During the declining phase of the solar cycle, geomagnetic storms, primarily driven by high-speed solar wind streams (HSSs) and associated co-rotation interaction regions (CIRs), become prominent. One of the major effects of these storms are the F region electron density perturbations, usually referred to as ionospheric storms. This study focuses on a positive ionospheric storm, characterized by an increase in electron density at mid-latitudes (40°- 60° MLAT) and observed during a moderate yet prolonged HSS/CIR-driven geomagnetic storm with a SYM-H minimum of -65 nT. The storm commenced on 14 March 2016 at 17:20 UT with a strong storm sudden commencement (SSC) and lasted until 21 March. This study uses global navigation satellite system (GNSS) total electron content (TEC) data for a global perspective of electron density variations and Millstone Hill incoherent scatter radar (52° MLAT, MLT=UT-4.6) data to provide local measurements of plasma parameters during the positive storm.In the global analysis of the TEC variations during the storm, a 6-h long strong positive ionospheric storm (TEC increase up to 50 %) at the mid-latitudes was observed in the day and dusk sectors, whereas a depletion in TEC (negative storm) prevailed at the high latitudes. The positive ionospheric storm initiated during the SSC and subsequently intensified with the onset of the main phase. The local electron density data from the Millstone Hill incoherent scatter radar showed an enhancement throughout the local evening MLTs. An uplift in the peak height together with an increased line-of-sight upward ion velocity was observed simultaneously as the traveling ionospheric disturbances (TIDs) reached Millstone Hill from the north-east direction with a phase velocity of 760 m/s. When the plasma is uplifted to greater altitudes in the F region, the recombination rate becomes slower and electron density may be enhanced. The TIDs were plausibly triggered by the Joule heating at high latitudes during the main phase of the geomagnetic storm. After the initial uplift, the peak height of electron density at Millstone Hill descended but electron densities were further enhanced. We will discuss the possible mechanisms including transportation of oxygen-rich air from high to mid latitudes when interpreting the measurements.

AbstractThis study has developed a new TEC‐based ionospheric data assimilation system for 3‐D regional ionospheric imaging over the South American sector (TIDAS‐SA) (45°S–15°N, 35°–85°W, and 100–800 km). The TIDAS‐SA data assimilation system utilizes a hybrid Ensemble‐Variational approach to incorporate a diverse set of ionospheric data sources, including dense ground‐based Global Navigation Satellite System (GNSS) line‐of‐sight Total Electron Content (TEC) data, radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2), and altimeter TEC data from the JASON‐3 satellite. TIDAS‐SA can produce a reanalyzed three‐dimensional (3‐D) electron density spatial variation with a high time cadence, yielding spatial‐temporal resolution of 1° (latitude) × 1° (longitude) × 20 km (altitude) × 5 min. This allows us to reconstruct and study the 3‐D ionospheric morphology with multi‐scale structures. The performance of the data assimilation system is validated against independent ionosonde and in situ measurements through an experiment for a strong geomagnetic storm event on 03–04 November 2021. The results demonstrate that TIDAS‐SA can provide detailed and altitude‐resolved information that accurately characterizes the storm‐time ionospheric disturbances in vertical and horizontal domains over the equatorial and low‐latitude regions of South America.

Eclipse studies for the 2017 total solar eclipse over the USA benefited greatly from the vast increase in fidelity and coverage of ground-based monitoring tools, especially GNSS monitoring of total electron content (TEC). During the 2017 eclipse, total electron content (TEC) depletions up to 60% in magnitude were reported (Coster et al. 2017). These were greater than those predicted by models. Wave responses in the form of traveling ionospheric disturbances (TIDs) following this eclipse were also reported (Zhang et al., 2017). Two major solar eclipses within the 2023-2024 time frame have crossed over North America. The first was an annular solar eclipse that occurred on 14 October 2023. It began in the United States, traveling from the coast of Oregon at approximately 9 am local time (PDT) and crossed into Nevada, Utah, New Mexico before reaching the Texas Gulf Coast at approximately noon local time (CDT). During an annular eclipse, the Moon is further away from the Earth than during a total solar eclipse. Because of this, the Moon does not totally obscure the Sun during the actual eclipse and a thin ring of the Sun's surface remains visible. The reduction in solar radiation is not 100%, so the effect on the ionosphere's TEC is somewhat less than during a total eclipse. On 8 April 2024, another total solar eclipse crossed North America, passing over Mexico, the United States, and Canada. The eclipse began in the United States in Texas, and then traveled through Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire, and Maine before entering Canada. For both of these eclipses, we have deployed numerous GNSS receivers collecting 1-second data along the path of the eclipse. In addition, we have deployed a specialized GNSS scintillation receiver outside of Austin, TX. These GNSS TEC observations have been included in the standard Millstone Hill Geospace Facility's GNSS TEC and differential TEC processing, utilizing 2000+ receivers in the continental U.S. We report here on initial observations following both of these eclipses, including a discussion of the TEC dynamics following the path of the eclipse, the percent of observed TEC depletions, and TID analysis. Comparisons of the size of observed TEC depletions will be made with those predicted by models.

The mid-latitude ionospheric trough (MLIT), an anomaly in the ionosphere's F layer caused by various mechanisms, affects radio wave propagation. In this study, we investigated the morphology and oscillations of the MLIT using global Global Positioning System total electron content map data between 1 January 2018, and 31 December 2020. The MLIT position varies longitudinally, reaching its farthest equatorward at 60 degrees ${}<^>{\circ}$W and its farthest poleward at 30 degrees ${}<^>{\circ}$E. The MLIT occurrence rates peak during the winter and equinoxes and dip in summer, while seasonal variations in MLIT position vary across longitude bands. Heightened geomagnetic activities, quantified by the SME6 index, promote MLIT occurrence, especially during pre-midnight hours in summer and equinoxes, and shift the MLIT equatorward, particularly during midnight and post-midnight hours. The MLIT position shows clear local time variation, with a gradual decrease before midnight, stabilization afterward, and a minor resurgence around dawn. Wavelet analysis reveals three distinct periodic components in the MLIT position: 27, 13.5, and 9, with the 27-day period being the most persistent. Cross-wavelet and wavelet coherence analyses suggest that solar wind (SW) velocity variations precede changes in the MLIT position. The main factors responsible for the equatorward movement of MLIT are the electric fields in high-speed SW that enhance the ionospheric convection pattern, and the intensified geomagnetic activities induced by interplanetary shocks. The minimum position of mid-latitude ionospheric trough shows discernible longitudinal variation Three distinct periodic components were observed in the trough's minimum position: 27, 13.5, and 9 days High latitude ionospheric plasma convection plays an important role in controlling the trough's minimum position

The latitudinal location of the Equatorial Ionization Anomaly (EIA) crest has seasonal variation, and there are disagreements on the interpretation of such seasonal characteristic in previous studies. Some studies suggested that this seasonal characteristic is determined by the seasonal characteristic of the equatorial electric field. Others suggested that this seasonal characteristic is determined by the seasonal changes of the thermospheric wind. The current paper uses Total Electron Content (TEC) data and the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) to analyze the seasonal variation of the northern EIA crest in the eastern Asian sector under low solar activity. Our results show that the monthly averaged latitudinal location of the northern EIA crest has a good linear relationship (r = 0.74) with the monthly averaged Equatorial Electrojet (EEJ) intensity, which is a commonly used proxy of the eastward electric field. However, TIEGCM simulations with and without F-region wind indicate that such a relationship might be attributed to wind effects. Additionally, the linear relationship between the EEJ intensity and the northern-southern EIA crest distance is not significant (r = 0.47) in the eastern Asian sector. Our results suggest that a good correspondence between the eastward electric field and the latitudinal location of the EIA crest is not assured annually, as the seasonally varying F-region wind significantly influences EIA evolution.

The effect of storms driven by solar wind high-speed streams (HSSs) on the high-latitude ionosphere is inadequately understood. We study the ionospheric F-region during a moderate magnetic storm on 14 March 2016 using the EISCAT Troms & oslash; and Svalbard radar latitude scans. AMPERE field-aligned current (FAC) measurements are also utilized. Long-duration 5-day electron density depletions (20%-80%) are the dominant feature outside of precipitation-dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12-21 magnetic local time, MLT) the depleted region is 10 degrees-18 degrees magnetic latitude (MLAT) in width, with the largest latitudinal extent 62 degrees-80 degrees MLAT in the afternoon. The second region is in the morning to pre-noon sector (04-10 MLT), where the depletion region occurs at 72 degrees-80 degrees MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion-frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion-neutral frictional heating transported by neutral winds. Even though depleted F-regions may occur within any of the large-scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.

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 degrees $\pm 40{}<^>{\circ}$ geomagnetic latitude. These midlatitude peaks exhibited a wide longitudinal extension exceeding 150 degrees ${}<^>{\circ}$ 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.

AbstractThis 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.

Joule heating is a major energy sink in the solar wind-magnetosphere-ionosphere system and modeling it is key to understanding the impact of space weather on the neutral atmosphere. Ion drifts and neutral wind velocities are key parameters when modeling Joule heating, however there is limited validation of the modeled ion and neutral velocities at mid-latitudes. We use the Blackstone Super Dual Auroral Radar Network radar and the Michigan North American Thermosphere Ionosphere Observing Network Fabry-Perot interferometer to obtain the local nightside ion and neutral velocities at similar to 40 degrees geographic latitude during the nighttime of 16 July 2014. Despite being a geomagnetically quiet period, we observe significant sub-auroral ion flows in excess of 200 ms(-1). We calculate an enhancement to the local Joule heating rate due to these ion flows and find that the neutrals impart a significant increase or decrease to the total Joule heating rate of >75% depending on their direction. We compare our observations to outputs from the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM). At such a low geomagnetic activity however, TIEGCM was not able to model significant sub-auroral ion flows and any resulting Joule heating enhancements equivalent to our observations. We found that the neutral winds were the primary contributor to the Joule heating rates modeled by TIEGCM rather than the ions as suggested by our observations.

Two interacting high-speed solar wind streams (HSSs) and associated stream interaction regions (SIR) caused a moderate geomagnetic storm during 14-20 March 2016. The spatio-temporal evolution of the total electron content (TEC) during the storm is studied by using Global Navigation Satellite System (GNSS) data. The moderate storm caused significant and long-lasting changes on TEC within the polar cap (70 degrees ${}<^>{\circ}$-90 degrees ${}<^>{\circ}$ MLAT), at auroral and sub-auroral latitudes (60 degrees ${}<^>{\circ}$-70 degrees ${}<^>{\circ}$ MLAT), and at mid-latitudes (40 degrees ${}<^>{\circ}$-60 degrees ${}<^>{\circ}$ MLAT). A 25%-50% depletion in TEC was observed for six days in the day, dusk and dawn sectors in the polar cap region and in the day and dusk sectors at the auroral and sub-auroral latitudes. Sub-auroral polarization streams observed by the Defense Meteorological Satellite Program satellite contributed to the sub-auroral dusk TEC decreases. At mid-latitudes, TEC depletion was observed in all local time sectors 21 hr after the storm onset. It is suggested that ion-neutral frictional heating causes the TEC depletions, which is further supported by the observed spatial correlation between TEC depletions and & sum; $\sum $O/N2 decreases at mid-latitudes observed by TIMED/GUVI. The storm induced a prolonged positive phase at mid-latitudes lasting 9 hr. In the polar cap, enhancements of TEC up to 200% were caused by polar cap patches. TEC increases were the dominant feature in the night and morning sectors within the auroral oval because of particle precipitation and resulted up to regionally averaged 6 TECU (200%) increases.

AbstractThe continental United States is well instrumented with facilities for mid‐latitude upper atmosphere research that operate on a continuous basis. In addition, citizen scientists provide a wealth of information when unusual events occur. We combine ionospheric total electron content (TEC) data from distributed arrays of GNSS receivers, magnetometer chains, and auroral observations obtained by citizen scientists, to provide a detailed view of the intense auroral breakup and westward surge occurring at the peak of the 10–11 May 2024 extreme geomagnetic storm. Over a 20‐min interval, vertical TEC (vTEC) increased at unusually low latitude (∼45°) and rapidly expanded azimuthally across the continent. Individual receiver/satellite data sets indicate sharp bursts of greatly elevated of vTEC (∼50 TECu). Intense red aurora was co‐located with the leading edge of the equatorward and westward TEC enhancements, indicating that the large TEC enhancement was created by extremely intense low‐energy precipitation during the rapid substorm breakup.

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 similar to ${\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. 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. The altitude-resolved Ne response to the solar eclipse in the 3-D domain was effectively reconstructed by TIDAS-SAMI3 data assimilation The eclipse led to a substantial ionospheric Ne reduction of 20%-50%, with the maximum depletion occurring in the F region of 200-250 km The 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

AbstractThe high latitude ionospheric evolution of the May 10‐11, 2024, geomagnetic storm is investigated in terms of Total Electron Content and contextualized with Incoherent Scatter Radar and ionosonde observations. Substantial plasma lifting is observed within the initial Storm Enhanced Density plume with ionospheric peak heights increasing by 150–300 km, reaching levels of up to 630 km. Scintillation is observed within the cusp during the initial expansion phase of the storm, spreading across the auroral oval thereafter. Patch transport into the polar cap produces broad regions of scintillation that are rapidly cleared from the region after a strong Interplanetary Magnetic Field reversal at 2230UT. Strong heating and composition changes result in the complete absence of the F2‐layer on the eleventh, suffocating high latitude convection from dense plasma necessary for Tongue of Ionization and patch formation, ultimately resulting in a suppression of polar cap scintillation on the eleventh.

This study investigates the ionospheric total electron content (TEC) responses in the 2-D spatial domain and electron density variations in the 3-D spatial domain during the annular solar eclipse on 14 October 2023, using ground-based Global Navigation Satellite System (GNSS) observations, a novel TEC-based ionospheric data assimilation system (TIDAS), ionosonde measurements, and satellite in situ data. The main results are summarized as follows: (a) The 2-D TEC responses exhibited distinct latitudinal differences. The mid-latitude ionosphere exhibited a more substantial TEC decrease of 25%-40% along with an extended recovery time of 3-4 hr. In contrast, the equatorial and low-latitude ionosphere experienced a smaller TEC reduction of 10%-25% and a faster recovery time of 20-50 min. The minimal eclipse effect was observed near the northern equatorial ionization anomaly crest region. (b) The ionospheric electron density variations during the eclipse were effectively reconstructed by TIDAS data assimilation in the 3-D domain, providing important altitude information with validity. (c) The ionospheric electron density variations showed a notable altitude-dependent feature. The eclipse led to a substantial electron density reduction of 30%-50%, with the maximum depletion occurring around the ionospheric F2-layer peak height (hmF2) of 250-350 km. The post-eclipse recovery of electron density exhibited a relatively slower pace near the F2-layer peak height than that at lower and higher altitudes. On 14 October 2023, the Great American annular solar eclipse traversed North, Central, and South America with dense observational network in place, presenting a valuable opportunity for exploring the eclipse-induced ionospheric responses from mid-latitude to equatorial regions. This paper presents a comprehensive analysis of the 2-D ionospheric TEC and 3-D electron density responses during the eclipse, utilizing dense ground-based GNSS observations, a new TEC-based ionospheric data assimilation system (TIDAS), and ionosonde and satellite data sets. The TIDAS data assimilation system provides accurate and reliable regional ionospheric electron density reconstruction, which can effectively reproduce the electron density variations during the eclipse in the 3-D domain with important altitude information and high-fidelity details. This multi-instrumental and data assimilation study highlights the latitudinal and altitudinal dependencies of the eclipse-induced ionospheric responses, advancing the current understanding of how a solar eclipse event impacts the ionosphere. The TEC response showed latitudinal variances, with a 25%-40% decrease in midlatitudes but only a 10%-25% reduction in the equatorial region The Ne response showed altitudinal dependencies, with a larger depletion and a slower recovery near the F2 peak height than below and above The NmF2 exhibited a 30%-50% reduction, and the hmF2 exhibited a 20-30 km decrease in the recovery phase after the maximum obscuration

We recommend that the community should emphasize and expand midlatitude and subauroral science in upcoming observation systems, satellite missions, and modeling efforts.Midlatitude studies require the best possible geospace modeling and observational capabilities.The community must synchronously push the boundaries of joint ground and space enhanced observational capabilities, and should create multiscale instrument networks throughout geospace and especially at midlatitudes for high fidelity and wide field ionospheric dynamic observations.We urge the community to adopt a joint analysis approach in all subauroral studies.Within these studies, statistically based observational and theoretical quantification of SED and midlatitude main trough dynamic variations, including its altitude and MLT variation, is essential for not only understanding ionospheric variations but for space weather assessment and forecast skill improvement.Characterization of SAPS, through modeling and understanding its formation and roles in the geospace system, remains very limited and needs substantial additional efforts.Continuing advances in understanding of the Geospace Plume as a global and cross-cutting feature are needed.The CEDAR and GEM communities should jointly emphasize coupled observational and theoretical studies of heavy ion mass flows originating in the ionosphere, ion upwellings and outflows, thermospheric changes, and ion outflow impacts on magnetospheric dynamics.Midlatitude electrodynamic coupling studies are essential for understanding the geospace system, including fine scale meteor radar and neutral wind observation networks, Farley-Buneman two-stream anomalous conductivity impacts on M-I coupling, kinetic effects, impacts of strongly elevated electric fields (e.g., SAPS) and plasma flow on neutral dynamics and plasma instability, and related subjects.Space weather knowledge, essential for technological society at midlatitudes, requires sustained emphasis on multi-scale observations of ionosphere and thermosphere dynamics combined with models capable of driving and accurately reproducing variability on km and larger scales.These observations must include not only ion but neutral parameters, and not just for a fixed height but across E and F regions and the topside ionosphere.

Abstract Assessing space weather modeling capability is a key element in improving existing models and developing new ones. In order to track improvement of the models and investigate impacts of forcing, from the lower atmosphere below and from the magnetosphere above, on the performance of ionosphere‐thermosphere models, we expand our previous assessment for 2013 March storm event (Shim et al., 2018, https://doi.org/10.1029/2018SW002034). In this study, we evaluate new simulations from upgraded models (the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model version 4.1 and the Global Ionosphere Thermosphere Model (GITM) version 21.11) and from the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) version 2.2 including eight simulations in the previous study. A simulation from the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model version 2 (TIE‐GCM 2.0) is also included for comparison with WACCM‐X. TEC and foF2 changes from quiet‐time background are considered to evaluate the model performance on the storm impacts. For evaluation, we employ four skill scores: Correlation coefficient (CC), root‐mean square error (RMSE), ratio of the modeled to observed maximum percentage changes (Yield), and timing error (TE). It is found that the models tend to underestimate the storm‐time enhancements of foF2 (F2‐layer critical frequency) and TEC (Total Electron Content) and to predict foF2 and/or TEC better in North America but worse in the Southern Hemisphere. The ensemble simulation for TEC is comparable to results from a data assimilation model (Utah State University‐Global Assimilation of Ionospheric Measurements (USU‐GAIM)) with differences in skill score less than 3% and 6% for CC and RMSE, respectively.