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A localized Pc4‐5 ultralow‐frequency (ULF) wave event associated with a plasmaspheric plume was observed by THEMIS‐E on the dawnside near L= 6, which was identified as a second harmonic poloidal wave. The plume was identified as a sudden density enhancement during an outbound pass. The charged particle populations in the plume have a variety of periodic modulation characteristics at different energies. First, there is an antiphase relationship between magnetic field Brand particle flux across a wide energy range both for ions and electrons (~50 keV to 1 MeV). Second, there is a 180° phase shift in the modulated ion flux within an energy range of ~2–6 keV, with negative slope dispersions of ion pitch angle distributions at ~2–6 keV and ~50–75 keV, which are characteristic of drift‐bounce resonances. Third, the lower‐energy (<32 eV) ion flux is modulated at double the wave frequency, which are the result of E× Beffect. Considering the generation mechanism of this poloidal mode wave within the plume, we find that it is likely generated by drift‐bounce resonance from an unstable population of ions, due to an inward radial phase space density gradient. We suggest that the localization of waves to the plume is because the high plasma density reduces the local poloidal mode eigenfrequency, enabling a match to the drift‐bounce frequencies of these ions, and resonant energy transfer from these particles to the eigenfunction at this location. This generates the Pc4‐5 second harmonic poloidal waves at a much lower Lregion than would otherwise be expected. The high‐mpoloidal waves in the dawnside plume were observed by THEMIS‐EThe inward radial gradient of ~57 keV and higher plasma density enable the second harmonic poloidal wave to occur in the dawnside plumeThree types of particle flux modulations are observed simultaneously along with the wave in different energy bands

The separatrix region is the region between the separatrix and the reconnection jet. Due to the E×Bdrift and velocity filter effect in which high‐energy particles with high parallel speed can be seen prior to low‐energy particles along the field line, electrons are separated from ions. The electron dynamics in this region is of interest; however it has not been studied in detail, because of the insufficient resolution of plasma data. We present a slow separatrix crossing event observed by Magnetospheric Multiscale (MMS) satellite constellation on 1 January 2016, from the magnetosheath side with high‐resolution burst mode data. The electron edge and ion edge are clearly distinguished in the separatrix region. Two types of electron dispersion, one with a short duration (~0.3 s) and the other with a longer duration (~13 s) were detected between the electron and ion edges. The rapid dispersion (with small time scale) is mainly in the parallel direction, which might originate from a thin layer with non‐frozen‐in electrons close to the separatrix. The gradual (long time scale) dispersion is seen from parallel to perpendicular directions, which comes from the E×Bdrift of a curved D‐shape distribution of electrons. The width of the electron diffusion region on the magnetosheath side is estimated based on MMS observation. The observation also reveals an unexpected parallel electron beam outside of the electron edge. Wave‐particle interaction or parallel potential in the inflow region may be responsible for the generation of this electron population. Magnetic reconnection is a process commonly occurred at the Sun, the dayside magnetopause, and in the magnetotail that changes magnetic topology, and converts electromagnetic energy into plasma kinetic and thermal energies. Due to the different parallel velocities of the particles and field line convection, electrons are separated from ions, and this creates the electron edge and ion edge in the separatrix region. These separations downstream of the X‐line widen the distance in the convection direction between the field lines involved in the reconnection. This, in turn, makes it easier for the satellite to observe the changes in the magnetic field topology and electronic dynamics during magnetic reconnection. However, high‐resolution satellite data are still required for the observation of the separatrix region in the downstream. The unprecedented high temporal and spatial resolution measurements of the Magnetospheric Multiscale (MMS) mission provide a unique opportunity to perform such studies. By analyzing the magnetic field and plasma data from MMS, this study has revealed the field line topology and electron dynamics between the electron and ion edges. Possible generation mechanisms based on the electron dynamics are also discussed in this paper. Rapid dispersion is observed at the electron edge, indicating that electrons are decoupled from the magnetic field close to separatrixThe width of the electron diffusion region on the magnetosheath side is estimated using in situ observationsA parallel electron beam is observed outside of the separatrix in the magnetosheath for the first time

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. 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 Effects of plasma density on the spatial and temporal scale sizes of plasmaspheric hiss amplitude is statistically investigated The plasmaspheric hiss amplitude is better correlated with the detrended electron density than the total electron density The spatial and temporal scale sizes of plasmaspheric hiss are mainly related to that of the relatively small‐scale density variations

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high‐speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin‐Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics. ULF wave power is systematically characterized as discrete or broadband, and the azimuthal wave number of discrete ULF waves is determinedStorm time discrete ULF waves can couple with the core radiation belt electron population via the drift resonanceStorm time ULF waves are the result of both external (Kelvin‐Helmholtz) and internal (plasma instabilities) generation mechanisms

During geomagnetic substorms, around 1015J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( X≾−9RE) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( X≿−9RE). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time. Lobe magnetic energy density increases prior to substorms and decreases following onsetThe change in magnetic energy density is first seen within 9 REand then propagates downtailOn average, the decrease in near‐tail energy density is observed before the ground onset

Typically, ultra‐low frequency (ULF) waves have historically been invoked for radial diffusive transport leading to acceleration and loss of outer radiation belt electrons. At higher frequencies, very low frequency waves are generally thought to provide a mechanism for localized acceleration and loss through precipitation into the ionosphere of radiation belt electrons. In this study we present a new mechanism for electron loss through precipitation into the ionosphere due to a direct modulation of the loss cone via localized compressional ULF waves. We present a case study of compressional wave activity in tandem with riometer and balloon‐borne electron precipitation across keV‐MeV energies to demonstrate that the experimental measurements can be explained by our new enhanced loss cone mechanism. Observational evidence is presented demonstrating that modulation of the equatorial loss cone can occur via localized compressional wave activity, which greatly exceeds the change in pitch angle through conservation of the first and second adiabatic invariants. The precipitation response can be a complex interplay between electron energy, the localization of the waves, the shape of the phase space density profile at low pitch angles, ionospheric decay time scales, and the time dependence of the electron source; we show that two pivotal components not usually considered are localized ULF wave fields and ionospheric decay time scales. We conclude that enhanced precipitation driven by compressional ULF wave modulation of the loss cone is a viable candidate for direct precipitation of radiation belt electrons without any additional requirement for gyroresonant wave‐particle interaction. Additional mechanisms would be complementary and additive in providing means to precipitate electrons from the radiation belts during storm times. We detail a new mechanism for direct modulation of electron precipitation via localized compressional wavesElectrons encountering a time‐varying and spatially localized ULF wave can break the third invariantThis localized mechanism has not previously been considered and may be important for radiation belt losses

The cold‐dense plasma sheet (CDPS) plays an important role in the entry process of the solar wind plasma into the magnetosphere. Investigating the seasonal variation of CDPS occurrences will help us better understand the long‐term variation of plasma exchange between the solar wind and magnetosphere, but any seasonal variation of CDPS occurrences has not yet been reported in the literature. In this paper, we investigate the seasonal variation of the occurrence rate of CDPS using Geotail data from 1996 to 2015 and find a semiannual variation of the CDPS occurrences. Given the higher probability of solar wind entry under stronger northward interplanetary magnetic field (IMF) conditions, 20 years of IMF data (1996–2015) are used to investigate the seasonal variation of IMF Bzunder northward IMF conditions. We find a semiannual variation of IMF Bz, which is consistent with the Russell‐McPherron (R‐M) effect. We therefore suggest that the semiannual variation of CDPS may be related to the R‐M effect. We present the spatial distribution of plasma sheet temperature and density in GSM X‐Yplane using 21 years of in situ Geotail dataCold‐dense plasma sheet occurrence rates have a semiannual variation, in keeping with the semiannual variation of the IMF + BzThe Russell‐McPherron effect may therefore control these variations and hence modulate solar wind entry into the magnetosphere

We conduct a superposed epoch analysis of Birkeland current densities from AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) using isolated substorm expansion phase onsets identified by an independently derived data set. In order to evaluate whether R1 and R2 currents contribute to the substorm current wedge, we rotate global maps of Birkeland currents into a common coordinate system centered on the magnetic local time of substorm onset. When the latitude of substorm is taken into account, it is clear that both R1 and R2 current systems play a role in substorm onset, contrary to previous studies which found that R2 current did not contribute. The latitude of substorm onset is colocated with the interface between R1 and R2 currents, allowing us to infer that R1 current closes just tailward and R2 current closes just earthward of the associated current disruption in the tail. AMPERE is the first data set to give near‐instantaneous measurements of Birkeland current across the whole polar cap, and this study addresses apparent discrepancies in previous studies which have used AMPERE to examine the morphology of the substorm current wedge. Finally, we present evidence for an extremely localized reduction in current density immediately prior to substorm onset, and we interpret this as the first statistical signature of auroral dimming in Birkeland current. The latitude of the substorm onset is colocated with the interface between the R1 and R2 systemsBoth Region 1 (R1) and Region 2 (R2) current densities enhance at substorm onsetUpward current density decreases prior to onset in a localized area, consistent with auroral dimming

Wave‐particle interactions play a crucial role in energetic particle dynamics in the Earth's radiation belts. However, the relative importance of different wave modes in these dynamics is poorly understood. Typically, this is assessed during geomagnetic storms using statistically averaged empirical wave models as a function of geomagnetic activity in advanced radiation belt simulations. However, statistical averages poorly characterize extreme events such as geomagnetic storms in that storm‐time ultralow frequency wave power is typically larger than that derived over a solar cycle and Kpis a poor proxy for storm‐time wave power. Storm‐time wave power varies significantly and is typically larger than that of empirical models derived from extended data setsGeomagnetic activity indices are a poor proxy for storm‐time wave powerMore accurate empirical models of wave power are required

We present a detailed ground‐based and in situ examination of two substorms on 9 April 2011 providing one of the most detailed observations of a series of activations to date. Using auroral observations we demonstrate that the initial signature of substorm onset is a localized brightening on closed field lines. In both cases auroral onset precedes any geosynchronous indications of substorm onset. Despite excellent in situ coverage of the near‐midnight plasma sheet there is no evidence of tail flows or topological changes which might indicate the initiation of reconnection and related disturbances prior to auroral onset. Similarly, no auroral streamers are observed prior to either onset. For the second substorm, following auroral onset and dipolarization of the geosynchronous field there is evidence of a rapid thinning of the plasma sheet followed by a secondary auroral activation and dipolarization of the magnetotail outside of geosynchronous orbit. This morphological change suggests that near‐Earth neutral line reconnection is eventually triggered in the tail, but only following the earlier auroral onset. These observations demonstrate that expansion phase onset is not always initiated by reconnection in the tail. Finally, our observations suggest that activations at the near‐Earth neutral line, or related to plasma instabilities and near‐Earth onset, can develop independently during expansion phase onset. In this paradigm, free energy stored during the growth phase can be stored in the stretched tail and the inner magnetosphere where the dipole field begins to stretch. During expansion phase onset either region could become unstable, independent of the other, leading to substorm initiation and onset. Substorms are initiated in the inner magnetosphereAuroral onset precedes magnetic reconnectionAuroral streamers are not a necessary condition of substorm onset

Chorus waves outside the plasmapause influence the Earth's radiation belt dynamics by interacting with energetic electrons via cyclotron and Landau resonance. Recent numerical diffusion experiments indicate that the diffusion process is sensitive to the spatial and temporal scale of variability in the wave‐particle interaction, which is reported to be more efficient than that based on the traditional average model. Using Van Allen Probes A and B data from November 2012 to July 2019, the spatial and temporal scale size of chorus waves are calculated by the correlation between the wave amplitudes detected by two satellites with varying spatial separation or time lag. We found that, the chorus wave is incoherent when the spatial extent is greater than 433 km or the time lag lasts ∼10 s, which are significantly smaller than that of plasmaspheric hiss. In addition, the spatial correlations of chorus tend to be higher near noon or with lower geomagnetic activity. The temporal correlations of chorus are always statistically near zero, which are not influenced by the location and geomagnetic activity. Our results can help refine the model of the interactions between energetic particles and chorus waves in the radiation belt. Chorus waves tend to be incoherent when the spatial extent is greater than 433 km or the time lag lasts 10 s at <∼6.5The spatial scale size of chorus wave is relatively larger near noon or with lower geomagnetic activityThe temporal scale size of chorus wave is always statistically near zero, not influenced by L‐shell, magnetic local time, or AL* index Chorus waves tend to be incoherent when the spatial extent is greater than 433 km or the time lag lasts 10 s at <∼6.5 The spatial scale size of chorus wave is relatively larger near noon or with lower geomagnetic activity The temporal scale size of chorus wave is always statistically near zero, not influenced by L‐shell, magnetic local time, or AL* index

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. Defense Meteorological Satellite Program (DMSP) and Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed transpolar arcs (TPAs) in both hemispheres even during a prolonged radial interplanetary magnetic field (IMF) intervalThe TPAs can be categorized into two different types according to the precipitating particles and plasma drift velocityTPA formation is not limited to northward IMF conditions, and TPAs can occur on both closed and open field lines Defense Meteorological Satellite Program (DMSP) and Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed transpolar arcs (TPAs) in both hemispheres even during a prolonged radial interplanetary magnetic field (IMF) interval The TPAs can be categorized into two different types according to the precipitating particles and plasma drift velocity TPA formation is not limited to northward IMF conditions, and TPAs can occur on both closed and open field lines

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW Pdyn) sudden increases, which are critical for understanding solar wind‐magnetosphere‐ionosphere coupling. We focus on two similar SW Pdynpulse events that occurred on January 24, 2012 and November 12, 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about 10 min after the pressure pulse arrival, with a counter‐clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground‐based All‐Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side and diminished on the dawn side. These observations provide direct evidence of the scenario proposed by Shi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry field‐aligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field‐aligned currents into/out of the ionosphere. Earth's magnetosphere and ionosphere are strongly affected by variations of the solar wind, especially the variation of solar wind dynamic pressure (SW Pdyn). A SW Pdynincrease/decrease leads to the compression/expansion of the Earth's magnetosphere. In response, field‐aligned currents, which are carried by precipitating or escaping plasma particles, are generated in the magnetosphere, and in turn lead to variations in the auroral intensity. In previous studies of events with similar variations of SW Pdyn, cases of both auroral brightening and dimming have been observed. Therefore, it has not been clear whether the aurorae become darker or brighter after SW Pdynincrease/decrease events. In this article, we clarify the relationship between the auroral intensity variation and positive SW Pdynpulses. We show that auroral brightening on the dusk side and dimming on dawn side during SW Pdynincrease events are due to oppositely directed field‐aligned currents in these regions. These are generated by oppositely rotating flow vortices on the dusk and dawn flanks of the magnetosphere, which are both produced by the passage of the SW Pdynpulse. Aurora and vortex on dawn/dusk side in response to a positive solar wind dynamic pressure (SW Pdyn) pulse are reportedPositive SW Pdynpulses generate current vortices with an opposite direction on the dawn and dusk side ionosphere, respectivelyAurorae dimmed/brightened after the appearance of the clockwise/counterclockwise current vortices Aurora and vortex on dawn/dusk side in response to a positive solar wind dynamic pressure (SW Pdyn) pulse are reported Positive SW Pdynpulses generate current vortices with an opposite direction on the dawn and dusk side ionosphere, respectively Aurorae dimmed/brightened after the appearance of the clockwise/counterclockwise current vortices

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. Plasmaspheric hiss waves are more spatially and temporally coherent at low‐L, and are incoherent where L> ∼4.5Plasmaspheric hiss becomes incoherent when spacecraft separation is greater than 0.23 RE or ∼10 min inside of L∼4.5Our results show that the timescale and spatial scales of plasmaspheric hiss are significantly less than previously thought Plasmaspheric hiss waves are more spatially and temporally coherent at low‐L, and are incoherent where L> ∼4.5 Plasmaspheric hiss becomes incoherent when spacecraft separation is greater than 0.23 RE or ∼10 min inside of L∼4.5 Our results show that the timescale and spatial scales of plasmaspheric hiss are significantly less than previously thought