Shasha Zou

We investigate global ionospheric responses to four X‐class solar‐disk flares that occurred in November 2025 from the same solar active region, using multi‐instrument observations including GNSS, ionosonde, geomagnetic and GOES solar irradiance data. Pronounced dayside TEC enhancements are observed for three flares (#2 X1.2, #3 X5.1, and #4 X4.0), with comparable durations of 20–30 min and TEC magnitudes increasing with flare intensity. A consistent time delay of 30–60 s is identified between peaks in EUV irradiance rate and rate of TEC (ROT), supporting the primary role of EUV radiation in the E/F‐region TEC enhancements. In contrast, no detectable TEC response was observed for flare #1 (X1.7), despite its higher GOES X‐ray class, common active‐region origin, and similar disk location to flare #2. We interpret this absence as mainly associated with its weak EUV irradiance‐rate enhancement and potentially insufficient to produce E/F‐region responses distinguishable from background ionospheric variability. Concurrent ionosonde observations reveal consistent increases in fmin and disappearance of foF2 across multiple sunlit stations, indicating strong high‐frequency (HF) radio wave absorption in the lower ionosphere. Additionally, GNSS TEC increased while ionosonde foF2/NmF2/hmF2 decreased during flare #2 (X1.2). This contrasting response coincided with short‐lived negative H‐component perturbations at nearby geomagnetic stations, suggesting a role for regional electrodynamic forcing.

On the nightside, the low‐altitude projection of the plasmapause is commonly associated with the ionospheric mid‐latitude trough, which is typically identified from ionospheric density measurements and is clearly manifested in total electron content (TEC) data. In the equatorial magnetosphere, the plasmapause, marks the outer boundary of the plasmasphere, which, in turn, is associated with intense whistler‐mode hiss waves. These waves efficiently scatter energetic electrons, producing a characteristic energy–‐shell structure in energetic electron fluxes. In this study, we identify this energy‐latitude structure in low‐altitude energetic electron measurements from the ELFIN CubeSats. We compare its latitudinal location with the position of the mid‐latitude trough inferred from TEC data and find a strong agreement between the two. Based on this relationship, we propose a new method for identifying the mid‐latitude trough, and thus the proxy of the low‐altitude projection of the plasmapause, using energetic electron measurements alone.

Nightside‐originating transpolar arcs (TPAs) represent a distinct mechanism of magnetosphere‐ionosphere energy transfer during northward IMF conditions. Nightside‐originating TPAs originate from the nightside auroral oval and grow across the polar cap to connect to the dayside, appearing to cross multiple topological boundaries as they develop. We study the magnetospheric conditions required for such a phenomenon by modeling the 10 January 1997 nightside‐originating TPA event observed by Polar UVI with a high resolution SWMF Geospace simulation. Our simulation produces a large central fold in the magnetotail neutral sheet that protrudes into the northern lobe. When magnetically mapped into the ionosphere, the development and subsequent dynamics of the folded neutral sheet show remarkable agreement to the observed nightside‐originating TPA. The folded neutral sheet bifurcates the northern lobe in the mid‐tail, around , and is located within a region of field‐aligned current (FAC) generation. We suggest that the large central fold in the neutral sheet forms as a response to multiple opposing twists of the magnetotail, driven by the changes in the IMF direction. We discuss the formation and decay of the neutral sheet fold during northward and southward IMF, and the relationship of the neutral fold to the ionospheric convection. Our results suggest that a neutral sheet fold with subsequent FAC generation is needed for transpolar arc activity, rather than just an extended closed field line region poleward of the main auroral oval.

Accurate forecasting of geomagnetic perturbations is essential for assessing space weather risk of geomagnetic activities. Thanks to decades of magnetometer observations from world‐wide distributed networks and upstream solar wind observations at L1, data‐driven modeling of global geomagnetic perturbations has become increasingly feasible. Such models are promising for operational use because they typically run at lower computational cost and can offer higher predictive skill than first‐principles, physics‐based simulations when observations available for training are abundant. However, this potential has not been fully realized as sparse observational coverage in the Southern Hemisphere (SH) has long limited model generalizability. In this work, we use a zeroth‐order mirror‐symmetry assumption in ionospheric electrodynamics under the joint sign reversal of magnetic latitude , interplanetary magnetic field and dipole tilt to improve SH predictions using information from the densely observed Northern Hemisphere (NH). Rather than enforcing exact symmetry, we build on our previous GeoDGP model by introducing a symmetry‐aware kernel for a deep Gaussian process that learns the symmetry‐asymmetry balance from data. For comparison, we also experiment with an alternative approach that imposes symmetry as a hard constraint via data augmentation. We evaluate the models on 22 geomagnetic storms and present a case study of the 2024‐05‐10 Gannon extreme storm. The results show that both methods significantly improve the SH predictions, while the kernel‐based approach achieves the best performance and maintains NH accuracy comparable to GeoDGP.

Ground‐based Global Navigation Satellite System (GNSS) receivers provide primary total electron content (TEC) measurements used to monitor ionospheric variability, yet their sparse distribution over oceanic and equatorial regions limits global coverage and model accuracy. A far‐ultraviolet spectrograph aboard the Global‐scale Observations of the Limb and Disk (GOLD) mission provides continuous observations of the OI 135.6 nm emission radiance, which can be used to infer TEC across these undersampled regions. We present a framework for filtering and calibrating TEC from GOLD observations using the retrieval technique developed by Qin et al. (2023, https://doi.org/10.1029/2023ea003222 ) to produce an enhanced, bias‐corrected data set (gTEC) that can be combined with well‐validated ground‐based TEC sources, such as the Madrigal TEC. After filtering GOLD observations to remove solar and auroral contamination, we apply an adaptive quantile‐mapping calibration method that aligns the cumulative distribution functions of GOLD and Madrigal TEC. The resulting gTEC achieves a mean absolute error of ∼1–2 TECU relative to Madrigal, reproducing ∼53% of its natural variability within one standard deviation. Daily averaged spatial coverage improves by ∼11.62 ± 3.65% for frames with corresponding GOLD data, especially over the equatorial Atlantic, where existing databases lack sufficient observations. The integration of gTEC into the Video Imputation with SoftImpute, Temporal smoothing and Auxiliary data (VISTA) ionospheric TEC algorithm yields considerable improvement in the reproduction of the Equatorial Ionization Anomaly (EIA) structure and asymmetry, as well as mesoscale Equatorial Plasma Bubble (EPB) features. These results demonstrate that gTEC can substantially enhance global ionospheric specification and modeling and improve data assimilation capabilities.

Storm‐time equatorial plasma bubble (EPB) dynamics play a critical role in space weather impacts on communication and navigation systems, yet the mechanisms governing their formation and evolution under storm conditions remain poorly understood. Here, we report two episodes of super EPBs observed during the April 23–24, 2023 interplanetary coronal mass ejection (ICME)‐driven storm, utilizing comprehensive observations together with coupled simulations of the Global Ionosphere Thermosphere Model and Sami3 is A Model of the Ionosphere (GITM‐SAMI3). Multi‐instrument data reveal that the first EPB episode emerged in the Africa‐Atlantic sector during the ICME sheath, while the second occurred over the America‐east Pacific region during magnetic cloud. Both EPBs evolved into super bubbles, ascending to magnetic latitudes beyond 35°, and were embedded within widened equatorial ionization anomalies and storm‐enhanced density. The coupled GITM–SAMI3 model, driven by high‐latitude conditions from the Space Weather Modeling Framework, successfully reproduced both observed EPB episodes. Quantitative analysis of the generalized linear Rayleigh–Taylor instability growth rates revealed that the E × B drift term dominated over thermospheric wind term in both magnitude and variability. Detailed investigation of EPB growth and suppression during the sheath and magnetic cloud periods highlights the controlling influence of external drivers. Notably, we identify an externally triggered EPB seeding mechanism linked to enhanced vertical velocity shear at the convergence of zonal eastward and westward prompt penetration electric fields in a longitudinally confined region. These results provide key new insights into storm‐time EPB evolution and pave the way for improved EPB forecast capability.

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Extreme conditions during storms profoundly alter the evolution of equatorial plasma bubbles (EPBs), yet the key dynamics remains poorly understood. We document the rapid expansion, contraction, and decay of super EPBs in South America during the 12 November 2025 storm with Interplanetary Magnetic Field (IMF) Bz/By magnitude exceeding ∼50 nT. These EPBs expanded to ∼37° magnetic latitude with pronounced hemispheric and longitudinal asymmetries and westward tilt. Notably, the equatorial ionization anomaly (EIA) crests retracted into an X‐pattern during suppression. We suggest that eastward prompt penetration electric fields (PPEF) driven by strong IMF Bz caused the rapid expansion, while fast equatorward winds (peaking at 488 m/s vs. typically ∼<100 m/s) together with westward PPEF triggered EIA merging and EPB decay. The entire EPB lifecycle unfolded within 3 hrs, considerably faster than previously reported events. Our findings reveal storm‐time mechanisms shaping the low‐latitude ionosphere under extreme conditions and advance understanding crucial for space weather prediction.

We study the impact of the evolving magnetosphere‐ionosphere‐thermosphere system on upward ion fluxes during the 23–24 April 2023 geomagnetic storm. This storm has a “double‐dip” structure where two southward IMF periods cause two dips in the SYM‐H index. The auroral electrojet indices and field‐aligned currents reveal comparable ionospheric energy inputs during both dips. We use global total electron content maps to track mid‐to‐high‐latitude ionospheric density structures, which are known to have significant effects on upward ion fluxes. We find that each southward turning triggers a positive ionospheric storm phase. However, thermospheric O/N ₂ depletions initiated during the 1st dip moderate the effects of magnetospheric driving on the ionosphere during the 2nd dip. Consequently, the density structures in the second positive phase are weaker and confined to lower latitudes than before. We use density and velocity measurements from DMSP satellites to identify regions with strong upward ion fluxes. We find that the storm‐enhanced density (SED) plume from the 1st dip drives significant upward fluxes when it crosses the open‐closed field line boundary. Comparable fluxes exist in the dawn sector due to large upward drifts driven by persistent energy inputs under strong negative conditions. We conclude that the evolution of storm‐time ion upflow flux has a local time dependency. The density‐regulated afternoon/evening sector fluxes are seeded by ionospheric high‐density structures and modulated by the thermospheric composition, while the velocity‐regulated dawn/morning sector fluxes are more consistent throughout the storm due to the steady magnetospheric driving under strong negative conditions.

Geomagnetically induced currents (GICs) are a major space weather hazard during geomagnetic storms and pose ongoing threats to technologies using long conductor systems. Accurate forecasting of geomagnetic perturbations is essential for estimating GIC risk. Thanks to decades of magnetometer and solar wind observations from the SuperMAG collaboration and upstream space instruments at L1, data-driven modeling of global geomagnetic perturbations has become increasingly feasible. Such models are promising for operational use because they typically run at lower computational cost and can offer higher predictive skill than first-principles, physics-based simulations when observations are abundant. However, this potential has not been fully realized as sparse observational coverage in the Southern Hemisphere (SH) has long limited the generalizability of data-driven models. In this work, we leverage mirror symmetry in ionospheric electrodynamics to improve SH predictions using information from the densely observed Northern Hemisphere (NH). Building on our previous GeoDGP model, we introduce a novel kernel that encodes mirror symmetry into the prior of the Bayesian model, enabling data-driven learning of covariance between interhemispheric conjugate locations. We also experiment with an alternative approach that imposes symmetry as a hard constraint via data augmentation. We evaluate the models on 22 geomagnetic storms and present a case study of the 2024-05-10 Gannon extreme storm. The results show that both methods significantly improve SH predictions, while the kernel-based approach achieves the best performance and maintains NH accuracy comparable to GeoDGP.