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Ultra‐low‐frequency (ULF) waves emerge as pivotal factors in elucidating the mechanisms that drive the intricate dynamics of radiation belt electrons within the plasmasphere and plasmaspheric plumes. Utilizing THEMIS data from September 2012 to September 2017, we conducted a comprehensive statistical analysis of Pc4‐5 ULF waves within and outside the plasmaspheric plume. Our findings reveal a distinctive dawn‐dusk asymmetry in occurrence rate and wave power of poloidal mode waves in the absence of the plume, resembling the toroidal mode asymmetry observed. Poloidal mode waves exhibit a higher likelihood of formation within the plume, while the toroidal mode waves show the opposite trend, contributing to the elevated dusk‐side occurrence rate of poloidal mode waves. Moreover, both wave modes within the plume demonstrate lower peak frequencies compared to waves outside the plume. The global distribution of wave power within the plume suggests higher power at noon than on the dusk side. Plain Language Summary: In this work, we report the statistical analysis of Pc4‐5 ULF waves both within and outside the plasmaspheric plume. We discovered a noticeable difference between the dawn and dusk sides in how often and how strongly poloidal and toroidal mode waves occur when the plasmaspheric plume is absent. Intriguingly, poloidal mode waves are more likely to form within the plume, while toroidal mode waves tend to do the opposite. This leads to a higher occurrence of poloidal mode waves on the dusk side. Additionally, both wave types within the plume have lower peak frequencies than those outside the plume. The power of both types of waves within the plasmaspheric plume, the toroidal and poloidal modes, is higher around noon compared to the dusk side. This indicates that the solar wind dynamic pressure and the plasmaspheric plume's connection with the flanks play a crucial role in creating and spreading ULF waves. Key Points: The Pc4‐5 ULF waves in the presence and absence of plasmaspheric plume at L ∼ 6–12 are statistically studied by THEMIS satellitesThe majority of waves within the plasmaspheric plume are poloidal waves, and the majority of waves outside the plasmaspheric plume are toroidal wavesThe plasmaspheric plume has a significant effect on the wave properties of poloidal mode waves and toroidal mode waves [ABSTRACT FROM AUTHOR]

The effects of plasma density on the plasmaspheric hiss amplitude and its spatial and temporal scale sizes are investigated using data from Van Allen Probes A and B. The plasmaspheric hiss amplitude and scale sizes are found to be more affected by the small‐scale density variations than the overall density profile of the plasmasphere. In detail, our results suggest that both the hiss wave amplitude and its scale sizes are (a) related to the total electron density when the density is less than ∼1,000 cm−3; (b) independent of the total electron density when the density is greater than ∼1,000 cm−3; and (c) better correlated with the detrended density (removing the background plasmasphere density profile from the total electron density) regardless of the density value. Key Points: Effects of plasma density on the spatial and temporal scale sizes of plasmaspheric hiss amplitude is statistically investigatedThe plasmaspheric hiss amplitude is better correlated with the detrended electron density than the total electron densityThe spatial and temporal scale sizes of plasmaspheric hiss are mainly related to that of the relatively small‐scale density variations [ABSTRACT FROM AUTHOR]

The geometrical structure of the Pc 5 compressional wave is important in judging its generation mechanism and the wave‐particle interaction process. In this work, 117 magnetic troughs (where magnetic field strength transiently decreases) identified from 50 Pc 5 compressional wave events in the duskside (15.5–18.5 local time) magnetosphere are studied based on the Magnetospheric Multiscale (MMS) data. We derived the three dimensional geometry of the magnetic trough by including the normal and velocity information at its boundaries from the multi‐spacecraft analysis method. The magnetic trough has a magnetic bottle shape along the magnetic field line with the most probable center (with weakest magnetic field) located at θ = 75° ${75}^{{}^{\circ}}$, while the widest part of the magnetic bottle located around θ=65° $={65}^{{}^{\circ}}$ (θ denotes the angle between spacecraft position vector and the ambient magnetic field). The cross section of the magnetic trough is eccentric and has a "wedge‐like" shape whose average open angle is ∼23° toward radial outward. It is found that the radial component of the current density is the dominant one at the boundaries, and the value is generally proportional to the depth of the magnetic trough. The generation of these Pc 5 compressional waves can be attributed to the drift Alfvén ballooning mirror instability. This work reveals the possible changes of magnetic field configuration caused by the Pc 5 compressional wave in the magnetosphere and may bring new ideas to the interaction way between wave field and ring current particles. Plain Language Summary: Ultra low frequency (Pc 5 band) compressional waves are considered to have an antisymmetric standing wave structure along the magnetic field lines in the outer magnetosphere. However, the complete picture of the wave structure has remained poorly understood because of the limited spatial coverage by spacecraft observations. Based on multi‐point observations by the four magnetospheric multiscale spacecraft during 2 months interval from September to October in 2015, we carry out a statistical analysis of the boundary features associated with observed magnetic trough signatures, which are the basic defining element of Pc5 compressional waves. In this study, we obtain a 3D geometry of the magnetic trough and find that the equatorial cross‐section of the magnetic trough has an 'eccentric wedge‐like' shape. These results are different from the conventional understandings, which predict an 'ellipsoidal' or 'cylindrical' shape for the mirror mode structure, as observed in the magnetosheath and/or solar wind. This work reveals the possible changes of magnetic field configuration caused by the Pc 5 compressional wave in the magnetosphere and may bring new ideas to the interaction way between wave field and ring current particles. Key Points: The 3D geometry of the Pc 5 compressional wave is revealed by a statistical study based on multispacecraft methods with MMS dataThe derived 3D geometry of the wave is characterized by a "wedge‐like" shape in the transverse section and a "bottle‐like" shape along the field lineThe statistical analysis provides evidence for the drift Alfvén ballooning mirror mode theory [ABSTRACT FROM AUTHOR]

Transpolar arcs (TPAs) are believed to predominantly occur under northward interplanetary magnetic field (IMF) conditions with their hemispheric asymmetry controlled by the Sun-Earth (radial) component of the IMF. In this study, we present observations of TPAs that appear in both the northern and southern hemispheres even during a prolonged interval of radially oriented IMF. The Defense Meteorological Satellite Program (DMSP) F16 and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed TPAs on the dawnside polar cap in both hemispheres (one TPA structure in the southern hemisphere and two in the northern hemisphere) during an interval of nearly earthward-oriented IMF on October 29, 2005. The southern hemisphere TPA and one of the northern hemisphere TPAs are associated with electron and ion precipitation and mostly sunward plasma flow (with shears) relative to their surroundings. Meanwhile, the other TPA in the northern hemisphere is associated with an electron-only precipitation and antisunward flow relative to its surroundings. Our observations indicate the following: (a) the TPA formation is not limited to northward IMF conditions; (b) the TPAs can be located on both closed field lines rooted in the polar cap of both hemispheres and open field lines connected to the northward field lines draped over one hemisphere of the magnetopause. We believe that the TPAs presented here are the result of both indirect and direct processes of solar wind energy transfer to the high-latitude ionosphere. [ABSTRACT FROM AUTHOR]

Plasmaspheric hiss is one of the most important plasma waves in the Earth's magnetosphere to contribute to radiation belt dynamics by pitch-angle scattering energetic electrons via wave-particle interactions. There is growing evidence that the temporal and spatial variability of wave-particle interactions are important factors in the construction of diffusion-based models of the radiation belts. Hiss amplitudes are thought to be coherent across large distances and on long timescales inside the plasmapause, which means that hiss can act on radiation belt electrons throughout their drift trajectories for many hours. In this study, we investigate both the spatial and temporal coherence of plasmaspheric hiss between the two Van Allen Probes from November 2012 to July 2019. We find ~3,264 events where we can determine the correlation of wave amplitudes as a function of both spatial distance and time lag in order to study the spatial and temporal coherence of plasmaspheric hiss. The statistical results show that both the spatial and temporal correlation of plasmaspheric hiss decrease with increasing L-shell, and become incoherent at L > ~4.5. Inside of L = ~4.5, we find that hiss is coherent to within a spatial extent of up to ~1,500 km and a time lag up to ~10 min. We find that the spatial and temporal coherence of plasmaspheric hiss does not depend strongly on the geomagnetic index (AL*) or magnetic local time. We discuss the ramifications of our results with relevance to understanding the global characteristics of plasmaspheric hiss waves and their role in radiation belt dynamics. [ABSTRACT FROM AUTHOR]

The terrestrial magnetosphere is perpetually exposed to and significantly deformed by the interplanetary magnetic field (IMF) in the solar wind. This deformation is typically detected at discrete locations by space‐ and ground‐based observations. Earth's aurora, on the other hand, is a globally distributed phenomenon that may be used to elucidate magnetospheric deformations caused by IMF variations, as well as plasma supply from the deformed magnetotail to the high‐latitude atmosphere. We report the utilization of an auroral form known as the transpolar arc (TPA) to diagnose the plasma dynamics of the globally deformed magnetosphere. Nine TPAs examined in this study have two types of a newly identified morphology, which are designated as "J"‐ and "L"‐shaped TPAs from their shapes and are shown to have antisymmetric morphologies in the Northern Hemisphere and Southern Hemisphere, depending on the IMF polarity. The TPA‐associated ionospheric current profiles suggest that electric currents flowing along the magnetic field lines (field‐aligned currents [FACs]), connecting the magnetotail and the ionosphere, may be related to the "J"‐ and "L"‐shaped TPA formations. The FACs can be generated by velocity shear between fast plasma flows associated with nightside magnetic reconnection and slower background magnetotail plasma flows. Complex large‐scale TPA FAC structures, previously unraveled by an magnetohydrodynamic (MHD) simulation, cannot be elucidated by our observations. However, our interpretation of TPA features in a global context facilitates the usage of TPA as a diagnostic tool to effectively remote sense globally deformed terrestrial and planetary magnetospheric processes in response to the IMF and solar wind plasma conditions. Plain Language Summary: In magnetospheric physics, the aurora is one of the most important phenomena in qualitatively and quantitatively understanding the transfer of plasma and energy from the solar wind to the high‐latitude atmosphere via terrestrial and other planetary magnetospheres. To understand the global picture of the plasma supply from the terrestrial magnetosphere, deformed by the interplanetary magnetic field (IMF) in the solar wind, to the auroral zone, the formation process of a new morphology of auroral transpolar arc (TPA) is investigated in this study. The source of these TPAs can be the electric currents flowing along magnetic field lines, induced by the plasma flows in the magnetosphere. The conventional TPA has a straight bar shape, which connects the nightside and dayside of the auroral oval. The new TPA morphologies, on the other hand, have significant "distortions" toward premidnight and postmidnight at their nightside ends, which may be caused by magnetic field line twisting and magnetosphere deformations due to the action of the IMF. Our results facilitate a paradigm shift in understanding the implications of TPA structure on global‐scale dynamics in the deformed magnetosphere, and as such, the usage of the auroral TPA shape as a tool to diagnose global‐scale magnetospheric effects. Key Points: A new morphological type of transpolar arc, characterized by large nightside distortions in the premidnight or postmidnight sector, is describedNightside reconnection and magnetotail deformation by IMF penetration play essential roles in the formation of nightside distorted TPANightside distorted TPAs can be used as a remote‐sensing tool to diagnose globally IMF‐deformed magnetospheric processes [ABSTRACT FROM AUTHOR]