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Abstract The drift motion of energetic charged particles can generate an azimuthal electric current around the planet known as the ring current, which regulates the field configuration of the magnetosphere. However, limited coverage of in-situ measurements makes it challenging to investigate the long-term variations of the global ring current. Taking advantage of the energetic neutral atom (ENA) imaging onboard the Cassini mission, we present a nearly 11-year cycle of the suprathermal ring current populations in Saturn’s magnetosphere. We find that the peak location of the suprathermal ring current in local time oscillates between post-midnight and pre-midnight sectors, and its intensity minimizes during the solar maximum. These results indicate that the modulation of the suprathermal ring current is closely related to the solar cycle. Our analysis also offers a preview of the ring current at other giant planets, such as Jupiter, which will be imaged by ENA cameras onboard the JUICE mission.

We present conjunctive observations to study the prompt responses of radiation belt electrons during the interplanetary shock (IPS) event on 7 September 2017. As the IPS impinged the Earth, the Time History of Events and Macroscale Interactions (THEMIS) E spacecraft located near the dayside bow shock observed alternating features of solar wind and magnetosheath, indicating that the magnetosphere was repeatedly compressed. Following each compression, rapid increases of relativistic electron fluxes and the corresponding drift echoes were well identified over the energy and pitch-angle spectra obtained by the Van Allen Probes (RBSP) in the inner magnetosphere. Meanwhile, oscillations in the Pc4 range are embedded in the flux variations, appearing as straight stripes in the pitch-angle distributions observed by RBSP-B inside the wave active region and “boomerang” stripes in the observations obtained by RBSP-A ∼6 MLT away. Such diverse signatures suggested an azimuthally confined ultralow frequency (ULF) wave. The patterns in the observed particle spectra agreed well with the theoretical predictions, by which we conclude that the surfing acceleration by the shock-induced pulse and the continuous modulation by the localized ULF wave conspired to cause significant disturbances to the Earth’s radiation belt.

Abstract The Shabansky orbit effect can exert a significant influence on the dynamics of particle motion. However, the north‐south asymmetry of the Shabansky orbits has received relatively little attention in previous research. This study investigates this asymmetry to elucidate the recently observed phenomenon of flux separation in the pitch angle distribution during drift echo events. The asymmetry in Shabansky orbits between the northern and southern hemispheres leads to two distinct drift periods for particles with specific equatorial pitch angles, resulting in the flux separation. To quantify this phenomenon, we develop a mathematical model for the magnetic field, enabling the generation of asymmetrical Shabansky orbits. The calculated drift periods for particles with different equatorial pitch angles corresponds with the characteristics of flux separation, providing evidence to support our hypothesis that the north‐south asymmetry of Shabansky orbits is responsible for flux separation.

Abstract Mars lacks a global magnetic field, and instead possesses small-scale crustal magnetic fields, making its magnetic environment fundamentally different from intrinsic magnetospheres like those of Earth or Saturn. Here we report the discovery of magnetospheric ion drift patterns, typical of intrinsic magnetospheres, at Mars using measurements from Mars Atmosphere and Volatile EvolutioN mission. Specifically, we observe wedge-like dispersion structures of hydrogen ions exhibiting butterfly-shaped distributions (pitch angle peaks at 22.5°−45° and 135°−157.5°) within the Martian crustal fields, a feature previously observed only in planetary-scale intrinsic magnetospheres. These dispersed structures are the results of drift motions that fundamentally resemble those observed in intrinsic magnetospheres. Our findings indicate that the Martian magnetosphere embodies an intermediate case where both the unmagnetized and magnetized ion behaviors could be observed because of the wide range of strengths and spatial scales of the crustal magnetic fields around Mars.

Abstract The cold ions, which are generally “invisible” to most instruments, have strong impacts on plasma wave and magnetic reconnection. Under particular situations, these cold ions could be accelerated and thus become detectable. In this study, we statistically investigated the properties of background cold ions based on Van Allen Probe observations. The cold ions could often be detected near the dusk sector, and a clear dawn‐dusk asymmetry is observed for all ion species with higher density at the dusk side, showing plasmaspheric plume‐like structures. Similar to the cold electrons, cold proton ions show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen increases, and the percentage of protons decreases as geomagnetic activity increases whereas the helium composition is generally small. Our results provide important information on ion compositions for the understanding of cold‐plasma dynamics in the inner magnetosphere.

In the space plasma environments, there are a variety of electromagnetic structures, including current sheets, magnetic holes and flux ropes, for which scales range from fluid down to ion or even electron kinetic scales. Our understanding of these structures, especially their electromagnetic field profiles and particle distributions, has been greatly improved due to the recent improvement in spacecraft technologies and enhancement in the observational database. To fully achieve the three-dimensional configurations of these structures, it is insufficient to use only the observations but also the theoretical models and numerical simulations. This paper briefly reviews the general framework of constructing theoretical models over these structures, which are self-consistent equilibrium solutions of Vlasov-Maxwell equations. In this framework, to ensure the satisfaction of the equilibrium Vlasov equation, particle distribution is expressed as a function of the particle invariant's motion. The moments of the particle distributions are then substituted into the Maxwell equations to solve the profiles of electromagnetic fields, which are further used to determine the particle phase space densities that can be compared directly to spacecraft observations. In recent years, this framework has been widely applied in the modeling and reconstruction of electromagnetic structures, demonstrating the ubiquitous presence of kinetic equilibrium structures in space plasmas. Moreover, the self-consistent models can be used as initial conditions of kinetic simulations. Finally, we discuss the open questions and future directions in studying kinetic structures in space plasmas.

One of the most important questions in space physics is how the energy of the solar wind is transmitted to energetic particles in the Earth's magnetosphere, part of which is in the form of ultra-low-frequency (ULF) electromagnetic waves in the mHz frequency range. More specifically, ULF waves can provide diagnostics of the magnetosphere. For example, ionospheric conductance and mass density structure can be derived; substorm onset can be timed; and geomagnetic field lines can be mapped. ULF waves can also modify the magnetosphere such as through nonlinear effects allowing Kelvin-Helmholtz surface wave energy at the magnetopause to penetrate into magnetosphere, and radial diffusion, which plays an essential role in flux enhancement and particle acceleration of the radiation belts. Hannes Alfvén first proposed the existence of transverse "electrohydrodynamic" waves in the magnetized plasma. The magnetohydrodynamic (MHD) theory was first applied to explain the observed geomagnetic pulsations in Earth's magnetosphere, which has been confirmed by spacecraft observations and ground-based magnetometers. ULF waves are usually categorized into poloidal and toroidal modes by different directions of the perturbed electromagnetic field. Magnetic line oscillations in the radial direction yields azimuthal electric fields (\begin{document}$ {E}_{\varphi } $\end{document}) and are referred to as poloidal waves, while the motion of field lines in the azimuthal direction yields electric field oscillations in the radial direction (\begin{document}$ {E}_{r} $\end{document}) which are referred to as toroidal waves. Efficient interaction between ULF waves and charged particles requires comparable periods of waves and particle's drift motion. An acceleration mechanism capable of a continuous energy exchange between ULF waves and charged particles is wave-particle drift resonance. When such resonance occurs, the azimuthal drift speed of a resonant particle matches the wave propagation speed, and the particle experiences a constant phase of the wave electric field. This process enables a sustained energy exchange between ULF waves and charged particles, which provides a major source of particle acceleration and diffusion in the Van Allen radiation belts. The conventional drift resonance theory assumes that the particle trajectories are unperturbed despite their energy gain or loss from ULF waves. This assumption is usually invalid in Earth's magnetosphere because ULF waves can have larger amplitudes and/or durations. The large wave-particle energy exchange can modify the particle trajectory and cause significant nonlinear effects. This paper mainly reviews the nonlinear drift resonance between poloidal/toroidal ULF waves and charged particles in the inner magnetosphere. The particle behavior can be described by a pendulum equation in poloidal ULF waves, with the nonlinear trapping frequency determined by the ULF wave amplitude. We further predict, based on the newly-developed theory, the observable signatures of nonlinear drift resonance such as rolled-up structures in the energy spectrum of particle fluxes. After considering how this manifests in particle data with finite energy resolution, we compare the predicted signatures with Van Allen Probes observations. Their good agreement provides the first observational identification of the nonlinear drift resonance, which highlights the importance of nonlinear effects in magnetospheric particle dynamics under ULF waves. Drift resonance between particles and toroidal ULF waves can occur even without the noon-midnight asymmetry of background magnetic field. This effect originates from the wave-carried compressional magnetic field oscillations, which turn out to play a key role in the energy exchange between toroidal ULF waves and charged particles. The resulting particle motion can be described by a modified pendulum equation with solutions depending on the wave number. These findings demonstrate that toroidal ULF waves, like their poloidal counterparts, play an important role in magnetospheric particle dynamics. This is significant because the new derivation allows a large body of existing work and understanding on the pendulum equation to be brought to bear on the problem of ULF wave-particle interactions in the inner magnetosphere. As particle detector technology improves, more nonlinear features will become observable, allowing further tests of the theory presented here.

The Jovian plasma sheet is a key region of the Jovian magnetosphere populated by a mix of warm and hot plasma. It is the main channel for radial transport of mass and energy in the Jovian magnetosphere and provides a favorable environment for magnetic reconnection and wave–particle interactions although the understanding of its plasma properties is incomplete. This study combines observations from the Jovian Auroral Distributions Experiment and Juno Energetic Particle Detector Instrument on board the Juno spacecraft during its first 31 orbits to analyze the plasma properties of the Jovian plasma sheet from 20 R _J to 100 R _J . Our results indicate that the plasma number density decreases from 1 cm ^−3 at 20 R _J to 0.005 cm ^−3 at 100 R _J , while the plasma pressure decreases from 2 nPa at 20 R _J to 0.02 nPa at 100 R _J . The plasma pressure inside the plasma sheet is comparable to the magnetic pressure in the lobe region, suggesting a rough balance between the two. In the plasma sheet with r > 70 R _J , the H ^+ density and pressure remain relatively constant, likely due to other plasma sources such as the solar wind. Additionally, we find that the pressure (density) ratios for heavy ions between the center and the edge of the plasma sheet are generally an order of magnitude, while that for H ^+ decreases with radial distance. These findings contribute to a more comprehensive understanding of the plasma properties of the Jovian plasma sheet.

The black aurora is a distinct phenomenon characterized by spatially well-defined regions where the diffuse auroral luminosity decreases notably. Typically, black auroras present as arcs moving at lower velocities, patches with higher moving speeds, and arc segments. However, the mechanism behind black auroras remains unclear. In this paper, we present a novel observation of a poleward-moving black auroral arc associated with impulse-excited field-line resonances in the dawnside sector from the multi-spacecraft THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission, equivalent ionospheric currents, and a conjugated all-sky imager. The field-line resonance velocities exhibit periodic vorticity, which correspond to periodic poleward-moving bands of enhanced FACs. Based on the relatively large reduction in luminosity, we conclude that the poleward-moving black auroral arc was most likely caused by downward FACs associated with field-line resonances.

The theory of equatorward moving east-west elongated auroral arcs associated with field line resonance (FLR) has been proposed for decades. However, confirming this theory requires in-situ observations of FLR within the magnetosphere and simultaneous all-sky imager observations of equatorward moving auroral arcs near satellite footpoints. In this study, we present the first observations of multiple equatorward moving auroral arcs related to impulse-excited FLR, using datasets from the WIND, Geotail satellites, and an all-sky imager at China’s Zhongshan Station (ZHS) in Antarctica. In the presented event, the ultra-low-frequency waves associated with solar wind dynamic pressure pulse was mainly toroidal mode, which is consistent with the theory that the toroidal mode waves usually related with external source. The all-sky imager located in Zhongshan station recorded several equatorward moving auroral arcs, followed by reverse propagating ones. The latitudinal width of the equatorward moving auroral arcs was on the order of 25 km and had an average equatorward propagation of ~0.37 km/s, which is very similar to the value from previous work. To better illustrate the observed evolution of auroral arcs related with the FLRs we proposed a simple model to evaluate the FACs induced by the FLRs in different latitudes. The latitudinal distribution evolution of FACs agrees well with the ground-based optical observations.

Kinetic-scale magnetic holes (KSMHs) are structures characterized by a significant magnetic depression with a length scale on the order of the proton gyroradius. These structures have been investigated in recent studies in near-Earth space, and found to be closely related to energy conversion and particle acceleration, wave-particle interactions, magnetic reconnection, and turbulence at the kinetic-scale. However, there are still several major issues of the KSMHs that need further study — including (a) the source of these structures (locally generated in near-Earth space, or carried by the solar wind), (b) the environmental conditions leading to their generation, and (c) their spatio-temporal characteristics. In this study, KSMHs in near-Earth space are investigated statistically using data from the Magnetospheric Multiscale mission. Approximately 200,000 events were observed from September 2015 to March 2020. Occurrence rates of such structures in the solar wind, magnetosheath, and magnetotail were obtained. We find that KSMHs occur in the magnetosheath at rates far above their occurrence in the solar wind. This indicates that most of the structures are generated locally in the magnetosheath, rather than advected with the solar wind. Moreover, KSMHs occur in the downstream region of the quasi-parallel shock at rates significantly higher than in the downstream region of the quasi-perpendicular shock, indicating a relationship with the turbulent plasma environment. Close to the magnetopause, we find that the depths of KSMHs decrease as their temporal-scale increases. We also find that the spatial-scales of the KSMHs near the subsolar magnetosheath are smaller than those in the flanks. Furthermore, their global distribution shows a significant dawn-dusk asymmetry (duskside dominating) in the magnetotail.

The Earth’s magnetospheric cusp, a region with an off-equatorial magnetic field minimum, is an important place which directly transports plasma and energy from the solar wind into the magnetosphere and ionosphere. Its magnetic topology and charged particles therein are known to respond to the solar wind and the interplanetary magnetic field. However, its dynamics in response to the interplanetary (IP) shock are still unknown, due to lack of direct spacecraft observations. This study first reports the observations of the cusp’s motion under the drive of an IP shock and both strong electric fields and outflowing energetic ions in the moving cusp. After an IP shock arrival on 7 September 2017, triple cusps were observed by Cluster C4 when it was crossing the high-altitude northern polar region to the sub-solar magnetosphere. The multiple cusps had a one-to-one correspondence with the dayside magnetosphere compression and relaxation detected by THEMIS E, indicating that one cusp moved back and forth three times due to the IP shock’s impact. In the moving cusp, there were strong impulsive electric fields with a peak of up to ∼40 mV/m and an ionospheric source population of upward propagating ions (O+, He+ and H+) with energies extending to MeV. However, the outflowing ions outside the cusp had energies of no more than 1 keV. An enhancement of energetic O+ appeared inside the cusp with the flux ratio of O+/H+ increasing from 10 keV to ∼ MeV, which implies the efficient acceleration of O+. These observations are shown to be consistent with the prompt acceleration by the impulsive electric fields, which is mass-dependent. This finding suggests a new acceleration mechanism for cusp energetic ions, especially for O+.

In many applications, such as the miniaturization and cooling of high-power electronics in aerospace, a new thermal management solution is needed, and metal foam radiators may be a valuable solution. In this work, X-ray scanning was applied to obtain the original structure of the metal foam. The real structure calculation model of the metal foam was obtained through a series of modeling, and high-precision numerical simulation was built to study heat and mass transfer in the original structure and homogeneous substitution model for three-dimensional porous metal foam. The distribution of velocity, pressure and temperature field is investigated. The results show that the heat transfer characteristics increase and flow resistance decreases with an increase in the Reynolds number. The heat transfer performance and flow resistance increase with the decrease of porosity. The porous media homogenization model can be consistent with the original real calculation results of metal foam by using appropriate values of resistance coefficient and porosity. The variation of resistance coefficient and porosity with the working condition in the porous homogenization model is identified.

Cluster satellites observed three successive outflowing ion beams on 28 March, 2001. It is generally accepted that these ion beams, composed of H+, He+, and O+ ions, with three inverted-V structures in their energy spectra, are produced by acceleration through U-shaped potential structures. By eliminating the background ion population and employing Maxwelling fitting, we find that ions coming from the center of the potential structure have higher temperature than those from the flanks. Higher temperature of O+ and He+ compared to that of H+ indicates that heavy ions are preferentially heated; we further infer that the heating efficiencies of O+ and He+ ions differ between the center and edges of the U-shaped potential structures. Estimation based on pitch angle observations shows that heating may also occur at an altitude above the upper boundary of the auroral acceleration region (AAR), where these beams are generally thought to be formed.

The purpose of this paper is to understand how low energy plasmaspheric electrons respond to ULF waves excited by interplanetary shocks impinging on magnetosphere. It is found that both energy and pitch angle dispersed plasmaspheric electrons with energy of a few eV to tens of eV can be generated simultaneously by the interplanetary shock. The subsequent period of successive dispersion signatures is around 40 s and is consistent with the ULF wave period (third harmonic). By tracing back the energy and pitch angle dispersion signatures, the position of the electron injection region is found to be off-equator at around –32° in the southern hemisphere. This can be explained as the result of injected electrons being accelerated by higher harmonic ULF waves (e.g. third harmonic) which carry a larger amplitude electric field off-equator. The dispersion signatures are due to the flux modulations (or accelerations) of " local” plasmaspheric electrons rather than electrons from the ionosphere. With the observed wave-borne large electric field excited by the interplanetary shock impact, the kinetic energy can increase to a maximum of 23 percent in one bouncing cycle for plasmaspheric electrons satisfying the drift-bounce resonance condition by taking account of both the corotating drift and bounce motion of the local plasmaspheric electron.

Electron resonant interaction with ultra-low-frequency (ULF) waves is considered to be a driver of electron radial transport in Earth's inner magnetosphere. Traditional concept of such interaction assumes the electron slow diffusive scattering by a broad-band ULF spectrum, but recent spacecraft observations reported a possibility for electrons to resonate nonlinearly with intense coherent ULF waves. This study proposes a theoretical model describing the main elements of such nonlinear resonant interactions. We implement a Hamiltonian approach to describe equatorial electron motion in a dipole magnetic field and electric ULF wavefield and demonstrate the ULF-electron interactions has two basic nonlinear resonant regimes: phase bunching and phase trapping. Finally, we discuss possible applications of the proposed theoretical approach and the importance of ULF-electron nonlinear resonances. [ABSTRACT FROM AUTHOR]