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Abstract The weakly ionized plasma in the Earth's ionosphere is controlled by a complex interplay between solar and magnetospheric inputs from above, atmospheric processes from below, and plasma electrodynamics from within. This interaction results in ionosphere structuring and variability that pose major challenges for accurate ionosphere prediction for global navigation satellite system (GNSS) related applications and space weather research. The ionospheric structuring and variability are often probed using the total electron content (TEC) and its relative perturbations (dTEC). Among dTEC variations observed at high latitudes, a unique modulation pattern has been linked to magnetospheric ultra‐low‐frequency (ULF) waves, yet its underlying mechanisms remain unclear. Here using magnetically conjugate observations from the THEMIS spacecraft and a ground‐based GPS receiver at Fairbanks, Alaska, we provide direct evidence that these dTEC modulations are driven by magnetospheric electron precipitation induced by ULF‐modulated whistler‐mode waves. We observed peak‐to‐peak dTEC amplitudes reaching 0.5 TECU (1 TECU is equal to electrons/) with modulations spanning scales of 5–100 km. The cross‐correlation between our modeled and observed dTEC reached 0.8 during the conjugacy period but decreased outside of it. The spectra of whistler‐mode waves and dTEC also matched closely at ULF frequencies during the conjugacy period but diverged outside of it. Our findings elucidate the high‐latitude dTEC generation from magnetospheric wave‐induced precipitation, addressing a significant gap in current physics‐based dTEC modeling. Theses results thus improve ionospheric dTEC prediction and enhance our understanding of magnetosphere‐ionosphere coupling via ULF waves. 

Abstract Dawnside auroral polarization streams (DAPS) are fast eastward flows in the dawn convection cell of Earth's ionosphere. With a steep flow gradient near the interface between Region 1 and 2 currents and a peak poleward of it, DAPS were suggested to be responsible for instabilities and dramatic events in the magnetosphere‐ionosphere (M‐I) system. To predict these events, it is important to investigate when and where DAPS prefer to occur and how they are related to other M‐I phenomena. We conduct this investigation statistically using 10 years of Swarm data and find that DAPS under sunlit and dark ionospheric conditions exhibit different dependencies on magnetic local times and geomagnetic activities, reflecting a complicated interplay between magnetotail dynamics and ionospheric conductance. The statistical results also reveal a strong correlation between DAPS and embedded Region 2 currents. These findings provide new insights into the DAPS generation mechanism. 

Abstract Energetic electron precipitation (EEP) during substorms significantly affects ionospheric chemistry and lower‐ionosphere (<100 km) conductance. Two mechanisms have been proposed to explain what causes EEP: whistler‐mode wave scattering, which dominates at low latitudes (mapping to the inner magnetosphere), and magnetic field‐line curvature scattering, which dominates poleward. In this case study, we analyzed a substorm event demonstrating the dominance of curvature scattering. Using ELFIN, POES, and THEMIS observations, we show that 50–1,000 keV EEP was driven by curvature scattering, initiated by an intensification and subsequent earthward motion of the magnetotail current sheet. Using a combination of Swarm, total electron content, and ELFIN measurements, we directly show the location of EEP with energies up to ∼1 MeV, which extended from the plasmapause to the near‐Earth plasma sheet (PS). The impact of this strong substorm EEP on ionospheric ionization is also estimated and compared with precipitation of PS (<30 keV) electrons. 

Abstract Disturbances in ionospheric Total Electron Content (dTEC) with frequencies of 1–100 mHz can be driven from above by processes in the magnetosphere and below by processes on the Earth's surface and lower atmosphere. Past studies showed the potential of dTEC as a diagnostic of magnetospheric Ultra Low Frequency (ULF) wave activity and demonstrated that ULF dTEC can impact space weather by, for example, changing ionospheric conductance. However, most past work has focused on single event studies, lacked magnetospheric context, or used sampling rates too low to capture most ULF waves. Here, we perform a statistical study using Time History of Events and Macrsoscale Interactions during Substorms (THEMIS) satellite conjunctions with a ground‐based magnetometer and Global Navigation Satellite System (GNSS) receiver at 65° magnetic latitude. We find that magnetospheric ULF waves generate dTEC variations across the broad range of frequencies examined in this study (2–50 mHz), and that ULF dTEC wave power is correlated with Kp, AE, solar wind speed, and magnetic field wave power observed in the magnetosphere and on the ground. We further find that magnetospheric ULF waves generate dTEC amplitudes up to TECU ( background), with the largest amplitudes occurring during geomagnetically active conditions, at frequencies below 7 mHz, and at local times near midnight. We finally discuss the implications of our results for magnetosphere‐ionosphere coupling and remote sensing techniques related to ULF waves. 

Abstract Recent observations show very near‐Earth reconnection (∼8–13R_E) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30R_E). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future. 

Abstract Abrupt variations of auroral electrojets can induce geomagnetically induced currents, and the ability to model and forecast them is a pressing goal of space weather research. We report an auroral electrojet spike event that is extreme in magnitude, explosive in nature, and global in spatial extent that occurred on 24 April 2023. The event serves as a fundamental test of our understanding of the response of the geospace system to solar wind dynamics. Our results illustrate new and important characteristics that are drastically different from existing knowledge. Most important findings include (a) the event was only of ∼5‐min duration and was limited to a narrow (2°–3°) band of diffuse aurora; (b) the longitudinal span covered the entire nightside sector, possibly extending to the dayside; (c) the trigger seems to be a transient solar wind dynamic pressure pulse. In comparison, substorms usually last 1–2 hr and span almost the entire latitudinal width of the auroral oval. Magnetic perturbation events (MPEs) span hundreds km in radius. Both substorms and MPEs are mainly driven by disturbances in the magnetotail. A possible explanation is that the pressure pulse compresses the magnetosphere and enhances diffuse precipitation of electrons and protons from the inner plasma sheet, which elevates the ionospheric conductivity and intensifies the auroral electrojet. Therefore, the event exhibits a potentially new type of geomagnetic disturbance and highlights a solar wind driver that is enormously influential in driving extreme space weather events. 

Abstract Magnetotail earthward‐propagating fast plasma flows provide important pathways for magnetosphere‐ionosphere coupling. This study reexamines a flow‐related red‐line diffuse‐like aurora event previously reported by Liang et al. (2011,https://doi.org/10.1029/2010ja015867), utilizing THEMIS and ground‐based auroral observations from Poker Flat. We find that time domain structures (TDSs) within the flow bursts efficiently drive electron precipitation below a few keV, aligning with predominantly red‐line auroral intensifications in this non‐substorm event. The diffuse‐like auroras sometimes coexisted with or potentially evolved from discrete forms. We forward model red‐line diffuse auroras due to TDS‐driven precipitation, employing the time‐dependent TREx‐ATM auroral transport code. The good correlation (∼0.77) between our modeled and observed red line emissions underscores that TDSs are a primary driver of the red‐line diffuse‐like auroras, though whistler‐mode wave contributions are needed to fully explain the most intense red‐line emissions. 

Abstract The Poynting vector (Poynting flux) from Earth's magnetosphere downward toward its ionosphere carries the energy that powers the Joule heating in the ionosphere and thermosphere. The Joule heating controls fundamental ionospheric properties affecting the entire magnetosphere‐ionosphere‐thermosphere system, so it is necessary to understand when and where the Poynting flux is significant. Taking advantage of new data sets generated from DMSP (Defense Meteorological Satellite Program) observations, we investigate the Poynting flux distribution within and around the auroral zone, where most magnetosphere‐ionosphere (M‐I) dynamics and thus Joule heating occurs. We find that the Poynting flux, which is generally larger under more active conditions, is concentrated in the sunlit cusp and near the interface between Region 1 and 2 currents. The former concentration suggests voltage generators drive the cusp dynamics. The latter concentration shows asymmetries with respect to the interface between the Region 1 and 2 currents. We show that these reflect the controlling impact of subauroral polarization streams and dawnside auroral polarization streams on the Poynting flux. 

Abstract Energetic (≳50 keV) electron precipitation from the magnetosphere to the ionosphere during substorms can be important for magnetosphere‐ionosphere coupling. Using conjugate observations between the THEMIS, ELFIN, and DMSP spacecraft during a substorm, we have analyzed the energetic electron precipitation, the magnetospheric injection, and the associated plasma waves to examine the role of waves in pitch‐angle scattering plasma sheet electrons into the loss cone. During the substorm expansion phase, ELFIN‐A observed 50–300 keV electron precipitation from the plasma sheet that was likely driven by wave‐particle interactions. The identification of the low‐altitude extent of the plasma sheet from ELFIN is aided by DMSP global auroral images. Combining quasi‐linear theory, numerical test particle simulations, and equatorial THEMIS measurements of particles and fields, we have evaluated the relative importance of kinetic Alfvén waves (KAWs) and whistler‐mode waves in driving the observed precipitation. We find that the KAW‐driven bounce‐averaged pitch‐angle diffusion coefficientsnear the edge of the loss cone are ∼10⁻⁶–10⁻⁵s⁻¹for these energetic electrons. Thedue to parallel whistler‐mode waves, observed at THEMIS ∼10‐min after the ELFIN observations, are ∼10⁻⁸–10⁻⁶s⁻¹. Thus, at least in this case, the observed KAWs dominate over the observed whistler‐mode waves in the scattering and precipitation of energetic plasma sheet electrons during the substorm injection. 

Abstract Low‐altitude observations of magnetospheric particles provide a unique opportunity for remote probing of the magnetospheric and plasma states during active times. We present the first statistical analysis of a specific pattern in such observations, energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. Using 3.5 years of data from the ELFIN CubeSats we report the occurrence distribution of 145 energetic electron flux dropout events and identify characteristics, including their prevalence in the dusk and premidnight sectors, their association with substorms and enhanced auroral activities, and their correlation with the region‐1 (R1) field‐aligned current region. We also investigate three representative dropout events which benefit from satellite conjunctions between ELFIN, GOES, and THEMIS, to better understand the magnetospheric drivers and magnetic field conditions that lead to such dropouts as viewed by ELFIN. One class of dropouts may be associated with magnetic field mapping distortions due to local enhancements and thinning of cross‐tail current sheets and amplification of R1 field‐aligned currents. The other class may be associated with the increase in perpendicular anisotropy of magnetospheric electrons due to magnetic field dipolarizations near premidnight. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, thereby improving the accuracy of field‐line mapping during substorms.