Libo Liu

The upper‐atmosphere cooling trend is well documented, while the long‐term trend of the ionospheric electrodynamics remains poorly understood. Using two geomagnetic observation data sets over Indian sector, we analyze the long‐term trend of the monthly‐median equatorial electrojet (EEJ) in 1967–2015 after removing the solar activity and seasonal signals. During 1967–1999, a period of slow geomagnetic‐equator motion, the residual of the EEJ shows a decreasing trend between 09 and 13 LT and an increasing trend between 13 and 16 LT. Following accelerated geomagnetic‐equator motion after 1999, the afternoon enhanced trend is suppressed, whereas the morning decreased trend is strengthened for the full‐period (1967–2015) data set. The semidiurnal‐like pattern in the EEJ long‐term trend is possibly related to the weakened migrating semidiurnal tide (SW2) caused by the enhanced greenhouse gases as reported in previous studies. Our results disentangle the influences of greenhouse gas and geomagnetic secular variation on equatorial ionospheric electrodynamics, and have potential implications for ionospheric climate and weather.

The atmosphere and ionosphere form a vertically coupled system in which disturbances in the lower atmosphere can modulate ionospheric variability. Using reanalysis data, ground‐based GNSS observations, and COSMIC‐2 profiles, we investigate gravity waves (GWs) generation and upward coupling during a southwest vortex (SWV) event over the Tibetan Plateau in June 2022. The results reveal a multi‐step process: primary gravity waves are excited near the tropopause by vortex‐induced dynamical disturbances and propagate upward. These primary waves dissipate at ∼45–50 km, depositing momentum and energy that may generate secondary gravity waves. The secondary waves may propagate further upward and contribute to the generation of higher‐order gravity waves, which penetrate into the ionosphere and generate persistent medium‐scale traveling ionospheric disturbances (MSTIDs). These findings demonstrate that ionospheric responses to tropospheric vortex systems cannot be explained solely by direct upward propagation, highlighting the critical role of multi‐step vertical coupling in atmosphere‐ionosphere.

Auroral visibility forecasting at mid‐latitudes is inherently challenging because these regions lie near the equatorward boundary of the auroral oval, where small boundary shifts can determine visibility. In East Asia, this sensitivity is further amplified by substantial deviations of the geomagnetic field from dipole approximations. We developed OVATION‐AACGM by integrating the Altitude‐Adjusted Corrected Geomagnetic (AACGM) v2 coordinate system into the publicly available OVATION Prime 2010 and evaluated its performance for forecasting at Mohe, China (53.5°N, 122.3°E; AACGM latitude ∼48.7°N). Using over 2 years of matched Defense Meteorological Satellite Program (DMSP)/Special Sensor Ultraviolet Spectrographic Imager observations and OVATION outputs, we derived optimal visibility thresholds for both coordinate systems. The AACGM system yields an optimal threshold of 1,040 km, corresponding to a physically realistic auroral elevation angle of ∼6.0°. In contrast, the dipole threshold by maximizing the F 1 score (1,730 km) corresponds to −1.3°, effectively placing the aurora below the geometric horizon. AACGM‐derived distances show improved agreement with DMSP observations (correlation 0.771 vs. 0.709; mean bias −154.5 km vs. +537.9 km). Independent validation of the OVATION‐AACGM model performance using 140 nights of ground‐based optical observations demonstrated operational skill, achieving 72.7% precision and 80.0% recall. A logistic regression model further provides a continuous probability‐distance relationship for auroral visibility. These results demonstrate that coordinate system selection fundamentally influences the physical interpretability of mid‐latitude aurora visibility thresholds and support the adoption of AACGM v2 for physically consistent and operationally reliable applications in the Asian sector.

The observations from GRACE‐FO and DMSP satellites are utilized to analyze the ion‐neutral interaction near dusk during the 10–12 May 2024 super‐storm. The horizontal plasma convection dragged the anti‐sunward wind in the polar cap and sunward winds around the auroral and subauroral regions during whole storm times. The strong westward winds were observed below subauroral latitudes (∼35°S–45°N Mlat) after storm onset, and these westward winds disappeared after 23 UT on 10 May. Meanwhile, the penetration of auroral and subauroral electric fields could induce the westward plasma drifts in lower latitudes before 23 UT on 10 May. We suggested that the ion‐drag force of the westward penetration plasma drift is the possible source for the westward disturbance winds below the subauroral latitudes at 17–23 UT on 10 May. Thus, the horizontal plasma drifts drag horizontal winds from polar cap to equator in the main phase, which has been never observed.

Martian ionosphere‐thermosphere (I‐T) coupling plays important roles in ionospheric variability. Although it has been examined in some literatures, coupling characteristics and mechanisms at different altitudes and the competition between the thermosphere and crustal magnetic fields for dominating ionospheric variations remain to be fully understood. In this study, these topics were discussed by investigating concurrent ionospheric and thermospheric longitudinal variations using MAVEN's measurements. The thermosphere can dominate ionospheric longitudinal variations directly or indirectly in the altitudinal range up to 300 km under weak crustal field conditions. Neutral density is the decisive factor for longitudinal variations of the topside electron density, whereas neutral scale height is decisive for those of the peak electron density. Longitudinal structures in electron and neutral densities remain in‐phase in the topside ionosphere, while they are anti‐phase below the electron density peak. These characteristics are consistent with in situ photochemical I‐T coupling. The region where the in situ coupling dominates can extend to altitudes above 200 km. Ionospheric longitudinal structures generated in this region can further extend to higher altitudes and amplify with increasing altitudes through upward plasma diffusion. The photochemical I‐T coupling can still dominate ionospheric longitudinal variations below ∼210 km under strong crustal field conditions. Crustal field effect gradually dominates over thermospheric effect as altitude increases. It causes a steady ionospheric structure relative to crustal field distribution, and that structure enhances with increasing altitudes. This study enhances our understanding of Martian I‐T coupling at different altitudes and its role in ionospheric variability.

The interplanetary environment exerts a persistent influence on the Earth system under both geomagnetic disturbed and quiet conditions. Multi‐day oscillations in thermospheric mass density (TMD) are an important signature of this influence. However, the impact of interplanetary environment variability is often confounded with the contributions from recurrent geomagnetic disturbances. We use a deep learning model to extract and reconstruct TMD variability from CHAllenging Minisatellite Payload observations and simulate multi‐day oscillations. Ablation experiments show that interplanetary environment variability alone can produce multi‐day TMD oscillations. The interaction of interplanetary environment variability with other drivers further shapes the oscillation patterns. The multi‐day TMD oscillations driven by interplanetary environment variability exhibit marked latitudinal and longitudinal dependence. In particular, the 9‐day TMD oscillations are stronger over the regions with Weddell Sea Anomaly like enhancement. Similar spatial features and seasonal dependence suggest that regions significantly influenced using dynamic processes are more susceptible to external forcing.

Regional ionospheric structures characterized by localized spatial variations in plasma density significantly impact radio signals and Global Navigation Satellite System (GNSS) applications. Using multi-instrument observations in the Asian sector, this study characterizes the regional ionospheric structures and their impacts on GNSS positioning. Steep longitudinal Total Electron Content (TEC) gradients exceeding 2 Total Electron Content Unit (TECU) per degree are identified within a narrow latitudinal range (20°–30°N). We reveal the limitations of current final Global Ionosphere Models (GIMs) products in capturing these steep TEC gradients. These GIMs fail to capture these gradients due to a low longitudinal resolution of 5°, among which the Chinese Academy of Sciences final GIM (CASG) and Jet Propulsion Laboratory final GIM (JPLG) products perform better than others (demonstrated by higher consistency with observed TEC gradients) due to the incorporation of new GNSS constellations and the adoption of a three-layer ionosphere model, respectively. Further statistical analysis of Standard Point Positioning (SPP) results using different GIMs shows that the positioning errors of SPP are larger in the present of steep TEC gradients. Since the CASG and JPLG are better in capturing these TEC gradients, the positioning errors of SPP using the CASG and JPLG are smaller than those using other GIMs. By combining Rate of TEC Index (ROTI) observations with kinematic Precise Point Positioning (PPP) solutions, we present the positioning errors induced by ionospheric irregularities during two geomagnetic storms (1 December 2023 and 10 May 2024 geomagnetic storms). Ionosonde and incoherent scatter radar measurements at Sanya (109.6°E, 18.3°N) are used to demonstrate how Penetration Electric Fields (PEFs) modulate post-sunset ionospheric irregularities and ultimately influence the performance of kinematic PPP. An under-shielding PEF enhances ionospheric irregularities, degrading PPP accuracy from < 10 cm to > 1 m. In contrast, an over-shielding PEF suppresses ionospheric irregularities, thereby prevent irregularity-induced PPP errors during the geomagnetic storms. These findings highlight the critical role of regional ionospheric structures and storm-time electrodynamics in GNSS positioning.

Atmospheric tides propagating upward from the lower atmosphere undergo nonlinear interactions and modulate ionospheric plasma redistribution, leading to pronounced day-to-day variability in ionospheric parameters. This variability reflects the superposition of multiple tidal components with different periods, zonal wavenumbers, and mode structures, yet the dominant modes remain unclear. A hybrid method that combines space-based observations (ICON/MIGHTI), ground-based measurements (Chinese meteor radar chain), and empirical tidal modes (ETMs) is applied to extract the short-term tidal variability. The method is validated during the 2021 sudden stratospheric warming event, capturing the enhancement of the SW2 tidal amplitude, a strengthened first antisymmetric mode, and the phase advance in E-region neutral winds. Future work will extend this approach to assess the modal contributions of tides to the variability of ionospheric plasma drift.

The spatial and temporal characteristics of meso‐scale zonal differences in ionospheric total electron content (TEC) and their relationship with geomagnetic conditions remain insufficiently understood. This study utilized one month of Beidou geostationary orbit (GEO) satellite observations of TEC in May 2024 to systematically investigate zonal TEC variations at mid/low latitudes in the Asian‐Australian sector. Pronounced zonal variations were observed in both hemispheres, with significant differences intermittently detected at receivers RTKV (31.0°N, 114.5°E), GDGZ (23.1°N, 113.3°E), and COCO (12.2°S, 96.8°E) during the month. These differences were strongly linked to the morphological evolution of the Equatorial Ionization Anomaly (EIA), particularly when its poleward edge approached the ionospheric pierce points. Moreover, a suppression of zonal differences was observed prior to the onset of the geomagnetic storm, which may be attributed to the diminished or obscured morphology of the EIA accompanied by the intensification of geomagnetic activity. Analysis of additional equinox storms in recent years indicated that the suppression process is not accidental. These findings highlighted the necessity of high‐resolution TEC observations for capturing meso‐scale zonal TEC differences, and further revealed that these zonal differences were driven by EIA dynamics and geomagnetic activity effects.