MIOS phased array is composed of 135 Yagi antennas, arranged in sword‐like grid and grouped into 15 identical subarrays, with distances separated by 2–19.5 times the wavelength for unambiguous interferometry measurements. The phased array can form narrow and wide beams, with half power width of 8° and 24° in azimuth, respectively, allowing narrow‐beam pulse‐to‐pulse steering and wide‐beam multi‐baseline imaging observations in the east‐west direction. Observational results show that the MIOS is capable of unambiguously locating various meteor echoes, that is, head, specular and non‐specular echoes, revealing the structural evolution of field‐aligned and non‐field‐aligned irregularity, and of determining the properties of meteoroids producing/not producing irregularities. Cases of bright meteors producing non‐field‐aligned irregularities and not producing field‐aligned irregularities respectively are presented, and possible factors affecting the generation of meteor trail irregularities are discussed based on current understanding. It is expected that the MIOS will provide an important tool to study the generation and evolution of various meteor trail irregularities and the properties of the corresponding meteoroids. Plain Language Summary: Meteoroids entering the Earth's atmosphere significantly disturb the background ionosphere, producing ionized columns and plasma density irregularities, change the ionospheric composition, and even seed large‐scale ionospheric irregularity structures. However, due to the limitations of previous measurements, some physical processes, for example, how efficiently various meteoroids affect the background ionosphere, either producing or not producing some specific meteor trail irregularity phenomena, and the related meteoroid properties and origins, are less well understood. This calls for a facility to measure the properties of both the meteor trail and its corresponding meteoroid, and thus to reveal potential factors affecting the generation of meteor trail irregularities and the disturbance of the background ionosphere. Under the support of national natural science foundation of China, a Meteor and ionospheric Irregularity Observation System (MIOS) was designed to simultaneously capture the radar and optical signatures of meteors and plasma density irregularities. Here we report the initial measurements of the MIOS, which demonstrate its capabilities for observing various meteor and irregularity phenomena. Key Points: An optical and radar facility (MIOS) intended to study how meteoroids change the Earth's background ionosphere has been developedFirst observations of field‐ and non field‐aligned meteor irregularities and the corresponding meteoroids from the facility are presentedThe facility is expected to play an important role in the study of various meteors, their origin and possible processes affecting ionosphere [ABSTRACT FROM AUTHOR]

Few observations investigated the interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB). This paper presents another interaction between a southwestward propagating EMSTID and an eastward drifting EPB in the equatorial ionization anomaly (EIA) crest region of China. When the EMSTID and the EPB touched each other, several depletions of the EMSTID (EPB) showed the eastward (westward) velocity disturbances of the EPB (EMSTID) depletions. Besides, phase elongations of the EPB depletions contrarotated as the EMSTID propagated southwestward. However, of important finding is that the interaction of the EMSTID and the EPB could have polarized one depletion of the postmidnight EPB that should have become a fossilized bubble. Inside that polarized EPB depletion were meter‐scale irregularities that caused activated radar echoes and enhanced ranged spread F (RSF). The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma. We propose that the EMSTID and the EPB could have electrically coupled with each other, causing an enhanced polarization electric field (PEF) that polarized that EPB depletion; the E × B gradient drift instability (Kelley, 1989) could have caused the meter‐scale irregularities when that enhanced PEF was imposed on that reactivated EPB depletion surrounded by that enhanced density plasma. This study provides observational evidence that how an electrical couple of EMSTID and EPB events can activate a postmidnight EPB depletion that should become a fossilized structure. Plain Language Summary: Otsuka et al. (2012, https://doi.org/10.1029/2012GL052286) provided the first report indicating the disappearance of an equatorial plasma bubble (EPB) airglow depletion after interaction with an electrical medium‐scale traveling ionospheric disturbance (EMSTID) airglow enhancement. For comparison, the current study reports a postmidnight airglow event in which an electrical coupling of an EMSTID and an EPB could have polarized one EPB airglow depletion that caused the activated radar echoes of very high frequency (VHF) radar and the enhanced ranged spread F (RSF) in descending ionosphere filled with an enhanced density plasma. Key Points: Interaction between an electrical medium‐scale traveling ionospheric disturbance (EMSTID) and an equatorial plasma bubble (EPB) in the equatorial ionization anomaly (EIA) crest region over ChinaElectrical coupling of the EMSTID and the EPB polarized one EPB depletion that caused activated radar echoes and enhanced ranged spread F (RSF)The interaction occurred in descending ionosphere and the lower density regions got filled up with an enhanced density plasma [ABSTRACT FROM AUTHOR]

The concurrent observation of the Hainan COherent Scatter Phased Array Radar (HCOPAR) and the 630 nm all‐sky airglow imager was carried out on the night of 13 April 2015 in low‐latitudes of China. The plasma bubble recorded by the airglow imager and the Field‐Aligned Irregularities (FAIs) observed by the HCOPAR kept a good consistency. At the beginning, the FAIs were found to fill the entire plasma bubble, and they were drifting eastward together. During the dissipation process, the plasma bubble became wider and wider in zonal direction, while the airglow gray gradient on the eastern and western walls of the bubble had no much changes. Simultaneously, the echo Signal‐to‐Noise Ratio (SNR) of the irregularities in the bubble became lower and lower. The SNR at the depletion center decreased fastest, and the SNR of the FAIs on the western wall of the bubble fell more slowly than those in other parts. Polarization electric field in the bubble is suggested to have influence on the distribution of the FAIs. Gradient drift instability on the western wall supported the FAIs to live longer, and the FAIs on the eastern wall, namely on the front edge of the bubble, dissipated more rapidly. Key Points: The irregularities were generated inside the plasma bubble and concentrated at the locations with steep gradientsWhile the bubble became wider and wider, the echo Signal‐to‐Noise Ratio (SNR) of the irregularities inside decreased quicklyIrregularities at the depletion center vanished first, and the echo SNR on the eastern wall fell faster than that on the western wall [ABSTRACT FROM AUTHOR]

We report for the first time a case of wavelike structures of E‐region irregularities observed by an all‐sky radar at Ledong (18.44°N, 108.97°E), China. The wavelike structures covered up to 250 km zonally, fluctuated in the vertical and horizontal planes, and were observed in the directions perpendicular and significantly off‐perpendicular to the geomagnetic field. The wavelength, and vertical and horizontal oscillating amplitudes were estimated at about 20–70, 5, and 2 km, respectively. Eventually, the large‐scale wavelike structure broke into spatially separated small‐scale patches and disappeared. The case provides observational evidence for the wave modulating sporadic E (Es) mechanism, which suggests that the wavelike structures of E‐region irregularity were primarily generated by wind shears and modulated by gravity waves. The wavelike structure was possibly an intermediate state between the continuous‐type and quasiperiodic‐type E‐region irregularities. The mechanism responsible for the echoing structure observed in off‐perpendicular directions is unclear. The possible contributions from refraction and reflection by intense Es layer/structure were discussed. Plain Language Summary: The E‐region irregularities were usually observed using narrow‐beam very high frequency radars. Due to the limited field‐of‐view (FOV) of the narrow‐beam radars, the spatial morphology of large‐scale irregularity structures could not be obtained. Over low latitudes, the E‐region irregularities were traditionally classified into two main types: the continuous type linked with the tidal sporadic E (Es) layer and the quasiperiodic (QP) type due to the drift of small irregularity patches. Facilitating from the very large FOV of the all‐sky radar at Ledong, we report a case of E‐region irregularities with different echo patterns from the continuous and the QP types. These E‐region irregularities showed wavelike structures covering up to 250 km in the zonal direction and were observed in directions both perpendicular and significantly off‐perpendicular to the geomagnetic field. In combination with the collocated Es and neutral wind measurements, the possible mechanisms responsible for the wavelike irregularity structures are discussed. Key Points: Wavelike structures of E‐region irregularities elongating up to 250 km are reported at low latitudes for the first timeThe wavelike structures were most likely generated by wind shears and modulated by gravity wavesThe wavelike structures may be an intermediate state between the continuous‐type and quasiperiodic‐type E‐region irregularities [ABSTRACT FROM AUTHOR]

Hainan COherent scatter Phased Array Radar has recorded four patches of daytime F‐region field‐aligned irregularities on 31 May 2021, and it is the first time to investigate the daytime irregularities by the radar imaging technique. In‐beam radar images are plotted on the curved plane transverse to the geomagnetic field lines. The displayed daytime irregularities present the obliquely elongated band‐like structures and the corresponding scatter regions extend to hundreds of kilometers in range and narrow to a few kilometers in the zonal direction. The diffuse and isolated structures were also observed in the radar images. The radar image sequences indicate that the irregularities were continuous in height as they drifted westward. The daytime irregularities maintained the oblique elongated structures that resemble the morphological features of the nighttime equatorial plasma bubbles. Combined with the in situ satellite measurements, these daytime F‐region irregularities are interpreted to be the remnants of the previous night's bubbles and come from the higher altitudes near the magnetic equator. Key Points: Daytime F‐region irregularities in low latitudes were detected by applying in‐beam radar imaging techniqueRadar images of the daytime irregularities exhibit narrow band‐like structures extending hundreds of kilometers verticallyThe band‐like structures are interpreted as remnants of bubbles developed on previous night [ABSTRACT FROM AUTHOR]

The total electron content (TEC) derived from ground‐based BeiDou geostationary orbit (GEO) satellite receivers lying on both sides of the eclipse path is used to study the ionospheric variations in southern China, particularly targeting eclipse‐induced gravity waves in the ionosphere during the annular solar eclipse on 21 June 2020. Eclipse‐induced gravity waves (GWs) signatures with periods of about 70 min, from wavelet analysis, were identified on both sides of TEC observations, which are most likely to be induced in the thermosphere. However, there were significant differences in the intensity of the gravity waves on both sides of the eclipse path, gravity waves are stronger on the northern side of the eclipse path than on the southern side. Moreover, a postponement of TEC depletion shows on the northern side of the eclipse path which is caused by the convergence effect of the wind field. And at the same obstruction rate, GWs possibly can slightly adjust the postponement of TEC depletion on a small scale. Key Points: The eclipse‐induced thermospheric gravity waves are observed by BeiDou‐geostationary orbit total electron content (TEC) on both sides of the solar eclipse pathGravity waves are stronger on the northern side of the eclipse path than that on the southern side due to convergence effectsThe gravity waves on the northern side may slightly contribute to the postponement of the TEC depletion on a small scale [ABSTRACT FROM AUTHOR]

We acquired measurements of four observable parameters of refracted signals received from the ionosphere (Doppler shift, Doppler spectrum, number of rays, and signal amplitude) with the oblique HF Doppler technique in the city of Harbin during the super typhoon Lekima event of 4–12 August 2019. The ionospheric response was observed by the Harbin Engineering University multifrequency multiple path coherent radio system in the People's Republic of China. The maximum distortion of radio‐wave characteristics and ionospheric disturbances were observed to occur during the day when the super typhoon Lekima energy gained a maximum value and when the super typhoon approached close to the ionospheric part of the radio‐wave propagation paths. The magnitude of the ionospheric disturbances decreased with the distance between the super typhoon and propagation path midpoints. Both aperiodic (chaotic) and quasi‐sinusoidal disturbances were observed to accompany the action of the super typhoon in the ionosphere. The action of the typhoon was often accompanied by up to ±1.5 Hz broadening of the Doppler spectra, 10–30 dBV variations in the signal amplitude, and quasi‐sinusoidal variations in the Doppler shift with 0.10‐ to 0.40‐Hz amplitudes and periods of 20–30 to 70–80 min. The quasi‐sinusoidal variations in the signal amplitude with a ∼24‐min period, T, are due to focusing and defocusing of rays by wavelike disturbances in the electron density. The periods of the quasi‐sinusoidal disturbances were observed to be ∼12–24 min. Such periods pertain to atmospheric gravity waves. The atmospheric gravity waves generated by the super typhoon gave rise to quasi‐sinusoidal variations in the electron density with relative amplitudes of ∼3%–∼19%. Plain Language Summary: Typhoons pertain to high‐energy sources that are capable of giving rise to perturbations in all geospheres. The disturbances produced in the atmosphere and geospace make a significant contribution to the state of space weather, whereas the ionospheric perturbations have a drastic effect on the propagation of HF radio waves. We have used super typhoon Lekima, which took place during the 4–12 August 2019 period, to demonstrate some of the many disturbing influences that typhoons have on the state of space weather and the propagation of HF radio waves. The multifrequency multi path observations have been taken by the coherent radio system located at the city of Harbin (People's Republic of China). Key Points: Atmospheric gravity waves launched by the typhoon Lekima produced ∼3%–∼19% quasi‐sinusoidal electron density variationsSpectrum broadening attained ±1.5 Hz, and Doppler shift varied with 0.10–0.40‐Hz amplitudes and 20–30‐ to 70–80‐min periodsThe action of the super typhoon Lekima was accompanied by 10–30 dBV variations in the signal amplitude [ABSTRACT FROM AUTHOR]

In this study, we examined the response of geomagnetic daily variation and ionospheric currents to the solar annular eclipse that passed through China at 10:00–17:30 LT on 21 June 2020. We utilized geomagnetic field data obtained from the Geomagnetic Network Center of China, Institute of Geophysics, China Earthquake Administration and International Real‐time Magnetic Observatory Network, which extend from middle latitudes to equatorial regions in China and surrounding areas. It is found that the eclipse obscuration reduces the intensity of geomagnetic daily variation at most stations as compared with the 5‐quiet‐day average. The response of geomagnetic daily variation to the solar eclipse depends on local time and latitude. In most shadowed areas of the eclipse, there is a reduction of northward (southward) component of the meridional magnetic daily field (ΔX) below (above) ∼30° magnetic latitudes. The solar eclipse can also induce a decrease of the zonal magnetic field variation (ΔY). Meanwhile, we observed that the solar eclipse reduces the intensity of vertical geomagnetic component (ΔZ) at the stations around the early afternoon aside the eclipse totality. In general, these geomagnetic responses imply that the solar eclipse can reduce the ionospheric solar quiet (Sq) current system. These ionospheric currents and ground geomagnetic field disturbances during the solar eclipse are also verified by a global physical model. The simulation demonstrates that the ionospheric currents in the southern hemisphere, where the solar eclipse does not cover, also have a weak response. Overall, the ionospheric current disturbance induced by the solar eclipse shows a counter‐Sq pattern. Key Points: The solar eclipse effects on global ionospheric electrodynamics are studied by extensive geomagnetic data in China and a numerical modelIonospheric electrodynamic response to the annular solar eclipse shows a local time and latitudinal dependenceThe ionospheric current system caused by the solar eclipse has a counter‐Sq pattern [ABSTRACT FROM AUTHOR]

Using meteor radar measurements at Mohe (MH, 53.5°N, 122.3°E) and the reanalysis data from 2011 to 2020, the characteristics of intraseasonal oscillations (ISOs) in the mesosphere and lower thermosphere (MLT) region over MH are investigated for the first time. The radar observation shows a prominent ISO with periods of 40–60 days in the zonal wind during the 2015/2016 winter. The observed ISO is the strongest in the oscillation period of 30–60 days during nearly a decade of observation. The ISO amplitude reaches a maximum of 16 m/s at 90 km. The vertical structure indicates that intraseasonal variabilities in the lower mesosphere contribute to the ISO in the MLT region. By bandpassing the zonal wind obtained from the reanalysis data along 122.5°E at 52 km in the Northern Hemisphere with cutoff periods of 40 and 60 days, we show that ISOs in the tropical and mid‐latitude mesosphere are related to each other. Our results also indicate that the tropical mesospheric ISO and the tropospheric Madden‐Julian Oscillations (MJO) are highly correlated. Our analysis suggests that the MJO in the equatorial troposphere may contribute to the enhancement of the ISO in the MLT region over MH. Additionally, a very strong El Niño during the 2015/2016 winter was accompanied by the positive Indian Ocean Dipole, which confines the active MJO in phases 2‒4 for a long time. This may be the reason that in the oscillation period of 30–60 days, the ISO observed at MH is the strongest during almost a decade of observation. Key Points: A strong intraseasonal oscillation (ISO) with a period of 40–60 days is first observed in the mesosphere and lower thermosphere at MoheThe ISO observed at Mohe is likely associated with the equatorial Madden‐Julian Oscillations in the troposphereStrong El Niño and the positive Indian Ocean Dipole indices may contribute to the enhancement of the ISO at MH [ABSTRACT FROM AUTHOR]

This study reports a special case of the propagation and morphology of medium scale traveling ionospheric disturbances (MSTIDs) over middle-latitude China on the night of August 8, 2016. The MSTIDs were simultaneously observed by multi-instruments, including the all-sky imager, Swarm satellite, and global positioning system (GPS). The MSTIDs lasted for about 6 h in the field view of airglow imager, showing typical wavelength, phase velocity of 272-296 km and 67-250 m/s, respectively. In addition, the imagers show that the inclination angles of phase fronts for some MSTIDs were decreasing during their propagation, resulting in the propagation direction changed from southwestward to nearly westward. More interestingly, the MSTIDs began to dissipate in the airglow observation when they propagated to lower latitudes (below ~40°N) whereas the MSTIDs at higher latitudes were still visible in the later local times. Simulation results from the Thermosphere-Ionosphere-Electrodynamics General Circulation Model are fairly consistent with the Fabry-Perot Interferometer (FPI) wind observations, which provide convincing explanation to show that the variations of ionospheric neutral winds might play important roles in the changes of propagation direction and the dissipation of MSTIDs. [ABSTRACT FROM AUTHOR]

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

The special daytime echoes, which present the V‐shaped echo pattern between 90–180 km ranges in the range‐time‐intensity (RTI) plot, were occasionally recorded by the Hainan coherent scatter phased array radar (HCOPAR) located at the low latitude of China. Four cases with distinct echo traces are presented to display the common characteristics of the V‐shaped echoes. They usually occur between 10:00–15:00 LT in the daytime with the enhanced top frequency and blanketing frequency of Es layer (ftEs and fbEs). Their left/right wings present negative/positive Doppler velocity. A case on 16 August 2015 was investigated by the joint observation of the HCOPAR, the Hainan Digisonde, and a scintillation receiver. The echo spectra show that the V‐shaped echoes were backscattered from the type 2 irregularities. The formation of the V‐shaped echo pattern was due to the leakage of the antenna sidelobe for the very intense scatter from Es layer. The irregularities were found traveling westward with the velocity of 41.12 ± 2.08 m/s. While the occurrence of the intense irregularities, there were periodic peaks appearing on the ftEs and fbEs curves, and the maximum ftEs‐fbEs reached 10.31 MHz. At the same time, the strong ionospheric scintillation event was recorded by the scintillation receiver. The wind shear is considered to produce the sharp density gradient in the Es plasma and then induce the gradient drift instability. The large‐scale gravity waves have the possibility to induce the E‐region wind shear. Key Points: Some scattered VHF echoes from daytime Es are so intense that they leak through antenna sidelobes to form V‐shaped echo pattern in RTI plotV‐shaped echoes on 16 August 2015 were accompanied with the strong scintillation event and the enhanced and periodic ftEs and fbEs peaksWind shear is considered to produce the sharp density gradient in the intense Es layer and then induce the gradient drift instability [ABSTRACT FROM AUTHOR]

We present an analysis of planetary‐scale oscillations during sudden stratospheric warming (SSW) events based on data obtained from a meteor radar located at Mohe (MH, 53.5°N, 122.3°E), the Aura satellite and Modern‐Era Retrospective analysis for Research and Applications, Version 2 data (MERRA2). The planetary‐scale oscillations in the mesosphere and lower thermosphere (MLT) region during eight SSW events from 2012 to 2019 have been statistically investigated. Our analysis reveals that the enhancement or the generation of westward propagating quasi 16‐day oscillation with wavenumber 1 (W1) is a common feature during SSWs over MH. A strong enhancement of the quasi 4‐day oscillation during the 2018/2019 SSW is captured by both radar and satellite observations. The amplified quasi 4‐day oscillation has a period of ~4.3 days in both meridional and zonal winds and with a wavenumber of W2 in the zonal component. Using the meteor radar and MERRA2 data, the vertical structure of the quasi 4‐day oscillation from the stratosphere to the lower thermosphere is derived. The upward propagating feature of the quasi 4‐day oscillation in the meridional component indicates that the oscillation is very likely generated in the lower mesosphere. The mesospheric zonal wind reversal after an elevated stratopause event is observed during the SSW, which results in a negative meridional gradient of the quasi‐geostrophic potential vorticity. Our results not only reveal that the amplified quasi 4‐day oscillation in the MLT region is associated with the 2018/2019 SSW but also suggest that the amplification is originally generated around 60 km due to barotropic/baroclinic instability and propagates upward to MLT region. Key Points: An enhancement of a quasi 4‐day oscillation is observed in the MLT region during the 2018/2019 SSW over MHThe enhanced quasi 4‐day oscillation during 2018/2019 SSW is originally generated from the lower mesosphereThe quasi 4‐day oscillation is likely generated by barotropic/baroclinic instability due to the large zonal wind reversal and the elevated stratopause during the SSW [ABSTRACT FROM AUTHOR]

The propagation features of three groups of multiple large‐scale traveling ionospheric disturbances (LSTIDs), which formed on 22 June 2015 during the passage of LSTIDs that propagated in different directions simultaneously over China, were investigated using the total electron content data from 341 Global Navigation Satellite System stations. The ranges of the periods and horizontal phase velocities of the LSTIDs were ~40–120 min and ~209–750 m/s, respectively. The first two groups were observed during 0700–1400 UT. The meeting of LSTIDs propagating in opposite directions yielded a differential of total electron content (DTEC) variation net structure, where the enhancement and reduction of DTEC amplitude appeared successively and reached a maximum amplitude of ~16% in the overlapping areas. DTEC series based on the observations of geostationary satellites from China's BeiDou Navigation Satellite System addressed the propagation and meeting of fronts. The third group contained two southward propagating LSTIDs, a northward LSTID, and a northwestward one. The joint propagation of the northward LSTID and northwestward LSTID led to a large‐amplitude mixed front, which was broadened both in latitudinal and meridional range. The mixed front moved northward at a distance of ~1,500‐km until it separated into two fronts, due to the difference in propagation velocities and directions. After separation, the northward LSTID continued to propagate and encountered a southward LSTID. The northwestward LSTID dissipated within 30 min in the northwest of China. Such multi‐LSTIDs have mainly been observed previously in equatorial regions, and our observations imply that the propagation of LSTIDs in the mid and low latitudes is more complicated than previously reported. Key Points: LSTIDs propagating in opposite directions produced localized enhancements of DTEC, with a maximum of 16%Two LSTIDs propagating in similar directions led to a mixed front with large amplitude, which was broadened both in zonal and meridional rangeObservations from BeiDou geostationary satellites addressed the propagation and meeting of fronts propagating in different directions [ABSTRACT FROM AUTHOR]

We report 19 coobserved enhanced sodium layers, including sporadic sodium layers (SSLs) and thermospheric enhanced sodium layers (TeSLs), over Hefei (31.8°N, 117.3°E) and Wuhan (30.5°N, 114.4°E) from 2011 to 2018. The total coobservation time at Hefei and Wuhan was 387.24 hr, and the average occurrence rates were 0.038 hr−1 and 0.028 hr−1 for the SSLs and TeSLs, respectively. The SSL and TeSL occurrence rates in summer were 0.078 and 0.039 hr−1, respectively, which were considerably higher than those in other seasons. Among all 19 cases, 16 cases, including 9 SSL cases and 7 TeSL cases, occurred almost simultaneously over Hefei and Wuhan without a time delay. Seven TeSLs and four out of the nine SSLs were accompanied by ionospheric sporadic E (Es), suggesting that an "Es‐SSL (TeSL)" chain formed via the wave‐induced wind shear mechanism. Three SSLs were modulated by waves and the two other cases were related with gravity wave overturning. In general, the correlation coefficients between Hefei and Wuhan for long‐duration cases (more than 2 hr) were high due to some large‐scale mechanism, and the short‐duration cases (less than 2 hr) had poor correlation due to different local characteristics. In addition, three cases were observed with an apparent time delay over Hefei and Wuhan, which might indicate the possible existence of long‐distance transport processes. Key Points: Enhanced sodium layers observed between Hefei and Wuhan showed the consistency of the overall propagating and the difference in detailMost of the enhanced sodium layers were related with ionospheric sporadic E, one of which extended horizontally for nearly 1,000 kmFive sporadic sodium layer cases in the upper mesosphere, instead of relate with Es, are mainly generated by neutral dynamic process [ABSTRACT FROM AUTHOR]