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In this paper, multi‐source perturbations during the evolution of an equatorial plasma bubble (EPB) event at low latitudes in China are studied by means of multi‐ground‐based instruments, including an all‐sky airglow imager, a very high frequency (VHF) radar and eight digisondes. We found that EPB event initially evolved from bottom perturbations (∼600 km scale) seeded by atmospheric gravity waves in a form of large‐scale wave‐like structure, accompanying smaller‐scale perturbations (∼150 km scale) mostly by collision‐shear instability (CSI); once formed, those seed perturbations further evolved into the ionospheric topside by the plasma instability. Observed and analyzed are two different instabilities: one is the Rayleigh‐Taylor instability (RTI) driven by a prereversal enhancement of the zonal electric field (PRE) occurred near sunset; the other is an equatorward wind‐induced secondary E × B gradient drift instability (GDI) around midnight. Accompanying the PRE‐induced RTI are freshly‐generated depletions with larger poleward (upward) velocities. The PRE‐driven RTI could elevate the bottom perturbations directly to form fast‐moving depletions/structures at the ionospheric topside. The E × B GDI was trigged by a vertical upward plasma jet caused by a seasonal equatorward wind in regions as far as 10°N (20°N) from the geomagnetic (geographic) equator. This equatorward wind‐induced E × B GDI continuously forced topside structures of those drifting‐type EPB depletions to extend poleward more slowly, resulting in active 3.2‐m irregularities around midnight. Besides, we present evidence that a westward polarization electric field generated in an adjacent trough region of the faster‐growing cluster‐type depletions inhibited the neighboring slower‐growing cluster‐type depletions. Plain Language Summary: Observational evidence is insufficient to understand how equatorial plasma bubbles (EPBs) form at low latitudes. The perturbation sources in the evolution of EPBs are various and controversial. This paper highlights the significance of multi‐source perturbations in the formation/evolution of a low‐latitudinal EPB event over China. We found perturbations on different scales induced by the atmospheric gravity wave in a form of large‐scale wave‐like structure and the potential collision‐shear instability (CSI) could seed the EPB event; once formed, they further extended into the ionospheric topside by the plasma instability. The Rayleigh‐Taylor instability driven by a prereversal enhancement of the zonal electric field well explains those freshly‐generated depletions that had faster poleward/upward velocities at/near sunset; those sustainably‐growing depletions (include those cluster‐type depletions) that should stop growing around midnight which evolved from drifting‐type EPBs had a more slower poleward/upward velocities and the accompanying active 3‐m irregularities were excited by a seasonal equatorward wind‐induced E × B gradient drift instability (GDI) at the ionospheric topside. Besides, we found that the faster growing cluster‐type depletions inhibited the development of the slower‐growing ones in neighboring regions, one of reasons for daily variation of EPBs. Key Points: Perturbations caused by atmospheric gravity waves and smaller‐scale perturbations by collision‐shear instability could seed the observed equatorial plasma bubble eventFreshly‐generated depletion near sunset had a faster poleward velocity, evolving from the bottom disturbance via prereversal enhancement of the zonal electric field‐induced Rayleigh‐Taylor instabilityEquatorward wind‐induced secondary E × B gradient drift instability is suggested to explain those evolving depletions around midnight over FukeThe faster‐growing cluster‐type depletions inhibited the neighboring slower‐growing cluster‐type depletions [ABSTRACT FROM AUTHOR]

Lower atmospheric gravity waves (GWs) can significantly impact waves in the middle and upper atmospheres and are vital for turbulence generation. This paper puts the spotlight on the spatial–temporal variability of tropospheric GW total energy (ET) and its potential sources above four regions of interest (ROIs) gathered from high‐resolution radiosonde observations from the China Radiosonde Network during the years 2016–2019. The seasonality of ET above four ROIs shows different characteristics and is dependent on latitudes and underlying terrains, reaching its maximum identified in the winter at middle latitudes. Interestingly, the annual cycles of the maximal ET shift from 35°N in October to 25°N in March of the next year, triggered by the shift in the winter subtropical jet. Based on the random forests regressor, the jet stream between 200 and 125 hPa likely serves as the primary source for the observed GWs above the ROIs with low and middle latitudes, with relative contributions of around 60%. However, the Kelvin–Helmholtz instability between 800 and 125 hPa could be the most recognized source of GWs and contributes around 68.4% to the observed energy. During the rainy season, the ET under scenarios of convective precipitation is around 20% larger than the other. As well, as the near‐surface or low‐level wind interacts with a mountain barrier over the Tibetan Plateau region, 12.4% of the observed ET is attributed to the strength of the low‐level wind. Plain Language Summary: The gravity wave (GW) is one of the most important waves in the atmosphere and acts as a triggering source to turbulence. However, the tropospheric GWs in the context of China has seldomly been investigated by using high‐resolution radiosonde data set. This analysis shows that the GW total energy exhibit obvious seasonal various at low and middle latitudes, with maximal identified in the winter and minimal in the summer. The jet stream in the upper troposphere is the most important source for GW at low and middle latitudes and gives rise to a southward propagation of the maximal GW energy in cold season. In the summer of southern China, the convective precipitation could contribute to the enhancement of GW energy. In addition, 12.4% of the observed GW energy is attributed to the strength of the low‐level wind over the Tibetan Plateau. Key Points: Jet stream is the dominant source for gravity waves (GWs) at low and middle latitudes and triggers southward movement of GW energy core during cold seasonsOver the Tibetan Plateau, Kelvin–Helmholtz instabilities and terrain‐induced flows contribute to the intensive GW activitiesDuring the summertime of southern China, convective precipitation could contribute to the enhancement of energy of about 20% [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]

Previous theoretical simulations showed the generation of double‐layer structures of E‐region field‐aligned irregularities (FAIs). In this study, we report the double‐layer structures of E‐region FAIs observed by an all‐sky radar at low latitude Ledong (18.4°N, 109°E), China. These FAIs appeared at the altitudes ∼90 and 110 km respectively. Both layers displayed quasi‐periodic patterns with synchronized spatial features, that is, being manifested as spatially separated patches or wavelike structures over similar longitudes at the two layers simultaneously. The neutral wind observations revealed the existence of wind shears at two separated altitudes where the double FAI layers occurred. The two wind shears created two vertically separated sporadic E layers and possibly drove the generation of the double‐layer FAIs via gradient drift instability. The synchronized spatial features of the FAIs at the two layers could be modulated by gravity waves. Plain Language Summary: The E‐region field‐aligned irregularities (FAIs) were observed usually in a form of single layer. Due to the limited field of view of narrow‐beam radars which were traditionally employed for observing ionospheric irregularity, the spatial morphology and evolution of E‐region FAIs in a small region were resolved. Owing to the recently developed capability of all‐sky radars in observing E‐region irregularities, the spatial features of E‐region irregularities over a larger zonal region up to hundreds of kilometers in the horizontal plane can be derived. Further, the spatial features of E‐region irregularities in the vertical direction can be unambiguously determined. Utilizing an all‐sky radar at low latitude Ledong, China, a case of double‐layer structures of E‐region FAIs, which occurred at ∼90 and 110 km altitudes respectively, was observed. The double layers of FAIs spanned a zonal coverage ∼250 km, showing as synchronized spatially separated quasi‐periodic patches or wavelike structures in two layers. Especially, the wavelike structures synchronized in the two layers could provide solid evidence for the crucial role of gravity waves in modulating E‐region irregularities. In combination with the collocated ionosonde Es and neutral wind observations, the physical mechanisms responsible for the generation of the double‐layer E‐region FAIs were discussed. Key Points: Double layer structures of E‐region field‐aligned irregularity (FAI) were observed at low latitude by an all‐sky radarBoth layers of FAIs were quasi‐periodic type and synchronously manifested as spatially separated patches or wavelike structuresThe double‐layer FAIs were likely generated by wind shear driven gradient drift instability and modulated by gravity waves [ABSTRACT FROM AUTHOR]

We used dense global navigation satellite system data from China to track the propagation of traveling ionospheric disturbances (TIDs) triggered by the 2022 January 15 Tonga volcanic eruption. We identified two TIDs originating from the eruption. One, which has been reported widely by a number of recent investigations, had a velocity of ∼361 m/s. However, another long‐distance propagating TID with a velocity of ∼264 m/s has not been widely discussed. The velocities of these TIDs coincide with previous simulation results of gravity‐wave L0 and L1 modes. We propose that these TIDs were caused by the L0 and L1 ducted modes of gravity waves excited by the volcanic eruption. However, the L1 mode is usually negligible due to its weak amplitude in comparison with that of the L0 mode. The enormous energy release resulted in a stronger amplitude of the L1 mode, which induced a detectable TID in the ionosphere. Plain Language Summary: A massive volcanic eruption occurred at 04:15 UT on 15 January 2022 in the Hunga Tonga‐Hunga Ha'apai volcano. The volcanic eruption generated atmospheric waves with circular wave fronts, similar to a "stone dropping in a pond leading to ripples." Atmospheric waves can propagate around the globe through waveguides, which guide waves from point to point like a transmission tube. We observed a group of atmospheric waves in China using global navigation satellite system receivers from 902 stations. Three disturbances events, induced by atmospheric waves, were observed in Eastern China 6 hr and 45 min after the eruption. In addition to the first disturbance that supports the long‐distance propagation, there are some slow‐velocities and small‐amplitude traveling ionospheric disturbances, which follow the first one with a velocity of 264 m/s. Upward energy leakage caused ionospheric disturbances, which possibly introduced errors in the positioning and navigation of global navigation satellite systems. Key Points: Two traveling ionospheric disturbances (TIDs) excited by the Tonga volcanic eruption were observed in Eastern ChinaThe velocities of these TIDs are consistent with previous simulation results of gravity‐wave L0 and L1 modesThe enormous energy release resulted in a stronger amplitude of the L1 mode, which induced a detectable TID in the ionosphere [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]

The Harbin Engineering University, the People's Republic of China, multifrequency multiple‐path coherent radio system operates continuously and provides data for post analysis. The data collected during the solar eclipse of 21 June 2020 have been chosen for this study with the objectives to interpret the variations in the Doppler spectra, Doppler shift, and in the reflected radio wave amplitudes that are associated with the solar eclipse, establish the magnitude and find physical significance of these variations, determine the reduction in the electron density caused by the solar eclipse, and to estimate an increase in wave activity in the ionosphere. The eclipse was accompanied by Doppler spectrum diffuseness resulting from an increase in the number of rays, the temporal variations in the Doppler shift were observed to be bi‐polar, asymmetrical, and anomalously small, with extreme Doppler shift magnitudes varying from −11 to −40 mHz and from 22 to 56 mHz. The duration of processes with negative Doppler shifts varied from 50 to 80 min, and the duration of processes with positive Doppler shifts changed from 30 to 80 min. The multi‐hop propagation (from two to five hops) took place along all propagation paths, with a 360 to 560‐km one‐hop length, due to the anomalous radio wave propagation via the sporadic‐E layer present about 80% of the time on 21 June 2020. The Doppler shift exhibited 4–18‐min period quasi‐sinusoidal variations with 20–10‐mHz amplitudes. Plain Language Summary: In the past, the study of the influence of eclipses on the ionosphere seemed to be a rather simple subject because a solar eclipse seemed to be just mimicking night, although over a shorter time interval. Subsequently, the movement of the solar terminators has been discovered to generate atmospheric gravity waves, which is also true for eclipses. Recent advances in instrumentation and measurement techniques have facilitated the revelation of unusual and interesting phenomena that do not necessarily belong to nighttime conditions. For example, in this study, we have discovered, along with the determination of specific changes in the atmospheric and radio wave parameters, that temporal variations in the Doppler shift were bi‐polar, asymmetrical, and anomalously small. Key Points: Doppler shift variations were bi‐polar, asymmetrical, and anomalously small, with extremes varying from −11 to −40 MHz and from 22 to 56 MHzMulti‐hop propagation (from two to five hops) took place along all propagation paths, with a length of one hop varying from 360 to 560 kmAn electron density decrease was determined to vary from –(12–16)% to –(20–26)% along different propagation paths [ABSTRACT FROM AUTHOR]

A sporadic case of noctilucent clouds (NLCs) was observed unexpectedly on the night of the 6–7 July 2020 in Beijing (40°2′N, 115°30′E). The noticeable wavy structures and observed ambient temperature (135.54K at the mesosphere and lower thermosphere [MLT]), both indicated that the increase in temperature oscillations could be the cold phase of gravity waves (GWs). The reasons for NLC formation were analyzed based on the observations and model data sets in our study. The real‐time synoptic analysis revealed that there were GWs originally generated by a squall line in the troposphere. Due to the blocked effect of a stable tropopause inversion layer (TIL), the GWs broke, leading to strong energy dissipation near the TIL. The reverse ray tracing analysis between the TIL and NLCs' layer revealed the travel distance (206.88 km) and time (49.91 min) of GWs. These findings show that the turbulence over the TIL (at approximately 14.64 km) excited secondary GWs, which propagated upwards toward the mesosphere and probably interacted with diurnal and semi‐diurnal tides. The cold phase of the larger‐amplitude waves can provide optimal conditions for NLCs forming. Our study highlights the significance of dynamic coupling mechanisms regarding the effects from troposphere to MLT thermal conditions and offers a case study for the increasing occurrences of NLCs at midlatitudes. Plain Language Summary: Noctilucent clouds (NLCs) are ice clouds forming at 80–86 km during summertime in the Northern Hemisphere. During the night of 6–7 July 2020, a Chinese photographer captured the occurrence of NLCs in Beijing (40°2′N, 115°30′E). The wavy structures in the photographs revealed that the occurrence of NLCs had a close relation with the atmospheric gravity waves (GWs), the significant transport of momentum and energy in the atmosphere. We used comprehensive data sets and models to locate the source of these GWs. The real‐time synoptic analysis in the troposphere revealed that the GWs were generated by a convective system, a squall line. However, a stable tropopause inversion layer inhibited the waves like a huge lid, resulting in strong energy dissipation and exciting new GWs propagating upwards. The secondary‐generation GWs can interact with large‐scale waves and cause severe cooling in the MLT, providing an ideal condition for the growth of ice particles in NLCs. These particles integrated the wave source at the tropopause and GWs that are visible in the NLCs for the first time at midlatitudes. The results inspired us to further explore the connection between the troposphere and the upper atmosphere in the future studies. Key Points: The mechanism of a sporadic case of formation of midlatitude noctilucent clouds (NLCs) in China is demonstratedThe interaction between gravity waves (GWs) and tidal waves substantially cooled the NLC layerTurbulence of the tropopause inversion layer excited secondary GWs which propagated upwards toward the mesosphere [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]

A mesoscale convective systems (MCSs) event occurring in northwest China is investigated to discuss the influences of interaction between different couplings of low‐ and mid‐level winds and orography on the evolution of MCSs, using high‐resolution reanalysis data. The formation of a low‐level anticyclonic circulation related to the plateau high caused the sequential occurrence of reverse and perpendicular low‐ and mid‐level winds couplings and the stagnation and expansion of a high‐humidity zone originating from the mountains. In the case of reverse coupling of low‐ and mid‐level winds, mid‐level winds were constantly lifted by low‐level winds ascending the mountains, resulting in strong updrafts and relevant inertia–gravity waves occurring continuously in the upstream direction of mid‐level winds. The unstable stratification generated by the high‐humidity zone prevented the inertia–gravity waves from propagating into the updrafts, leading to the maintenance and connection of the updrafts. Meanwhile, a strong downdraft occurred and was maintained on the leeward side of the updrafts due to the coupling of the inertia–gravity waves associated with the uplift of mid‐level winds with the inertia–gravity wave triggered by low‐level winds ascending the mountains. Consequently, MCSs developed as they propagated into the updrafts, and perished abruptly when they reached the leeward‐side downdraft. In the case of perpendicular coupling of low‐ and mid‐level winds, a broad and strong low‐level updraft band formed and stayed on the windward side of the mountain ridge due to terrain‐induced uplift. Significant mid‐level updraft and an associated inertia–gravity wave were triggered on the side of the low‐level updraft band blown by mid‐level winds. The unstable stratification generated by the high‐humidity zone hindered the propagation of the gravity wave into the low‐level updraft band, leading to the maintenance of strength and continuity of the low‐level updraft band. Consequently, MCSs were maintained during their entire propagation along the low‐level updraft band. [ABSTRACT FROM AUTHOR]