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Abstract Poleward boundary intensifications (PBIs) are one of the most common auroral disturbances and have been shown to play a key role in substorm dynamics. PBIs are commonly interpreted as the ionospheric signature of magnetotail reconnection, providing insight into X‐line evolution. While PBIs have been linked to ionospheric flow channels using line‐of‐sight observations, their two‐dimensional flow dynamics remain poorly understood. Using coordinated THEMIS All‐Sky Imager and SuperDARN observations, we present high‐resolution two‐dimension flows derived by the spherical elementary current systems (SECS) method in association with PBI morphology. AMPERE magnetic perturbation data are presented for one event to analyze the current system associated with a dawn‐cell PBI. Ionospheric flow channel observations are used to infer plasma inflow in the magnetotail, and our results show they play a determining role in PBI morphology, including spatial extent, number, and propagation. These results suggest that inflow properties strongly influence X‐line dynamics. 

Abstract This study investigates the impact of high‐latitude ionospheric irregularities on global positioning system (GPS) position accuracy during the 10–11 May 2024 geomagnetic storm. Using data from ground‐based GPS receivers and an all‐sky camera in Antarctica, we examined evolution of GPS vertical total electron content, phase fluctuations, and precise point positioning (PPP) errors. Results reveal that auroral precipitation associated with the coronal mass ejection shock induced significant GPS phase fluctuations and positioning errors in the dayside cusp region. Tongues of ionization (TOIs) had the largest and most sustained impact on signal disruptions and degraded position accuracy, with a peak position error reaching 28 m. Furthermore, dawnside auroral activity during the substorms and poleward boundary intensifications (PBIs) caused localized disturbances in the auroral oval. Despite sufficient satellite visibility, these irregularities significantly compromised GPS positioning performance. 

Abstract Using high‐time‐resolution Super Dual Auroral Radar Network (SuperDARN) radar data, we investigated the dynamic evolution of ionospheric convection under various conditions, including an interplanetary magnetic field (IMF) southward turning, a substorm onset, quasi‐steady southward IMF, and Pc5 ultra‐low frequency (ULF) waves. The SuperDARN high‐resolution convection revealed that during an IMF southward turning, convection enhancements form narrow, transient flow channels in the polar cap rather than broad, uniform flows, with velocity peaks moving equatorward and crossing the nightside open‐closed boundary. These flow peaks are linked to poleward boundary intensifications. The convection response was nearly simultaneous across latitudes. During a substorm onset, enhanced equatorward flow across the polar cap boundary preceded auroral onset, suggesting that flow observations may be a more sensitive indicator of pre‐onset conditions than auroral data. Under quasi‐steady southward IMF, both aurora and plasma velocity show significant unsteady behavior, with flow channels moving equatorward beyond the extent of their corresponding auroral streamers. For Pc5 ULF waves, the data reveal fine‐scale velocity structures within larger poleward‐moving enhancements, which correspond to breaks in the ULF magnetic field. This study emphasizes that high‐time‐resolution convection observations are essential for accurately capturing the rapid evolution of ionospheric convection, which is often underestimated using conventional lower‐resolution methods. 

Abstract Whistler‐mode chorus waves have been widely examined in the Earth's magnetosphere since they play an important role in the dynamics of the radiation belts around the Earth and energetic electron precipitation into the upper atmosphere. In this study, we examine the relationship between the low‐energy (<800 eV) electrons and the linear instability of chorus waves through a statistical analysis of 12 years of in situ observations from the THEMIS mission. Our results show that 31% of all lower‐band chorus wave events observed near the magnetic equator (|MLAT| < 10°) are linearly unstable if the low‐energy electron density is considered, whereas only 4% are unstable when the low‐energy electrons are ignored. Additionally, we identify a critical transition in chorus wave properties, where emission types shift from predominantly electromagnetic ( < 0.03) to a mixture of electromagnetic and quasi‐electrostatic components ( ∼ 0.1) depending on the low‐energy electron density ratio. Moreover, an analysis of correlation coefficients () associated with temporal variations of wave amplitude reveals that the low‐energy electron density is the dominant factor (∼43% with  > 0.2) of chorus wave modulation, while the hot electron temperature (>800 eV) ratio and electron parallel beta contribute only 15% and 21%, respectively. These results emphasize the important contribution of the low‐energy electrons to the instability and modulations of chorus waves in the Earth's magnetosphere. 

Abstract This study investigates impacts of the May 2024 superstorm on the mid‐latitude Global Positioning System (GPS) scintillation and position errors. Using 1‐Hz GPS receiver data, we identified position errors in PPP mode reaching up to 70 m in the Central United States during the storm main phase on May 10. The PPK solution becomes unstable following the arrival of storm and lasted till the recovery phase, coinciding with reported GPS outages of farming equipment. The large position errors were attributed to strong scintillation and carrier phase cycle slips around the equatorward boundary of the ionosphere trough, where large total electron content (TEC) gradients and irregularities were present. In the Southwestern United States, position errors of 10–20 m were associated with the storm‐enhanced density and equatorial ionization anomaly. Scintillation and cycle slips in this region were minor, and bending of the GPS signal paths (refractive effect) is suggested to cause the position errors. PPP outages were also associated with sudden changes in the geometric distributions of available GPS satellites used in position calculations. On May 11, energetic particle precipitation during substorms led to abrupt jumps in TEC and scintillation, resulting in rapidly evolving position errors of up to 10 m. These findings highlight the critical role of storm‐time plasma transport, precipitation, and irregularity formation in degrading GPS performance. The study underscores the need for accurate ionospheric state specification, improved signal processing technique, real‐time ionospheric corrections, and optimized satellite selection algorithms, to enhance navigation resilience during severe space weather events. 

Abstract This study investigates the evolution of substorm onset beads into poleward expansion, surge, and streamer formation during the substorm expansion phase. Using optical observations, we infer the transition from near‐Earth instability to the formation of a near‐Earth neutral line (NENL). We found that a thin, faint arc appeared immediately poleward of the onset arc shortly after substorm onset but prior to significant poleward expansion. Beads within the longitudinal extent of this poleward arc expanded poleward more rapidly than those outside this region. The western edge of the poleward‐expanding beads formed the surge, and streamers emanated from the poleward‐expanding arc. Poleward expansion occurred stepwise, with each step associated with a re‐intensification of the poleward arc. Analysis of an event with simultaneous observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite and THEMIS all‐sky imager showed a near‐simultaneous occurrence of stepwise poleward expansion and dipolarization fronts. The lack of a significant time delay suggests that an X‐line initiates in the near‐Earth plasma sheet at approximately 11.8 R_Eafter onset. This stepwise poleward expansion suggests a corresponding stepwise tailward retreat of the X‐line toward NENL locations observed further tailward in earlier studies. 

Abstract We report the first simultaneous observations of total electron content (TEC), radio signal scintillation, and precise point positioning (PPP) variation associated with Strong Thermal Emission Velocity Enhancement (STEVE) emissions during a 26 March 2008 storm‐time substorm. Despite that the mid‐latitude trough TEC decreases during the substorm overall, interestingly, we found an unexpected TEC enhancement (by ∼2 TECU) during STEVE. Enhancement of vertical TEC and phase scintillation was highly localized to STEVE within a thin latitudinal band of 1°. As STEVE shifted equatorward, TEC enhancement was found at and slightly poleward of the optical emission. PPP exhibited enhanced variation across a 3° latitudinal range around STEVE and indicated increased GNSS positioning error. We suggest that TEC enhancement during STEVE creates local TEC structures in the ionosphere that degrade Global Navigation Satellite Systems (GNSS) signals and PPP performance. The TEC enhancement may be created by particle precipitation, Pedersen drift across STEVE, neutral wind, or plasma instability. 

Abstract Although Strong Thermal Emission Velocity Enhancement (STEVE) and subauroral ion drifts (SAID) are often considered in the context of geomagnetically disturbed times, we found that STEVE and SAID can occur even during quiet times. Quiet‐time STEVE has the same properties as substorm‐time STEVE, including its purple/mauve color and occurrence near the equatorward boundary of the pre‐midnight auroral oval. Quiet‐time STEVE and SAID emerged during a non‐substorm auroral intensification at or near the poleward boundary of the auroral oval followed by a streamer. Quiet‐time STEVE only lasted a few minutes but can reappear multiple times, and its latitude was much higher than substorm‐time STEVE due to the contracted auroral oval. The THEMIS satellites in the plasma sheet detected dipolarization fronts and fast flows associated with the auroral intensification, indicating that the transient energy release in the magnetotail was the source of quiet‐time STEVE and SAID. Particle injection was weaker and electron temperature was lower than the events without quiet‐time STEVE. The plasmapause extended beyond the geosynchronous orbit, and the ring current and tail current were weak. The interplanetary magnetic field (IMF)B_zwas close to zero, while the IMFB_xwas dominant. We suggest that the small energy release in the quiet magnetosphere can significantly impact the flow and field‐aligned current system. 

Abstract Following the auroral substorm onset, the active aurora undergoes expansion, which can vary in spatial and temporal extent. The spatiotemporal development of the expansion phase active aurora is controlled by new auroral intensifications that often follow the initial onset. Using seven examples, we investigate the nature of these new auroral intensifications and address a question: are they new auroral onsets, that is, “successive onsets” or poleward‐boundary intensifications (PBIs) and ensuing auroral streamers? We observed events that included both types of auroral features—successive onsets and PBIs—and their combinations. For multiple‐onset substorms, successive onsets may occur eastward, westward, and poleward of the initial onset, resulting in a diverse range of expansion phase spatial extent and durations. Single‐onset substorms show only one auroral onset, but their spatiotemporal development can resemble that of multiple‐onset substorms. However, the additional activations are mainly PBIs and subsequent streamers. In some cases, PBIs undergo explosion, leading to a rapid poleward and azimuthal expansion of the aurora, resembling the auroral substorm onset. A prolonged sequence of PBIs and its longitudinal extension can contribute significantly to the spatiotemporal development of substorms expansion phase. Results suggest that post‐onset flow channels drive the spatiotemporal development of the substorm expansion phase by (a) triggering successive onsets and (b) inducing bursts of PBIs and their prolonged sequence. We speculate that post‐onset flow channels likely originate from the polar cap, but more evaluation is required. Our findings highlight the significance of examining imager data before solely relying on magnetometers to identify substorm onsets. 

Abstract Space‐based observations of the signatures associated with STEVE show how this phenomenon might be closely related to an extreme version of a SAID channel. Measurements show high velocities (>4 km/s), high temperatures (>4,000 K), and very large current density drivers (up to 1 μA/m²). This phenomena happens in a small range of latitudes, less than a degree, but with a large longitudinal span. In this study, we utilize the GEMINI model to simulate an extreme SAID/STEVE. We assume a FAC density coming from the magnetosphere as the main driver, allowing all other parameters to adjust accordingly. We have two main objectives with this work: show how an extreme SAID can have velocity values comparable or larger than the ones measured under STEVE, and to display the limitations and missing physics that arise due to the extreme values of temperature and velocity. Changes had to be made to GEMINI due to the extreme conditions, particularly some neutral‐collision frequencies. The importance of the temperature threshold at which some collision frequencies go outside their respective bounds, as well as significance of the energies that would cause inelastic collisions and impact ionization are displayed and discussed. We illustrate complex structures and behaviors, emphasizing the importance of 3D simulations in capturing these phenomena. Longitudinal structure is emphasized, as the channel develops differently depending on MLT. However, these simulations should be viewed as approximations due to the limited observations available to constrain the model inputs and the assumptions made to achieve sensible results.