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The present study examines the negative ionospheric response over Europe during two geomagnetic storms on 10–13 May 2024, known as the Mother’s Day geomagnetic superstorm. The first storm, with a peak SYM-H value of −436 nT, occurred in the interval 10–11 May, while the second, less intense storm (SYM-H~−103 nT), followed in the interval 12–13 May. Using data from four European locations, temporal and spatial variations in ionospheric parameters (TEC, foF2, and hmF2) were analyzed to investigate the morphology of the strong negative response. Sharp electron density (Ne) depletion is associated with the equatorward displacement of the Midlatitude Ionospheric Trough (MIT), confirmed by Swarm satellite data. A key finding was the absence of foF2 and hmF2 values over all ionosonde stations during the recovery phase of the storms, likely due to the coupling between the Equatorial Ionization Anomaly (EIA) crests and the auroral ionosphere influenced by the intense uplift of the F layer. Relevant distinct features such as Large-scale Travelling Ionospheric Disturbance (LSTID) signatures and Spread F were also noted, particularly during the initial and main phase of the first storm over high midlatitude regions. Regional effects varied, with high European midlatitudes exhibiting different features compared to lower European latitude areas.

The separation and classification of ionospheric troughs in the winter evening and morning ionospheres of the southern hemisphere were performed using CHAMP satellite data for high solar activity (2000–2002). In the high-latitude ionosphere, the main ionospheric trough (MIT) was separated from the high-latitude trough (HLT). The separation was carried out using a thorough analysis of all the characteristic structures of the ionosphere in the framework of the auroral diffuse particle precipitation model. Two types of high-latitude troughs were identified: (1) a wide trough associated with zone II of diffuse precipitation on the poleward edge of the auroral oval and (2) a narrow trough of ionization, which is presumably associated with an electric field action. The poleward wall of MIT is as ever formed by diffuse precipitation in zone I on the equatorward edge of the auroral oval. The HLT and MIT separation is most difficult at the longitudes of the eastern hemisphere, where all structures are located at the highest latitudes and partially overlap. In the mid-latitude ionosphere, all the characteristic structures of the ionosphere were also identified and considered. MIT was separated from the ring ionospheric trough (RIT), which is formed by the decay processes of the magnetospheric ring current. The separation of MIT and RIT was performed based on an analysis of the prehistory of all geomagnetic disturbances during the period under study. In addition to the RIT, a decrease in the electron density equatorward of the MIT was found to be often formed at the America–Atlantic longitudes, which masks the MIT minimum. For completeness, all cases of a clearly defined polar cavity are also presented.

This study examines the ionospheric response to the intense geomagnetic storm of 9–12 October 2024 over the European sector. Digisonde data from mid-latitude European stations and in situ electron density measurements from Swarm A and B satellites were used to analyze variations in key ionospheric characteristics, including the critical frequency (foF2), peak height (hmF2) and plasma drift velocities. Significant uplift of the F2 layer and a corresponding reduction in foF2 were observed across latitudes, primarily driven by prompt penetration electric fields (PPEFs) and storm-induced thermospheric winds. Horizontal and vertical ion drifts showed large asymmetries and reversals, with zonal drift velocities exceeding 1000 m/s at some stations. Swarm observations confirmed plasma density enhancements during the main phase and notable depletions during recovery, particularly after 1:00 UT on 11 October. The midlatitude ionospheric trough (MIT) intensified during the recovery phase, as can be seen from Swarm B. These variations were shaped by electrodynamic forcing, compositional changes and disturbance dynamo electric fields (DDEFs). The results emphasize the role of solar wind drivers, latitude-dependent electrodynamic coupling and thermospheric dynamics in mid-latitude ionospheric variability during geomagnetic storms.

The dynamics of ionospheric troughs during intense geomagnetic storms is considered in this paper. The study is based on electron density measurements at CHAMP satellite altitudes of 405–465 km in the period from 2000 to 2002. A detailed analysis of four storms with Kp from 5+ to 9− is presented. Three troughs were identified: sub-auroral, mid-latitude, and low-latitude. The sub-auroral trough is usually defined as the main ionospheric trough (MIT). The mid-latitude trough is observed equatorward of the MIT and is associated with the magnetospheric ring current; therefore, it is named the ring ionospheric trough (RIT). The RIT appears at the beginning of the storm recovery phase at geomagnetic latitudes of 40–45° GMLat (L = 1.75–2.0) and exists, for a long time, at the late stage of the recovery phase at latitudes of the residual ring current 50–55° GMLat (L ~ 2.5–3.0). The low-latitude trough (LLT) is discovered for the first time. It forms only during great storms at the latitudes of the internal radiation belt (IRB), 34–45° GMLat (L = 1.45–2.0). The LLT’s lowest latitude of 34° GMLat was recorded in the night sector (2–3 LT). The occurrence probability and position of the RIT and LLT depend on the hemisphere and longitude.

The injection of energy into the Earth’s magnetosphere during geomagnetic storms and substorms has a direct impact on the ionosphere in the auroral and sub-auroral regions. This influence can be observed through phenomena such as the expansion of the auroral oval, and fluctuations in plasma density and the intensity of the field-aligned current (FAC) system. Changes in the geomagnetic environment significantly affect the main ionospheric trough (MIT) and field-aligned currents (FACs). In this study, we analysed two geomagnetic storms of different origins—October 2015 and September 2017 that occurred during solar cycle 24. Both storms were characterised by two minima of the Dst index and classified as two-step storms. In this work, we investigate the evolution of FACs density, the displacement of MIT and FACs during the development of geomagnetic storms, and we quest for the relationship between field-aligned currents and the main ionospheric trough. We noticed a connection between the positions of the main ionospheric trough minima and the Dst index, found cases especially during the main and recovery phase of the storms when FACs increase after leaving the ionospheric trough, and observed the predominance of downward currents in the area of the MIT which suggest their influence on the formation of this region of depleted plasma density.

The ionosphere is a layer of weakly ionized plasma bathed in Earth’s geomagnetic field extending about 50–1,500 kilometres above Earth 1 . The ionospheric total electron content varies in response to Earth’s space environment, interfering with Global Satellite Navigation System (GNSS) signals, resulting in one of the largest sources of error for position, navigation and timing services 2 . Networks of high-quality ground-based GNSS stations provide maps of ionospheric total electron content to correct these errors, but large spatiotemporal gaps in data from these stations mean that these maps may contain errors 3 . Here we demonstrate that a distributed network of noisy sensors—in the form of millions of Android phones—can fill in many of these gaps and double the measurement coverage, providing an accurate picture of the ionosphere in areas of the world underserved by conventional infrastructure. Using smartphone measurements, we resolve features such as plasma bubbles over India and South America, solar-storm-enhanced density over North America and a mid-latitude ionospheric trough over Europe. We also show that the resulting ionosphere maps can improve location accuracy, which is our primary aim. This work demonstrates the potential of using a large distributed network of smartphones as a powerful scientific instrument for monitoring Earth. Data from millions of smartphones are used to map the ionosphere in greater detail, leading to improved smartphone location accuracy, particularly in parts of the world with few monitoring stations.

We report the first observation of the plasmasphere's periodic response to solar wind high‐speed streams (HSS) during the declining phase of Solar Cycle 23, based on plasmapause location data from the IMAGE and THEMIS satellites. In both 2005 and 2008, the daily variability of the plasmapause exhibits a strong anti‐correlation with solar wind speed, oscillating coherently at specific timescales. A similar anti‐correlated variation is identified in the latitude of the midlatitude ionospheric trough (MIT) minimum, derived from electron density measurements by the DMSP F16 satellite. Periodogram analysis reveals a distinct 9‐day periodicity in 2005, and both 9‐ and 13.5‐day periodicities in 2008 across all parameters. These findings provide direct evidence of magnetospheric modulation by recurring solar wind drivers and establish a clear connection between the plasmasphere and the midlatitude ionosphere under periodic solar forcing.

The structure of the winter noon ionosphere of the southern hemisphere was studied. This structure includes the dayside cusp, associated high-latitude ionospheric trough (HLT), main ionospheric trough (MIT), electron density (Ne) peak at latitudes about 70°, mid-latitude ring ionospheric trough (RIT), and low-latitude quasi-trough. Data from the CHAMP satellite in the southern hemisphere for quiet geomagnetic conditions under high solar activity were selected for analysis. The DMSP satellite data and a model of auroral diffuse precipitation were also used. This model represents two zones of auroral diffuse precipitation on the equatorward and poleward edges of the auroral oval. It is shown that the situation in the winter noon ionosphere of the southern hemisphere depends cardinally on longitude. At sunlit longitudes, only the HLT is observed, and MIT is formed in the shadow region. At intermediate longitudes, both troughs can be observed and, therefore, there is a problem of their separation. The positions of all structures of the ionosphere depend on the longitude; in particular, the positions of the daytime MIT are changed by 6°−7°. At latitudes of the dayside cusp, both the peak and the minimum of Ne can be observed. A high and narrow peak of Ne is regularly recorded in the CHAMP data at latitudes of the equatorward zone of diffuse precipitation (68°−72°). In the shadow region, this peak forms the MIT poleward wall, and at sunlit longitudes a quasi-trough equatorward of this peak is sometimes observed. The RIT is rarely formed during the day, only at the American and Atlantic longitudes.

— The structure of the winter morning (0500–0900 LT) ionosphere in the Northern and Southern hemispheres is studied in detail. For this, CHAMP satellite data for quiet conditions during the period of high solar activity of 2000–2002 are used. Careful analysis is used to identify electron concentration troughs: the high-latitude ionospheric trough; subauroral, or main, ionospheric trough; and mid-latitude ring ionospheric trough. In order to identify and separate the high-latitude and main ionospheric troughs, the model of auroral diffuse precipitation of the Polar Geophysical Institute is used, which describes the boundaries of low-latitude zone I and high-latitude zone II of auroral diffuse precipitation. The longitudinal variations of the precipitation boundaries are corrected using the DMSP satellite data. The problem of separating the troughs becomes more complicated with the passage of local time, because the main ionospheric trough is more strongly displaced to the pole than the auroral oval; therefore, its area of existence begins to overlap the area of existence of the high-latitude trough. In order to identify and separate the main and ring troughs, all, even weak, geomagnetic disturbances for the observation period are analyzed in detail. The asymmetry of the Northern and Southern hemispheres is considered, and similar and different characteristics are identified. Therefore, a more complete and accurate pattern of the structure of the morning ionosphere is obtained.

We present the first ground‐based measurements of daytime stable auroral red (SAR) arc using OI 630.0 nm emissions. SAR arc is a direct consequence of heat conduction from the inner‐magnetosphere to the ionospheric regions characterized by increased electron temperatures and low electron density in the region of mid‐latitude trough. So far, SAR arc emissions have only been reported for nighttime conditions. For the present study, daytime optical measurements were enabled using a high‐resolution imaging echelle spectrograph from Boston (42.36°N, 71.05°W, MLAT ≈ 53°). Simultaneous Millstone Hill Incoherent Scatter Radar and Defense Meteorological Satellite Program measurements of electron density and temperature confirm our findings. Forward modeling approach enabled estimation of daytime electron temperatures in 200–650 km altitude during this SAR arc event to be varying between 3,500 and 4,400 K. These observations open numerous possibilities of optical investigations of magnetospheric‐ionospheric interactions during daytime, when the upper atmosphere is dynamic with large gradients in ionospheric conductivities, temperatures, and winds. Plain Language Summary A latitudinally narrow and zonally elongated channel consisting of enhanced red‐line emissions are known to be present, on occasions, in the optical measurements in the sub‐auroral latitudes and are referred to as the stable auroral red (SAR) arcs. These are now known to be caused by the conduction of heat from the Ring current region into the ionosphere. Earlier studies suggested an increased electron temperature (above ∼300 km altitude) collocated with the low electron density region of ionospheric trough. So far, observations of SAR arcs have only been reported for the night time. In the daytime, due to the presence of sunlight the atmospheric conditions are different, so also are the upper atmospheric dynamics. The measurements of SAR arcs in the daytime are also challenging as the background solar brightness is very large. In this work, by making use of high spectral resolution optical measurements in the daytime, we present the first measurements of daytime SAR arcs and perform model calculations to estimate the values of electron temperature in the 200–650 km altitude range to be varying in the range of 3,500–4400 K. Key Points Ground‐based measurements of O(1D) dayglow showed enhancement in emissions on a geomagnetically disturbed day at a mid‐latitude location Complementary data sets revealed the increase in optical emissions to be stable auroral red (SAR) arcs, which are associated with elevated electron temperatures This study presents the first ground‐based detection of daytime SAR arcs in OI 630.0 nm red‐line emissions