Abstract Plasmaspheric hiss plays an important role in radiation belt electron dynamics, and its excitation and propagation have long attracted attention. During a substorm, Van Allen Probe B observed the disappearance of plasmaspheric hiss at the magnetic dip, which was driven by the injection of energetic protons. The perpendicular (to the magnetic field) components of both the wave vector and Poynting vector were directed mainly radially outward. We analyzed the event from two perspectives: excitation and propagation. The growth rate of plasmaspheric hiss remained below the threshold both inside and outside the dip, indicating that the waves were not locally excited. Regarding propagation, theoretical calculations suggest that the observed whistler‐mode hiss waves were reflected by the magnetic dip in a broad frequency range. Our results indicate the important role that the magnetic structures play in the propagation of plasmaspheric hiss.

The banded "zebra-stripe" structures, widely observed in energy spectrograms of inner radiation belt electron fluxes, provide an effective diagnostic of large-scale electric-field perturbations. However, the generation mechanism and spatiotemporal distributions of such perturbed fields remain unclear. Using high-resolution data of electron fluxes from Macau Science Satellite-1 (MSS-1) within L < 2.5, we analyze the zebra-stripe observations to extract parameters of the electric-field perturbations, with emphasis on perturbation onset times across different L-shells. The results indicate that in some events, the electric-field perturbations at different L-shells does not occur simultaneously. A statistical survey further indicates that the field perturbations could propagate either inward or outward, with a speed of the order of 1 hr per Earth radius. In each event category, the directions of the perturbed electric fields always display a broad distribution, with a statistical enhancement toward local noon. These findings provide new constraints on the spatiotemporal structure of inner-belt electric fields and the mechanisms governing energetic-electron transport.

In regions of the Solar System distant from planetary magnetic fields, galactic cosmic rays (GCRs) have generally been assumed to be uniformly distributed over the Earth-Moon distance. However, our analysis of data from the LND (Lunar Lander Neutron and Dosimetry) experiment onboard the Chang'E-4 lander revealed a region of reduced GCR flux in the prenoon sector of the lunar orbit. Further investigation suggests the presence of an energetic particle cavity, formed by Earth's magnetic field acting as an obstacle to GCR propagation. This cavity indicates that the influence of Earth's magnetic field within the space environment extends unexpectedly up to and far beyond the lunar orbit. This finding offers the potential to avoid high radiation levels during future lunar exploration and deep-space missions.

In Saturn's magnetosphere, the inward transport of magnetic flux is largely carried by localized injection flux tubes filled with warm, tenuous plasma, although their inflow speeds and spatio-temporal properties remain poorly constrained. Here, we propose that these flux tubes can modify electron microsignatures, the small-scale, absorption-induced flux depletions downstream of Saturn's moons, by twisting them into zigzag-shaped structures in the energy-time spectrograms of energetic electrons. Using observations from the Cassini spacecraft, we identify zigzag-shaped microsignatures and reproduce their morphology through test-particle simulations based on this scenario. The inferred radial velocities at low -shells (L similar to 5) are on the order of 1 km/s, suggesting significant braking of injection flux tubes and consistent with the observed decline in their occurrence inside L similar to 7. The ability to resolve such low inflow velocities remotely from microsignature distortions offers a new diagnostic for constraining mass and flux circulation in the magnetospheres of giant planets.

Chorus waves play a critical role in the acceleration and loss of energetic electrons in the Earth’s inner magnetosphere. Based on the wave normal angle (θ), lower band (LB) chorus waves are commonly classified by the Gendrin angle (θg) into quasi-parallel (θ<θg) and oblique (θ>θg) LB chorus waves. In this study, we have statistically investigated the characteristics of quasi-parallel and oblique LB chorus waves based on observations by Van Allen Probe B. We found that quasi-parallel LB chorus waves exhibit pronounced day-night asymmetry in both spatial distribution and the variation of wave properties as a function of magnetic latitude (MLAT). They appear over a wide range of MLATs at high L shells on the dayside, whereas they are more confined on the nightside. In contrast, oblique LB chorus waves show no significant day-night asymmetry in either spatial distribution or latitudinal variations of wave properties. Moreover, our statistical results reveal that equatorward propagating quasi-parallel waves can extend to higher MLATs (up to MLAT=±20°) at higher L shells on the dayside, but are limited to within ±5° on the nightside. Oblique waves, by comparison, show restricted equatorward propagation within ∣MLAT∣<5° on both dayside and nightside. These findings suggest that the source region of quasi-parallel LB chorus waves could extend to higher MLATs at high L shells on the dayside.

Abstract Discrete energy bands of ion fluxes, typically organized in velocity with equal separations, have been frequently observed in Jupiter's magnetosphere either in association with Galilean moons or in regions far from their influence. Here, we focus on the latter type and propose that these bands are manifestations of bounce‐phase structuring, analogous to drift‐phase structuring responsible for the well‐known zebra‐stripe patterns. In our proposed framework, latitude‐dependent electric‐field perturbations interact with particles at different bounce phases, producing phase‐dependent energy modulations. As the particles continue to bounce at their respective bounce frequencies, which scale with velocity, these modulations naturally evolve into discrete, velocity‐ordered banded structures. We examine several potential sources of latitude‐dependent electric fields, including impulsive disturbances and wave‐related processes, and perform test‐particle simulations showing that the key observational features can be reproduced. These results support bounce‐phase structuring as a unifying interpretation of discrete bands, providing potential diagnostic perspectives on Jovian magnetospheric dynamics.

Abstract Wave‐particle resonance is a key mechanism for energy transfer in collisionless plasmas with important implications for wave excitation and particle acceleration. Cyclotron resonance occurs when a particle's gyromotion synchronizes with the wave fields, with their interactions usually describable by quasilinear diffusion or nonlinear trapping. The nonlinear wave‐particle energy transfer is facilitated by background inhomogeneities such as magnetic field gradients and/or frequency sweeping. When wave amplitudes become large, the particle's gyromotion can be strongly perturbed, altering the classical cyclotron resonance condition and giving rise to a nonlinear process known as anomalous resonance or anomalous trapping. This mechanism has been explored in idealized, uniform settings; however, its behavior in realistic inhomogeneous environments remains poorly understood. Here, we present in situ evidence of simultaneous cyclotron and anomalous resonances between electromagnetic ion cyclotron (EMIC) waves and charged particles in Earth's dayside magnetosphere. The local magnetic‐field gradient enables bidirectional wave‐particle energy transfer: high‐energy ions drive the waves through cyclotron resonance, while lower‐energy ions absorb energy from the same waves via anomalous resonance. We also identify signatures consistent with anomalous resonance of sub‐keV electrons, suggesting that efficient wave‐particle coupling may extend far below the classical cyclotron‐resonant energy. This interplay of resonances reveals an inhomogeneity‐driven pathway for energy redistribution across a broader energy range than previously recognized. These findings highlight the need to incorporate anomalous resonance into models of magnetospheric dynamics, especially in regions with strong inhomogeneities, and open new avenues for understanding energy transfer in complex plasma environments.

Abstract The coupling between inner‐belt energetic electron precipitation (EEP) and ionospheric disturbances at low‐to‐mid latitudes during extreme geomagnetic storms remains an unresolved aspect of magnetosphere‐ionosphere coupling. We investigated this coupling using high‐resolution observations from the medium‐energy electron spectrometer (MES) aboard the Macao Science Satellite‐1A (MSS‐1A), during the “Gannon Storm” of 10–11 May 2024 (minimum Dst nT; minimum SYM‐H nT), integrated with global vertical total electron content (VTEC) maps from the CAS Global Ionosphere Map (GIM) and magnetic field line mapping using IGRF‐13 and T96. MSS‐1A recorded intense flux enhancements (40–754 keV) penetrating to unusually low ‐shells during the storm main phase, with significant spectral hardening (power‐law index from 1.89 to 1.6), indicating deep injection and adiabatic transport into the inner magnetosphere and slot region. Concurrently, GIM observations revealed large‐scale VTEC restructuring, including storm‐enhanced density (SED) exceeding 80 TECU and poleward displacement of the equatorial ionization anomaly (EIA). While MSS‐1A confirms bounce loss cone filling at 55–507 keV, three independent lines of evidence—temporal precedence, global spatial scale, and multi‐hour persistence—demonstrate that the large‐scale VTEC response was driven by prompt penetration electric fields (PPEF) and the super‐fountain effect rather than direct EEP. Our findings show that, although EEP may enhance localized ionization in the E‐region under extreme storm conditions, its contribution to large‐scale VTEC perturbations is secondary to storm‐time electrodynamic forcing, remaining below the detection capability of current global ionospheric maps due to altitude decoupling from the F‐region and spatial‐temporal smoothing in the gridded data.

Abstract Electrons of several hundred keV in Saturn's ring current are important seed components of the radiation belt. In this study, we have statistically analyzed the spatial distribution of energetic electrons on the equatorial plane of the inner magnetosphere based on the Cassini in situ observations. We found for all energy channels, the peak position of energetic electron flux shifts from the midnight sector to the afternoon sector as L shell increases. At specific L shells, the transitional energy (Et), which separates the peaks of energetic electron flux in azimuthal direction, decreases as L shell increases and is consistent with the theoretical prediction of corotation drift resonant energy (ECDR). Further analysis indicates that the day‐night asymmetry of energetic electron flux is caused by the noon‐to‐midnight electric field, with its direction deviates from the noon‐midnight line. These findings advance our understanding of the energization mechanism of inward radial transport.

Abstract During substorms, magnetospheric convection and magnetotail current sheet disturbances are two critical processes, both driving ionospheric electrojets. Dusk‐side SMU and dawn‐side SML (SuperMAG Electrojet indices) enhancements reflect two‐cell convection. Using this relation, we categorize substorms with different convection properties near onset. When primary SML enhancements occur near midnight, the presence or absence of dusk‐side SMU/dawn‐side SML activity distinguishes strong from weak convection events. Additionally, we identify “dawn‐dominant” events characterized by primary SML enhancements in the dawn sector as a separate category. “Strong convection” events feature stronger and longer southward IMF and faster solar wind speeds than “weak convection” events. “Dawn‐dominant” events share similar conditions with the “strong convection” category plus a tendency of negative B y . “Strong convection” events statistically have lower onset magnetic latitudes (MLAT), indicating current sheet instabilities closer to the Earth. Field‐aligned current (FAC) evolution reveals that the two‐cell convection pattern extends from dayside to nightside before onset, possibly contributing to flux depletion and current sheet thinning. An event with strong convection exhibits continued pressure loading, with unloading occurring only after onset. This indicates a significant contribution of open flux accumulation to current sheet thinning. The example dawn‐dominant event has strong global convection dominating the SMU/L profile, and aurora are brightened in a wide MLT range. On top of this, localized bursts in SML, aurora, dipolarizations, and flux ropes suggest that meso‐scale current sheet disturbances occur. These classifications and analyses help clarify how convection and current sheet disturbances develop and interact, potentially valuable for elucidating the descriptions of substorm processes.

Studies have long suggested that shocks can undergo cyclical self-reformation as a type of shock nonstationarity. Until now, providing solid evidence for shock reformation in spacecraft observation and identifying its generating mechanisms remain challenging. In this Letter, by analyzing magnetospheric multiscale observations, we unambiguously identified shock reformation occurring in a quasiperpendicular shock. A 2D PIC simulation reproduces and explains the observed shock reformation. The result reveals that the shock reformation is driven by the ion-scale whistler waves, manifested in two distinct types: whistler precursor and modified two-stream instability whistlers. An interesting finding is that the two mechanisms compete and dominate the reformation at early and later stages, respectively. Our results not only provide evidence of ion-scale whistlers as mechanism driving shock reformation, but also demonstrate the detailed kinetic processes how it happens, offering valuable insights into shock dynamics.

Abstract Geomagnetic substorms are a major path for energy storage and release in the magnetosphere, which typically occur in the midnight sector with a strong enhancement of westward electrojets. However, by analyzing 37,940 events from a substorm list, about 3% of events unexpectedly initiated in the noon sector. These dayside events occur more frequently during northern‐hemisphere summer and solar maximum years, and are predominantly associated with a strongly negative IMF By component, accompanied by a IMF southward turning shortly before enhanced westward electrojet. During these events, both westward electrojets and downward Region‐0 Field‐Aligned Currents (FACs) intensify near ∼80° MLAT on the dayside within 10 min after onset, with westward electrojets gradually drifting to the dawnside. We propose that these events are triggered by dayside reconnection during southward IMF conditions at high latitudes, thereby enhancing the convection electric field, downward FACs, and the westward electrojets.

Abstract Although cross‐scale wave‐particle interactions play a critical role in energy transfer in the magnetosphere, the influence of geomagnetic activity on the physical mechanisms involved remains unclear. In this study, we investigate this question by analyzing Van Allen Probes‐A observations of an event occurring on 25 July 2016. Ultra‐low frequency (ULF) waves, chorus waves, and enhancements of electron fluxes with substorm‐induced energy‐dispersive characteristics were detected simultaneously during this event. Cross‐wavelet analysis suggests that the chorus intensity is modulated by ULF waves. The frequency of chorus elements decreases concurrently with substorm injections. We perform linear instability analysis to interpret the frequency drop. It is found that substorm‐injected electrons with energies of tens to ∼200 keV have reduced perpendicular anisotropy, thereby changing the energy range responsible for chorus growth and modifying the generation frequency. Our results enhance knowledge of cross‐scale wave‐particle interactions and their roles in auroral evolution during substorm processes.

Substorms are often described by a loading-unloading cycle, where onset follows gradual accumulation of solar wind magnetic flux in the magnetosphere. Yet observations indicate that intense substorms can also be directly driven, though the underlying mechanism remains unresolved. For the first time, global observations strongly indicate that substorm triggering is linked to enhanced dayside-driven convection and Region 1 FAC, supported by simulations. At 17:17UT during the May 2024 superstorm, a shock-compressed southward interplanetary magnetic field enhanced sunward convection and auroral currents. These rapidly extended to the nightside, initiating substorm expansion within 6 min. Simulations reproduce this response, revealing that dayside-driven convection of closed field lines depleted nightside flux and thinned the current sheet. This lowered onset threshold and triggered substorm expansion with negligible flux loading. Following onset, nightside flux loading became significant as a reconnection X-line formed near 10 Earth radii, extended azimuthally, and supported a global substorm current wedge.

Abstract Energetic electron spectra in Earth's inner radiation belt often exhibit regular stripe‐like features, known as “zebra stripes,” which are typically attributed to the drift motion of stably‐trapped electrons disturbed by electric field perturbations. Using high‐quality electron measurements from the Macao Science Satellite‐1, we report the first observation of zebra stripe structures in quasi‐trapped electrons within the inner radiation belt. Through a combination of data analysis and test particle simulations, we show that the formation of quasi‐trapped stripes follows a two‐step radial transport driven by dual‐pulse electric fields. The first electric field generates zebra stripes in the stably‐trapped region, while the second moves them further inward into the quasi‐trapped zone, where they are identified as quasi‐trapped zebra stripes. These findings offer new insights into utilizing electron observations to diagnose the morphology and temporal profile of large‐scale electric fields within the inner radiation belt.

Abstract Geomagnetic disturbances in polar regions can be decomposed into two main patterns: the Disturbance Polar (DP) 1 and 2. How their relative contribution reshapes global current system remains unclear. In this study, we classified substorms from 2010 to 2022 into weak and strong DP‐2 events based on the ratio of the eastward to westward auroral electrojets variations. Auroral electrojets, ring current, and field‐aligned currents (FACs) were analyzed via superposed epoch. The FAC enhancements after substorm onset are found primarily around midnight (dusk) for weak (strong) DP‐2 events. Furthermore, dawn‐dusk asymmetries in ring current variations and an additional duskside SME index peak indicate the potential formation of additional current wedges under strong DP‐2. These findings demonstrate that DP‐2 currents could reshape the current topology during substorms, which may lead to a change of global current topology with different DP‐1/2 relative contribution.

Anthropogenic radio signals substantially reshape Earth's inner radiation belt by scouring away energetic electrons that pose persistent risks to satellites and astronauts. While the potential for active radiation belt remediation is recognized, existing paradigms largely treat it as a "point-source" effect centered on a single powerful transmitter in Australia. The collective impact of globally distributed transmitter networks has remained obscure. Here we show that a cluster of moderate-power transmitters across the European sector acts as a synergistic source, driving significant electron loss. Using high-resolution particle measurements from the Macao Science Satellite-1 and Fengyun-3E missions, we report the first direct identification of structured precipitation features unambiguously induced by the European cluster. These structures—visible only when Europe is on the nightside—separate into discrete bands corresponding to distinct frequency groups, revealing a coordinated multi-frequency driver that dramatically expands the electron scavenging scope beyond the limitation of single-station effects. Our analysis demonstrates that this regional cluster collectively rivals the clearing efficiency of the most powerful individual facilities. This discovery shifts the paradigm from point-source interactions to coordinated regional effects, providing the missing link to a global phenomenon. Together, our results suggest that existing radio infrastructure can be orchestrated as a tunable, planetary-scale network capable of remodeling the inner radiation belt, representing a decisive advance in our perspective on actively mitigating radiation hazards near Earth.

Resonant interactions between ultra-low-frequency (ULF) waves and particles are critical for energy transfer in the Earth's magnetosphere, and have been extensively investigated in the inner magnetosphere. However, resonant processes between ULF waves and electrons in the dayside outer magnetosphere remain largely unexplored. Using MMS data, we investigate their bounce resonance in the dayside outer magnetosphere. Quasi-periodic oscillations of electron fluxes below 1 keV and ULF waves with periods comparable to the electron bounce periods are observed simultaneously. Test-particle simulations demonstrate that off-equatorial magnetic field minima can increase the number of resonant pitch angles for electrons at a given energy. Overlap of stripes corresponding to different resonant energies gives rise to observed 360 phase shifts. Furthermore, off-equatorial minima and temporal evolution of ULF waves broaden the conventional bounce resonance condition , which links the harmonic type of ULF waves to the parity of .

Auroral and energetic electron precipitation (EEP) are key diagnostics for the coupling between the solar wind, magnetosphere, and ionosphere. We report coordinated observations of structured, discontinuous EEP near the outer radiation belt boundary (oRB) using FY-3E and DMSP measurements. These events are characterized by latitudinally separated precipitations for electrons up to hundreds of keV and spatial coincidence between EEP and localized auroral structures (double ovals, transpolar arcs, diffuse aurora superposed with discrete filaments, etc.). Two principal scattering pathways for EEPs are identified: (a) field-line curvature scattering (FLCS) in the locally stretched plasma sheet (PS); and (b) wave-particle interaction (WPI), for example, by whistler waves. Statistics over 114 events show that the discontinuous EEP and auroral precipitations can result from all types of mechanisms mentioned above. Since EEPs are confined to closed field lines, their co-existence indicates the closed field line topology. Main auroral ovals lie roughly along the oRB, while poleward auroral structures mostly occur within 10 degrees MLAT from oRB. In a discrete arc event, the FLCS-driven EEP shows a distinct flux-energy profile from the monoenergetic auroral electrons. The observations suggest a plausible mechanism where localized structures in the equatorial plane both stretch field lines to scatter energetic PS electrons and generate parallel potential to accelerate auroral electrons. In a diffuse aurora event, the diffuse flux-energy profiles extend from auroral to FY energies (1-100 s keV), suggesting broadband scattering by waves. The coordinated observations of correlation between discontinuous energetic and auroral electron precipitations help reveal the mesoscale magnetosphere-ionosphere coupling along field lines.

For the measurement of neutron in the complex radiation environment of low Earth orbit (LEO), a neutron spectrometer (NS) incorporating background suppression techniques was constructed and launched on board the “Weiming-1” CubeSat in January 2024. The NS is based on silicon solid-state detectors with a high anti-coincidence efficiency for background charged particles, which can enhance the precision of fast neutron flux quantification. This study comprehensively details the energy calibration, efficiency calibration, and stability test of the NS, through a series of controlled particle sources, including monoenergetic fast neutron beams, a thermal neutron field, and gamma-ray radioactive sources. The experimental results are well reproduced by Geant4 simulation. Unfolding results of monoenergetic neutron spectra and preliminary analysis of on-orbit data are presented as well.