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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 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 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 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. 

Abstract An approach for creating continental‐scale, multi‐scale plasma convection maps in the nightside high‐latitude ionosphere using the spherical elementary current systems technique has been developed and evaluated. The capability to reconstruct meso‐scale flow channels improved dramatically, and the velocity errors were reduced by ∼30% compared to the spherical harmonic fitting method. Uncertainties of velocity vectors estimated by varying the model setup was also low. Convection maps for a substorm event revealed multiple flow channels in the polar cap, dominating the convection in the quiet time and early growth phase. The meso‐scale flows extended toward the nightside auroral oval and had continuous flow channels over >20° of latitude, and the flow channels dynamically merged and bifurcated. The substorm onset occurred along one of the flow channels, and the azimuthal extent of the enhanced flows coincided with the initial width of the auroral breakup. During the expansion phase, the meso‐scale flows repetitively crossed the oval poleward boundary, and some of them contributed to subauroral polarization streams enhancements. Increased flows extended duskward, along with the westward traveling surge. Then, flows near midnight weakened and evolved to the Harang flow shear. The meso‐scale flow channels had significant (∼10%–40% on average) contributions to the total plasma transport. The meso‐scale flows were highly variable on ∼10 min time scales and their individual maximum contributions reached upto 73%. These results demonstrate the capability of specifying realistic convection patterns, quantifying the contribution of meso‐scale transport, and evaluating the relationship between meso‐scale flows and localized auroral forms. 

Abstract We utilized a 4K imaging to examine properties of fine‐scale structures of Strong Thermal Emission Velocity Enhancement (STEVE) near the magnetic zenith. Its high spatial (0.09 km at 200 km altitude) and temporal (24 Hz) resolution provided unprecedented details of fine‐scale structures in the subauroral ionosphere. Although the STEVE emission was seen as a homogeneous purple/mauve arc in the all‐sky images, the high‐speed imaging revealed that STEVE contained substantial multi‐scale structures. The characteristic wavelength and period were 12.4 ± 7.4 km and 1.4 ± 0.8 s, and they drifted westward at 8.9 ± 0.7 km/s. The speed is comparable to the reported magnitude of the intense subauroral ion drifts (SAID), suggesting that the fine‐scale structures are an optical manifestation of theE × Bdrift in the intense SAID. A spectral analysis identified multiple peaks at >10, 4, 2, 1.1, and <1/5 s period (>83, 33, 16, 9, and <1.7 km wavelength). Although most of the fine‐scale structures were stable during the drift across the field of view, some of the structures dynamically evolved within a few tens of km. The fine‐scale structures have a power law spectrum with a slope of −1, indicating that shear flow turbulence cascade structures to smaller scales. The fine‐scale structures pose a challenge to the subauroral ionosphere‐thermosphere interaction about how the ionosphere creates such fine‐scale structures and how the thermosphere reacts much faster than expected from a typical chemical reaction time. 

Abstract We present observations during two substorms using simultaneous Time History of Events and Macroscale Interactions During Substorms satellites and all‐sky imagers to determine plasma sheet dynamics associated with substorm auroral onset beads. The multi‐satellite observations showed that the cross‐tail current decreased and the field‐aligned currents increased at the substorm auroral onset, indicating that the satellites detected an initiation of the currents being deflected to the ionosphere. For duskward‐propagating beads, the electric field was tailward, and ions were accumulated closer to the Earth than electrons. The mapped bead propagation speed was close to energetic ion drift speed. Theand electron drift speeds increased duskward and reduced the cross‐tail current at the onset. For dawnward‐propagating beads, the electric field was equatorward/earthward, and electrons were inferred to accumulate earthward of ions. The mapped bead propagation speed was comparable to the dawnwardand electron drift speeds. The duskward ion drift and tail current were reduced, and electrons became the dominant current carrier. We suggest that the plasma species that is responsible for the bead propagation changes with the electric field configuration and that the tail current reduction by the enhanceddrift at onset destabilizes the plasma sheet. Ion and electron outflows substantially increased low‐energy plasma density and may have increased the role ofdrifts. The bead wavelength was comparable to ion gyroradius and thus ion kinetic effects are important for determining the wavelength. In the dawnward‐propagating event, the mode of oscillation in the plasma sheet was suggested to be the sausage‐mode flapping oscillations. 

Abstract Inner‐magnetospheric conditions for subauroral polarization streams (SAPS) and subauroral ion drifts (SAID) have been investigated statistically using Time History of Events and Macroscale Interactions during Substorms and RBSP observations. We found that plasma sheet electron fluxes at its earthward edge are larger for SAID than SAPS. The ring current ion flux for SAID formed a local maximum near SAID, but the ion flux for SAID was not necessarily larger than for SAPS. The median potential drop across SAID and SAPS is nearly the same, but the potential drop for intense SAID is substantially larger than that for SAPS. The plasmapause is sharper and electromagnetic waves were more intense for SAID. The SAID velocity peak does not strongly correlate with solar wind or geomagnetic indices. These results indicate that local plasma structures are more important for SAPS/SAID velocity characteristics as compared to global magnetospheric conditions.