Abstract. A global climatology of diurnal tides in the mesosphere and lower thermosphere (MLT) is constructed using multiyear observations from fifteen meteor radars distributed worldwide. The results show that diurnal tidal amplitudes are strongest at low and mid-latitudes (10°–50° N/S), with peak values of about 60 m s-1 near 20°–30° N/S, and are comparatively weak near the Equator and at polar latitudes. The seasonal variations of the diurnal tide are characterized by maxima around the equinoxes and minima during the solstices. In addition to these global climatological features, we identify a clear modulation of the vertical structure of diurnal tidal amplitude and phase by seasonal variations in solar forcing, represented here by the solar zenith angle (SZA). This modulation is particularly evident at northern low and mid-latitudes, but is much weaker in the Southern Hemisphere. The hemispheric asymmetry suggests that the tidal response to solar forcing is not globally uniform. To further explore the possible cause of this asymmetry, we examine the meridional fluxes of zonal tidal momentum. The results suggest that background zonal winds can influence tidal propagation through filtering effects and momentum drag, thereby contributing to the observed hemispheric differences in tidal structure. These results provide new observational evidence for the coupling between solar forcing and diurnal tides in the MLT region and offer useful constraints for the evaluation of general circulation models. They also improve our understanding of tidal propagation and variability in the middle and upper atmosphere.

Using the Low lAtitude long Range Ionospheric raDar (LARID) at Dongfang (19.2 degrees N, 108.8 degrees E), Hainan Island, China, we report a unique case of complex ionospheric irregularities observed on 9 June 2024. The most interesting aspect is the first-time long-range (similar to 2,000 km) detection of daytime ionospheric irregularities and post-sunset band-like irregularity structures by HF radar at low latitudes. By incorporating VHF radar and global navigation satellite system (GNSS) rate of total electron content index (ROTI) observations, the radio wave propagation modes of the ionospheric irregularity events observed by LARID and evolution of these ionospheric irregularities were analyzed. The daytime ionospheric irregularity echoes observed by LARID over the Indian Ocean exhibited westward drift, evident in both echo patterns and Doppler velocities. These ionospheric echoes were likely backscattered from E region field aligned irregularities by the "downleg" of the 1-hop HF ray path. The post-sunset irregularity echoes observed by LARID over Indian Ocean manifested as a band-like structure. This structure was closely attached to the bottom of ground/sea scatter echoes and showed the same range variation as the ground/sea scatters. GNSS ROTI observation revealed that irregularities primarily appeared south of the magnetic equator and drifted eastward across the eastern longitudes in later hours. Analysis indicates that the displacement of small-scale irregularities within the equatorial plasma bubble event along the magnetic field likely contributed to the band-like irregularity structure observed by LARID and the asymmetric irregularity distribution observed by GNSS.

High-frequency (HF) radio communication is essential for long-range connectivity in remote and maritime regions lacking conventional communication infrastructure. The maximum usable frequency (MUF), which is critical to HF effectiveness, is sensitive to dynamic ionospheric conditions. Traditional MUF observations and global models suffer from limited coverage or reduced accuracy, particularly over low-latitude oceanic regions where ionospheric variability is pronounced. This study introduces a practical MUF inversion method for low-latitude regions, utilizing ground/sea scatter echoes from the low-latitude long-range ionospheric radar (LARID) at Dongfang (19.2 degrees N, 108.8 degrees E). By combining ray-path geometry with ionospheric characteristic parameters, the proposed technique generates MUF profiles through nonlinear fitting of ground distance and operating frequency. This method demonstrates better performance in low-latitude regions compared to the linear fitting approach traditionally used by HF radars at high latitudes. Validation against ionosonde data at Ledong (18.4 degrees N, 109 degrees E) shows close agreement in diurnal variation trends. The results reveal MUF asymmetries driven by the equatorial ionization anomaly (EIA) and local time effects-features that cannot be captured by empirical models or single ionosonde measurements. This approach addresses a critical gap in real-time MUF monitoring over oceanic regions, enhancing the reliability of HF communication links in maritime and emergency scenarios.

The mesosphere and lower thermosphere (MLT) region plays a crucial role in the vertical coupling between the lower and upper atmosphere, but continuous regional observations of its wind field remain rare. In this study, we utilize observations from the Eastern China Multistatic Meteor Radar Network (ECMMET) to evaluate two 3-dimensional wind retrieval methods: the newly developed Volume Velocity Processing (VVP) (NDVVP) and the 3-dimensional variational assimilation using the divergence constraint (3DVAR+DIV). Both methods maintain stable performance at temporal resolutions as high as 10 min and yield highly consistent horizontal winds, with mean wind correlation coefficients exceeding 0.9 and wind velocity mean differences within 1 m/s. Comparisons with simultaneous lidar observations further confirm the reliability of the ECMMET-derived winds, particularly the 3DVAR regional winds, which exhibit a correlation coefficient of 0.93 and a root-mean-square (RMS) difference of 14.0 m/s relative to the lidar observations at 88 km. The results showed that the 3DVAR+DIV method effectively reveals more detailed spatial variability, whereas the NDVVP performs better for retrieving the overall trend of the wind field. The non-random discrepancies between the two retrievals originate from their inherent assumptions. NDVVP's linear wind assumption smooths higher-order and nonlinear fluctuations, while 3DVAR+DIV captures them more effectively through spatial gridding. These findings emphasize the advantages of both methods and demonstrate ECMMET's capability to provide reliable, high-resolution, and refined wind measurements essential for studies of MLT dynamics.

Abstract The irregularities of ionospheric plasma bubbles (EPBs) sequentially generated over wide longitudes are expected to share similar lifetime under normal conditions. In this study, we report a special case where the EPB irregularity lifetime over wide longitudes showed a quasi‐wavelike undulation pattern with wavelength of ∼3,000 km, that is, the lifetime of EPB irregularities sequentially generated at different longitudes experienced several gradually decreasing/increasing cycles. Irregularities with longer (shorter) lifetime corresponded to the EPBs with larger (smaller) development in latitude/altitude. Further observations revealed that smaller‐scale (hundred‐kilometer) and larger‐scale (thousand‐kilometer) waves may coexist at the bottomside of ionosphere around sunset. The former acted as the seeding source for EPB generation, whereas the latter superimposed on PRE and modulated EPB development in altitude over different longitudes, leading to the wavelike undulation morphology in irregularity lifetime ultimately. This study provides implications on how the complex wave structures influence EPB morphology over wide longitudes.

Electron Bernstein waves (EBWs) are electrostatic modes that propagate perpendicular to the magnetic field in magnetized plasmas, serving as a diagnostic tool for understanding plasma properties and the energetic particles that drive energy transport. However, their naturally occurring harmonic structures have long eluded ground-based radar detection due to their faint signals and the requirement for an observational geometry nearly perpendicular to the geomagnetic field. Here, we report high-resolution broadband spectral observations of naturally excited EBWs in Earth's ionosphere using the Sanya Incoherent Scatter Radar. Our measurements reveal a series of discrete, well-defined spectral lines corresponding to the first seven electron gyroharmonics, a result consistent with theoretical predictions. Our analysis suggests that these EBW modes, enhanced by suprathermal photoelectrons, become observable under near-perpendicular geometry, where Landau damping is reduced. The frequency structure of these harmonics acts as a precise diagnostic ruler that enables the retrieval of electron density and perpendicular electron temperature, parameters that are difficult to retrieve with ion line techniques under such geometric constraints. This work provides ground-based radar observations of naturally excited EBWs and shows their use for diagnosing plasma parameters in magnetized plasmas.

In this study, daytime F-region irregularities were observed at low latitudes during the main phase of the 12 November 2025 strong magnetic storm, causing intense very high frequency radar echoes ranging hundreds of kilometers in altitude. They were generated mainly at the F-region topside, associated with significant plasma density enhancement possibly driven by storm-time electric field polarity transition from eastward to westward, or meridional wind. The enhanced plasma density could provide favorable downward density gradient for generating the daytime irregularities via gradient drift instability at F-region topside under westward storm-time electric field. The irregularities occurred over a large zonal region spanning thousands of kilometers. The study provides new possibility of low-latitude F-region irregularities different from the major irregularity types over low latitudes, which could be the first report of its kind at this region.

Abstract High‐altitude optical meteors initiating above 150 km are exceedingly rare, with confirmed observations largely confined to the Leonids. Using the Meteor and ionospheric Irregularity Observation System, we recorded a bright 43‐Cassiopeiids fireball with heterogeneous material. It initiated luminously at an exceptional altitude of 157.8 0.34 km and began ablating at ∼112.0 km. Multiple flares between 101.6 and 79.5 km were accompanied by a ∼160 s non‐field‐aligned plasma irregularity (NFAI), directly linking irregularity formation to meteoroid fragmentation. The >4 s delay between the optical event and the radar NFAI echo indicates that dust‐driven NFAIs can develop on a timescale of seconds, as ablated material vapor re‐condenses, charges, and organizes into Bragg‐scale structures. This observation provides the first direct evidence for second‐scale NFAI formation by meteors, and offers new constraints on meteor dust‐driven plasma irregularity processes in the mesosphere.

Sporadic E (Es) layer plays essential role in ionosphere-atmosphere coupling. Theoretical simulations show that the atmospheric gravity waves (GWs) in the lower thermosphere are the main modulatory source on Es dynamics at small spatial scales and short periods. Based on Sanya Incoherent Scatter Radar, direct observational evidence for the mesoscale GW modulations on low-latitude Es layer is firstly obtained by four-dimensional electron density with extreme fine resolutions of 37.5 m in range and 50 s in time. The wave-like horizontal structures, with similar to 10 min period and similar to 55 km wavelength, indicate the typical features of mesoscale GWs. Our results confirm that, interacting with the background tidal winds, the upward propagating GWs modulate the horizontal structures and drift velocity of Es layers at different altitudes with increasing amplitude to cause the rapid oscillations in range.

The all-sky meteor radars are primarily designed for observations of mesospheric neutral wind. Recently, the capability of all-sky radars in investigating ionospheric irregularities of the Bragg scales has been developed. However, limited by the relatively small range gate span of the all-sky radar, this capability is mainly constrained to the observations of irregularities at lower altitudes, e.g., E-region irregularities. The F-region irregularities at higher altitudes up to hundreds of kilometers, e.g., equatorial plasma bubbles (EPBs), are usually considered beyond the detection range of all-sky radars. In this paper, we try to extend the capability of a conventional interferometric all-sky meteor radar located at Sanya (18.4°N, 109.7°E) for investigating the spatial features of EPBs. Based on the arriving angles of irregularity backscatter echoes obtained by the radar interferometry technique, and according to the magnetic sensitivity of the EPB field-aligned irregularities, the true ranges of the irregularity structures could be determined to further reveal the spatial features of EPB structures. The results are confirmed by the collocated narrow-beam very high frequency (VHF) radar and the Low lAtitude long Range Ionospheric raDar (LARID). It is revealed that the all-sky radar could be employed to investigate EPB irregularities in a larger zonal region than narrow-beam VHF radars, which could be up to ∼2000 km. The observations could well cover the blind area of the LARID field-of-view, and thus could be employed to continuously trace EPB occurrences and evolutions over thousands of kilometers by combining with LARID in future studies.

Abstract Intense Global Navigation Satellite System (GNSS) amplitude scintillations usually occur in nighttime at low latitudes due to ionospheric plasma bubble irregularities. During daytime at middle latitudes, previous studies found that the sporadic E (Es) layer could lead to GNSS amplitude scintillations, which however, is relatively weak, for example, S4 generally below 0.3. In this study, relatively intense daytime GNSS amplitude scintillations with S4 up to ∼0.5 were observed linked with strong Es with a top frequency up to 20 MHz. The strong Es layer, manifested as pulse‐like disturbances in total electron content time series and narrow‐band‐like structures elongating ∼2,000 km in the horizontal plane, caused short‐duration signal fading up to ∼3 dB in the raw GNSS data, thus resulting in the relatively intense scintillations. Further analysis found that the strong Es was possibly multi‐layered in the vertical direction, which may complicate radio‐wave propagation between multiple layers and further enhance signal fading and scintillation.

Observations of meteoroids and their associated phenomena in the atmosphere provide crucial insights into the composition and structure of their parent bodies, and enhance our understanding of their impact on the near-Earth space environment. In this study, we present observations of meteor M20221224_183010 by the meteor and ionospheric irregularity observation system at Hainan, China. This meteoroid underwent extensive, multi-stage fragmentation (with apparent visual debris but without any sharp spikes and flares) between altitudes of 81.0 and 41.1 km, with an entry velocity of 13.23 km/s. The right ascension and declination of the meteor are 16.27 degrees and -6.8 degrees, respectively, being originated possibly from debris of an Apollo asteroid. The meteor is primarily composed of fragile and comet-like materials, but also contains regions with mechanical strengths comparable to carbonaceous meteorites, presenting significant internal heterogeneity due to variations in fragmentation resistance. The observation of a continuous emission spectrum indicates that the material distribution remained relatively homogeneous with a high content of Na. This meteoroid produced non-field-aligned plasma irregularities around 64-60 km altitudes. We surmise that the meteoroid's continuous fragmentation could produce dust particles, which rapidly acquire charge and form Bragg-scale structures in low mesosphere, manifesting as non-field-aligned plasma irregularity backscatter echoes.

The all-sky radar, also known as all-sky meteor radar, is primarily used to study mesospheric neutral wind by observing specular meteors, and has been recently developed to investigate the spatial features of ionospheric E-region irregularities. However, the interferometry result by a single all-sky radar may contain potential errors under certain conditions, e.g., radio signal refraction. This article proposes a new method to investigate the spatial features of low-latitude E-region irregularities based on a bistatic all-sky radar system installed in Hainan, China, including two all-sky radars located at Ledong and Sanya, respectively, which are separated by similar to 70 km. Ledong radar is for radio signal transmission and reception, while Sanya radar is only for reception. By employing the interferometry technique in the bistatic all-sky radar system, the spatial features of E-region irregularities could be investigated in a larger field-of-view (FOV). Preliminary interferometry cases of E-region irregularities under different conditions are presented, showing a good performance of this technique. The observations obtained by the two subradars of the bistatic system could verify the interferometry results to further determine whether there are obvious detection errors or complement each other to acquire more information about the spatial location, morphology, and evolution of E-region irregularities.

Previous studies suggest that the planetary waves in mesosphere and low thermosphere (MLT) could modulate the occurrence of equatorial plasma bubbles (EPBs) via altering post‐sunset F layer height. Using simultaneous observations by Global Navigation Satellite System receiver networks, two ionosondes separated by about 10° in longitude, high frequency and very high frequency radars, we investigated the day‐to‐day variations of post‐sunset F layer height and EPB occurrence in southeast Asia during the quasi‐2‐day planetary wave (QTDW) event in July 2023. The results showed that the post‐sunset F layer height over Bac Lieu (9.3°N, 105.7°E) and EPB occurrence had a quasi‐2‐day (QTD) variation. However, such a 2 day variation of F layer height was confined in a very limited longitude, that is contradictory to the planetary scale characteristics of QTDW. We suggest that the QTD variations of post‐sunset F layer height and EPB occurrence over the specific location were not necessarily due to the QTDW in MLT. The local seeding source, as characterized by satellite traces in ionosonde ionograms, could drive the small‐scale longitudinal structure of F layer height and play an important role in shaping the QTD variation of EPB. The results implicate that the connection between planetary waves and the EPB occurrence over a specific location should be interpreted carefully, even if the day‐to‐day variation of post‐sunset F layer height shows periodic behavior with planetary wave scale.

Using the Low-lAtitude long-Range Ionospheric raDar (LARID) at Hainan Island, we present a statistical survey of medium-scale traveling ionospheric disturbances (MSTIDs) at low latitudes in the East and Southeast Asia sector. MSTID signatures manifested as structured enhancements in the backscatter echo power maps, with dominant periods of 30-40 min and wavelengths of 150-250 km. The MSTIDs were observed to propagate quasi-zonally, with a higher occurrence rate in the quasi-eastward direction (74%) than in the quasi-westward direction (26%), a feature corroborated by nearby total electron content observations. The possible causes of the east-west asymmetry are discussed in terms of the geomagnetic activity, the atmospheric gravity wave (AGW) source and the neutral wind effects on the vertical propagation of AGW. We propose that the filtering effect of daytime westward thermospheric wind could contribute to the observed quasi-eastward propagation preference of MSTIDs. The propagation direction of quasi-zonally propagating MSTIDs in LARID and GNSS observations was slightly shifted poleward from the due zonal direction. The poleward shift could be contributed by the enhanced convection activities in the lower latitudes close to the equator, which could serve as an important source of AGWs.

An increasing amount of evidence indicates that lunar water ice exists in permanently shadowed regions at the poles and will soon become an important resource for lunar exploration.However,the water ice content and distribution are still uncertain.We report a new 70-cm-wavelength radar image of the lunar south pole obtained by an Earth-based bistatic radar system consisting of the Sanya incoherent scatter radar(SYISR)and the five-hundred-meter aperture spherical radio telescope(FAST).The upper limit of water ice content(0 wt.％-6 wt.％)and its potential distribution are determined from a radar circular polarization ratio(CPR)map by considering the coherent backscatter opposition effect(CBOE)of water ice and ignoring the contribution of roughness to the CPR.This result is advantageous for future lunar exploration missions.

Abstract Equatorial plasma bubble (EPB) irregularities can significantly impact satellite‐based communication and navigation systems. Accurate prediction of EPB occurrence is essential for mitigating these impacts. Using the GNSS receiver network and ionosonde data from East and Southeast Asia during 2010–2021, and the rate of TEC change index to characterize the occurrence of EPB irregularities, we developed a novel Spatio‐Temporal deep learning model for regional EPB irregularities short‐term Prediction (STEP). The model integrates the convolutional neural network and long short‐term memory (LSTM) network, together with attention mechanisms, to capture both spatial and temporal features of regional ionospheric irregularities. The results show that for 5‐min forecast, the STEP model achieves a root mean square error (RMSE) of 0.062 TECU/min and an R2 of 0.818, reducing RMSE by 19.48% compared to LSTM and 27.06% compared to gated recurrent unit model. For 60‐min prediction, the STEP model can still achieve reasonable accuracy with an RMSE of 0.110 TECU/min and an R2 of 0.482, showing significant improvement over traditional models. The equatorial F layer height and regional TEC fluctuations were identified as the most critical factors for predicting the generation and duration of EPB irregularities, respectively. The spatial and temporal distributions of EPB irregularities, including their latitudinal variation and delayed onset after sunset, and the occurrence across different days in East and Southeast Asia, were well predicted by the STEP. It is expected that the STEP model would provide a valuable tool for improving the resilience of GNSS against ionospheric scintillations induced by EPB irregularities.

F‐region bottom‐type scattering layers (BSLs) occurring over equatorial and low latitudes may act as the precursor of plasma bubbles, usually observed by narrow‐beam very high frequency radars. However, their spatial features remain unknown due to the radar narrow field‐of‐view. Here we report a case of localized BSL not accompanying plasma bubbles firstly observed by an all‐sky radar at low latitude. Based on radar interferometry over a large field‐of‐view, the BSL was revealed to occur over a limited area northeastward of the radar and did not cause scintillation when Global Navigation Satellite System satellites passed through. The BSL vertical displacement precisely followed the F‐layer bottom fluctuation, without obvious horizontal movement. Interestingly, the localized BSL caused weak spread‐F traces in the ionograms indicating irregularities from specific directions, which are distinct from the satellite traces or range spread F related to plasma bubble development thus could serve as a new signature of BSL in future studies.

Rocket-launching can trigger traveling ionospheric disturbances (TIDs) which often show V-shaped structures. Using a dense Global Navigation Satellite System total electron content (TEC) receiver network, we investigated the ionospheric response to the waves triggered by the launches of Long March (LM) series of rockets, the LM-2D on 30 March 2023 and LM-6A on 10 September 2023. Results show that during the two launches, large ionospheric holes and TIDs with unusual semicircular and circular structures were observed. The ionospheric disturbances traveled for a distance of more than 1,500 km with amplitudes gradually decreased from about 0.1 to 0.03 TEC unit. The period and horizontal speed of disturbances were similar to 7-8 min and around 700-800 m/s, respectively, which fell into the acoustic mode. We suggest that the TIDs were induced by shock acoustic waves (SAWs). The semicircular and circular structures could be attributed to the rocket flight tilt angle and the corresponding altitude of conic SAWs.

Thunderstorm‐related activities in the lower atmosphere are usually observed and investigated based on atmospheric weather radars. In this study, the occurrence of thunderstorms has been observed and investigated for the first time by all‐sky meteor radars, which are primarily employed in mesospheric/ionospheric observations. By applying the radar interferometry technique to a bi‐static all‐sky radar system installed on Hainan Island, China, the thunderstorm activities on 5 June 2024 were captured and tracked. The thunderstorm echoes occurred at the altitudes ∼5–20 km along the southern coast of Hainan Island, lasted for ∼2 hr, and migrated northeastward at a speed of ∼15 m/s. The occurrence time, spatial locations in both the horizontal and vertical planes, and evolution of the echoes all corresponded well with the thunderstorm development process confirmed by multiple other kinds of lower‐atmospheric observations. The study highlights the extended capability of all‐sky radars in investigating lower‐atmospheric activities, i.e., thunderstorms, which may favor the investigation on tropospheric‐ionospheric coupling process during extreme convective activities in the future.