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On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

Theoretical analysis has revealed a specific resonance that shares the same condition as Landau resonance, but instead involves wave electromagnetic fields rather than traditionally electrostatic fields. While this resonance, referred to as electromagnetic Landau resonance due to its properties, is considered significant for magnetospheric dynamics, rare reports or evaluations based on observations have been made thus far. Here, we present an event detected by the Magnetospheric Multiscale mission near the dayside magnetopause. During this event, ∼748‐eV protons are observed to be in resonance with a wave. Detailed data analysis demonstrates the resonant velocity closely matches the wave's parallel phase speed, which, combined with the significant work done by wave perpendicular electric field, confirms this interaction as electromagnetic Landau resonance. Further investigation indicates these protons are being secularly accelerated within this resonance. Consequently, our observations provide the first empirical evidence supporting the previously suggested theoretical importance of the electromagnetic Landau resonance. Plain Language Summary: The electromagnetic Landau resonance shares the same resonance condition as the normal Landau resonance, but differs in that it involves the electromagnetic components of wave fields instead of the electrostatic components found in the case of the latter. Although it has been examined in theories and confirmed to occur in space plasmas experimentally, this resonance, especially the accompanying evolution of particle velocity distributions, has yet to be evaluated quantitatively through spacecraft observations. In this paper, we report the first observation of this resonance obtained by the Magnetospheric Multiscale mission near the dayside magnetosphere. In the reported case, protons of ∼400–1,000 eV are found to be in resonance with a wave. A more detailed examination of the observed fields and protons shows the corresponding resonant velocity is nearly identical to the wave's parallel phase speed. This observation, combined with the significant work done by wave perpendicular electric field, conclusively confirms the observed interaction as the electromagnetic Landau resonance. Further investigation indicates the protons are secularly accelerated by the wave via this resonance. Hence, our observations provide empirical evidence supporting the previously suggested theoretical significance of the electromagnetic Landau resonance. Key Points: MMS observations of electromagnetic Landau resonance near the dayside magnetopause are presentedThis resonance follows the same condition as the Landau resonance, but modulates particles via the electromagnetic components of wave fieldsAnalysis of the observed fields and protons provide conclusive evidence for this resonance accelerating protons [ABSTRACT FROM AUTHOR]

Field Line Resonances (FLRs) are a critical component in Earth's magnetospheric dynamics, associated with the transfer of energy between Ultra Low Frequency waves and local plasma populations. In this study we investigate how the polarisation of FLRs are impacted by cold plasma density distributions during geomagnetic storms. We present an analysis of Van Allen Probe A observations, where the spacecraft traversed a storm time plasmaspheric plume. We show that the polarisation of the FLR is significantly altered at the sharp azimuthal density gradient of the plume boundary, where the polarisation is intermediate with significant poloidal and toroidal components. These signatures are consistent with magnetohydrodynamic modeling results, providing the first observational evidence of a 3D FLR associated with a plume in Earth's magnetosphere. These results demonstrate the importance of cold plasma in controlling wave dynamics in the magnetosphere, and have important implications for wave‐particle interactions at a range of energies. Plain Language Summary: Earth's space environment is home to electrons and ions across a wide range of energies, trapped in the region by our global geomagnetic field. Energy can be transferred to and from the trapped particles through oscillations in the magnetic field, and these processes are responsible for the extreme energization of trapped electrons to hazardous levels for local spacecraft. In this paper we explore a type of magnetic field oscillation termed Field Line Resonances (FLRs): standing waves on a field line analogous to the oscillatory motion of guitar strings. We use spacecraft observations to show that the direction of the field line oscillations changes significantly depending on the density of the background plasma. The results confirm previous modeling work, and are the first observational evidence of 3D FLRs at a plume. The findings have important consequences for how FLRs transfer energy between the electrons and ions. Key Points: We present the first observational evidence of a 3D Field Line Resonance at the sharp density gradient of a plume edgeThe observed polarisation change confirms magnetohydrodynamic modeling results and predictions made by Elsden and Wright (2022)The presence of 3D Field Line Resonances during storm times has impacts for how Ultra Low Frequency waves couple and interact with local plasma [ABSTRACT FROM AUTHOR]

Optical remote sensing observations have suggested that the top layer of Europa's icy surface is heavily affected by external weathering agents. To model and understand these effects, it is necessary to characterize the environment as fully as possible. In this paper, we focus on one agent in the environment (energetic electrons). We show Juno electron data from its 2022 Europa flyby and other time periods. While the Juno sensor used here (Jupiter Energetic Particle Detector Instrument) was not designed to obtain high quality electron data in an intense radiation environment, it is possible to extract information such as how Europa blocks energetic particles from accessing some of the surrounding space. The decrease in charged particle flux in Europa's wake provides an upper limit on the precipitation fluxes of the same particles. We also report that electron pitch angle distributions near Europa for the single energy channel considered here are time variable and not isotropic. Plain Language Summary: We present energetic electron data obtained when the Juno spacecraft crossed the plasma wake of the moon Europa in 2022. A major goal of this work is to estimate the amount of blockage of MeV electrons near the moon to inform radiation estimates for future Europa orbiters and landers. While the Jupiter Energetic Particle Detector Instrument does not directly detect particles between 1 and 20 MeV, using knowledge gained since the instrument began obtaining data we were able to estimate that about 80% of the particles in question are blocked by Europa in its wake at low altitude. Research on the weathering of Europa's surface by electrons is also discussed. Key Points: Jupiter Energetic Particle Detector Instrument electron data relevant to Europa are presentedWe estimate Europa's blockage causes an ∼80% reduction of ∼1–20 MeV electrons in the immediate plasma wakePitch angle distributions are computed from 70 keV electron data and show time variability [ABSTRACT FROM AUTHOR]

In Saturn's magnetosphere, the radially‐inward transport of magnetic fluxes is usually carried by localized flux tubes with sharply‐enhanced equatorial magnetic fields. The flux tubes also bring energetic particles inward, which are expected to drift azimuthally and produce energy‐dispersive signatures. Spacecraft observations, however, indicate the occurrence of energy‐dispersionless signatures for perpendicular‐moving particles. These unexpected features are attributed to the sharp magnetic gradient at the edge of the flux tubes, which significantly modifies the drift trajectories of perpendicular‐moving particles to enable their trapping motion within the flux tubes. The bouncing particles are less affected by the gradient, and therefore, still display energy‐dispersive signatures. It is the distinct particle behavior, together with different spacecraft traversal paths, that underlies the observational diversity. The results improve our understanding of particle dynamics in the magnetospheres of giant planets and indicate that pitch‐angle information should be considered in the extraction of flux‐tube properties from energetic particle observations. Plain Language Summary: The conservation of magnetic fluxes in Saturn's magnetosphere requires that the outward convection is compensated by a return process of magnetic fluxes, which has been observed in the form of localized flux tubes associated with sharply‐enhanced equatorial magnetic field and hot plasma population. The azimuthal drift of energetic particles within the flux tubes produces energy‐dispersive signatures, which have been utilized to estimate the age and starting position of the returning flux tubes. In this paper, we are motivated by Cassini observations of energy‐dispersionless signatures for perpendicular‐moving particles, to demonstrate that their drift paths can be significantly modified by the sharp magnetic gradient to cause their trapping within the flux tubes. The bouncing particles, on the other hand, are less affected by the gradient and, therefore, can leave the flux tubes to continue their drift around the planet. We further construct the magnetic configuration associated with the flux tubes, to illustrate the origin of the diverse observational signatures depending on particle pitch angle, spacecraft traversal path, and the trapping extent. These results have important implications for the interpretation of observational data in the injection flux tubes, and therefore improve our understanding of giant planet's magnetosphere and the associated particle dynamics. Key Points: In Saturn's injection flux tubes, charged particles often show distinct energy‐dispersive or dispersionless signals depending on pitch angleThe unexpected, energy‐dispersionless features for perpendicular‐moving particles are formed by their trapping motion within the flux tubesThe bouncing particles can hardly be trapped, and therefore, exhibit the characteristic energy dispersion and particle leaking signatures [ABSTRACT FROM AUTHOR]

The electron butterfly distribution, characterized by pitch angles (PA) primarily at 45° and 135°, was rarely observed in Earth's magnetotail. Here using the high‐resolution measurements from Magnetospheric Multiscale mission, we present the observation of electron butterfly distribution in a contracting dipolarization front (DF), and propose a new physical mechanism to explain its formation. Specifically, we discover that the electron butterfly distribution only exhibited in the locally contracted DF and was observed above 1.7 keV. We infer that local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure, and the butterfly distribution was formed by the magnetic mirror effect of this magnetic hourglass. Additionally, the theoretically estimated loss cone of the magnetic hourglass fits well with the observations of electrons, validating our inference about the formation mechanism. These findings can improve our understanding of electron dynamics in Earth's magnetosphere. Plain Language Summary: Examining the pitch‐angle (PA) distribution of electrons can help us understand the electron dynamic process in space. In this paper, we present the observation of electron butterfly distribution, characterized by PA primarily around 45° and 135°, in Earth's magnetotail. We find that the electron butterfly distribution was observed only above 1.7 keV, and exhibited in a locally contracting dipolarization front (DF). We propose a new formation mechanism for this distribution, and perform the theoretical calculations to validate it. Our findings can significantly improve the knowledge of electron dynamics in Earth's magnetosphere. Key Points: The electron butterfly distribution was observed above 1.7 keV and only exhibited in the contracted dipolarization front (DF)The local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structureThe butterfly distribution is formed by the magnetic mirror effect of the hourglass‐shaped structure [ABSTRACT FROM AUTHOR]

The Galileo spacecraft passed close to Europa on 11 encounters, two of which (E12 and E26) came within 400 km of the surface. In E12 data, there are perturbations in field and plasma data consistent with effects of a nearby plume (Jia et al., 2018). Huybrighs et al. (2020, https://doi.org/10.1029/2020GL087806) report depletions of proton flux in one channel of the Galileo Energetic Particle Detector (EPD) as Galileo passed close to Europa on E26. They trace particle trajectories in the magnetic field provided by a magnetohydrodynamic simulation and conclude that the spacecraft probably also passed through or close to a vapor plume on E26. However, the absence of a related signature in the measured magnetic field led us to question this conclusion. Examination of the EPD data remote from Europa on the E26 flyby reveals that the putative plume signature in the EPD data is an artifact. Plain Language Summary: In recent years, there have been reports that plumes, or extraterrestrial geysers, rise hundreds of kilometers above the surfaces of Saturn's moon, Enceladus, and Jupiter's moon, Europa. A very recent paper examines data from a close pass by Europa (E26 flyby) made by the Galileo spacecraft on January 3, 2000. The paper identifies a localized decrease in the count rate of energetic protons lasting about 20 s very near closest approach to Europa's surface and attributes the decrease to an interaction with a plume rising above Europa's surface. In this "comment" we demonstrate that a localized decrease of proton count rates is recorded at the same point in almost each measurement cycle (every 280 s) even very far from Europa on this pass due to an anomaly in the Energetic Particle Detector (EPD) channel in question. Therefore, the use by the authors of the EPD data to establish the presence of a plume during this pass is erroneous. Our conclusion is that during E26 the Galileo EPD data has to date not shown evidence of a plume. Key Points: The energetic proton flux decrease previously interpreted as the signature of a plume on the Galileo E26 flyby is an artifactThere is yet no convincing evidence for a plume encounter on Galileo's E26 pass by Europa [ABSTRACT FROM AUTHOR]

Recent observations have reported that magnetosonic waves can exhibit rising‐tone structures in the frequency‐time spectrogram. However, the generation mechanism has not been identified yet. In this paper, we investigate the generation of rising‐tone magnetosonic waves in the terrestrial magnetosphere using 1‐D particle‐in‐cell (PIC) simulations, in which the plasma consists of three components: cool electrons, cool protons and ring distribution protons. We find that the magnetosonic waves excited by the ring distribution protons can form a rising‐tone structure with frequency of the structure ranging from about 0.5Ωlh to nearly Ωlh, where Ωlh is the lower hybrid frequency. It is further demonstrated that the rising frequency of magnetosonic waves can be accounted for by the scattering of ring distribution protons. Moreover, the rising‐tone timescale obtained by PIC simulation is compared with the satellite observation. Our findings provide some new insights to understand the nonlinear evolution of plasma waves in the Earth's magnetosphere. Plain Language Summary: Magnetosonic waves in the Earth's inner magnetosphere often exhibit a series of harmonics of the local proton gyrofrequency. Recent observations found that they also have some other features, such as rising‐tone spectra and quasiperiodicity in time. Since these phenomena are similar to the rising‐tone chorus and electromagnetic ion cyclotron waves, many researchers speculated the generation of rising‐tone magnetosonic waves may be also related to the nonlinear wave‐particle interaction. In spite of the speculation, the specific generation mechanism of rising‐tone magnetosonic waves has not been identified yet. In this paper, we perform 1‐D particle‐in‐cell (PIC) simulations to investigate the generation of rising‐tone magnetosonic waves. It is found that the magnetosonic waves excited by the ring distribution protons can form a rising‐tone structure. In addition, we also demonstrate that the rising‐tone magnetosonic waves can be accounted for by the scattering of ring distribution protons. Key Points: Using 1‐D PIC simulations, we have investigated the generation of rising‐tone magnetosonic waves excited by ring distribution protonsThe rising‐tone structure of magnetosonic waves can be accounted for by the scattering of ring distribution protonsPIC simulations show that the rising‐tone timescale depends on the initial ring proton density and thermal speed [ABSTRACT FROM AUTHOR]

Whistler‐mode waves can modulate the electron distributions through wave‐particle interactions in the Earth's inner magnetosphere. In this paper, we have investigated the effects of the whistler waves on the electron distributions by a combination of electron magnetohydrodynamics and test‐particle simulations. Our simulations show a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to higher latitudes. Such a whistler wave causes distinct flux increases of tens of keV electrons at large pitch angles to form electron butterfly distributions. This whistler wave traps numerous resonant electrons and transports them from lower latitudes to higher latitudes through nonlinear Landau trapping. Meanwhile, these trapped electrons are efficiently accelerated to higher energies contributing to the rapid formation of electron butterfly distributions at higher latitudes. Our study suggests nonlinear Landau trapping is a potential formation mechanism of the electron butterfly distributions observed in the Earth's inner magnetosphere. Plain Language Summary: Wave‐particle interactions are known to play an important role in regulating energetic electron distributions in the Earth's inner magnetosphere. In particular, whistler‐mode waves can efficiently modulate the electron distributions by cyclotron and Landau resonant interactions. In this paper, we used a 2‐D electron magnetohydrodynamics model to simulate a whistler wave in the dipole geomagnetic field. The simulation results indicate a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to high latitudes. Furthermore, a test‐particle simulation shows that such a whistler wave can trap and accelerate electrons from the lower latitudes to higher latitudes by nonlinear Landau trapping, which leads to the rapid formation of electron butterfly distributions at tens of keV. Previous studies suggested that nonlinear cyclotron trapping can cause electron butterfly distributions at tens of keV observed in the Earth's magnetosphere. Our study provides a new potential formation mechanism of electron butterfly distributions in the Earth's magnetosphere. Key Points: Effects of whistler waves on electron distributions have been studied by a combination of electron magnetohydrodynamics and test‐particle simulationsA whistler wave causes distinct flux enhancements of 10s keV electrons at large pitch angles to form electron butterfly distributionsNonlinear Landau trapping contributes to the rapid formation of the electron butterfly distributions [ABSTRACT FROM AUTHOR]

The westward traveling surge (WTS) is one of the most notable manifestations of auroral substorms. Using the Rice Convection Model, previous study confirmed that a single plasma‐sheet bubble injection can result in the formation of WTS. In reality, however, earthward‐propagating bubbles are persistently injected from the magnetotail during substorms; hence it is worth further investigating how sequential bubble injections would affect the features of WTS. In this study, we find that the successive WTSs observed by all‐sky imagers correspond well with continual plasma‐sheet bubble injections detected by the Time History of Events and Macroscale Interaction during Substorms satellite in the magnetosphere. Numerical simulations are carried out to reveal that the multiple plasma‐sheet bubble injections lead to sequential surges in the ionosphere. Interestingly, our results indicate that the propagation speeds of WTSs are well correlated with the magnetic field dipolarization in the magnetosphere. Plain Language Summary: During geomagnetic active times, peaceful auroral arcs brighten suddenly and expand poleward. This is known as auroral substorms. In its expansion phase, a bulge structure continues to move westward rapidly, leading to a splendid auroral phenomenon which is referred to as westward traveling surge (WTS). Our previous study has shown that a low entropy structure injected from the nightside of the near‐Earth space, called "bubble," plays a central role in the formation and evolution of a WTS. In this paper, we report that a series of transient bubble injection processes are responsible for the sequential formation of WTSs in the ionosphere. Using numerical simulations, we find that multiple bubble injections can make the WTS appear one after another. Therefore, they are essential ingredients for successive WTSs in prolonged substorms. Key Points: Five westward traveling surges (WTSs) correspond well with successive dipolarizations and fast plasma flows in the magnetosphere from observationsRice Convection Model confirms that continual plasma‐sheet bubble injections can lead to the sequential westward traveling surgesThe expansion speeds of WTSs are linearly proportional to the increase of Bz in the magnetosphere [ABSTRACT FROM AUTHOR]

We report on plasma observations from Juno/Jovian Auroral Distributions Experiment during the Ganymede flyby on 7 June 2021. Juno approached Ganymede from southern latitudes, passed through the wake region, then through its magnetosphere to closest approach (1,046 km from the surface) on the night side, and then back into Jupiter's plasma disk. We describe general plasma properties in the regions explored along the trajectory. We infer that Juno traversed a region of open field lines where one end intercepts Ganymede and the other Jupiter. The observations do not support Juno crossing into the closed field line region. The ion composition near Ganymede is very different than that of the nearby plasma environment. H2+ and H3+ ions were detected near Ganymede and in the wake region. Low energy (∼0.1–1 keV) electrons are enhanced just outside the magnetopause, in the wake (inbound trajectory) and in the magnetopause boundary layer (outbound trajectory). Plain Language Summary: On 7 June 2021 the Juno mission came as close as 1,046 km from the surface of Ganymede, the largest moon in the solar system. Similar close encounters were previously made by the Galileo mission, from which we learned much of the interaction of the moon, with its own intrinsic magnetic field, and Jupiter's magnetosphere. In this paper, we present an overview of the plasma observations, that is, ions and electrons in the lower part of the energy spectrum, made by the Jovian Auroral Distributions Experiment. We find that the ion composition near Ganymede is very different than that from Jupiter's magnetosphere. Near Ganymede, the plasma composition is dominated by molecules and ions that originate from water in the atmosphere or the surface. One surprising observation is the presence of the molecular ion H3+ inside Ganymede's magnetosphere and in a region just outside and downstream, that we call the wake. H3+ was not included in various models of Ganymede's atmosphere. Key Points: The plasma observations show that Juno crossed into the open field line region, but do not support crossing into a closed field line regionThe ion composition near Ganymede is very different from its local plasma environmentH2+ and H3+ ions were detected inside Ganymede's magnetopause and outside in the wake region [ABSTRACT FROM AUTHOR]

We present initial results of low‐energy ion observations from BepiColombo's first Mercury flyby. Unprecedentedly high time resolution measurements of low energy ions at Mercury by BepiColombo Mio reveal rapid (a few seconds) and large (1–2 orders of magnitude) fluctuations of ion flux around the magnetopause and within the magnetosphere. Around the magnetic equator in the pre‐midnight magnetotail, Mio observed plasma sheet ions consistent with previous observations. In the midnight magnetotail near the closest approach, Mio observed the co‐existence of high‐energy (∼keV/q) and low‐energy (<∼300 eV/q) ion components. The low‐energy component is inferred to be cold with a temperature well below 100 eV and have a major contribution to the total density as opposed to previously reported cold tenuous ions. Future observations by Mio will provide insights into the sources, transport, and acceleration of the newly identified ion components. Plain Language Summary: BepiColombo, launched on 20 October 2018, is now en route to Mercury. Along its 7‐year journey to the Mercury orbit insertion, BepiColombo conducted, and will conduct, nine planetary flybys in total. This paper presents brand‐new observations of ions (positively charged particles) in Mercury's magnetosphere by BepiColombo Mio during its first encounter with Mercury. Thanks to high‐time resolution measurements by Mio, it is revealed that ions around Mercury change dramatically and rapidly just over a few seconds. Mio also observed an unexpectedly dense ion population with low energies, for which we do not have a definitive explanation yet. Future flyby and in‐orbit observations by BepiColombo will provide key information for unraveling the dynamics and sources of ions in Mercury's magnetosphere. Key Points: We present initial reports on low energy ion observations during BepiColombo's first Mercury flybyMio observed large fluctuations of ion flux with time scales down to a few seconds around the magnetopause and within the magnetosphereIon energy spectra obtained in the midnight magnetotail suggest the presence of an unexpectedly dense cold component [ABSTRACT FROM AUTHOR]

This paper presents observations of dayside auroral signatures resulting from a massive localized compression of the magnetopause, magnetosheath, and bow shock. On 10 December 2016, the Magnetospheric Multiscale Mission (MMS) spacecraft observed a compression of the entire magnetosheath and bow shock, which moved past the spacecraft in 1 min and 45 s. Shortly afterward, the resulting auroral signature was observed at the Kjell Henriksen Observatory located in Longyearbyen, Norway. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Plain Language Summary: The aurora borealis and australis are the visible signatures of interactions between the Earth's magnetic field and the solar wind. When the solar wind's magnetic field interacts with the Earth's magnetic field, there can be large fluctuations in its positioning and shape. This paper presents observations from satellites of a very large compression of the magnetic field, resulting in unique ionospheric signatures directly over an all‐sky camera at Kjell Henricksen Observatory in Svalbard, Norway. One minute and 57 s after the compression at the magnetosphere, previously unstructured aurora began to intensify and organize, first propagating eastward and then brightening westward. The auroral arcs then began to spiral in a counterclockwise direction forming a spiral structure over the field of view. This defined shape only lasted for a few seconds before dispersing and moving poleward. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Key Points: Analysis of rapid compression of the magnetopause, magnetosphere, and bow shockObservations of unusual auroral signature over Svalbard, Norway resulting from compression [ABSTRACT FROM AUTHOR]

A new radio component namely Saturn Anomalous Myriametric Radiation (SAM) is reported. A total of 193 SAM events have been identified by using all the Cassini Saturn orbital data. SAM emissions are L‐O mode radio emission and occasionally accompanied by a first harmonic in R‐X mode. SAM's intensities decrease with increasing distance from Saturn, suggesting a source near Saturn. SAM has a typical central frequency near 13 kHz, a bandwidth greater than 8 kHz and usually drifts in frequency over time. SAM's duration can extend to near 11 hr and even longer. These features distinguish SAM from the regular narrowband emissions observed in the nearby frequency range, hence the name anomalous. The high occurrence rate of SAM after low frequency extensions of Saturn Kilometric Radiation and the SAM cases observed during compressions of Saturn's magnetosphere suggest a special connection to solar wind dynamics and magnetospheric conditions at Saturn. Plain Language Summary: This paper reports a new type of radio emission at Saturn. We name this emission "Saturn Anomalous Myriametric Radiation" (SAM). Due to the different morphological behavior of SAM emissions in the spectrogram, compared to the regularly observed narrowband emissions, SAM emissions are identified as a new radio component. We search for SAM events by using all the available data acquired by the Cassini spacecraft. A total of 193 SAM events are found and we summarized its characteristics. The spatial distribution of SAM shows a high latitude preference in occurrence. The observed intensities of SAM increase as the spacecraft approaches Saturn, suggesting a source near Saturn. The typical central frequency of SAM is near 13 kHz and the duration of SAM can extend to near 11 hr and even longer. The majority of the 193 SAM emissions are observed after the enhancement of the dominant Saturn radio emission Saturn Kilometric Radiation (SKR), which is deemed to be closely connected to the solar wind dynamics at Saturn. Therefore, SAM emissions are also possibly related to the conditions that cause the enhancement of SKR. This newly identified SAM emission may be used to study the magnetospheric dynamics of Saturn in the future. Key Points: A new radio component namely the Saturn Anomalous Myriametric Radiation (SAM) is reportedThe characteristics of SAM are given including its spatial distribution, typical frequency, bandwidth and time durationSAM shows a possible connection to the compression of the magnetosphere [ABSTRACT FROM AUTHOR]

In situ conjugate electromagnetic ion cyclotron (EMIC) waves observed by the Swarm mission in both hemispheres are presented. A complex and unusual pattern of Alfvénic EMIC wave energy is observed, with a mid‐latitude peak close to the source at L =3.3, as well as a secondary lower L‐peak. A wave propagation model reveals that the secondary peak at L =1.7 may be explained by wave power being redirected equatorward due to the Buchsbaum resonance, crossing and interfering with the same EMIC wave power propagating equatorwards from the opposite hemisphere. This interference creates a coherent equatorial driver for a low‐L field line resonance at the secondary peak, and which is associated with strong shear‐to‐fast mode coupling in the ionosphere. This behavior complicates the interpretation of low‐Earth orbit EMIC data for applications assessing radiation belt loss. Combined low Earth orbit observations and modeling enable these novel and localized magnetosphere‐ionosphere EMIC wave propagation pathways to be identified. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are important in near‐Earth space due to their role in reducing the amount of radiation in the Earth's radiation belts following geomagnetic storms. They are also often observed during non‐storm times. They are studied using satellites and ground observatories. Our paper reveals how these waves can follow complicated and previously unknown pathways to reach the upper atmosphere where they can be detected on the ground. This study shows a new and unusual effect where some EMIC wave energy is reflected and diverted toward the equator, where it meets its opposite‐hemisphere counterpart, interferes with it and sets up a resonance. This resonance then creates a new signal peak in the upper atmosphere at lower latitudes, far away from the location of the initial source. This presents a new and hitherto unseen pathway for wave energy to travel from their generation region in near‐Earth space down to the ionosphere. Understanding such pathways is very important for correctly diagnosing the location of these wave populations in space, and assessing their role in causing reductions in the levels of space radiation. Key Points: Electromagnetic ion cyclotron wave propagation from the magnetosphere to the ionosphere is complicated by reflection from the Buchsbaum resonance and interferenceWaves reflected from the Buchsbaum resonance interfere to generate a coherent driver for a secondary lower latitude field line resonanceThis generates a field‐guided secondary lower‐latitude peak associated with strong shear‐to‐fast mode energy conversion in the ionosphere [ABSTRACT FROM AUTHOR]

The most important driver of geomagnetic activity is the north–south (Bz) component of the interplanetary magnetic field (IMF), which dominates the solar wind‐magnetosphere coupling and all solar wind coupling functions. While the east–west (By) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for By>0 than for By<0 in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the By sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar‐Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit By dependence. This By dependence is strong in the midnight and dawn sectors where it causes a related By effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity. Key Points: Fluxes of energetic electrons precipitating into atmosphere are strongly dependent on the sign and amplitude of IMF ByThis IMF By effect on electron fluxes is strong around midnight and at dawn but weak at duskThis IMF By effect is expected to modulate ionospheric conductivity and to affect a similar effect in geomagnetic activity [ABSTRACT FROM AUTHOR]

Kinetic structures of electron diffusion regions (EDRs) under finite guide fields in magnetotail reconnection are reported. The EDRs with guide fields 0.14–0.5 (in unit of the reconnecting component) are detected by the Magnetospheric Multiscale spacecraft. The key new features include the following: (1) cold inflowing electrons accelerated along the guide field and demagnetized at the magnetic field minimum while remaining a coherent population with a low perpendicular temperature, (2) wave fluctuations generating strong perpendicular electron flows followed by alternating parallel flows inside the reconnecting current sheet under an intermediate guide field, and (3) gyrophase bunched electrons with high parallel speeds leaving the X‐line region. The normalized reconnection rates for the three EDRs range from 0.05 to 0.3. The measurements reveal that finite guide fields introduce new mechanisms to break the electron frozen‐in condition. Plain Language Summary: Magnetic reconnection plays a crucial role in the dynamics of the terrestrial magnetotail. For reconnection to occur, the plasma must decouple from the magnetic field. The bounce motion of particles in the magnetotail current sheet is regarded as a key to this decoupling for cases when the current sheet has no magnetic field along the direction of the current. This paper reports that while bounce motion remains relevant when a finite magnetic field is present along the current, new mechanisms to decouple electrons from the magnetic field are introduced, and new open questions unfold. The observations are based on measurements from the Magnetospheric Multiscale mission. The mission's unprecedented high cadence electron data make possible the revelation of the new mechanisms. The results reported in this paper expand the frontiers of our knowledge on magnetotail reconnection and have major implications on the fundamental physics of magnetic reconnection in all plasma systems where binary collisions are not effective, including solar, astrophysical, and laboratory plasmas. Rapid dissemination of the results will set the ground for advances in magnetic reconnection research. Key Points: Kinetic structures of three electron diffusion regions under finite guide fields show that meandering remains relevant in all three casesCold inflowing electrons are accelerated along the weak guide field yet fail to follow the local magnetic field at the field minimumFor an intermediate guide field, wave fluctuations strongly influence electron dynamics in the reconnecting current sheet [ABSTRACT FROM AUTHOR]

A three‐dimensional global hybrid simulation is conducted for the interaction of the Earth's bow shock with an interplanetary directional tangential discontinuity (TD) whose normal lies in the equatorial plane. Although the convection electric field points into the TD from its trailing side both north and south of the equator, the interaction only generates a hot flow anomaly (HFA) in the southern hemisphere. HFA generation results from bow shock‐reflected ions energized by the inward‐pointing electric field on one side of the TD and inward ion gyration on the other side. The latter only occurs south of the equator due to the global bow shock geometry. The global HFA asymmetry is driven by a reversal in the north‐south component of reflected ion velocities in the two hemispheres, such that ions gyrate into (away from) the TD in the southern (northern) hemisphere. Our results indicate HFAs must generally exhibit north‐south and dawn‐dusk asymmetries. Plain Language Summary: Interplanetary magnetic field (IMF) discontinuities frequently occur in the solar wind. When interplanetary tangential discontinuities (TDs) interact with the terrestrial bow shock, localized large‐amplitude transient structures known as hot flow anomalies (HFAs) can be generated and cause significant perturbations in the magnetosphere, even though the pristine TD exhibits no plasma variations. Previous studies based on satellite observations and global hybrid simulations suggest that HFAs occur when convection electric fields point toward the TD on at least one side, a condition under which the bow shock ions are likely to be injected and accumulated in the TD. The previous global simulations, however, were based on 2‐D assumptions. In this paper, we conduct a 3‐D global hybrid simulation for the interaction of the bow shock with an interplanetary directional TD, in which the IMF is symmetric about the equator on both sides of the TD. Although the convection electric field points into the TD from one side of the discontinuity at locations both north and south of the equator, the interaction only generates an HFA in the southern hemisphere. By examining ion velocity distributions, a mechanism is proposed for the global asymmetry of the HFA. Our simulation indicates that HFAs must generally exhibit north‐south and dawn‐dusk asymmetries. Key Points: Global hybrid model predicts that hot flow anomalies (HFAs) generally exhibit north‐south and dawn‐dusk global asymmetries due to the 3‐D bow shock geometryHFAs generated by tangential discontinuity impact only appear in one hemisphere even when electric fields point inwards in both hemispheresThe north‐south global HFA asymmetry is driven by a reversal in the north‐south component of ion velocity across the equatorial bow shock [ABSTRACT FROM AUTHOR]

The electromagnetic ion cyclotron (EMIC) wave usually observed in the Earth's magnetosheath is thought to be generated through the ion temperature anisotropy instability. This paper presents an observation of a long‐lasting large‐amplitude EMIC wave event in the dawnside flank of the magnetosheath by the Magnetospheric Multiscale mission, lasting from 06:33:00 to 12:35:00 UT on 16 April 2018. The wave amplitude is around 0.2 nT as compared to the ambient magnetic field ~12 nT. The characteristic frequency and scale size are around 0.2 Hz and 1,028 km, respectively. Accompanying EMIC waves are density fluctuations, which exhibit both positive and negative correlations with the longitudinal magnetic field. Using the fitted parameters for the ion and electron phase space densities, plasma kinetic theory predicts local excitations of both EMIC and mirror instabilities, which provide the energy of these observed EMIC waves and density fluctuations. Plain Language Summary: The Earth's Magnetosheath is a region slowing down high‐speed particles originating from the solar wind, protecting the Earth from the hazards of high‐energy charged particles. A considerable fraction of the energy of energetic particles is transferred into plasma waves. This paper presents an observational event of large‐amplitude low‐frequency electromagnetic waves lasting nearly 6 hr, propagating from the outer magnetosheath to the inner magnetosheath. Density fluctuations are found to coexist with these electromagnetic waves. Applying the measurements of charged particles to modeling, we show the observed waves are likely excited by free energy provided by ions with an anisotropic temperature distribution, consistent with wave theory. Key Points: MMS observes a long‐lasting large‐amplitude electromagnetic ion cyclotron wave event in the dawnside flank of the magnetosheathDensity fluctuations are both positively and negatively correlated with the parallel magnetic fieldLocal excitation of the electromagnetic ion cyclotron instability and mirror instability contributes to the observed fluctuations [ABSTRACT FROM AUTHOR]

Electron cyclotron harmonic (ECH) waves are effective in precipitating electrons into the loss cone to form diffuse aurora. Recent observations have shown that ECH waves at Earth could exhibit frequency chirping, which is an interesting feature of various plasma waves and whose mechanism is under intensive research. Whether chirping of ECH waves is also present in other planetary magnetospheres, however, is unknown. In this paper, we report the first observation of quasi‐periodic ECH waves with frequency chirping at Saturn using Cassini data. The identified six falling tone and one rising tone ECH wave events occur at L<9 near the magnetic equator. The duration of ECH wave chirping elements lasts from seconds to minutes. Our findings suggest that frequency chirping of ECH waves could be observed at other planetary magnetosphere, and are useful to understand the fundamental mechanisms of frequency chirping of various plasma waves. Plain Language Summary: Electron cyclotron harmonic (ECH) waves play an important role in electron acceleration and pitch angle scattering loss to the atmosphere at Earth. Here we report the periodic ECH waves with discrete elements observed at Saturn. We found six ECH wave events with falling tones and one event with rising tones. The detected ECH wave events with discrete elements occur near the magnetic equator in the range of 5

The moon Ganymede has a strong internally generated magnetic field that separates the surface into two regions, with the polar surface magnetically connected to the Jovian environment. Consequently, the weathering of Ganymede's surface by plasma and energetic charged particles trapped in the Jovian magnetosphere is not uniform. At the same time, optical data suggest differences in the surface ice between the poles and the equator, with further refinements in the equatorial surface that have a longitudinal dependence. Here, we use Juno spacecraft data to characterize the charged particle environment along Ganymede's orbit (from about 50 eV to 1 MeV for electrons and 10 eV to 6 MeV for protons). These results put us into a better position to test the hypothesis that space weathering by electrons causes the brighter poles of Ganymede, given that electron fluxes are likely to be more clearly separated between the polar and equatorial regions. Plain Language Summary: Electron weathering of ice may be the reason Ganymede's poles are brighter. These particles are well separated by the two magnetic topologies of Ganymede, whereas ions are not as well separated. We suggest ions may be converted to energetic neutral atoms near Ganymede and reach the surface despite the magnetic barrier. We quantify the environmental inputs in this paper. Key Points: Juno orbital evolution has allowed better characterization of the region near Jupiter's magnetic equator and Ganymede's orbital distanceElectron and proton energy spectra from that region derived from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic‐particle Detector Instrument (JEDI) data are presentedWe propose that polar versus equatorial albedo differences on Ganymede are due to changes resulting from electron precipitation [ABSTRACT FROM AUTHOR]

Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north‐south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres. Plain Language Summary: Occasionally, the aurora suddenly brightens in local areas of the Northern and Southern Hemispheres, causing explosive and widespread phenomena. At this time, it is known that a part of the aurora travels toward east or west from the point of the explosion; however, the formation mechanism of this traveling aurora is unknown. Recently, this traveling aurora has been reproduced by computer simulation, and it has been shown that the direction of its traveling is determined by the state of the ionosphere rather than by the projected motion of the source region in the magnetosphere. In this paper, we observed traveling auroras with two identical high‐speed cameras installed in the Northern and Southern Hemispheres simultaneously and clarified for the first time that the state of the ionosphere determines the direction of the traveling aurora. Understanding the mechanism of such connection between the space and the Earth is important for predicting the space environment required for various human activities and technologies using the space. Key Points: An auroral breakup was captured simultaneously by ground‐based all‐sky cameras at magnetically conjugate stations in both hemispheresThe auroral surge evolved asymmetrically regarding the longitudinal direction in both hemispheresThe surge evolved in the direction of the ionospheric Hall current, which is consistent with the expectation from a global simulation [ABSTRACT FROM AUTHOR]

Energetic electron spectra in the Earth's inner belt and slot region usually display extrema at some energies that form uninterrupted lines when color‐coded in two‐dimensional maps in which one axis represents the normalized equatorial distance, L, and the other axis represents kinetic energy. These patterns are called "zebra stripes." This letter presents the first observations of zebra stripe inversions: Lines of local minima in the electron spectra at low L values (below L = 2) turn into lines of local maxima in the electron spectra at higher L values, and vice versa. The observations are interpreted by specifying the connection between fluctuations in directional differential fluxes and radial disturbances of trapped population drift motion. The analysis highlights the role of flux energy‐ and radial‐gradients when it comes to determining radial motion from particle data. The underlying method applies to a variety of problems, whether at Earth or at the outer planets. Plain Language Summary: The "zebra stripes" are a special feature of electron flux measurements that have been detected at Earth and Saturn. Specifically, uninterrupted bands appear when differential fluxes are color‐coded in two‐dimensional maps in which one axis represents the equatorial distance to the center of the planet and the other axis represents kinetic energy. They are one signature of the couplings that exist between the magnetosphere and the ionosphere. This work presents the first observations of "zebra stripes" whose pattern intertwines. These observations are appealing because they do not appear consistent with existing theories for zebra stripe generation. Indeed, theoretical and numerical works on zebra stripe generation have so far focused on reproducing continuous, uninterrupted stripes, not intertwined ones. We propose an explanation for zebra stripe inversions that remains consistent with existing theories for zebra stripe generation. While instances of zebra stripe pattern inversions are relatively exceptional at altitudes below three Earth radii, the method discussed in the paper remains important because it is relevant to various magnetospheric research topics, whether at Earth or at the outer planets. Key Points: The zebra stripe pattern usually present in energetic electron spectra below L = 3 can be disrupted and inversed in the slot regionA formula to relate radial displacement and fluctuations in directional differential fluxes is detailed in the dipole fieldThe inversion of the zebra stripe pattern is related to a strong decrease in energetic electron fluxes with radial distance [ABSTRACT FROM AUTHOR]

We use measurements from NASA's Van Allen Probes to calculate the decay time constants for electrons over a wide range of energies (30 keV to 4 MeV) and L values (L = 1.3–6.0) in the Earth's radiation belts. Using an automated routine to identify flux decay events, we construct a large database of lifetimes for near‐equatorially mirroring electrons over a 5‐year interval. We provide the first accurate estimates of the long decay timescales in the inner zone (∼100 days), which are highly resolved in energy and free from proton contamination. In the slot region and outer zone, we compare our lifetime calculations with prior empirical estimates and find good quantitative agreement (lifetimes ∼1–20 days). The comparisons suggest that some prior estimates may overestimate electron lifetimes between L≈ 2.5–4.5 due to instrumental effects and/or background contamination. Previously reported two‐stage decays are explicitly demonstrated to be a consequence of using integral fluxes. Plain Language Summary: The Earth is surrounded by two invisible, donut‐shaped belts of charged particle radiation (think electrons and protons) called the Van Allen belts. The particles in these belts orbit rapidly around the Earth in the same region where spacecraft fly, like GPS and weather satellites. Since the particles in the belts can damage satellites, we need to understand what specific processes make the intensity of the belts go up and down. Knowing which processes are important for changing the belt intensity helps us build better computer models that can be used to predict the future state of the belts (much like weather prediction models). This letter uses spacecraft observations to estimate the loss timescales in the radiation belt region, which are then used in a companion paper to better understand the processes that make the belt intensity go down. Key Points: A large database of radiation belt electron decay timescales is calculated from Van Allen Probes MagEIS measurementsWe provide the first accurate estimates of these timescales over a wide range of energies in the inner zone, free from proton contaminationOuter zone decay timescales generally agree well with prior estimates; some differences exist and may be due to instrumental effects [ABSTRACT FROM AUTHOR]

This paper aims to extend the understanding of Strong Thermal Emission Velocity Enhancement (STEVE) and the Picket Fence related to strong subauroral ion drifts (SAID). We numerically demonstrated that precipitating energetic electrons are critical for the structuring of the Picket Fence. It is created by feedback‐unstable magnetosphere‐ionosphere interactions driven by the SAID electric field when the Hall conductance created by energetic (≥1 keV) electrons exceeds the Pedersen conductance. We show that thermal excitation of the red‐line emission in STEVE is inhibited by inelastic collisions with molecular nitrogen. Suprathermal (≤500 eV) electrons coming from the turbulent plasmasphere appear to be the major source. We also show that the chemiluminescencent and radiative attachment reactions do not explain the short‐wavelength part of the STEVE continuum and argue that accounting for vibrational excitation may resolve the problem. Atmosphere's upwelling due to enhanced ion‐neutral drag leads to the increased abundance of molecular components relative to atomic oxygen. Key Points: Feedback‐unstable magnetosphere‐ionosphere interaction in the presence of energetic electron precipitation explains the Picket Fence formationThermal excitation of the red‐line emission in the low‐density SAID channel is inhibited by inelastic collisions with molecular nitrogenTo understand the STEVE continuum, transitions between vibrationally excited molecular states in the SAID channel should be taken into account [ABSTRACT FROM AUTHOR]

On 10 January 2001, Cassini briefly entered into the magnetosphere of Jupiter, en route to Saturn. During this excursion into the Jovian magnetosphere, the Cassini Magnetosphere Imaging Instrument/Charge‐Energy‐Mass Spectrometer detected oxygen and sulfur ions. While Charge‐Energy‐Mass Spectrometer can distinguish between oxygen and sulfur charge states directly, only 95.9 ± 2.9 keV/e ions were sampled during this interval, allowing for a long time integration of the tenuous outer magnetospheric (~200 RJ) plasma at one energy. For this brief interval for the 95.9 keV/e ions, 96% of oxygen ions were O+, with the other 4% as O2+, while 25% of the energetic sulfur ions were S+, 42% S2+, and 33% S3+. The S2+/O+ flux ratio was observed to be 0.35 (±0.06 Poisson error). Plain Language Summary: While en route to Saturn, Cassini received a gravity assist from Jupiter. During this Jupiter flyby, Cassini briefly entered into the Jovian magnetosphere, due to an especially extended period of low solar wind pressure. In this paper, we focus on observations of the relative flux abundance of energetic (95.9 keV/e) heavy ions (oxygen and sulfur) by ion charge state (i.e., On+ and Sn+). These heavy ions would have largely originated from the moon Io, located 5.9 Jupiter radii (RJ) from Jupiter, while the observations from Cassini are near 200 RJ. By comparing the relative charge state zabundances of these heavy ions in the outer magnetosphere to previous observations closer to Io, we can study the evolution of plasma as it is transported away from Jupiter. These observations also serve to constrain modeling efforts and provide context to instruments incapable of independent ion mass and charge state determination. Key Points: Cassini measured the relative charge state abundances of 95.9 keV/e oxygen and sulfur in the outer magnetosphere (~200 RJ) of JupiterThe flux of 95.9 keV/e O+ was higher than that of S2+, with S2+ being the most abundant charge state among sulfur ionsThe relative abundances of 95.9 keV/e heavy ions are compared to thermal ions in the inner to middle magnetosphere [ABSTRACT FROM AUTHOR]

In this letter, we have investigated the correlation between lower band chorus waves and electron beam‐like distribution with long‐term THEMIS data. Cold, hot, and beam electrons are assumed in the plasma. For quasi‐parallel chorus waves, they are usually observed along with a relatively weak electron beam. Moreover, the density ratio between beam and hot electrons nb/nh is positively correlated with the wave parallel electric field and inversely correlated with the hot electron density, in agreement with a formation of the beam by Landau acceleration during wave propagation, while, along with oblique waves, there typically exists a strong electron beam. Notably, the nb/nh is found to be independent of both the parallel electric field and hot electron density, indicating that the electron beam‐like distribution should be a precondition for exciting oblique waves rather than caused by waves. Our study provides some new insights into understanding the generation and evolution of chorus waves. Plain Language Summary: The interaction between whistler‐mode chorus waves and electrons has long been a hot topic, which is important for understanding the evolution and generation of chorus waves. Both theoretical and simulation studies have indicated that quasi‐parallel chorus waves are initially excited near the magnetic equator and then propagate toward high‐latitude regions with increasing wave normal angles and enhanced parallel electric fields. During the propagation, the Landau resonance between chorus waves and electrons begins to take effect, causing the damping of waves and the formation of local electron beam. Within the equatorial region, there also exist a number of oblique chorus waves with large wave normal angles, which should be also locally excited. To generate oblique waves, a beam‐like electron distribution is needed to suppress the Landau damping of oblique chorus waves, which should already exist before those waves. Although the two physical pictures are well known in this community, actually, there is still a lack of observational support. In this paper, we have made some new progress in understanding these two different scenarios by studying the correlation between chorus waves and electron beam‐like distribution with long‐term THEMIS data. Key Points: We have investigated the correlation between lower‐band chorus waves and electron beam‐like distribution using THEMIS data.Along with quasi‐parallel chorus waves, a weak elelctron beam can be observed, which is caused by Landau resonance with waves.But for oblique chorus waves, the electron beam‐like distribution should be a precondition for exciting waves rather than caused by waves. [ABSTRACT FROM AUTHOR]

Data are analyzed from a Magnetospheric Multiscale encounter with a dayside magnetopause reconnection region on 29 December 2016. The uniqueness of the event stems from the small (~7 km) average spacecraft separation and the sequential sampling of an electron diffusion region with electron crescent distributions. We quantitatively investigate the earthward acceleration of magnetosheath electrons through the in‐plane null by the polarization electric field EN that points radially outward from the magnetopause. The results compare favorably with previous plasma simulations with one important difference that the reconnection electric field (EM) extends throughout the region of strong EN so that both fields energize electrons in the same region. This acceleration is quantified here for the first time. As the spacecraft penetrate deeper into the region of enhanced EN, the magnetic reflection of lower‐energy electrons produces a thinner crescent. Plain Language Summary: Reconnection causes the conversion of magnetic energy to particle energy and the breaking and reconnection of solar wind and geomagnetic field lines. In asymmetric reconnection at the dayside magnetopause with different plasma densities and magnetic field strengths on the Earth and Sun sides of an X‐line, this energy conversion is displaced to the earthward side of the current layer. An electric field pointing outward normal to the dayside magnetopause drives reconnection by accelerating solar wind electrons across the reconnection current sheet. These electrons take up dawnward‐directed meandering orbits, and they are ultimately the carriers of the reconnection current. This paper reports the first direct measurement of this acceleration. Key Points: Parallel and perpendicular energy conversion and electron acceleration are studied with data from four spacecraft with average separations of 7 kmThe magnetopause normal electric field (EN) accelerates magnetosheath electrons across the reconnection current sheet to form crescent distributionsMagnetic reflection causes the crescent energy peaks to become narrower with increasing depth into the diffusion region [ABSTRACT FROM AUTHOR]

Fast interplanetary shocks occurrence, their parameters, and drivers near Jupiter's orbit are determined in this paper. It was found that 70% of the fast shocks are forward (FS) and 30% are reverse (RS). Interplanetary coronal mass ejection‐driven FS occur more frequently in all solar cycle phases except in the declining phase, when corotating interaction region‐driven shocks predominate. Most of the shocks were quasi‐perpendicularly (65° to 70°) propagating relative to the ambient interplanetary magnetic field. The average shock magnetosonic Mach number is slightly higher for FS (2.6) than for RS (2.4), which in turn are stronger than shocks near 1 and 10 AU reported in previous works. This occurs because of the full development of corotating interaction region shocks and higher occurrence of strong interplanetary coronal mass ejections near 5 AU and that the magnetosonic speed at 5 AU has only 60% of its value at 1 AU. Plan Language Summary: Interplanetary shock waves are sudden variations in solar wind plasma and magnetic field parameters. They propagate in the interplanetary space and have important effects on planetary magnetospheres. For instance, shocks cause expansions and compressions of Jupiter's magnetosphere and trigger its auroral emissions. In this work plasma and magnetic field data from several spacecraft that crossed interplanetary space near Jupiter's orbit space are analyzed. A list of interplanetary shocks is compiled, and their parameters and drivers are determined. It was found that shocks at 5 AU have preferentially a quasi‐perpendicular propagation to the magnetic field direction and are on average stronger than shocks observed near Earth's orbit. Key Points: Interplanetary shock occurrence, parameters, and drivers near Jupiter's orbit are determinedThere are about 70% fast forward and 30% reverse shocks; about 90% of RS are CIR driven, while 45% of FS are CIR driven and 55% are ICME drivenShocks at 5 AU are mostly quasi‐perpendicularly propagating and stronger on average than shocks at 1 AU [ABSTRACT FROM AUTHOR]

Energetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of "flux‐reduced injections" resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection. Plain Language Summary: Energetic particles are suddenly and abruptly pushed earthward through near‐Earth space via processes called "injections." These are commonly seen by spacecraft as sudden time‐ and energy‐dependent increases in the number of observed particles and are known to play an important role in sourcing the planet's radiation belts. Investigating the processes governing energetic particle transport is critical to better understand fundamental plasma processes, the dynamics of Earth's radiation belts, and the effects of space weather. This paper investigates a set of similar, but unique, signatures where the number of particles suddenly decreases instead of increasing. The analysis presented here surprisingly reveals that the particles observed in these signatures come from the magnetotail, which raises new questions about what generates these signatures. We investigate two hypotheses to explain the observed decrease in particles: (1) that a process removes the particles from the system and (2) that a process similar to "normal" injections moves a population of decreased particles into an area where there are more particles (hence generating a local decrease). The analysis finds that the secondary hypothesis is far more likely to be the primary driver while the first cannot be ruled out entirely as a secondary contributor. Key Points: MMS/FEEPS has repeatedly observed energy/time‐dispersed transient reductions in energetic electron flux in the dawnside magnetosphereDrift tracing consistently indicates that these signatures are from a postmidnight magnetotail sourceResults suggest that these features result from earthward injection in the presence of a negative radial gradient in phase space density [ABSTRACT FROM AUTHOR]

Pc3 range frequency (22–100 mHz) auroral pulsations often occur at daytime high latitudes, equatorward of the cusp/cleft and typically map to the dayside outer magnetosphere. In this paper we present simultaneous observations of compressional Pc3 magnetic pulsations in the dayside outer magnetosphere that occurred in direct association with daytime Pc3 auroral pulsations at South Pole Station (−74.4° magnetic latitude). The pulsations were almost identical at the two locations, and their correlation was clearest when the magnetospheric pulsations were highly monochromatic. Lower‐band chorus waves and keV electron fluxes were also modulated in the Pc3 range, likely by the compressional magnetic pulsations. The common Pc3 frequency in the magnetosphere and aurora matched well with the predicted frequency of upstream ultralow frequency waves. These results provide the first compelling evidence for the direct dayside outer magnetosphere‐ionosphere linkage between upstream‐generated compressional Pc3 waves, Pc3 range modulations of chorus waves and keV electrons, and Pc3 auroral pulsations. Plain Language Summary: South Pole Station, Antarctica (SPA), is a unique polar observatory, which can monitor dayside aurora from the ground throughout austral winter. High time resolution optical instruments at SPA frequently capture auroral pulsations with Pc3 frequencies (22–100 mHz) around the midday hours. Here we use unique conjunctions of ~2‐Hz SPA all‐sky camera and Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft to identify the origin of the periodicity of daytime Pc3 auroral pulsations observed at SPA and their driver in the magnetosphere. Simultaneous in situ measurements in the dayside outer magnetosphere observed identical Pc3 magnetic pulsations in the compressional component. Lower‐band chorus waves and keV electron fluxes were observed to be modulated also with the Pc3 range. The common Pc3 frequency in the magnetosphere and ionosphere matched well with the expected frequency of upstream ultralow frequency (ULF) waves generated in the foreshock region. The simultaneous SPA‐THEMIS spacecraft conjunctions provide, for the first time, direct evidence that upstream‐origin compressional Pc3 magnetic pulsations drive daytime auroral pulsations through wave‐particle interactions. One important implication of this study is that the high temporal cadence 2‐D observations of daytime auroral pulsations can be a useful tool for remote sensing the dynamics of outer magnetospheric Pc3 ULF waves. Key Points: Daytime auroral pulsations have a one‐to‐one correspondence with compressional Pc3 magnetic pulsations in the outer magnetosphereIn situ lower‐band chorus waves and keV electron fluxes are modulated by the compressional Pc3 magnetic pulsationsUpstream ULF waves are one of the most influential sources that control the frequency of the daytime auroral pulsations [ABSTRACT FROM AUTHOR]

The scattering of charged particles by oxygen ion cyclotron harmonic (OCH) waves in the inner magnetosphere is investigated by evaluating the relevant quasi‐linear diffusion coefficients. Recent studies demonstrated that OCH waves are oxygen ion Bernstein modes and their complex kinetic dispersion relation has made it challenging to assess their role in scattering charged particles. The present study calculates the quasi‐linear diffusion coefficients for the scattering of electrons and ions by OCH waves using their kinetic dispersion relation. The results show that OCH waves can effectively scatter electrons between ∼100 eV and 100s keV via Landau resonance. They are also capable of heating cold helium and oxygen ions through cyclotron resonances. Specially, it is found that the 4th harmonic of OCH waves can lead to effective heating of helium ions, while oxygen ions would interact more efficiently with lower harmonics of OCH waves. Plain Language Summary: Oxygen ion cyclotron harmonic (OCH) waves observed in the inner magnetosphere often have multiple spectral peaks at harmonics of the local oxygen ion cyclotron frequency. They have been shown to be excited by hot oxygen ion loss‐cone or ring/ring‐like distributions and follow a complicated kinetic dispersion relation for oxygen ion Bernstein waves. Since OCH waves cannot be described by the relatively simple cold plasma dispersion relation, it has been difficult to calculate their diffusion coefficients in scattering charged particles in quasi‐linear theory. The present study numerically solves the kinetic dispersion relation for OCH waves and then uses the results to calculate the corresponding quasi‐linear diffusion coefficients for electrons and ions. The diffusion coefficients obtained show that OCH waves can effectively interact with ∼100 eV to 100s keV electrons and are capable of heating cold helium and oxygen ions. Thus, OCH waves have their own unique contribution to the particle dynamics in the inner magnetosphere. Key Points: Quasi‐linear diffusion coefficients are evaluated for particle scattering by oxygen ion cyclotron harmonic (OCH) waves for the first timeOCH waves can scatter electrons in a wide energy range (∼100 eV–100s keV) via Landau resonanceOCH waves are capable of heating cold helium and oxygen ions through cyclotron resonance [ABSTRACT FROM AUTHOR]

Previous studies have shown that auroral kilometric radiation (AKR) can play an important role in the magnetosphere‐atmosphere coupling and has the right‐handed extraordinary (R‐X), left‐handed ordinary (L‐O) and left‐handed extraordinary (L‐X) modes. However, the L‐X mode has not been directly observed in the lower latitude magnetosphere yet, probably because of its very limited frequency range. Here, using observations of the Arase satellite on 6 September 2018, we present an AKR event with two distinct bands (8–20 and 300–1000 kHz) around the location: L = 8 and latitude = −37°. The low (high) band is identified as the L‐X (R‐X) mode based on the polarization and frequency ranges. Simulations of 3‐D ray tracing show that most of ray paths with 14 (11 and 18) kHz pass (miss) the location of Arase, basically consistent with observations. Our study provides direct evidence that the L‐X mode can propagate from high latitudes downward to lower latitudes. Plain Language Summary: Auroral kilometric radiation (AKR) is a widely existing radio emission with kilometric wavelength at the Earth, contributing to the magnetosphere‐atmosphere coupling. Similar emissions have been observed on all magnetic planets of the solar system. Previous studies have shown that AKR primarily occurs in the R‐X mode, with a small contribution in the L‐O and L‐X modes. The L‐X mode at lower latitudes has not been directly observed so far, most likely due to its extremely limited frequency range. Here, we present an L‐X mode (peak frequency ∼14 kHz) in the lower latitude magnetosphere observed by the Arase satellite. Using the 3‐D ray tracing method, we simulate ray paths with different initial wave parameters and source locations. Simulations show that ray paths with 14 (11 and 18) kHz pass (miss) the location of the Arase satellite and are highly dependent on initial wave parameters and the location of source. Our results provide a direct evidence that the L‐X mode from high latitude source regions can propagate downward to lower latitudes under suitable conditions. This study enriches the understanding of AKR propagation characteristics in the magnetosphere. Key Points: An auroral kilometric radiation (AKR) event with two distinct bands (8–20 kHz and 300–1000 kHz) is observed around the location: L = 8 and latitude = −37°Based on the polarization and frequency ranges, the low (high) band AKR is identified as the L‐X (R‐X) mode3‐D ray tracing simulations show that L‐X mode can propagate downward to lower latitudes, basically consistent with observations [ABSTRACT FROM AUTHOR]

Two‐dimensional (2‐D) hybrid model is developed to investigate the second harmonic (SH) generation of electromagnetic ion cyclotron (EMIC) waves. Applying the singular value decomposition method to simulated fields, we show that the SH exhibits wave properties analogous to typical EMIC waves generated by ion cyclotron instabilities, that is, left‐hand polarization and small wave normal angle. However, the bicoherence index inferred from simulated fields reflects a strong phase coupling between the fundamental wave (FW) and the SH, illustrating the nonlinear generation of the SH by the FW. The necessary conditions, especially for the wave vector relation, are further verified from a 2‐D perspective. The simulated amplitude ratios well meet the theoretical results only in the SH saturation stage, while the necessary conditions remain satisfied almost throughout the simulation. This study provides a comprehensive analysis of the SH excitation in a 2‐D simulation domain, contributing to a deeper understanding of EMIC wave nonlinear generation. Plain Language Summary: Recent studies have unveiled the generation of nonlinear second harmonics (SH) of electromagnetic ion cyclotron (EMIC) waves. The SH is nonlinearly driven by the fundamental wave (FW) and satisfies the necessary conditions: ω2 = 2ω1 and k2 = 2k1, where ω1 (ω2) and k1 (k2) are the frequency and the wave vector of the FW (SH), respectively. Previous studies have relied on one‐dimensional (1‐D) hybrid simulations to investigate the SH. However, 1‐D simulations allow waves to propagate along a single dimension and lack spatial variation of the field orthogonal to this dimension, which impedes the complete verification of the relation k2//k1. Thus, in this study, the SH generation is modeled by two‐dimensional (2‐D) hybrid codes. The simulated SH exhibits characteristics similar to typical EMIC waves with left‐hand polarization and small wave normal angle. The bicoherence index is utilized to reveal the phase coupling between the SH and FW. The necessary conditions, especially for the wave vector relation, are verified from a 2‐D perspective for the first time. Additionally, the amplitude ratios of the SH to the FW and their phase velocities are compared with theoretical results. The comprehensive analyses of this study provide substantial evidence for the SH generation mechanism. Key Points: Two‐dimensional (2‐D) hybrid codes are developed to model the nonlinear second‐harmonic (SH) generation of electromagnetic ion cyclotron wavesThe necessary conditions required by the SH generation mechanism are verified from a 2‐D perspectiveThe theoretical amplitude ratios between harmonics are only satisfied in the SH saturation stage [ABSTRACT FROM AUTHOR]

The temperatures of ions in the magnetospheres of Jupiter and Saturn were observed to increase substantially from about 10 to 30 planet radii. Different heating mechanisms have been proposed to explain such observations, including a heating model for Jupiter based on magnetohydrodynamic (MHD) turbulence with flux‐tube diffusion. More recently, an MHD turbulent heating model based on advection was shown to also explain the temperature increase at Jupiter and Saturn. We further develop this turbulent heating model by combining effects from both diffusion and advection. The combined model resolves the physical consistency requirement that diffusion should dominate over advection when the radial flow velocity is small and vice versa when it is large. Comparisons with observations show that previous agreements, using the advection only model, are still valid for larger radial distance. Moreover, the additional heating by diffusion results in a better agreement with the temperature observations for smaller radial distance. Plain Language Summary: The temperatures of ions in the magnetospheres of Jupiter and Saturn were observed to increase substantially near the planet. This suggests that there should be some heating sources to counter the cooling effect due to expansion. There have been several models trying to explain such observation using different heating mechanisms, including a heating model for Jupiter based on turbulence and diffusion effects, as well as a model based on advection effects for Jupiter and Saturn. We further develop a heating model by combining effects from both diffusion and advection. The combined model resolves the physical consistency requirement that diffusion should be stronger than advection nearer to the planet, but shifting to the opposite farther away. Comparisons with observations show that previous agreements using the advection only model are still valid, and are improved by including diffusion nearer to the planet. Key Points: A new model for the heating of the magnetospheres of Jupiter and Saturn by magnetohydrodynamic turbulence is developedThe model combines effects from diffusion and advection such that each is dominant when the radial velocity is small or large, respectivelyPredictions of the temperature and radial velocity profiles agree better with Jupiter and Saturn observations than previous models [ABSTRACT FROM AUTHOR]

The bright proton aurora is generally understood to be the projection of the Central Plasma Sheet where there is sufficient particle energy to cause auroral luminosity and strong pitch angle scattering (presumably due to field line curvature). This region is often interpreted as the transition region between dipolar and tail-like magnetic topologies. The location of auroral features relative to the peak in the proton aurora has been used, for example, to argue that the substorm onset arc lies on field lines that thread this transition region. In this paper, we present statistics of proton aurora luminosity computed from Time History of Events and Macroscale Interactions in Substorms electrostatic analyzer measurements for various radial distances in the magnetotail. These results are compared to ground observations of proton auroral luminosity and used to derive a statistical source region of the bright proton aurora. [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation. Plain Language Summary: The Earth's magnetosphere is a dynamic region shaped by the interplay between the solar wind and Earth's magnetic field. This interaction occurs at the boundary of the magnetosphere (magnetopause) through a process known as magnetic reconnection, giving rise to Flux Transfer Events (FTEs), which are magnetic structures that carry flux and energy into the magnetosphere. These FTEs form either in sudden bursts, patchy patterns or in a continuous, and relatively stable way making the magnetopause surface dynamic. As the FTEs move along the boundary of the magnetosphere, they create compressed regions and lead to wave generation that can extend into the magnetosphere. The study uses an advanced 3D hybrid‐Vlasov simulation model to analyze waves originated from FTE formation and propagation at the magnetopause. We find that rapidly moving and large FTEs have a significant impact on the magnetopause, leading to the generation of ULF waves with frequency above 0.1 Hz. This shows first direct evidence supporting previous theoretical speculations regarding the ability of FTEs to generate waves near the magnetopause. Key Points: Dayside Pc2 waves (>0.1 Hz) have been detected in a 3D hybrid‐Vlasov simulationThese waves exhibit lower intensity within the magnetosphere at noon, compared to the prenoon and postnoon sectorsPc2 waves observed in the simulation are associated with largest and fast moving flux transfer events initiated by subsolar reconnection [ABSTRACT FROM AUTHOR]

Using the high‐quality measurements of plasma by the Magnetospheric Multiscale (MMS) spacecraft, we investigate the wave‐particle interaction between cold protons and Electromagnetic ion cyclotron (EMIC) waves of different polarizations in the inner magnetosphere. Both the left‐hand and right‐hand polarized EMIC waves can interact with cold protons via non‐resonance interaction, and energize the cold protons (∼eV) up to suprathermal energies (∼tens of eV), resulting in a series of bridge‐like arcs in the ion energy spectrogram. Cold protons experience different types of phase bunching in left‐hand and right‐hand polarized EMIC waves. However, the relative angles between the velocities of bunched protons and wave magnetic or electric fields are independent of the polarizations of EMIC waves. Thus, the energy transfer process is not affected by the polarization of EMIC waves. The observations indicate that EMIC waves can affect the magnetospheric particle dynamics by energizing the cold protons. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves play an important role in the magnetospheric particle dynamics via wave‐particle interactions. Previous studies mainly focus on the resonance interaction between particles and EMIC waves. The resonance interaction is effective but it needs the energy of particles to be above the minimum resonant energy. In our observation, we find that the cold protons (∼eV) are energized up to suprathermal energies (∼tens of eV) by the EMIC waves, resulting in a series of bridge‐like arcs in the ion energy spectrogram. The energy of cold protons is much less than the minimum resonant energy, suggesting that the cold protons should be energized by the non‐resonance interaction. The polarization of EMIC waves could change the gyrophase distribution of bunched cold protons, but the energy transfer process is not affected by the polarization of EMIC waves. The observations suggest EMIC waves can affect the magnetospheric particle dynamics by energizing the cold protons. Key Points: Both the left‐hand and right‐hand polarized electromagnetic ion cyclotron (EMIC) waves can interact with cold protons via non‐resonance interactionThe cold protons experience different types of phase bunching with different polarizations of EMIC wavesThe energy transfer process is not affected by the polarization of EMIC waves [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) instabilities in the inner magnetosphere are investigated with considering the effect of temperature anisotropic electrons. The perpendicular anisotropic electrons (Ae > 1) impose an inhibiting effect on EMIC instabilities, whereas parallel anisotropic electrons (Ae < 1) may generate a promoting or inhibiting effect depending on the direction of wave group velocity. The electron firehose modes can be coupled with EMIC modes. Three unanticipated secondary waves nearby the ion cyclotron frequencies are aroused by the electrons resonating with EMIC waves. The strong wave growth at proton gyrofrequency indicates that the electrons as an important energy provider for EMIC modes have been largely overlooked in previous studies. For the first time, we report a backward EMIC wave with the group velocity in opposite direction of the phase velocity. We suggest the new‐found effects from hot anisotropic electrons may improve our understanding of EMIC instabilities in the inner magnetosphere. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are produced by the anisotropic ions. It is generally believed that electrons have nothing to do with the generation of these waves. However, recent studies for the solar wind have demonstrated that temperature anisotropic electrons can affect the EMIC instability. We extend the study into the inner magnetosphere. The wave growth rate and dispersion relation are calculated in a multicomponent plasma consisting of H+, He+, and O+. Our studies suggest that MeV electrons with Ae > 1 have a strong inhibiting effect on the EMIC instability. Whereas, Ae < 1 electrons may generate a promoting or inhibiting effect depending on the direction of wave group velocity. A wave mode coupling occurs between the EFH and EMIC waves. The dispersion curves of EMIC waves in the three bands may turn over from below the ion cyclotron frequencies to above them due to a reversal of the group velocity. The special wave mode modulation greatly changes the dispersion relations, and brings the characteristic frequency of EMIC waves into better agreement with the statistical data, such as the occasional presence of hydrogen waves at ω > ΩH and helium waves at ω < 0.5ΩHe, and the infrequent occurrence of oxygen waves at ω < ΩO. Key Points: Ae > 1 electrons inhibit electromagnetic ion cyclotron (EMIC) instabilities, whereas Ae < 1 electrons promote or inhibit EMIC instabilities depending on the Vg directionThe resonating electrons can provide energy for EMIC modes and generate secondary waves nearby the ion cyclotron frequenciesFor the first time, we report a backward EMIC wave with a group velocity in opposite direction of the phase velocity [ABSTRACT FROM AUTHOR]

Chorus subpackets/subelements are the wave packets occurring at intervals of ∼10–100 msec and are suggested to play a crucial role in the formation of substructures within pulsating aurora. In this study, we investigate the evolution of subpackets from the upstream to downstream regions. Using Van Allen Probe A measurements, we have found that the frequency of the upstream subpackets increases smoothly, but that of the downstream subpackets remains almost unchanged. Through a simulation in the real‐size magnetosphere, we have reproduced the subpackets with characteristics similar to those in observations, and revealed that the frequency chirping is influenced by both resonant current of electrons and wave amplitude due to nonlinear physics. Although the resonant currents in the upstream and downstream regions are comparable, the wave amplitude increases significantly during evolution, resulting in lower sweep rate in the downstream region. Our findings provide a fresh insight into the evolution of chorus subpackets. Plain Language Summary: Subpackets within chorus waves are suggested to play a significant role in producing the substructures within pulsating aurora. How does the frequency change inside subpackets is still an open question. In this study, the subpackets are found with Van Allen Probe A observation to be excited upstream of the magnetic equator, and propagate toward downstream. The frequency of subpackets increases with time in the upstream region, while it keeps almost unchanged in the downstream region. Meanwhile, a particle‐in‐cell simulation has been performed to study the characteristics of subpackets, and the simulation results agree well with those in observations. The frequency variation of subpackets is influenced by both resonant electrons and wave amplitude. Our study provides a clue for better understanding the nonlinear wave‐particle interactions in the evolution of chorus subpackets. Key Points: The source region of chorus subpackets has been observed by Van Allen Probe AThe chorus subpackets have been investigated via the general curvilinear particle‐in‐cell simulation in the real‐size magnetosphereNonlinear physics is a dominant process in the evolution of chorus subpackets [ABSTRACT FROM AUTHOR]

Chorus waves are electromagnetic emissions in the magnetosphere and play a crucial role in energetic electron dynamics. Observations show that these waves often exhibit a two‐band structure with a power gap near half the electron cyclotron frequency. However, the mechanism behind this gap remains an open question, despite the existence of multiple theories. Here we propose a model to explain the banded structure by the different propagation properties of the lower and upper band waves within a density crest. This requires independent sources for lower and upper band waves, and unducted propagation of upper band waves to the observing location. The model naturally generates a gap near half the electron cyclotron frequency and can be extended to explain lower‐band‐only, upper‐band‐only, and no‐gap whistler waves. The model calls for further research on density irregularities in the magnetosphere and their effects on shaping wave properties. Plain Language Summary: Whistler mode waves are intense electromagnetic emissions naturally occurring in the magnetosphere. These waves can produce relativistic electrons in hazardous radiation belts and cause the precipitation of energetic electrons into the atmosphere. Satellite observations reveal that whistler waves often exhibit a two‐band structure with a power minimum near half the electron cyclotron frequency, dividing the waves into lower and upper bands. Despite several theoretical models, a consensus on the origin of this banded structure is still lacking. In this study, we propose a model that explains the banded form of whistler waves based on the different propagation properties of lower and upper band waves in the presence of a density irregularity. Our model can also be applied to explain other types of whistler waves, such as those with a lower or upper band only, or waves with no power gap. Key Points: Lower and upper band waves have different sources, and upper band waves reach the observing location through unducted propagationDensity irregularities in the magnetosphere play important roles in the formation of banded chorus wavesThe model can be extended to explain the formation of no‐gap, lower band only, and upper band only whistler waves [ABSTRACT FROM AUTHOR]

Since the discovery of the large‐scale field‐aligned currents it is widely acknowledged that gaps exist between the Region 1 (R1) and Region 2 (R2) currents in which the current values are relatively small as compared to neighboring regions. Assuming that the field‐aligned currents are generated by plasma pressure gradients, we analyzed data collected by the THEMIS satellites between 2007 and 2011 to identify regions with very low plasma pressure gradients (pressure plateaus), which could be responsible for the appearance of these gaps. It was found that the pressure profiles with low radial gradients are typically located between 8 and 10 Radii around the Earth. Projections of pressure plateau regions onto ionospheric altitudes, for both individual events and on a statistical basis, coincide with the locations of gaps between Iijima and Potemra field‐aligned currents. The role played by identified pressure plateaus in shaping the pattern of large‐scale field‐aligned currents is discussed. Plain Language Summary: Field‐aligned currents that flow between the magnetosphere and ionosphere along magnetic field lines play a crucial role in magnetosphere‐ionosphere interactions. Despite being discovered at the start of the space era, their origins are still subject to debate. In our current study, we have established that the observed gaps between the upward and downward currents correspond to the plateau regions of constant plasma pressure, as obtained using data from the THEMIS mission. This finding provides strong evidence in favor of the generation of field‐aligned currents by plasma pressure gradients in the transition region between the dipole and tail‐like geomagnetic field. This is a critical piece of information for understanding the global dynamics of Earth's magnetosphere. Key Points: During quiet geomagnetic conditions, the regions of plasma pressure plateau are observed in the inner magnetosphereThese regions are located at geocentric distances of 8–10 RE forming a ring surrounding the EarthThe localization of the plasma pressure plateau region and gaps between the Region 1 and 2 currents should be located at the same distance [ABSTRACT FROM AUTHOR]

The magnetic dips can create the electron butterfly pitch angle distribution (PAD) in the inner magnetosphere. However, its universality is still unconfirmed. We statistically investigate the global distribution of the occurrence rate of the magnetic dip‐related butterfly PAD for 466 keV and 2.1 MeV electrons using the measurements of the twin Van Allen Probes spanning from September 2012 to November 2018. The statistical results show that the magnetic dip‐related butterfly PADs are mainly confined to duskside to midnight within 4.5 < L < 6, and the peak occurrence rate (30% for 466 KeV and 70% for 2.1 MeV) occurs at the duskside equator around L = 5. The parametric study results suggest that the butterfly PADs preferably occur for high energy electrons with a negative initial radial flux gradient and a nearly isotropic initial PAD, which is consistent with the statistic results. This work confirms the vital importance of the magnetic dips on the evolution of the electron PADs. Plain Language Summary: A previous case study has shown that small localized depression of the background magnetic field can lead to the formation of butterfly pitch angle distribution (PAD) of the energetic electrons in the outer radiation belt. We present a statistical study to investigate the occurrence rate of the electron butterfly PAD with magnetic field depression at different locations in the near‐Earth space. The statistical result shows that the magnetic field depression‐related electron butterfly PAD preferably takes place at duskside equator with altitude ~4 Earth radius at which the occurrence rate reaches 30% for low energy electrons (~400 keV) and 70% for high energy electrons (~2 MeV). We also apply a parametric study to understand in detail how the electron butterfly PAD is created by magnetic field depression and the result suggests that the butterfly PAD preferably occurs for high energy electrons with a negative initial radial flux gradient and a nearly isotropic initial PAD, which is consistent with Van Allen Probe observations. The results of this paper have confirmed the generality of the magnetic field depression‐related electron butterfly PAD in the inner magnetosphere and improved the understating of the influence of magnetic field variation on electron PAD in the space environment. Key Points: The occurrence rate of bz dip‐related electron butterfly PADs peaks (30% for 466 keV and 70% for 2.1 MeV) at duskside equator around L = 5Bz dip events are common at night side and duskside within L = 4–6 and the depth of bz dips peaks (~10%) around L = 5 at MLT = 18Butterfly PADs preferably occur for electron with large negative radial flux gradient and isotropic initial PAD in large bz dip at high L [ABSTRACT FROM AUTHOR]

On 05 December 2015 the Magnetospheric MultiScale constellation observed interstellar pickup ion distributions during an inbound crossing of Earth's perpendicular bow shock near the subsolar point, which provides new insights into shock acceleration of pickup ions throughout the heliosphere. In this study we analyze the upstream and downstream velocity distributions of H+ and He+ using data from the Hot Plasma Composition Analyzer on Magnetospheric MultiScale. We derive average two‐dimensional pitch angle distributions in the upstream and downstream field‐aligned bulk plasma frame, as well as integrated one‐dimensional velocity distributions. By comparing the upstream and downstream distributions, we find evidence of accelerated H+ and He+ downstream of the shock. By comparing measured and theoretical reflection ratios for He+, we attribute a significant part of this acceleration to a single reflection at the shock. Plain Language Summary: By studying distributions of ions before and after a shock in space, we gain valuable insights into shock acceleration mechanisms that produce high‐energy ion populations, observed throughout the heliosphere. On 05 December 2015 the Magnetospheric MultiScale constellation crossed Earth's perpendicular bow shock and measured velocity distributions of He+ ions before and after the shock. This study analyzes these distributions in search for signs of accelerated He+. We find that He+ is accelerated mainly perpendicular to the local magnetic field, and that a significant fraction of the accelerated He+ is consistent with the theory of shock reflection at the bow shock, in which an ion is reflected at the shock and then energized by the solar wind electric field (pointing parallel to the shock surface) as the ion gyrates back to the shock. Key Points: MMS observed interstellar pickup He+ upstream and downstream of the Earth's perpendicular bow shockComparison of upstream and downstream velocity distributions revealed accelerated pickup He+The observations are consistent with single reflection shock acceleration [ABSTRACT FROM AUTHOR]

We obtain current densities from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), alongside By and Bz from the Interplanetary Magnetic Field (IMF) for March 2010. For each AMPERE spatial coordinate, we cross‐correlate current density with By and Bz, finding the maximum correlation for lags up to 360 min. The patterns of maximum correlation contain large‐scale structures consistent with the literature. For the correlation with By, the lags on the dayside are 10 min at high latitudes but up to 240 min at lower latitudes. Lags on the nightside are 90–150 min. For Bz, the shortest lags on the dayside are 10–20 min; on the equatorward edge of the current oval, 60–90 min; and on the nightside, predominantly 90–150 min. This novel approach enables us to see statistically the timescales on which information is electrodynamically communicated to the ionosphere after magnetic field lines reconnect on the dayside and nightside. Plain Language Summary: We take the minute‐by‐minute behavior of the interplanetary magnetic field (which is in the solar wind) and electric currents flowing along Earth's magnetic field lines in the Northern Hemisphere. We move them with respect to one another to find the time lag required to make them agree best, and then note the best agreement and the time lag that we found. We plot both of these quantities on maps of the Earth's Northern Hemisphere, and then analyze these maps to uncover new information about Earth's reaction to the solar wind. Key Points: We cross‐correlate Birkeland current density with IMF By and Bz and find morphology of R1/R2 and other current systemsBz direct driving timescales are 10‐20 min; expanded polar cap timescales, 60‐90 min; and nightside timescales, 120‐150 minBy shows 10‐min timescales on the dayside at high latitudes, but lags up to 240 min elsewhere [ABSTRACT FROM AUTHOR]

Using the Van Allen Probe‐A observations, we obtained the global distribution of proton rings and calculated the linear wave growth rate of fast magnetosonic (MS) waves in the region L ~3–6. Our statistical and calculated results demonstrate that MS waves can be locally excited on the dayside outside the plasmapause, as well as in the dusk sector inside the plasmapause. The frequency range of unstable MS waves is strongly modulated by the ratio of the proton ring velocity (Vr) to the local Alfvén speed (VA). High harmonic MS waves (ω > 20ΩH+) can be excited outside the plasmapause where Vr/VA < 1, while low harmonic MS waves (ω < 10ΩH+) with frequencies less than ~30 Hz are found to be excited both outside and inside the plasmapause where 1 < Vr/VA < 2. Plain Language Summary: Since MS waves can propagate radially and azimuthally in the inner magnetosphere, it is difficult to attribute observed MS waves to propagation or local excitation. Because MS waves can be excited by proton ring distributions, it will be an effective method to research the characteristics of locally excited MS waves through global distribution of observed proton rings. With observations of Van Allen Probe‐A spacecraft between October 2012 and August 2015, we obtain the global statistical distribution of the occurrences of proton rings both outside and inside the plasmapause. Proton rings, as a source of exciting fast magnetosonic (MS) waves, are distributed mainly on the dayside at L ~3–6. In order to find the characteristics of locally excited MS waves, through combining observed particle properties and a quantitative calculation of linear wave growth rate, we demonstrate that MS waves can be locally excited on the dayside outside the plasmapause, as well as in the dusk sector inside the plasmapause. The frequency range of unstable MS waves is strongly modulated by the ratio of the proton ring velocity to the local Alfvén speed. This paper provides the global distribution of proton rings and locally excited MS waves. Key Points: This paper provides the global statistical distribution of proton rings both outside and inside the plasmapauseMS waves are found to be locally excited on the dayside outside the plasmapause, as well as in the dusk sector inside the plasmapauseThe manuscript is helpful to understanding the global distribution characteristics of locally excited MS waves [ABSTRACT FROM AUTHOR]

The Kolmogorov scaling in the inertial range of scales is a distinct characteristic of fully developed turbulence, and studying it offers valuable insights into the evolution of turbulence. In this work, we perform a statistical survey of the power spectra with the Kolmogorov scaling in Saturn's magnetosphere using Cassini measurements. Two cases study show that both magnetic‐field and electron density spectra exhibit f−5/3 at the MHD scales. The statistical analysis reveals a wide‐ranging and abundant presence of Kolmogorov spectra throughout magnetosphere, observed across all local times. Interestingly, the occurrence rate of these Kolmogorov‐like events within Saturn's magnetosphere surpasses that observed in the planetary magnetosheath. The measurements of magnetic compressibility for the Kolmogorov‐like events show the dominance of incompressible Alfvénic turbulence (44.64%) with respect to magnetosonic‐like one (6.94%). In addition, the source and evolution of the turbulent fluctuations are further discussed. Plain Language Summary: Turbulence is ubiquitous in space and astrophysical plasmas, such as the solar wind, planetary magnetospheres, and the interstellar medium. Plasma turbulence has been widely studied in the solar wind and planetary magnetosheaths, but much less in the planetary magnetospheres. In the solar wind, power spectral density of the magnetic field fluctuations generally follows the so‐called Kolmogorov spectrum f−5/3 at the magnetohydrodynamic (MHD) scales, which suggests a fully developed turbulent state. In this study, we have discovered the widespread presence of Kolmogorov spectra in the Saturn's magnetosphere. The spatial distribution and nature of turbulent fluctuation for the Kolmogorov‐like events are also investigated in detail. Key Points: Magnetic field and electron density spectra have Kolmogorov scaling of f−5/3 at MHD scales in Saturn's magnetosphereThe spatial distribution of the Kolmogorov spectra within Saturn's magnetosphere reveals the extensive occurrence of Kolmogorov‐like eventsThe fluctuations for Kolmogorov‐like events are dominated by Alfvénic modes (44.64%) with respect to magnetosonic‐like one (6.94%) [ABSTRACT FROM AUTHOR]

The off‐equatorial minimum‐B‐pocket (OEMBP) at high latitudes in the dayside outer magnetosphere is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves, in addition to the magnetic equator. However, its crucial role in the excitation of the dayside ECH waves has never been directly investigated due to its spatiotemporal‐varying and difficulty to be located. Based on the observations from Magnetospheric Multiscale satellites, we statistically analyze the properties of dayside ECH waves by organizing them according to the latitude distance from the OEMBP located by the spatiotemporal‐varying geomagnetic field model. Such mapping confirms larger occurrence rates and electric field amplitudes of dayside ECH waves in the vicinity of the OEMBP, which gradually fall down nearby with the latitude distance increasing. Our results provide statistical evidence for the OEMBP as another source region of ECH waves besides the magnetic equator. Plain Language Summary: The off‐equatorial minimum‐B‐pocket is the local minimum of magnetic field strength at high latitude in the dayside outer magnetosphere, which is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves besides the magnetic equator. However, its importance for the dayside ECH waves at high latitudes has never been directly investigated since the off‐equatorial minimum‐B‐pocket is spatiotemporal‐varying and is difficult to be located. We use the solar wind‐dependent geomagnetic field model to locate the off‐equatorial minimum‐B‐pocket along the field line threading the observational point, and subsequently introduce the latitude distance to quantify the distance from the off‐equatorial minimum‐B‐pocket. Our results prove that dayside ECH waves in the outer magnetosphere are more frequent and stronger in the vicinity of the off‐equatorial minimum‐B‐pocket, demonstrating that it is indeed a source region of ECH waves. Key Points: Dayside electron cyclotron harmonic (ECH) waves on bifurcated magnetic field lines are statistically analyzed according to their latitude distance from minimum‐B‐pocketThese dayside ECH waves have larger occurrence rates and amplitude in the vicinity of the off‐equatorial minimum‐B‐pocketThe occurrence and amplitude of these dayside ECH waves are asymmetric relative to the off‐equatorial minimum‐B‐pocket [ABSTRACT FROM AUTHOR]

In this study employing data from the Swarm satellites at an altitude of ≈500 km, we present evidence of field‐line resonances (FLRs) in the Pc2‐3 (0.02–0.2 Hz) frequency band in the undisturbed post‐sunset region of the equatorial ionosphere. We identify 26 events extending to magnetic latitudes as low as 2°, which was previously thought to be too low for FLRs to exist due to ionospheric damping. Identification of FLRs is supported by a narrow‐band fundamental and first harmonics, an oscillating field‐aligned Poynting vector, a relative phase shift between E and δB of ≈90°, and in one case observation of one wave in conjugate hemispheres by two satellites separated by 4 min. We show that, contrary to the non‐uniform trend of ground‐based observations, the fundamental frequencies of these occurrences decline monotonically with increasing field‐line length. Their frequencies differ from those of ground‐based magnetometer observations reported in the literature. Plain Language Summary: Ultra‐low‐frequency (ULF) waves play a crucial role in the coupling of the magnetosphere‐ionosphere system. These waves have been reported abundantly in the high latitude ionosphere, where they are driven directly by processes in the magnetosphere and solar wind. ULF waves have also been reported at the equatorial ionosphere, but less frequently. In some cases, low‐latitude waves are thought to be driven by high‐latitude waves ducting through the ionosphere or by compressional waves, which can propagate across geomagnetic field lines from high altitudes in the equatorial region. These waves can then excite interhemispheric field‐line resonances (FLRs), in which Earth's magnetic field lines vibrate like a guitar string, with longer field lines oscillating at lower frequencies. In this work, we use the Swarm satellites to study low‐latitude FLRs at an altitude of ≈500 km in the quiet ionosphere at geomagnetic latitudes below 3°. We demonstrate the existence of FLRs with periods between 5 and 50 s which increase with magnetic latitude, as expected when field line lengths increase. Key Points: Field line resonances (FLRs) are identified at the lowest magnetic latitude ever reportedThe fundamental frequency of our events decreases with increasing magnetic latitude in a uniform trendFLRs detected at conjugate latitudes by two spacecraft at the equatorial ionosphere [ABSTRACT FROM AUTHOR]

We use the Kelvin‐Helmholtz instability (KHI) condition with particle and magnetic field observations from Jovian Auroral Distributions Experiment and MAG on Juno along the dawn flank of Jupiter's magnetosphere. We identify the occurrence of magnetopause crossings that show evidence of being KH (Kelvin‐Helmholtz) unstable. When estimating the k vector to be parallel to the velocity shear, we find that 25 of 62 (40%) magnetopause crossings satisfy the KHI condition. When considering the k vector of the maximum growth rate through a solid angle approach, we find that 60 of 62 (97%) events are KH unstable. This study shows evidence of KH waves at Jupiter's dawn flank, including primary drivers such as high velocity shears and changes in plasma pressure. Signatures of magnetic reconnection were also observed in ∼25% of the KH unstable crossings. We discuss these results and their implication for the prevalence of KHI at Juno's dawn magnetopause as measured by Juno. Plain Language Summary: The Kelvin‐Helmholtz instability occurs when a boundary separating two fluids of different densities is perturbed and these fluids are moving at different speeds, directions, or both. The difference in speeds across the perturbed boundary that separates the fluids creates wave structures as these fluids diffuse into each other. The Kelvin‐Helmholtz instability may be observed at the boundary that separates a planetary magnetic field (magnetosphere) from the stream of charged particles emitted by the Sun (solar wind); this boundary is known as the magnetopause. This instability is confirmed to occur at Earth and Saturn, but is not confirmed at Jupiter. This study analyzes the properties of the plasma and magnetic field in Jupiter's magnetosphere and the surrounding solar wind to identify signatures of the Kelvin‐Helmholtz instability. We find that out of 62 occurrences where the Juno spacecraft crosses the magnetopause, 25 events signify that the Kelvin‐Helmholtz instability is possible—primarily due to large differences in velocities—and 37 events do not. Key Points: There is evidence of Kelvin‐Helmholtz instability (KHI)‐driven waves along Jupiter's dawn flank magnetopause during the Juno prime mission24 (38.7%) crossings satisfied the KHI condition and 38 (61.3%) crossings did not satisfy the KHI conditionMagnetopause crossings that satisfied the KHI condition had, in general, larger velocity shears than those that did not [ABSTRACT FROM AUTHOR]