Jiawei Kuai

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.

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.

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.

We conducted observational studies of ionospheric responses to the geomagnetic storm on 12 May 2021. We selected three cases that the electron density altitude profiles observed by Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) exhibited significant longitudinal differences in less than 30° longitudes. These cases occurred over the Atlantic Ocean and during the storm's recovery phase. All three cases show relatively weaker variations on the west side between disturbed and quiet time (<10%), while displaying pronounced positive/negative electron density variations on the east side (as large as 100%). GOLD and ICON observations illustrate that the thermospheric composition plays a minor role in the longitudinal differences, while neutral winds and disturbed electric fields make major contributions. Furthermore, significant longitudinal differences in E×B drift and neutral winds have also been observed, which will be part of the future work. This study provides unique insights into ionospheric responses within 30‐degree longitude range over the ocean.

Longitudinal discontinuity is observed over nighttime equatorial ionization anomaly (EIA) crests, characterized by a significant depletion between longitudes. Multiple observations, especially GOLD 135.6‐nm airglow images, are used to investigate the EIA discontinuity in details. Discontinuity events are found more likely to occur in one hemisphere, with a longitudinal width of tens of degrees and prolonged durations of several hours from 19 to 24 LT. The plasma depletion is most conspicuous at low altitudes, near or below the peak height. For the Atlantic sector, the EIA discontinuity is most prevalent between the March Equinox and the September Equinox, with an equinoctial asymmetry of higher occurrence rates around September Equinox. The climatological results indicate that field‐aligned trans‐hemispheric wind plays a predominant role in promoting the discontinuity, which transports the plasma from one hemisphere to the other hemisphere, resulting in potential localized depletion over the EIA crest.

The strip‐like bulge is a storm‐time conjugate ionospheric plasma density enhancement, constituted by the plasmaspheric H⁺/He⁺, that extends widely (over 150° in longitude) in the zonal dimension but occupies only 1°–5° in latitude. Based on in‐situ measurements of 11 low earth orbit satellites, this study statistically investigates the bulge structures of geomagnetic storms driven by 136 interplanetary coronal mass ejections during 2000–2021. The statistical results show that the strip‐like bulges are observed at the end of the storm main phase and can persist for more than 60 hr. The spatial and temporal coverage of the strip‐like bulge varies from storm to storm. However, the bulges do exhibit occurrence preferences: stronger storms (for the ICME‐driven) during solar minimum periods, the Asian‐Pacific sector (with eastward magnetic declination), and the nightside of the dawn‐dusk terminator. A quiet time density enhancement called mid‐latitude enhancement could be recognized as a precursor of the strip‐like bulge. The evolution features of the plasmapause height exhibit similarities with the strip‐like bulge, indicating a field‐aligned downward and cross‐L inward intrusion of the plasmaspheric ions. The local net ion drifts partly support this scenario with downward/inward being the most dominant but not unique pattern, the other diverse net ion drift configurations exist but their impact on the strip‐like bulges remains unclear.

Previous studies paid little attention to the ionospheric storm‐time altitudinal differences due to insufficiency of ionospheric measurements. In this work, multiple instrumental observations were used to investigate the ionospheric storm‐time response at low latitudes in the American and Asian‐Australian sectors during the May 2021 geomagnetic storm. The ground‐based Global Navigation Satellite Systems (GNSS) total electron content (TEC) and Low Earth Orbit (LEO) satellite topside TEC presented opposite (positive/negative) variations in the low‐latitude and equatorial region of both sectors during this storm. The electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) and the Sanya Incoherent Scatter Radar showed a good agreement and well explained the opposite variations between GNSS TEC and LEO satellite topside TEC. The F2‐layer peak height (hmF2) and peak density (NmF2) displayed inverse variations, and the feature was present mostly in the regions between equatorial ionization anomaly crests. The combined modulation effects of the storm‐time zonal electric fields and the field‐aligned transports possibly resulted in the contrary variations of hmF2 and NmF2 in the low‐latitude and equatorial region, leading to the storm‐time altitudinal differences during this storm. Relatively, the storm‐time thermospheric composition disturbances might be a minor factor responsible for these differences.

The strip-like bulge is a storm-time conjugate ionospheric plasma density enhancement that extends widely (over 150° in longitude) in the zonal dimension but occupies only 1°~5° in latitude. Based on in-situ measurements of 11 low earth orbit (LEO) satellites, this study statistically investigates the bulge structures of geomagnetic storms driven by 136 interplanetary coronal mass ejections (ICMEs) during 2000~2021. The statistical results show that the strip-like bulges are initiated at the end of the storm main phase and can persist for more than 60 hours. The spatial and temporal coverage of the strip-like bulge varies from storm to storm. However, the bulges do exhibit occurrence preferences: stronger storms, the Asian-Pacific sector (with eastward magnetic declination), the nightside of the dawn-dusk terminator, and solar minimum periods. A quiet time density enhancement called midlatitude extra peaks could be recognized as a precursor of the strip-like bulge. The plasmaspheric compression shares some similar occurrence features with the strip-like bulge, indicating a field-aligned downward and cross-L inward intrusion of the plasmaspheric ions. The local net ion drifts partly support this scenario with downward/inward being the most dominant but not unique pattern, other diverse net ion drift configurations exist but their impact on the strip-like bulges remains unclear.

The abundant observations and research established a detailed category of the terrestrial ionospheric irregularities, which significantly advanced our understanding of how the Earth system's complicated physical and chemical process generates the intermediate‐scale structures of the charged particles. Motivated by a future attempt at categorizing the Martian ionospheric irregularity, this study designs a method for naive classification of the plasma density depletion, enhancement, and oscillation based on the in situ measurements of the Martian ionosphere. The technique consists of several procedures: trend estimation, detrending and candidate extraction, and parameterization. The classification is achieved through a machine‐learning‐like process using some testing artificial density profiles. A preliminary credence test shows a good performance in separating the terrestrial low‐latitude Equatorial Plasma bubble (depletion) and mid‐latitude Median‐scale Traveling Ionospheric Disturbance (oscillation). Another detection experiment of the Martian plasma depletion events (collected by Basuvaraj et al. (2022a, https://doi.org/10.1029/2022je007302)) showed a recall rate (i.e., true positive) of 38% but with a high precision of 67.8%. Therefore, we believe the proposed method could convincingly extract different Martian ionospheric irregularities and help uncover the climatological characteristics in the future.

Peak electron density data derived from GOLD measurements during 2018–2022 are used to analyze the magnitudes and correlations of ionospheric diurnal variability at low latitudes. The correlation distance describes the correlations between spatial locations and is defined in this paper as the angular separation at which the correlation coefficient decreases from 1 to 0.7. Variations in correlation distance with local time, season, magnetic latitude, solar activity, and geomagnetic activity are discussed in this study. The average value of the zonal correlation distance is approximately 8.55° and 3.56° for the meridional direction at low geomagnetic latitudes (magnetic latitudes <30°). The statistical results indicate that both zonal and meridional correlation distances vary little with local time premidnight, while they show pronounced seasonal and latitudinal variations. Both zonal and meridional correlation distances increase with increasing solar activity and decrease with enhancing geomagnetic activity. The EIA strength and gradient mainly modulate the distributions of correlation distances related to magnetic latitude, season, and solar flux level. An empirical model is constructed to describe the temporal and spatial variations in the correlation distance at low latitudes. The study of correlation distances would contribute to a better understanding of ionospheric variability and improvements in data assimilation.