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The plasmapause is the outer boundary of the plasmasphere and plays a crucial role in the propagation of plasma waves. We statistically investigate the relationship between the distribution of banded hiss and plasmapause locations. Wave power distributions of banded hiss are analyzed in terms of two ways: (a) the distance away from the plasmapause (ΔL) and (b) the equatorial distance away from the Earth. Statistical results show both bands of banded hiss have larger wave powers and occurrence rates near the plasmapause. The frequencies of two banded hiss waves both decrease discernably with increasing L‐shell at most magnetic local time sectors and geomagnetic activities, but remain nearly constant with increasing ΔL. The highly consistent distribution suggests both bands may be generated in the plume region. The correlation between banded hiss waves and plasmapause locations sheds new light on the generation mechanisms of banded hiss waves. Plain Language Summary Plasmaspheric hiss is a whistler‐mode emission with a broad frequency range from ∼20 Hz to several kHz. The wave is generally observed inside the dayside plasmasphere and plumes. Hiss wave powers are modulated by the combining effect of many parameters such as L (the distance of observation away from the Earth), MLT (magnetic local time), geomagnetic activities, and plasmapause locations. Owing to the difference in electron density inside and outside the plasmapause, the chorus may be damped when propagating into the plasmasphere, and hiss waves in the plasmasphere can be reflected when they propagate near the plasmapause. Recently, A new banded structure of hiss has been reported with a lower band below ∼100 Hz and an upper band above ∼200 Hz. Using ∼7 years measurements of Van Allen Probes, we statistically investigate the distribution of banded hiss wave power with respect to the plasmapause location. The L, MLT, and geomagnetic activities are also considered. Our results suggest that banded hiss wave power shows a strong correlation with the location of plasmapause. Banded hiss waves closer to the plasmapause show larger powers. Plasmapause‐sorted banded hiss wave power can be used to better understand the generation and propagation mechanisms of banded hiss. Key Points The intensity distributions of banded hiss with plasmapause locations at different MLTs under various geomagnetic levels are investigated In statistics, the frequencies of two banded hiss waves decrease discernably with increasing L, but remain nearly constant with ΔL Banded hiss waves with larger amplitudes usually occur close to the plasmapause, implying both bands may be generated in the plume

Plasmapause surface waves (PSWs) near the plasmapause boundary are regarded to be the magnetospheric source of ionospheric auroral giant undulations (GUs) located at the equatorward boundary of diffuse aurora. However, the observational evidence of wave‐particle interaction connecting PSWs and GUs is absent. In this letter, we demonstrate GUs are driven by pitch‐angle scattering of time domain structures modulated by the PSWs, based on the conjugated ionospheric and magnetospheric observations. Specifically, ionospheric GUs are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV near the plasmapause. Further, the total fluxes during one PSW period and energy of scattered electron and ions determine the size and luminosity of GUs. Our research provides observational evidence that PSWs cause periodic electron precipitation via modulating the time domain structures rather than the previously predicted chorus or electron cyclotron harmonic waves. Plain Language Summary Boundary surface waves usually act as a kind of special oscillation along the boundary layer and are the widely existing physical phenomena in the universe. In our Earth, there are magnetopause surface wave and plasmapause surface wave. For the latter, the plasmapause surface wave has been confirmed to be a kind of sawtooth‐type auroral structures locating on the equatorial edge of aurora oval, named as giant undulations. But how can the plasmapause surface wave produce the auroral giant undulations is still unknown. Based on this question, we have provided the observational evidence of auroral giant undulations being driven by the periodic pitch‐angle scattering of time domain structures modulated by plasmapause surface waves. Our new results in this research would help us to better understand the energy conversion controlled by boundary dynamics and the crucial effect of boundary dynamics on the near‐surface space environment. Key Points Giant undulations (GUs) are lighted by the pitch‐angle scattering of <1 keV thermal electron and ions and energetic ions with energy up to dozens of keV Total fluxes during one plasmapause surface wave (PSW) period and energy of scattered electron and ions determine the size and luminosity of GUs PSWs can cause periodic electron precipitation by modulating time domain structures

Hiss waves frequently occur in the plasmasphere or plumes, playing a key role in energetic electron loss in the Earth's inner magnetosphere. While previous studies have linked hiss wave enhancements in the outer plasmasphere (just inside the plasmapause) to electron injections during substorms, their evolution across various substorm phases remains unclear. Using Van Allen Probes observations over 2013–2019, we evaluate hiss wave evolution during various phases of substorm activity. At L > 4, both hiss wave intensity and energetic electron flux increase shortly after substorm onset, first on the morning side, then progress to later magnetic local times (MLTs) at a rate of ∼1–3 hr MLT per hr in universal time (UT), eventually stabilizing near 13 MLT. Stronger substorms result in larger and faster intensification in hiss wave intensity and have more significant impact at lower L‐shells. Our results highlight the global variation of hiss waves during substorms.

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. Plain Language Summary During magnetospheric substorms, energetic electrons in the Earth's plasma sheet (PS), the night‐side magnetosphere region filled by hot plasma, precipitate to the ionosphere. Energetic electron precipitation (EEP) affects the density, temperature, and composition of the ionosphere. However, the exact process that causes such precipitation is not well understood due to observational constraints. The challenge lies in simultaneously measuring the EEP properties at the ionosphere and the plasma and wave properties in the PS. We analyze a fortuitous satellite conjunction during a substorm, during which EEP was simultaneously captured by ELFIN, Swarm, and POES at low altitudes, and THEMIS in the equatorial PS. EEP was observed to extend across a broad equatorial domain, projecting into a wide ionospheric region and encompassing the PS region and a significant portion of the inner magnetosphere. High‐energy‐resolution measurements from ELFIN reveal that the main driver of precipitation is the scattering of energetic electrons by strongly curved magnetic field lines in the PS, as opposed to the more commonly suggested scattering mechanisms associated with wave‐particle interactions. We also show that the EEP drastically altered the ionization profile of the ionosphere. Key Points We investigate the radial location of energetic (50–1,000 keV) electron precipitation (EEP) during a substorm We compare the impact of plasma sheet electron precipitation (<30 keV) and EEP (50–1,000 keV) on the altitudinal profile of ionization Our results underscore the key role of curvature scattering in energetic electron precipitation during substorms

We report a new population of outer belt electron acceleration events ranging from hundreds of keV to ∼1.5 MeV that occurred inside the plasmasphere, which we named “Inside Events” (IEs). Based on 6 year observations from Van Allen Probes, we compare the statistical distributions of IEs with electron acceleration events outside the plasmasphere (OEs). We find that most IEs were observed at L < 4.0 at energies below ∼1.5 MeV, with weaker acceleration ratio (<10) and larger event numbers (peaking value reaching >200), compared to stronger but less frequently occurred (peaking event numbers only reaching ∼80) OEs that were mostly observed at L > 4.0. The evolution of electron phase space density of a typical IE shows signature of inward radial diffusion or transport. Our study provides a feasible mechanism for IE, which is the results of the inward radial transport of the electron acceleration in the outer region of outer belt. Plain Language Summary Since the discovery of the Earth's Van Allen radiation belts in 1958, extensive studies have advanced our understanding of outer belt electron acceleration mechanisms. However, most previous studies focused on the electron acceleration process occurring in the low electron density region, outside the plasmasphere. Although limited previous studies reported the electron flux enhancements penetrated down to very low L‐shells (L = 2.5), involving flux enhancements inside the plasmasphere, these studies did not specify the plasmapause location. In this letter, we report a new population of outer belt electron acceleration events ranging from hundreds of keV to ∼1.5 MeV observed inside the plasmasphere. The “inside electron acceleration events” (IEs) are weaker but occur much more frequently compared to the stronger acceleration events observed outside plasmasphere, and cannot be neglected when investigating radiation belt electron dynamics. The evolution of electron phase space density (PSD) of a typical IE event demonstrates signature of inward radial transport, showing gradual flux enhancements over several days and monotonically increasing radial profile of electron PSD. Our study provides convincing evidence that this observed IE in the low L‐shell region (L = 2.5) was dominantly caused by inward radial transport of electron acceleration in the outer region of the outer belt. Key Points We report a new population of outer belt electron acceleration events from 300 keV to ∼1.5 MeV observed inside the plasmasphere (IE) At >300 keV, the IEs are weaker but occur more frequently compared to the stronger acceleration events observed outside the plasmasphere The evolution of electron phase space density of a typical IE shows monotonically increasing radial profile, suggesting the crucial role of radial transport

Energy circulation in geospace lies at the heart of space weather research. In the inner magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere, which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role in magnetospheric dynamics have not yet been detected. Here, we show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based measurements. The wave oscillates the plasmapause in the afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras occurred in more than 90% of geomagnetic storms during 2014–2018, indicating that they are a systematic and crucial process in driving space energy dissipation. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations. Here, the authors show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based observations.

Plasmaspheric density structures are considered to control the propagation trajectories of fast magnetosonic (MS) waves in the inner magnetosphere. However, whether the plasmaspheric plume can effectively alter the propagation of MS waves remains unknown. Based on the analytical model of plasma density, ray tracing simulations are performed to investigate the propagation of exactly perpendicular MS waves in the equatorial plane in the magnetosphere containing a plasmaspheric plume. We find that plasmatrough and plume MS waves propagating toward the plasmaspheric plume can be reflected into the plasmaspheric core by the plume, then potentially migrating globally and thus quasi‐trapped inside the plasmaspheric core. The simulations also indicate that lower‐frequency MS waves approaching the plasmaspheric plume are more easily reflected and quasi‐trapped inside the plasmaspheric core. Our findings illustrate a previously unexplored way that plasmatrough MS waves could access and be trapped inside the plasmaspheric core via azimuthal plasmaspheric density structures. Plain Language Summary Fast magnetosonic (MS) waves are one of the most common and intense electromagnetic emissions observed both inside and outside the plasmasphere. Portions of MS waves observed inside the plasmasphere are supposed to come from the propagation of those waves outside the plasmasphere. It is well known that the radial plasmaspheric density structures can strongly alter the propagation trajectories of MS waves in the inner magnetosphere. However, a typical azimuthal plasmaspheric density structure, the plasmaspheric plume, can form in the magnetosphere during the geomagnetic disturbed times. How the plasmaspheric plume affects the propagation of MS waves remains unknown. Based on the ray tracing simulations, we show that MS waves generated outside the plasmapause and inside the plume can be reflected into the plasmaspheric core by the plasmaspheric plume and a few of the reflected waves can migrate globally inside the plasmaspheric core. Our findings provide a new potential way for MS waves to access and be trapped inside the plasmaspheric core. Key Points Propagation of magnetosonic waves in the magnetosphere containing a plasmaspheric plume is examined using the ray tracing simulations Plasmatrough and plume magnetosonic waves can be reflected into the plasmaspheric core by the plasmaspheric plume Those reflected magnetosonic waves can be quasi‐trapped inside the plasmaspheric core or propagate radially down to low altitudes

We present the first global geospace simulation to reproduce auroral giant undulations (GUs). To identify their magnetospheric drivers, we employ the MAGE (Multiscale Atmosphere‐Geospace Environment) model in a case study of a geomagnetic storm for which there were spacecraft‐ and ground‐based observations of GUs. The model reproduces the spatial and temporal scales of the GUs as well as the presence of duskside subauroral polarization streams (SAPS) and plasmapause undulations. Based on our modeling, we are able to identify the magnetospheric drivers of GUs as mesoscale ring current injections which, after drifting westward, create inverted regions of flux‐tube entropy (FTE) and subsequent interchange instability. Outward‐protruding interchange fingers disrupt shielding of the inner magnetosphere, creating longitudinally localized ripples in magnetospheric convection equatorward of the magnetospheric instability, which structure the plasmapause and duskside diffuse precipitation. While not causal, SAPS and plasmapause undulations are a consequence of the unstable magnetospheric configuration. Plain Language Summary The visually dazzling display of the aurora during active periods is caused primarily by the precipitation of energetic electrons from the magnetosphere into the ionosphere. The auroral oval plays host to a variety of morphological features, or auroral forms, that are a reflection of magnetospheric processes and therefore a powerful tool for understanding the cross‐scale processes that bind together different geospace domains. Unlocking that power, however, requires an understanding of how magnetospheric processes are reflected in the aurora. Despite decades of study, that understanding has remained elusive, primarily due to limited in situ measurements and uncertainty in the magnetic mapping connecting them to the ionosphere. Only recently have new global geospace models emerged that can provide this understanding. In this letter we identify the magnetospheric driver of auroral giant undulations (GUs), wave‐like trains of undulations that form on the equatorward edge of the diffuse aurora with typical spatial scales of 100 km. We show that GUs are the consequence of a “buoyancy imbalance” formed during the buildup of the ring current and the subsequent disruption of the ionospheric current systems that typically shield the inner magnetosphere. Key Points We present the first global geospace simulation to reproduce auroral giant undulations (GUs) Model shows GUs result from localized under‐shielding as a consequence of interchange instability during the buildup of the ring current Interchange‐unstable regions drive ripples in magnetospheric convection, structuring the plasmapause and duskside diffuse precipitation

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.

Hiss waves play a critical role in shaping Earth's radiation belts and mediating magnetosphere‐ionosphere energy transfer. Intense hiss emissions are frequently generated within dynamic plasmaspheric plumes through linear and nonlinear wave‐particle interactions. However, the contribution of plume hiss to the spatial distribution of hiss throughout the plasmasphere is not yet well quantified. In this study, we perform ray‐tracing simulations to investigate the global propagation of plume hiss under varying plume morphologies, including different widths and levels of density lumpiness. We find that most hiss power is confined near the local time sector of the plume. Narrower plumes with embedded density ducts significantly enhance earthward wave guidance into the plasmaspheric core, compared to wide, smooth plumes. Furthermore, a subset of rays guided azimuthally along the plasmapause can serve as seed waves for intense dayside hiss. Our results highlight the role of plume hiss in shaping the global‐scale distribution of hiss waves.