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Ionospheric topside O+ ${O}^{+}$ diffusive flux is derived using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation data, to investigate its global distribution and also its role in winter nighttime enhancement (WNE) of electron density. The flux of the winter hemisphere maintains downward throughout the night. It is much larger between 30° $30{}^{\circ}$ and 50° $50{}^{\circ}$ geomagnetic latitudes and keeps increasing until 22:00–00:00 LT. It peaks at 60° $60{}^{\circ}$W and 60° $60{}^{\circ}$E–120° $120{}^{\circ}$E geographic longitudes during the December solstice, and at 180° $180{}^{\circ}$E during the June solstice. These features are similar to those of WNE in NmF2. Furthermore, the derived flux is applied as the upper boundary condition to run the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). The simulated spatial‐temporal variations of WNE are consistent with the observations. The results indicate that downward plasma diffusion from the plasmasphere is the major mechanism of WNE, and the simulation quantifies its contribution. Plain Language Summary: Solar radiation ionizes the atmosphere to produce the ionosphere. However, ionospheric electron density in the midlatitude of the winter hemisphere has been observed to increase at night with the absence of photoionization, which is referred to as winter nighttime enhancement (WNE). Many studies have suggested that the plasma causing WNE comes from the overlying plasmasphere via downward diffusion, but so far the global distribution of ionospheric topside diffusive flux has not been systematically examined because it cannot be measured directly. In this study, the topside O+ ${O}^{+}$ diffusive flux is derived based on observational data. The flux is downward at night and varies with geographical location and local time. These characteristics are similar to those of WNE. Furthermore, the theoretical model Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) is used to conduct a modeling of WNE, with the upper boundary condition modified by incorporating the derived diffusive flux. WNE is well reproduced, providing direct evidence that downward plasma diffusion is the major mechanism of WNE. This study provides new insight into the physical processes in the nighttime ionosphere, and has implication for future development and improvement of ionospheric models. Key Points: Global distribution of ionospheric topside O+ diffusive flux is derived for the first time using COSMIC radio occultation dataThe Thermosphere Ionosphere Electrodynamics General Circulation Model simulation driven by the derived flux effectively reproduces the midlatitude electron density enhancement in winter nighttimeFirst global‐scale evidence indicating downward plasma diffusion as the dominant mechanism for electron density enhancement is provided [ABSTRACT FROM AUTHOR]

This study investigates the responses of the equatorial and low-latitude ionosphere in the American–Atlantic longitude sector during the super geomagnetic storm that occurred on 10–11 May 2024. The investigation utilizes multi-instrument datasets, including ground-based observations (GNSS TEC, ionosonde, and Fabry–Perot interferometer) as well as space-borne satellite measurements (GOLD, Swarm, DMSP, and TIMED). Our findings reveal significant day-to-day variations in the storm-time equatorial ionization anomaly (EIA), summarized as follows: (1) During the main phase of the storm, the low- and mid-latitude ionosphere experienced a positive storm, with TEC drastically enhanced by 50–100% within a few hours. The EIA crests exhibited a substantial poleward expansion, reaching as high as ±35° MLAT. This expansion was caused by the enhanced fountain effect driven by penetration electric fields, along with increased ambipolar diffusion due to transient meridional wind surges. (2) During the recovery phase of the storm, the global ionosphere was characterized by a substantial negative storm with a 50–80% depletion in TEC. The EIA crests were notably suppressed and merged into a single equatorial band, which can be attributed to the composition change effect and the influence of disturbance dynamo electric fields. These results illustrate the complex processes of magnetosphere–ionosphere–thermosphere coupling during a superstorm, highlighting the significant impacts of space weather on the global ionosphere. [ABSTRACT FROM AUTHOR]

Satellite drag coefficients are crucial for determining the neutral mass densities that affect spacecraft operations in the thermosphere. Many studies typically utilize a constant drag coefficient of 2.2 to calculate the neutral density. However, due to the variability of space environment, uncertainties in the drag coefficient can lead to significant systematic discrepancies in neutral density measurements. Satellite drag coefficient may fluctuate in the thermosphere under various geomagnetic activities and altitudes. For the first time, we calculate the spherical satellite drag coefficient using data from the "Orbital Atmospheric Density Detection Experimental Satellite," referred to as the QX satellite. Our findings reveal that the drag coefficient can be estimated by thermospheric temperature and density, which are dependent on geomagnetic activity and altitude. At an altitude of ∼510 km, drag coefficients are adjusted to around 2.425, instead of the constant value of 2.2. Furthermore, the drag coefficient may decrease due to the significant influence of increasing geomagnetic activity, such as geomagnetic storms, on thermospheric density and temperature. These estimates of the drag coefficient can also be used to reduce discrepancies when deducing the ballistic coefficient. Consequently, using the estimated drag coefficient can accurately determine the QX‐derived neutral density, which agrees well with the density from Swarm‐B satellite. Plain Language Summary: Satellite drag coefficient is a key factor in advancing space engineering and understanding the impacts of space weather conditions on satellites in the thermosphere. It helps in calculating the neutral mass density associated with atmospheric drag force and estimating the satellite orbital lifetime. In this study, it is the first time calculating the spherical satellite drag coefficient using data from QX satellite, collected from 28 July 2022 to 30 June 2023. We investigate the response of the drag coefficient to thermospheric temperature and density under different geomagnetic activities and altitudes. These relationships allow us to estimate the spherical satellite drag coefficient, which in turn helps us improve the accuracy of determining the ballistic coefficient and the corresponding neutral mass density. Compared with the results of the drag constant of 2.2, the QX‐derived densities determined using the estimated drag coefficients, align well with the in‐situ observations from Swarm Precise Orbit Determination data. Overall, our results enhance the understanding of how the spherical satellite drag coefficient changes in response to variations in the thermosphere under different space weather conditions. We also suggest a method for estimating the drag coefficient, which has valuable applications in the calculation of neutral mass density. Key Points: We put forward a new method for determining drag coefficients using temperature and density affected by geomagnetic activity and altitudeGeomagnetic activity increases, associated with the enhancements in temperature and density, leading to a reduction of the drag coefficientUsing the estimated drag coefficients allows for accurate determination of QX‐derived density, which aligns well with Swarm‐derived density [ABSTRACT FROM AUTHOR]

This study reports a peculiar nighttime ionospheric enhancement in the total electron content (TEC) from the geostationary (GEO) satellites in response to the geomagnetic superstorm of May 2024. The enhancements occurred at low to mid‐latitudes during the storm's recovery phase on 11–12 May, with the TEC values almost twice as high as the quiet ones. Surprisingly, the nighttime ionospheric enhancements sub‐rotated westward with a speed of ∼130.5 m/s. The nighttime TEC enhancements lasted ∼5–7 hr for a given location and persisted over half a day over wide longitude ranges. Meanwhile, ionosonde data from two equatorial ionization anomaly (EIA) stations showed a higher peak height of the F2 layer, and a significant double‐crest EIA structure was observed by both GEO TECs and in situ electron densities from China Seismo‐Electromagnetic Satellite during this period of interest, indicating a possible contribution from disturbance electric fields producing nighttime eastward equatorial electric fields. Nevertheless, it remains a mystery whether the nighttime TEC enhancement and its movement were associated with the wind disturbance dynamo. Plain Language Summary: Geomagnetic storms can greatly disturb the ionosphere, resulting in various abnormal ionospheric structures affecting the radio signal propagations of different frequencies. During the most intense storm of May 2024 in the past 20 years, we observed unusual nighttime ionospheric enhancements at low to mid‐latitudes. Unlike the fixed localized enhancements reported in previous studies, these enhancements were moving structures. They were observed in a wide longitudinal extension over the Pacific Ocean and Asia and persisted for more than half a day, even extending beyond sunrise, with a westward propagation speed of about 130.5 m/s. This paper presents a newly observed space weather event and highlights the less understood aspects of this phenomenon. Key Points: Abnormal nighttime ionospheric enhancements were observed by the GEO TECs during the May 2024 superstorm's recovery phaseThe nighttime ionospheric enhancements had movement structures with a westward velocity of ∼130.5 m/sWhether the disturbance electric field is the cause for the moved nighttime TEC enhancements remains a mystery to resolve [ABSTRACT FROM AUTHOR]

This study investigates midlatitude ionospheric variations during the super geomagnetic storm on 10–11 May 2024, utilizing multi‐instrument data from ground‐based sources (Global Navigation Satellite Systems receivers and a Fabry–Perot Interferometer) and space‐based measurements (Swarm and DMSP). We observed several distinct density gradient structures in the midlatitude ionosphere, with the main findings summarized as follows: (a) Significant zonal plasma density enhancements developed continuously in local dusk across the American‐Pacific‐Asian longitude sectors around ±40° $\pm 40{}^{\circ}$ geomagnetic latitude. These midlatitude peaks exhibited a wide longitudinal extension exceeding 150° ${}^{\circ}$ and a prolonged duration of 12–15 hr during the late main phase and early recovery phase of the storm. (b) Strong storm‐enhanced density (SED) was observed in both hemispheres yet with different longitudinal and universal time preferences. In the Northern Hemisphere, significant SED occurred over the American longitude sector during 20:30–22:30 UT on May 10. In the Southern Hemisphere, pronounced SED was observed not only in the American longitudes during 20:30–22:30 UT on May 10 but also in the Australian longitude sector during 02:00–04:00 UT on May 11. Plain Language Summary: The super geomagnetic storm on 10–12 May 2024, known as the Mother's Day Storm, was one of the strongest storms recorded in the Space Age era. This storm induced a series of significant disturbances in the global midlatitude ionosphere, revealing unexpected plasma density gradient structures. Notably, distinct midlatitude plasma density peaks were observed on the poleward side of the equatorial ionization anomaly crests during local dusk. These zonal plasma density enhancements extended across the wide American‐Pacific‐Asian longitude sectors, covering more than 150° ${}^{\circ}$, and persisted for 12–15 hr. Furthermore, strong storm‐enhanced density plume appeared in both hemispheres though with considerable asymmetry. In the Northern Hemisphere, the plume enhancement was more pronounced in the American longitude sector, while in the Southern Hemisphere, the plume intensity was more significant in the Australian longitude sector. These salient and dynamic density gradient structures highlight the complex nature of magnetosphere‐ionosphere‐thermosphere coupling processes during super geomagnetic storms. Key Points: Distinct midlatitude plasma density peaks continuously developed in local dusk across American‐Pacific‐Asian sector around ±40° MLATMidlatitude plasma density enhancements showed a wide zonal extension exceeding 150° in longitude and a prolonged duration of 12–15 hrStrong storm‐enhanced density occurred in both hemispheres, with the Northern (Southern) one being more significant over American (Australian) longitudes [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of ∼ ${\sim} $30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration. Plain Language Summary: On 8 April 2024, a total solar eclipse traversed across North America with a dense network of observational equipment in place, providing a great opportunity for analyzing ionospheric effects during the eclipse. This paper presents a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density response to this solar eclipse, utilizing Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm satellite in situ measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS) over continental US and adjacent regions with the SAMI3 as the background model. The observations and SAMI3‐TIDAS data assimilation reveals the time‐evolving 3‐D spatial distribution of the ionospheric electron density during the eclipse, highlighting key features of altitude‐dependent ionospheric variation with significant discrepancies and time delays between the bottomside and topside ionosphere. Key Points: The altitude‐resolved Ne response to the solar eclipse in the 3‐D domain was effectively reconstructed by TIDAS‐SAMI3 data assimilationThe eclipse led to a substantial ionospheric Ne reduction of 20%–50%, with the maximum depletion occurring in the F region of 200–250 kmThe Ne showed a time‐delayed variation with increasing altitude, from 5 to 10 min in the bottomside to 1–2.5 hr in the topside ionosphere [ABSTRACT FROM AUTHOR]

Using the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIEGCM), a comparison of the ionospheric total electron content (TEC) between the South Atlantic anomaly (SAA) and the Indian Ocean (IO) at solar maximum is performed in this study. The results show that the average total electron content in the SAA is greater than that in the Indian Ocean in general. In order to further analyze the difference between the two regions, the empirical orthogonal function (EOF) are used to investigate the temporal and spatial characteristics of TEC. The empirical orthogonal function method separate part of the global four-peak structure (an equatorial ionization anomaly structure, distributed in Southeast Asia, South America, Africa, and central Pacific) and spatial variations in both regions. Moreover, the first mode of EOF shows the different distribution of Equatorial ionization anomaly in South America and central Pacific caused by deviation of geomagnetic field and tides between two regions, and the enhancement of TEC in SAA region at dusk is emphasized, but the enhancement of TEC in IO region at dawn is emphasized. The second mode performs the distribution of EIA in Africa related to solar radiation and E χ B drift. The third mode indicates the similar spatiotemporal variations from the geomagnetic field. Besides, the correlation between TEC and Dst in two regions indicate that there are some deficiencies in simulation to the specificity of SAA, and the deficiencies are likely caused by the model's inaccurate simulation of the magnetic field and particle deposition in the SAA region. [ABSTRACT FROM AUTHOR]

This paper investigates the midlatitude ionospheric disturbances over the American/Atlantic longitude sector during an intense geomagnetic storm on 23 April 2023. The study utilized a combination of ground‐based observations (Global Navigation Satellite System total electron content and ionosonde) along with measurements from multiple satellite missions (GOLD, Swarm, Defense Meteorological Satellite Program, and TIMED/GUVI) to analyze storm‐time electrodynamics and neutral dynamics. We found that the storm main phase was characterized by distinct midlatitude ionospheric density gradient structures as follows: (a) In the European‐Atlantic longitude sector, a significant midlatitude bubble‐like ionospheric super‐depletion structure (BLISS) was observed after sunset. This BLISS appeared as a low‐density channel extending poleward/westward and reached ∼40° geomagnetic latitude, corresponding to an APEX height of ∼5,000 km. (b) Coincident with the BLISS, a dynamic storm‐enhanced density plume rapidly formed and decayed at local afternoon in the North American sector, with the plume intensity being doubled and halved in just a few hours. (c) The simultaneous occurrence of these strong yet opposite midlatitude gradient structures could be mainly attributed to common key drivers of prompt penetration electric fields and subauroral polarization stream electric fields. This shed light on the important role of storm‐time electrodynamic processes in shaping global ionospheric disturbances. Plain Language Summary: The storm‐time midlatitude ionosphere is a highly dynamic region that can exhibit much more pronounced electron density gradients and disturbances than expected. The combined data from ground‐based and satellite measurements provided valuable insights into the intricate behavior of the midlatitude ionosphere during an intense geomagnetic storm on 23 April 2023. The midlatitude ionosphere was characterized by significant electron density gradient structures, comprising both phenomenal density depletion and exceptional enhancements. Notably, a remarkable band‐like ionospheric super‐depletion structure was observed in local dusk, which extended to ∼40° geomagnetic latitude, corresponding to an altitude of ∼5,000 km above the geomagnetic equator. In contrast, a significant midlatitude ionospheric density enhancement plume appeared simultaneously in local afternoon, which exhibited a dynamic variation with its intensity being quickly doubled and halved in just a few hours. The synchronous occurrence of these striking yet opposing density gradients highlights the importance of electrodynamic effect driven by common key drivers of storm‐time perturbed electric fields. This underscores the complex and interconnected nature of the ionospheric response during intense geomagnetic storms. Key Points: The storm main phase was characterized by coincident nightside bubble‐like ionospheric super‐depletion structure (BLISS) and dayside storm‐enhanced density (SED)The BLISS manifested as a poleward/westward‐streaming channel and extended to ∼40 MLAT, corresponding to an APEX height of ∼5,000 kmThe SED plume experienced a dynamic variation with the plume intensity being quickly doubled and halved in just a few hours [ABSTRACT FROM AUTHOR]

This work investigates mid‐ and low‐latitude ionospheric disturbances over the American sector during a moderate but geo‐effective geomagnetic storm on 13–14 March 2022 (π‐Day storm), using ground‐based Global Navigation Satellite System total electron content data, ionosonde observations, and space‐borne measurements from the Global‐scale Observations of Limb and Disk (GOLD), Swarm, the Defense Meteorological Satellite Program (DMSP), and the Ionospheric Connection Explorer (ICON) satellites. Our results show that this modest but geo‐effective storm created a number of large ionospheric disturbances, especially the dynamic multi‐scale electron density gradient features in the storm main phase as follows: (a) The low‐latitude equatorial ionization anomaly (EIA) exhibited a dramatic storm‐time deformation and reformation, where the EIA crests evolved into a bright equatorial band for 1–2 hr and then quickly separated back into the typical double‐crest structure with a broad crest width and deep equatorial trough. (b) Strong equatorial plasma bubbles (EPBs) occurred with an abnormally high latitude/altitude extension, reaching the geomagnetic latitude of ∼30°, corresponding to an Apex height of 2,600 km above the dip equator. (c) The midlatitude ionosphere experienced a conspicuous storm‐enhanced density (SED) plume structure associated with the subauroral polarization stream (SAPS). This SED/SAPS feature showed an unusual temporal variation that intensified and diminished twice. These distinct mid‐ and low‐latitude ionospheric disturbances could be attributed to the storm‐time electrodynamic effect of electric field perturbation, along with contributions from neutral dynamics and thermospheric composition change. Key Points: A common key driver of storm‐time electric field generated three dynamic ionospheric changes simultaneously from low to subauroral latitudesEquatorial ionization anomaly crests showed merging and separation during interplanetary magnetic field Bz oscillation, and strong equatorial plasma bubbles reached 30° MLAT corresponding to 2,600 km Apex heightSalient storm‐enhanced density associated with subauroral polarization stream occurred in midlatitudes, showing a dynamic temporal variation that intensified and diminished twice [ABSTRACT FROM AUTHOR]