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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 High‐intensity long‐duration continuous auroral electrojet (AE) activity (HILDCAA) events are associated with intensification of relativistic electron fluxes in the inner magnetosphere. The physical mechanisms of this intensification are not well established yet. We study observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the near earth plasma sheet at radial distances of 10 Earth radii, at the transition region between tail and dipole‐like magnetic configurations, referred to as the nightside transition region (NTR), during a HILDCAA event. The observations revealed recurrent dipolarizations accompanied by plasma flow vortices, impulsive electric field enhancements, and increases in electron fluxes at energies of 100 keV up to 1 MeV. Electron pitch angle (PA) distributions at THEMIS showed field‐aligned flux enhancements at energies of 100 keV. This indicates a Fermi‐type energization. Arguably, electrons gain energy up to MeV via repetitive bouncing through the acceleration region. Energization of ions was insignificant which led to 1. We suggest that the increased ratio leads to a local increase of the Hall conductivity in the conjugate ionosphere, which causes ionospheric current intensification and strong , consistent with observations. 

Abstract Magnetic field‐line curvature scattering (FLCS) of energetic particles in the equatorial magnetotail results in isotropization of pitch‐angle distributions, loss‐cone filling, and precipitation above a minimum energy at a given latitude. At a fixed energy, the lowest latitude of isotropization is the isotropy boundary (IB) for that energy. Nominally, the IB (latitude) exhibits a characteristic energy dependence due to the monotonic variation of the equatorial magnetic field intensity with radial distance. Deviations from this nominal IB dispersion can occur if the radial variation (spatial or temporal) is non‐mononotic and/or if other precipitation mechanisms prevail. With its sensitive and detailed measurements of electron spectra up to relativistic energies, ELFIN's recent observations reveal a variety of electron IBe patterns near magnetic midnight which are repeatable enough to warrant classification. This study aims to categorize the various IBe patterns observed by ELFIN's high‐fidelity but short lived dataset (a few months), compare them with simultaneous nearby POES observations, which are made with a limited energy coverage and resolution but last for decades, and discuss their possible interpretation. The general agreement between ELFIN and POES IB observations indicate a relatively large‐scale nature of IBe patterns. Surprisingly, there exists a large number (up to 2/3 of all events) of non‐monotonic‐or steep/multiple‐IB patterns. This suggest an abundance of non‐trivial tail current sheet structures or a mixed contribution of two mechanisms in the vicinity of IBe in these cases. 

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 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. 

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. 

Electromagnetic ion cyclotron (EMIC) waves are a key plasma mode affecting radiation belt dynamics. These waves are important for relativistic electron losses through scattering and precipitation into Earth's ionosphere. Although theoretical models of such resonant scattering predict a low-energy cut-off of ∼1 MeV for precipitating electrons, observations from low-altitude spacecraft often show simultaneous relativistic and sub-relativistic electron precipitation associated with EMIC waves. Recently, nonresonant electron scattering by EMIC waves has been proposed as a possible solution to the above discrepancy. We employ this model and a large database of EMIC waves to develop a universal treatment of electron interactions with EMIC waves, including nonresonant effects. We use the Green's function approach to generalize EMIC diffusion rates foregoing the need to modify existing codes or recompute empirical wave databases. Comparison with observations from the electron losses and fields investigation mission demonstrates the efficacy of the proposed method for explaining sub-relativistic electron losses by EMIC waves. 

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 In planetary radiation belts, the Kennel‐Petschek flux limit is expected to set an upper limit on trapped electron fluxes at 80–600 keV in the presence of efficient electron loss through pitch‐angle diffusion by whistler‐mode chorus waves generated around the magnetic equator by the same 80–600 keV electron population. Comparisons with maximum measured fluxes have been relatively successful, but several key assumptions of the Kennel‐Petschek model have not been experimentally tested. The Kennel‐Petschek model notably assumes an exponential growth of chorus waves as the trapped electron flux increases, and a fixed maximum wave power gain of about 3. Here, we describe a method for inferring the near‐equatorial wave power gain using only measurements of trapped, precipitating, and backscattered electron fluxes at low altitude. Next, we make use of Electron Losses and Fields Investigation (ELFIN) CubeSats measurements of such electron fluxes during two moderate geomagnetic storms with sustained electron injections to infer the corresponding chorus wave power gains as a function of time, energy, and equatorial trapped electron flux. We show that wave power increases exponentially with trapped flux, with a wave power gain roughly proportional to the theoretical linear convective gain, and that the maximum inferred gain near the upper flux limit is roughly 10, with a factor of 2 uncertainty. Therefore, two key theoretical underpinnings of the Kennel‐Petschek model are borne out by the present results, although the strong inferred gains should correspond to higher flux limits than in traditional estimates. 

Abstract Energetic electron precipitation (EEP) from the radiation belts into Earth's atmosphere leads to several profound effects (e.g., enhancement of ionospheric conductivity, possible acceleration of ozone destruction processes). An accurate quantification of the energy input and ionization due to EEP is still lacking due to instrument limitations of low‐Earth‐orbit satellites capable of detecting EEP. The deployment of the Electron Losses and Fields InvestigatioN (ELFIN) CubeSats marks a new era of observations of EEP with an improved pitch‐angle (0°–180°) and energy (50 keV–6 MeV) resolution. Here, we focus on the EEP recorded by ELFIN coincident with electromagnetic ion cyclotron (EMIC) waves, which play a major role in radiation belt electron losses. The EMIC‐driven EEP (∼200 keV–∼2 MeV) exhibits a pitch‐angle distribution (PAD) that flattens with increasing energy, indicating more efficient high‐energy precipitation. Leveraging the combination of unique electron measurements from ELFIN and a comprehensive ionization model known as Boulder Electron Radiation to Ionization (BERI), we quantify the energy input of EMIC‐driven precipitation (on average, ∼3.3 × 10⁻² erg/cm²/s), identify its location (any longitude, 50°–70° latitude), and provide the expected range of ion‐electron production rate (on average, 100–200 pairs/cm³/s), peaking in the mesosphere—a region often overlooked. Our findings are crucial for improving our understanding of the magnetosphere‐ionosphere‐atmosphere system as they accurately specify the contribution of EMIC‐driven EEP, which serves as a crucial input to state‐of‐the‐art atmospheric models (e.g., WACCM) to quantify the accurate impact of EMIC waves on both the atmospheric chemistry and dynamics.