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Abstract The Quasi‐Biennial Oscillation (QBO) is a dominant mode of stratospheric variability and is known to modulate the variability of the ionosphere‐thermosphere (IT) system. However, the extent of its influence on the ionosphere‐thermosphere system remains uncertain due to weak signals and confounding with similar periodicities in solar flux. In this study, we investigated QBO signatures in ionosonde derived peak electron density (NmF2), GNSS total electron content (TEC), and thermospheric composition (O/N₂) from the Global Ultraviolet Imager on NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Local empirical models are used to isolate the stratospheric QBO signature in NmF2 and TEC. Multi‐channel singular spectrum analysis is used to reveal seasonal modulation of theO/N₂response to QBO. We found that the amplitude of non‐solar origin QBO in NmF2, TEC, andO/N₂reaches up to 4% and exhibits an out‐of‐phase relation with stratospheric QBO phase at 30 hPa (QBO30). The TEC QBO signal shows strong regional variability, peaking over Europe. TheO/N₂QBO signal shows clear seasonality with maximum correlation with QBO30 around the equinoxes. The NmF2 response to stratospheric QBO is enhanced at most stations during the September equinox. The QBO signal inO/N₂at different latitudes shows maximum correlation with the stratospheric QBO at different pressure levels. Overall, the global reduction in NmF2, TEC, andO/N₂during the eastward QBO phase, along with their seasonal structure, is consistent with enhanced mixing driven by migrating diurnal tide. However, the regional structure in the TEC response implies additional mechanisms with varying spatial influence and vertical extent. 

Abstract Mesoscale ionospheric irregularities are statistically investigated using an unprecedented 19‐year long‐term global GNSS (Global Navigation Satellite System) TEC (total electron content) data set. These irregularities are represented by ionospheric fluctuations within a 5 3.75 (latitude by longitude) region and a timeframe of 15 min. These fluctuations are derived from the GNSS differential TEC data and are primarily caused by medium scale traveling ionospheric disturbances (MSTIDs). This analysis focuses on Eastern American longitude sectors while comparing them to Asian and African sectors. (a) Global mesoscale irregularities at midlatitudes are characterized by the enhanced intensity during solstice seasons. In the winter hemisphere, the intensity peaks by day, and in summer, by night. The enhanced wintertime irregularity is consistent with gravity wave (GW) activities observed in the stratosphere. This study also explores the potential influence of the South American GW hotspot. (b) At equatorial latitudes, the absolute intensity exhibits semiannual variations, maximizing in equinox at Asian and African longitudes; in Eastern American sectors, highly elevated intensities persist throughout the entire September‐March period, peaking in the December solstice. The relative intensity, however, is much enhanced at night during solstices. (c) Hemispheric conjugacy of the enhanced intensity of nighttime irregularities extends from mid‐to equatorial latitudes during solstices, particularly under low solar activity. These enhanced solstitial irregularities constitute a fundamental mode of global ionospheric variability, and the increased relative intensity of equatorial irregularities may therefore reflect modulation of this underlying background state. 

Abstract Medium‐Scale Traveling Ionospheric Disturbances (MSTIDs) have long been a subject of interest in ionospheric research. However, their spatiotemporal variability across regions, local times, seasons, and solar cycles is very complicated and remains not well established. Using Total Electron Content (TEC) data from global GNSS receiver networks processed at MIT Haystack Observatory, we perform a detailed statistical analysis of MSTIDs over the Continental US (CONUS). Differential TEC data every day from 2012 to 2023 are processed using a keogram‐based image processing technique to identify MSTID wave properties, including the occurrence, propagation direction, phase speed, wavelength, and period. Focusing on eastern US midlatitudes (80°W, 40°N), we extend comparisons longitudinally and latitudinally across CONUS. Our results reveal significant variability in MSTID occurrence rates and propagation directions, notably linked to solar terminators. MSTID occurrence peaks after summer sunrise (with minor maxima near winter daytime), around summer sunset, and after summer midnight. Occurrence generally correlates positively with solar activity in summer but can become negative after winter midnight. In winter, MSTIDs propagate southeastward in the morning and rotate clockwise to west‐northwestward after midnight; in summer, propagation is more variable. Comparisons across the CONUS highlight strong regional differences. Our findings reflect complex drivers behind MSTIDs, including gravity waves, electrodynamic processes, and solar terminators. Their relative influences vary with local time, season, and location. This long‐term analysis provides critical insights into MSTID climatology and forms a basis for in‐depth investigations of MSTID generation mechanisms. 

Abstract Atmospheric gravity waves (GWs) are believed to transport energy and momentum between different regions of the atmosphere. Historically, observations of these waves from both ground and space have been relatively abundant at altitudes up to the lower thermosphere, and somewhat less abundant in the upper thermosphere and F‐region ionosphere altitudes. Much of what is known of the typical properties and occurrence of these waves at thermospheric altitudes has been inferred from their impacts on the ionospheric density and motion, as direct observations of the neutral atmosphere have been less prevalent. Gravity waves in the middle thermosphere, from ∼120–200 km altitude, have rarely been observed directly and as such, their properties at these altitudes are less well documented. NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission makes observation of the middle thermosphere during daytime. During dedicated campaigns, GOLD has been able to observe GWs in this region. This study leverages 22 such campaigns during quiet geomagnetic conditions and low to moderate solar activity levels. Waves were observed with typical periods ∼2–4 hr. Leveraging ground‐based observations, the wavelengths were identified to be between ∼1,500–5,000 km, with phase speeds ∼150–600 m/s. The waves observed were seen to propagate primarily meridionally, in agreement with prior daytime mid‐latitude observations. Using observations of the background wind, the energy and momentum fluxes carried by these waves were found. During the quiet conditions observed, the waves were seen to transport energy flux over a wide range of latitudes. 

Abstract The study reports a long‐lasting nighttime TEC (Total Electron Content) enhancement and the violent outburst of scintillations in response to the recovery phase of the May 2024 Superstorm over the northeast Asian sector using comprehensive observations mainly from the Shandong Peninsula of China and the madrigal GPS TEC in global. Since ∼11:30 UT on 11 May 2024 of the geomagnetic superstorm, a unique nighttime TEC enhancement (above 40 TECU) appeared in Northeast Asia with almost double TEC values than that in the quiet condition, which is lasting more than 12 hr. It is interesting to emphasize that the nighttime TEC enhancement drifted westward overall from Japan to the west of China. Moreover, a sharp escalation of phase and amplitude scintillation indices (>0.4) has been captured at Weihai since ∼20:00 UT at 11 May 2024, which is coincident with the arrival of spread F precisely along with the nighttime TEC enhancement very likely. In contrast, normally, the scintillation indices just fluctuate around 0.1, in which the amplitude scintillation index is slightly greater than the phase scintillation index in general. Therefore, the findings detail the responses to 2024 May superstorm at mid‐latitudes of Northeast Asia, deepening the recognization on space weather effects. 

Abstract This study investigates the four‐peak electron density structure in the Equatorial Ionization Anomaly (EIA) during the intense geomagnetic storms of 10–20 May 2024, utilizing dayside data at ∼500 km from the China Seismo‐Electromagnetic Satellite (CSES‐01). Observations revealed distinct four peaks in latitudinal profiles of electron density, symmetrically distributed across both hemispheres. Particularly, the poleward crests can expand beyond ±30° quasi‐dipole latitude during the 10 May superstorm. To reveal the potential driver of such unique EIA structure, statistical analysis is conducted on 35 CSES‐01 orbits which identified the four‐peak phenomenon. The enhanced equatorial electrojet activity, inferred from the scalar magnetic field residuals, indicates that the four‐peak morphology is likely related to the prompt penetration electric field (PPEF). Superposed epoch analysis also highlighted that intensification of the mean SuperMAG |SML| index preceded the onset of four‐peak structures by ∼20 min, whereas the mean eastward equatorial electric field, derived from an empirical model of the interplanetary electric field driven penetration, exhibits no statistically significant enhancement, implying that substorm‐associated PPEF may be the primary driver of this type of EIA structure. This study advances understanding of EIA variability under extreme space weather conditions and the findings underscore the critical role of magnetosphere‐ionosphere coupling via substorm‐driven electric fields in reshaping storm‐time ionospheric plasma distribution. 

ABSTRACT The Earth's ionosphere plays a critical role in radio wave transmission, reflection, and scattering, directly affecting communication, navigation, and positioning systems. However, the comprehensive impacts of space weather remain to be fully established in cases where the ionosphere experiences strong disturbances during geomagnetic storms. We reported unprecedented observational evidence of extreme ionospheric electron density depletion and its hemispheric asymmetry during the May 10–12, 2024 super geomagnetic storm, utilizing multi-instrument ground-based and spaceborne in-situ observations. The ionospheric electron density significantly decreased, with a maximum reduction of 98% over the whole northern hemisphere for more than 2 days, causing backscatter echo failures in multiple ionosondes within the Chinese Meridian Project (CMP) monitoring network. In contrast, mid-to-low latitude regions in the southern hemisphere exhibited electron density enhancements. Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) simulations demonstrated strong consistency with northern hemispheric observations. The vertical drift and the column integrated ratio of O and N2 (ΣO/N2) from observations and simulations indicated the deep reduction of total electron content (TEC) mainly generated by severe ion recombination associated with neutral composition changes that interacted with the disturbed electric field. The summer to winter neutral wind and asymmetry of O/N₂ were possibly responsible for the asymmetry in electron density between the northern and southern hemispheres. These results advance understanding of ionospheric storm physics by establishing causal links between magnetosphere-thermosphere coupling processes and extreme electron density variations, while providing critical observational constraints for space weather model refinement. 

Abstract Prior observational uncertainties have hindered the clear understanding of the link between tropospheric Lamb waves and ionospheric disturbances. In this study, we precisely extracted ionospheric Lamb waves originating from the epicenter of the 15 January 2022 Tonga eruption, propagating upward in a conical structure. This was achieved by using line‐of‐sight observations from the BeiDou geostationary satellites, which eliminated the spatiotemporal ambiguity introduced by the relative motion of Global Positioning System satellites, enabling the clear extraction of the Lamb signal in the ionosphere. The observed L0 mode speed (∼323 m/s) and period (∼30 min) were consistent with those of the tropospheric Lamb wave. It suggested that the ionospheric Lamb wave is likely driven by the surface Lamb wave, leading to a conical wave‐front that extends in altitude. This study highlights the significant role of Lamb waves in transmitting energy from epicenters through Earth's atmosphere and plasma systems. 

Abstract Solar flares are a rapid increase in solar irradiance, specifically in X‐ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 3–30 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub‐auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare‐induced density gradient likely favors the formation of gradient‐drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line‐of‐sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 2–3 times compared to a geomagnetically quiet and non‐flare day. The observed irregularities, attributed to flare‐driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross‐polar‐cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity. 

Abstract Hypotheses concerning processes related to medium‐scale traveling ionospheric disturbances (MSTIDs) are investigated with the application of models and the analysis of observational data. Wave‐packet parameters for MSTIDs from 2011 through 2022 are obtained from OI 6300 Å observations from the Boston University all‐sky imager (ASI) at the Millstone Hill Observatory during periods for which concurrent Millstone Hill (MH) incoherent scatter radar (ISR) observations are available. A combination of a numerical multi‐layer (NML) model for gravity waves (GW) in the thermosphere with the Field‐Line Interhemispheric Plasma (FLIP) model for ionospheric processes and upper‐atmospheric emissions is applied to generate perturbation electron‐density values, which are compared with ISR‐observed perturbation electron‐density values. A detailed comparison is made between model‐generated and ISR‐observed electron density for two cases, and the comparisons show notably good agreement. Twelve other MSTID cases are also described, giving a total of 14 cases. The results confirm that some nighttime MSTIDs at midlatitudes directly correspond to local GWs. They also suggest that some MSTIDs occurring over MH primarily consist of plasma fluctuations without corresponding local neutral fluctuations and that such MSTIDs are more common during winter months. The phase relationship between electron density and neutral vertical velocity variations is examined for two cases. Additionally, the hypothesis that standard thermospheric dynamic molecular viscosity values should be reduced is evaluated, and it is found that this is not supported by the results.