Jiahao Zhong

The westward electric field plays an important role in the hmF2-declining type (where hmF2 denotes the peak height of the F2 layer) of postmidnight electron density (Ne) enhancements at equatorial ionization anomaly (EIA) latitudes through the compression effects, whereas its effectiveness in causing ionospheric storms has not been carefully investigated. As the counter equatorial electrojet (CEJ) is an indicator of the westward electric field, in this study, we focused on the American sector during the long-lasting (~10 h) intense CEJ event on 26 August 2018, based on observations from magnetometers, ionosonde-derived Ne profiles, ground-based and satellite-borne total electron content (TEC) measurements, and ∑O/N₂. The peak density of the F2 layer (NmF2) and TEC at the ionosondes presented severe long-duration enhancements at southern EIA latitudes, accompanied by decreases in hmF2 and scale height (Hm) of the Ne profile, similar to the hmF2-declining type of postmidnight NmF2 enhancements. The dominant contribution to the TEC enhancements came from the topside ionosphere. Moreover, the TEC map displayed prominent enhancements spanning a wide range of longitudes in the American sector. This study suggests that the electric-field-driven compression and convergence played important roles in the F-region Ne enhancements by changing the shape of the F layer. Meanwhile, elevated O/N₂ and equatorward winds may also have contributed to the TEC enhancements.

This study develops and examines an integrated automated method for identifying Martian photoelectrons, suprathermal electron void and Photoelectron Boundary (PEB) using measurements from the Mars Atmospheric and Volatile EvolutioN (MAVEN) Solar Wind Electron Analyzer (SWEA). The core of the approach is grounded by a robust Support Vector Machine (SVM) classification model, trained on a large manually vetted data set of photoelectron (PHE) and non‐photoelectron (nPHE) spectra. To eliminate sporadic misclassifications inherent in the SVM classifier, particularly near the PEB or under nightside conditions, a rigorous calibration routine is implemented. Applying this method facilitates a statistical analysis of photoelectron, electron voids, and PEB characteristics. The results are consistent with the existing literature on the occurrence feature modulated by solar zenith angle, altitude, and crustal magnetic field strength, thereby enhancing the credibility of the method. Comparative analysis further demonstrates the advantages of this approach over previous studies by resolving ambiguities and presenting several novel occurrence features, which will shed light on our understanding of the Martian ionosphere.

Prior studies identified a fine structure in the middle latitude ionosphere known as the strip-like plasma density bulge. These bulges emerge during geomagnetic storms, exhibiting a broad longitudinal span of over 150° and a narrow latitudinal extent of 1°~5°. The observations from the DMSP and ICON satellites reveal stronger equatorward ion drifts and neutral winds on the poleward side of bulges compared to the equatorward side. Using the Sami2 is Another Model of the Ionosphere (SAMI2), the bulge feature was reproduced for the storm of 4~6 November 2021 by amplifying the default meridional winds. Numerical simulations indicate that global wind disturbances establish a sharp meridional wind gradient within the lower mid-latitude region. This gradient, in turn, drives a divergence in ion transport parallel and perpendicular to the magnetic field lines, which ultimately results in the localized accumulation of plasma. The phenomenon is most pronounced in the vicinity of ±30° quasi-dipole latitude. This region is characterized by a magnetic inclination angle of approximately 45°, a configuration where the meridional wind component acts most efficiently to elevate ions vertically.

Prior studies identified a fine structure in the middle latitude ionosphere known as the strip‐like plasma density bulge. These bulges emerge during geomagnetic storms, exhibiting a broad longitudinal span of over 150° and a narrow latitudinal extent of 1°–5°. The observations from the DMSP and ICON satellites reveal stronger equatorward ion drifts and neutral winds on the poleward side of bulges compared to the equatorward side. Using the Sami2 is Another Model of the Ionosphere (SAMI2), the bulge feature was reproduced for the storm of 4∼6 November 2021 by amplifying the default meridional winds. Numerical simulations indicate that global wind disturbances establish a sharp meridional wind gradient within the lower mid‐latitude region. This gradient, in turn, drives a divergence in ion transport parallel and perpendicular to the magnetic field lines, which ultimately results in the localized accumulation of plasma. The phenomenon is most pronounced in the vicinity of ±30° quasi‐dipole latitude. This region is characterized by a magnetic inclination angle of approximately 45°, a configuration where the meridional wind component acts most efficiently to elevate ions vertically.

During the recovery phase of March 31, 2001, geomagnetic storm, post-midnight equatorial plasma depletions (EPDs) associated with abnormal background ion density enhancement were identified by ROCSAT-1 satellite near 180°E. The ion vertical drift within the EPDs was downward and shared a similar pattern to the reference days. Together with the Thermosphere–Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulation, we inferred that traditional cognitions of the electrodynamics and neutral disturbance were not able to explain either the background density enhancement or the EPDs. With the same temporal and spatial coverage, the Polar-orbiting Operational Environmental Satellite (POES) observed significant energetic electron flux enhancement. We suggest that abundant quasi-trapped electrons that originated from inner radiation belt might be responsible for the abnormal ion density enhancement and further facilitate the occurrence of post-midnight EPDs at the topside ionosphere.

Using data from multiple Low‐Earth‐Orbit (LEO) satellites, including Swarm‐A/B, TanDEM‐X (TDX), MetOp‐B/C, Defense Meteorological Satellite Project and Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2), we focused on the prominent ionospheric altitudinal responses in the Asian‐Australian sector during the May 2024 superstorm. During the minimum symmetric component of the ring current (SYM‐H) period from ∼22:00 UT on 10 May to ∼12:00 UT on 11 May, LEO‐based total electron content (TEC) revealed a prolonged and large‐scale daytime meridional enhancement, with maximum increases of 250%–450% and extending to above 800 km, likely driven by storm‐time neutral winds and disturbed electric fields. For the topside nighttime enhancement on 11 May in the northern Asian‐Australian sector, both enhanced H ⁺ /Ni and vertical drifts suggested that the plasmasphere refilling made major contributions. LEO‐based and ground‐based Global Navigation Satellite System (GNSS) TECs exhibited opposite responses in the southern Asian‐Australian region. Two distinct electron density profiles in the Australian and Indonesian regions were depicted, highlighting the differences linked to thermospheric compositions and dynamics/electrodynamics. These physical processes occurred throughout multiple different altitudes from bottomside to topside ionosphere, showing the severe impact of superstorms.

Equatorial plasma bubbles (EPBs) typically form after sunset at the bottom of the F layer in the equatorial region and then grow upward and elongate along magnetic flux tubes. According to optical observations, in many cases, the poleward ends of EPBs tilt westward with respect to the magnetic north, but in a few cases, eastward tilts are observed. In this work, the EPB tilt directions, drift velocities, and morphology are investigated from an all-sky airglow imager over South China during the geomagnetically quiet night of April 3, 2022. During this event, the poleward ends of the EPBs in the off-equatorial region initially tilted westward after sunset (20:04–23:27 LT), but gradually turned to tilt eastward around and after midnight (23:27–03:38 LT). Successive airglow images revealed different groups of EPBs with westward tilts, westward-eastward tilts, and eastward tilts throughout a single night. The changes in morphology, tilt angles, and drift velocities of EPBs are analyzed according to the airglow images. The tilt angles ranged from −12° to 25°, which were calculated by Principal Component Analysis (PCA) through Singular Value Decomposition (SVD). The westward-to-eastward tilt direction changed in less than 17 min, and the eastward tilt of the EPBs lasted 4 h. When the EPBs drifted eastward over the observation region, the ionogram displayed distinct spread F signatures, and satellite signals manifested significant scintillation phenomena. The drift velocities of the EPBs at relatively high latitudes clearly increased and were greater than those at lower latitudes when the EPBs turned to tilt eastward at approximately 23:27 LT (15:53 UT), with a velocity differential of 20 m/s. The changes in tilt direction may result from latitudinal variations in ion drift velocities, which might be related to the latitudinal/altitudinal variations in F-region Pedersen conductivity and the presence of greater zonal winds or electric fields along the geomagnetic field line.

In this work, topside Total Electron Content (TEC) obtained from CHAMP, GRACE, and SAC‐C, along with in situ electron density (Ne) measurements from CHAMP and GRACE, were utilized to investigate the response of the topside ionosphere to the November 2003 superstorm, particularly during its recovery phase. Observations from multiple satellites revealed unexpected and persistent enhancements in the topside ionosphere during the late recovery phase (22–24 November 2003), which were more pronounced in the Eastern Pacific sector than in the Asian sector, indicating notable longitudinal differences. Intriguingly, the opposite inter‐hemispheric asymmetries of the enhancement were observed near the peak height of F2 and the topside ionosphere on the dayside in the Eastern Pacific sector. During the late recovery phase, the storm‐time enhancement in the eastward electric field and trans‐equatorial wind likely contributed to the observed positive ionospheric storm effects and inter‐hemispheric asymmetry, respectively. Additionally, this study confirmed substantial enhancements in the dayside topside ionosphere during the storm's main phase on 20 November 2003. These findings underscore the need for a global, multi‐instrumental approach to fully understand the diverse responses of the topside ionosphere to geomagnetic storms.

In this study, the typical latitudinal structure of the topside ionosphere and its response to the geomagnetic storm were investigated, based on the in‐situ electron density detection around ∼860 km and upward‐looking total electron content (TEC) above ∼830 km on dawn and dusk sides. It was found that the topside TEC had an arc‐shaped latitudinal structure on both dawn and dusk sectors, regardless of longitude, local time, and season. The dusk side in‐situ electron density displayed a narrow, Mexican hat‐shaped structure with the peak at the magnetic equator, while this structure disappeared on the dawn side. The magnitude of this hat‐shaped structure was highly dependent on longitude. In addition, the hat‐shaped structure was partly masked by strong hemispheric asymmetry during the solstice, while the topside TEC remained largely symmetric between the northern and southern hemispheres. During the main phase of geomagnetic storms, topside ionospheric electron density was enhanced at middle latitudes. However, the arc‐shaped structure in the topside TEC appeared less sensitive to geomagnetic disturbances than that of in‐situ electron density. The discrepancies between in‐situ electron density and up‐looking TEC suggested that topside TEC were more representative of plasmasphere than the in‐situ electron density at ∼860 km, which thus highlighted the complex altitude‐dependent behavior of the topside ionosphere.