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We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons. Plain Language Summary: During magnetic storms and substorms, energetic ions are sporadically injected into the geospace, which distorts the stable population and velocity distributions of ions in space. At these moments, various plasma instabilities lead to ultra‐low frequency (ULF) wave excitations. The lowest‐frequency waves in the ULF range have a wavelength comparable to the size of the Earth and are typically analyzed using magnetohydrodynamic principles. This approach considers the plasma environment using macroscale parameters such as pressure and density. In this paper, we report a spacecraft observation of a broadband compressional ULF wave that cannot be interpreted using magnetohydrodynamics. Such waves have rarely been reported and analyzed; however, their interaction with energetic ions is important to understand magnetospheric energy dynamics. The plasma conditions were described using the kinetic theory, which involves particle velocity distributions. We observed that a drift resonance occurred between the energetic protons and waves, while the gradient instability condition was satisfied for a part of time. Therefore, we concluded that the wave was in a drift‐compressional mode excited through drift resonance and gradient instability. The interpretation of compressional waves via satellite observations of energetic ions has been receiving increasing attention to understand their excitation mechanism. Key Points: Pc4–5 compressional ultra‐low frequency waves with an azimuthal wave number of −130 were observed in the nose structure on dusksideTheoretically predicted values of drift‐compressional mode frequency match the observed wave frequencyBoth radial ion temperature gradient and drift resonance of 20–25 keV protons serve as energy sources of the wave [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]

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]

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]

Omega bands are mesoscale auroral structures emerging as eastward moving quasi‐periodic poleward protrusions well within the closed field line region of the auroral oval. Neither specific conditions of their appearance nor their causes are well understood. We perform a superposed epoch analysis of OMNI and SuperMAG measurements taken during 28 omega band events recorded by auroral all‐sky imager observations from 2006 to 2013 to identify their solar wind drivers. We find local enhancements in the solar wind flow speed, magnetic field, pressure, and proton density at the time of the omega band observation. In the magnetosphere‐ionosphere, we see enhancements in the ring current, partial ring current, and auroral electrojets. These features are consistent with geomagnetic activity caused by stream interaction regions (SIRs). 19 of our events overlap with SIRs from published event catalogs. Our findings suggest that omega bands are driven by compression regions commonly associated with SIR events. Plain Language Summary: Omega bands are eastward moving wave‐like structures in the aurora that typically appear at the equatorward border of the auroral oval during periods of enhanced activity in Earth's magnetosphere. However, the specific drivers of these structures are not well understood. In this work, we perform a statistical analysis of spacecraft observations taken from multiple omega band events to identify potential drivers of these structures. We find that the solar wind exhibits increased speed, pressure, and particle density when omega bands appear overhead. These features are consistent with localized compression in the solar wind generated when a fast solar wind stream interacts with a slower leading stream. Our work suggests that the appearance of omega bands is driven by this compression. Key Points: A statistical analysis of 28 omega band events shows that they are driven by extended periods of enhanced solar wind density19 of the 28 omega band events studied occurred during documented stream interaction region events in the solar windAnalysis of the flow speed during all 28 events indicate that omega bands are driven by compression regions rather than high speed streams [ABSTRACT FROM AUTHOR]

We report a rare regime of Earth's magnetosphere interaction with sub‐Alfvénic solar wind in which the windsock‐like magnetosphere transforms into one with Alfvén wings. In the magnetic cloud of a Coronal Mass Ejection (CME) on 24 April 2023, NASA's Magnetospheric Multiscale mission distinguishes the following features: (a) unshocked and accelerated low‐beta CME plasma coming directly against Earth's dayside magnetosphere; (b) dynamical wing filaments representing new channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope; (c) cold CME ions observed with energized counter‐streaming electrons, evidence of CME plasma captured due to by reconnection between magnetic‐cloud and Alfvén‐wing field lines. The reported measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to understand how sub‐Alfvénic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets close to their host stars. Plain Language Summary: Like supersonically fast fighter jets creating sonic shocks in the air, planet Earth typically moves in the magnetized solar wind at super‐Alfvénic speeds and generates a bow shock. Here we report unprecedented observations of Earth's magnetosphere interacting with a sub‐Alfvénic solar wind brought by an erupted magnetic flux rope from the Sun, called a coronal mass ejection (CME). The terrestrial bow shock disappears, leaving the magnetosphere exposed directly to the cold CME plasma and the strong magnetic field from the Sun's corona. Our results show that the magnetosphere transforms from its typical windsock‐like configuration to having wings that magnetically connect our planet to the Sun. The wings are highways for Earth's plasma to be lost to the Sun, and for the plasma from the foot points of the Sun's erupted flux rope to access Earth's ionosphere. Our work indicates highly dynamic generation and interaction of the wing filaments, shedding new light on how sub‐Alfvénic plasma wind may impact astrophysical bodies in our solar and other stellar systems. Key Points: MMS observed a rare regime of magnetosphere interaction with unshocked low‐beta CME plasmaWing filaments represent dynamical channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux ropeCold CME ions observed on closed field lines, likely generated by dual‐wing reconnection [ABSTRACT FROM AUTHOR]

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary: The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points: On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphereMMS's observations represent the first in situ measurements of Alfvén Wing current structuresThe current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary [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]

Interplanetary shocks (IPS) can initiate prompt acceleration of relativistic electrons in the Earth's radiation belt, which is related to the generation and propagation of impulsive electric field (IEF). We investigate the effect of IEF on accelerating radiation belt electrons in the 6 September 2017 IPS event. A "surfing" effect of electrons with respect to the electric field, referring to electrons that drift together with the tailward‐propagating IEF in the duskside, is investigated in this study. Our results show that the maximum increase of electron differential flux is at 3.4 MeV by a factor of 2.2, corresponding to a drift velocity of 531 km/s, which is more consistent with the IPS propagating speed of 621 km/s rather than the fast‐mode speed of 1,074 km/s. We suggest that the effect of IPS propagation is important for radiation belt dynamics, and we highlight the potential importance of the parameter of IPS propagation speed. Plain Language Summary: Acceleration and injection of the relativistic electrons are frequently occurred when interplanetary shocks impinge on the Earth's magnetosphere. The impulsive electric field is the direct driver and its propagation may strongly affect the efficiency of the acceleration. In this study, we use the Van Allen Probes observation to investigate the interplanetary shock‐induced acceleration of the relativistic electrons occurred on 6 September 2017. It is found that the drift velocity of electrons in the energy ranges that experience a maximum increase in differential flux is better correlated with the propagation speed of the interplanetary shock rather than the fast‐mode speed in the inner magnetosphere. A new mechanism of the effect of interplanetary shocks' propagation on radiation belt dynamics is proposed, and the potential importance of the propagation speed of the interplanetary shock on electron acceleration is highlighted. Key Points: The effect of the propagation of the shock‐induced electric field on accelerating radiation belt electrons is investigatedThe drift velocity of electron experiencing maximum flux increase is consistent with the interplanetary shock's propagation speedA new mechanism of how interplanetary shocks' propagation influence the radiation belt dynamics is proposed in this study [ABSTRACT FROM AUTHOR]

The plasma density is one of the most fundamental quantities of any plasma yet measuring it in space is exceptionally difficult when the density is low. Measurements from particle detectors are contaminated by spacecraft photoelectrons and methods using plasma wave emissions are hampered by natural plasma instabilities which dominate the wave spectrum. Here we present a new method which calculates the density from magnetosonic waves near the lower hybrid resonance frequency. The method works most effectively when the ratio of the plasma to cyclotron frequency is fpe/fce < 3.5. The method provides a lower bound on the plasma density. Using the new method we show that wave acceleration of electrons to relativistic energies is increased by orders of magnitude. The method enables years of satellite data to be re‐analyzed for the Earth and the effectiveness of wave acceleration at the Earth, Jupiter and Saturn to be re‐assessed. Plain Language Summary: The electron plasma density is a fundamental quantity of any plasma, but it is very difficult to measure in space using satellites. Satellites charge to different potentials with repel or attract electrons making the true measurement very difficult. The plasma density can be determined from wave oscillations at the plasma frequency, but the waves are difficult to identify as the wave spectrum is often dominated by other much stronger waves. Here we analyze satellite data and show that magnetosonic waves near the lower hybrid resonance frequency can be used to calculate the plasma density. This method provides a lower bound on the density. We show that this lower density leads to much faster electron acceleration by wave‐particle interactions, accelerates electrons to much higher energies and increases the electron flux at one MeV by two orders of magnitude or more. The method enables the importance of electron acceleration at the Earth, Jupiter and Saturn to be re‐evaluated. Key Points: New method to calculate plasma density from magnetosonic waves near the lower hybrid resonance frequencyMethod enables low densities to be measured near the magnetic equatorLow density increases wave acceleration of electrons by orders of magnitude [ABSTRACT FROM AUTHOR]

Using a two‐way coupled magnetohydrodynamics with embedded kinetic physics model, we perform a substorm event simulation to study electron velocity distribution functions (VDFs) evolution associated with Bursty Bulk Flows (BBFs). The substorm was observed by Magnetospheric Multiscale satellite on 16 May 2017. The simulated BBF macroscopic characteristics and electron VDFs agree well with observations. The VDFs from the BBF tail to its dipolarization front (DF) during its earthward propagation are revealed and they show clear energization and heating. The electron pitch angle distributions (PADs) at the DF are also tracked, which show interesting energy dependent features. Lower energy electrons develop a "two‐hump" PAD while the higher energy ones show persist "pancake" distribution. Our study reveals for the first time the evolution of electron VDFs as a BBF moves earthward using a two‐way coupled global and kinetic model, and provides valuable contextual understanding for the interpretation of satellite observations. Plain Language Summary: Bursty bulk flows (BBFs) are identified as the fast earthward‐propagating flows from magnetic reconnection in the Earth's magnetotail. BBFs are related to particle energization and heating processes as reported by satellite observations. For the first time, we use a novel numerical model that simulates kinetic physics directly in a global model. The electron velocity distribution functions extracted from multiple locations associated with BBF show good agreements with the satellite in situ observations. The energization and heating of the electrons associated with BBF and their energy‐dependent pitch angle distributions are revealed. Key Points: For the first time, we simulate the bursty bulk flow (BBF) events with kinetic physics embedded in a global magnetosphere modelThe electron velocity distribution functions exhibit different anisotropy features at different locations of the BBFEnergy dependent electron pitch angle distribution evolutions are identified in the simulation [ABSTRACT FROM AUTHOR]

The electric fields of subauroral polarization streams (SAPS) have been suggested to affect energetic charged particles' dynamics in the inner magnetosphere, though their role on radiation belt electrons has never been properly quantified. A moderate geomagnetic storm on 2015‐09‐07 caused the deep injection of 10–100s of keV electrons in Earth's inner magnetosphere to low L* (L* < 4). Using a 2‐D test particle tracer, we present the effects of electric fields given by the Volland‐Stern model, a SAPS (Goldstein et al., 2005, https://doi.org/10.1029/2005ja011135) model, and a modified SAPS model on the energetic electron deep injections. The modified SAPS model reflects the SAPS electric field observations by the Van Allen Probes and is supported by Defense Meteorological Satellite Program observations. Simulations suggest that the SAPS electric field pushes 10–20 MeV/G electrons Earthward to L* ∼ 2.7 in 2.5 hr, much deeper compared to the Volland‐Stern electric field. Plain Language Summary: The study of energetic charged particle dynamics in Earth's inner magnetosphere is important as these charged particles pose a danger to the human technology systems (satellites, etc.) traversing through the region. The moderate to intense geomagnetic storms cause added complexity by transporting these charged particles into and out of the inner magnetospheric region. To understand this complexity, a study is conducted using Van Allen Probes observations for a moderate storm on 2015‐09‐07 which pushed 10–100 s of keV electrons deep into Earth's inner magnetosphere. It is suggested that the electric fields generated due to subauroral polarization streams (SAPS) are one of the major causes behind such deep injections, and that SAPS cause lower energy electrons to penetrate deeper into the inner magnetosphere than higher energy electrons. Key Points: A moderate storm on 2015‐09‐07 injected energetic charged particles into Earth's lower L* (≤4)The electric fields generated by subauroral polarization streams (SAPS) are a potential cause behind these deep injectionsThe quantitative fitting of the SAPS model to Van Allen Probes observations highly affects the simulated lower energy electron fluxes [ABSTRACT FROM AUTHOR]

Plasmaspheric density structures are considered to control the propagation trajectories of fast magnetosonic (MS) waves in the inner magnetosphere. However, whether the plasmaspheric plume can effectively alter the propagation of MS waves remains unknown. Based on the analytical model of plasma density, ray tracing simulations are performed to investigate the propagation of exactly perpendicular MS waves in the equatorial plane in the magnetosphere containing a plasmaspheric plume. We find that plasmatrough and plume MS waves propagating toward the plasmaspheric plume can be reflected into the plasmaspheric core by the plume, then potentially migrating globally and thus quasi‐trapped inside the plasmaspheric core. The simulations also indicate that lower‐frequency MS waves approaching the plasmaspheric plume are more easily reflected and quasi‐trapped inside the plasmaspheric core. Our findings illustrate a previously unexplored way that plasmatrough MS waves could access and be trapped inside the plasmaspheric core via azimuthal plasmaspheric density structures. Plain Language Summary: Fast magnetosonic (MS) waves are one of the most common and intense electromagnetic emissions observed both inside and outside the plasmasphere. Portions of MS waves observed inside the plasmasphere are supposed to come from the propagation of those waves outside the plasmasphere. It is well known that the radial plasmaspheric density structures can strongly alter the propagation trajectories of MS waves in the inner magnetosphere. However, a typical azimuthal plasmaspheric density structure, the plasmaspheric plume, can form in the magnetosphere during the geomagnetic disturbed times. How the plasmaspheric plume affects the propagation of MS waves remains unknown. Based on the ray tracing simulations, we show that MS waves generated outside the plasmapause and inside the plume can be reflected into the plasmaspheric core by the plasmaspheric plume and a few of the reflected waves can migrate globally inside the plasmaspheric core. Our findings provide a new potential way for MS waves to access and be trapped inside the plasmaspheric core. Key Points: Propagation of magnetosonic waves in the magnetosphere containing a plasmaspheric plume is examined using the ray tracing simulationsPlasmatrough and plume magnetosonic waves can be reflected into the plasmaspheric core by the plasmaspheric plumeThose reflected magnetosonic waves can be quasi‐trapped inside the plasmaspheric core or propagate radially down to low altitudes [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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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 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]

Jupiter's auroral emission is a spectacular phenomenon that provides insight into energy release processes related to the coupling of its magnetosphere and ionosphere. This energy release is influenced by solar wind conditions. Using joint observations from Juno and the Hubble Space Telescope (HST), we statistically investigate the relationship between auroral power and current sheet variations under different solar wind conditions. In this study, we reveal that during global main auroral brightening events that are closely connected to solar wind compressions, the dawn side current sheet is substantially thinner than during times when a quiet auroral morphology is present. Furthermore, the total current intensity in the current sheet is found to increase under solar wind compression conditions compared to the quiet period. These findings provide important observational evidence for how magnetospheric dynamics driven by solar wind behavior affect auroral activity, deepening our understanding of the coupling between Jupiter's magnetosphere and ionosphere. Plain Language Summary: Jupiter, the largest planet in our solar system, has a fascinating and powerful auroral emission that can help us understand the interactions between its magnetic field and the charged particles in its atmosphere. These auroral emissions are influenced by solar wind conditions, which are streams of charged particles coming from the Sun. In this study, we used observations from the Juno spacecraft and the Hubble Space Telescope to investigate the relationship between Jupiter's auroral emissions and changes in the planet's magnetic field. We found that during periods of increased solar wind pressure, the magnetic field layer, known as the current sheet, becomes thinner compared to times when the aurora is quiet. These findings offer valuable evidence of how Jupiter's magnetic field is affected by solar wind behavior and improve our understanding of the relationship between the planet's magnetic field and its aurora. This research helps us better comprehend the complex processes occurring in Jupiter's magnetosphere and can potentially enhance our knowledge of similar phenomena occurring on other planets. Key Points: The features of the current sheet in Jupiter's dawnside magnetosphere are highly relevant to auroral contextsDuring a solar wind compression event, the dawnside current sheet becomes substantially thinner than during quiet timesThe total current intensity of the current sheet is much higher during solar wind compression events than the quiet times [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]

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 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]

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]

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]

By using the magnetic field data from Van Allen Probes, we analyzed the distribution characteristics of the electromagnetic environment in the inner magnetosphere on different Dst* index and magnetic local time. Our results show that for the response of different current systems, the dawn‐dusk and noon‐midnight asymmetry distribution of the residual magnetic field δB increases with Dst* index. When Dst* < −60 nT, a "banana"‐shaped geomagnetic field negative disturbance peak region appears in the sector from midnight to dusk. Then, we obtained the azimuthal current density and found the asymmetric internal eastward and external westward ring current. Through the vector analysis of three‐dimensional current density, the current density vector distribution in the magnetic equatorial plane is completely displayed for the first time, which directly proves the existence of banana current near r = 3.0–4.0 RE during strong geomagnetic storms. Plain Language Summary: By using the magnetic field data from Van Allen Probes, we find that the electromagnetic environment of the Earth's inner magnetosphere exhibits different distributions on different geomagnetic disturbance strengths. During the occurrence of strong magnetic disturbances, the residual magnetic field δB will exhibit distinct dawn‐dusk and noon‐midnight asymmetric distributions. And a "banana"‐shaped peak region of negative geomagnetic field perturbation will appear in the midnight to dusk sector, while both the eastward and westward ring currents exhibit asymmetry. The presence of the banana current near r = 3.0–4.0 RE during strong geomagnetic storms is then directly demonstrated by vector analysis of the 3D current density. Key Points: The negative perturbation response of space currents to the geomagnetic field in the inner magnetosphere is analyzedBoth the eastward ring current and westward ring current is asymmetricThe vector distribution diagram of banana current density is obtained [ABSTRACT FROM AUTHOR]

We show the possible existence of a significant axial asymmetry in the reconnection separator at the Jovian magnetopause using first‐principle, physics‐based global simulations. Under eastward interplanetary magnetic field (IMF) conditions near Jupiter's orbit, reconnection occurs at the southern‐dusk and northern magnetopause with large shear angles between the magnetospheric and magnetosheath magnetic fields. When driven by the westward IMF, the reconnection position switches to the northern‐dusk and southern magnetopause. Component reconnection at the southern‐dusk/northern‐dusk magnetopause is associated with the interaction of the IMF with the nearly‐dipolar background fields. Nearly‐antiparallel reconnection near the noon‐midnight plane at the northern/southern magnetopause is related to the dawn‐dusk asymmetric, helical, closed lobe magnetic fields, which is a consequence of significant planetary corotation effects and not expected at Earth. Such configuration is testable as Juno has proceeded its orbit to the high‐altitude cusps and provides new insight into the interpretation of measurements from other rotationally‐driven systems. Plain Language Summary: Magnetic reconnection is a fundamental plasma process for understanding the dynamic evolution of planetary magnetospheres. It is related to the breaking and reconnecting of magnetic fields from two different sources. Depending on the orientation of the interplanetary magnetic field (IMF), reconnection at a planetary magnetopause is a combination of antiparallel and component reconnection of magnetic fields in the outer magnetosphere and the inner magnetosheath. In the terrestrial magnetosphere, magnetopause reconnection that is dominated by solar wind‐magnetosphere interactions is generally symmetric about the Sun‐Earth axis in solar‐magnetospheric coordinates regardless of the IMF orientation. However, this axial symmetry may not exist in planetary magnetospheres where the time scale of planetary rotation is much shorter than the solar wind transition, especially at Jupiter that has a faster planetary rotation and a significantly larger magnetosphere than Earth's. This study focuses on an unusual asymmetry in magnetospheric configuration and magnetopause reconnection at Jupiter and its relationship with fast planetary rotation. Key Points: Unlike at Earth, magnetopause reconnection at Jupiter may exhibit a significant axial asymmetry under east‐west interplanetary magnetic field (IMF) conditionsThis asymmetry is caused by the helical, closed lobe magnetic field with dawn‐dusk asymmetry due to fast Jovian rotationReconnection may occur at northern and southern‐dusk magnetopause under eastward IMF and has a north‐south reversal for westward IMF [ABSTRACT FROM AUTHOR]

Precipitation into the Earth's atmosphere due to pitch angle scattering by plasma waves has been recognized as one of the major loss mechanisms for energetic electrons. In this study, we quantitatively evaluate their roles in precipitating electrons during a conjunction event with modulated electron precipitation observed at low altitudes by Electron Loss and Fields INvestigation and three types of whistler mode waves (hiss, plume hiss, and chorus) measured near the equator by Time History of Events and Macroscale Interactions during Substorms. Electron precipitation was observed from ∼50 keV to <1 MeV with a spatial modulation, suggested by a good correlation between L shell‐sorted precipitation fluxes and wave intensities. A quasi‐linear analysis supports the observed energy range of precipitation and the ratio of precipitating‐to‐trapped flux. Our findings reveal that the modulated energetic electron precipitation is driven by hiss, plume hiss, and chorus waves. Plain Language Summary: Energetic electrons precipitated from the inner magnetosphere into the upper atmosphere can form diffuse and discrete aurora and modulate the ionospheric conductance. One of the major drivers of electron precipitation is wave‐particle interaction with whistler mode waves. In this study, we use the Electron Loss and Fields INvestigation CubeSats to measure electron precipitation at low altitudes and the Time History of Events and Macroscale Interactions during Substorms to provide wave and plasma measurements near the magnetic equator in the conjugate locations. We find that the electron precipitation rate is highly correlated to the whistler mode wave intensity near the equator. Through a quasi‐linear analysis, we demonstrate that the modulation of electron precipitation is driven by whistler mode hiss, plume hiss, and chorus waves that occur in an extensive region of the Earth's magnetosphere. Key Points: Modulated electron precipitation from tens to hundreds of keV over L shells of 4–9 is observed by Electron Loss and Fields INvestigation at low altitudesA good correlation is observed between the spatial variations of electron precipitation and wave intensities of hiss, plume hiss, and chorusQuasi‐linear modeling based on the observed wave and plasma parameters reproduced the observed electron precipitation [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]

Juno satellite observations have illustrated substantial examples of broadband electron energization up to 105–106 eV levels. In order to explain these observations, we use a hybrid gyrofluid kinetic‐electron model in an untilted dipolar topology to illustrate energization to high levels in weak current conditions by inertial Alfvén waves close to the Jupiter ionosphere for ambient plasma densities and magnetic field perturbations inferred from Juno satellite observations. The key to the high energization is the extremely low densities evident in the observations which necessitates the acceleration of electrons to very high velocities in order to carry the field‐aligned current. Plain Language Summary: Juno satellite observations have illustrated broadband energization that is associated with Alfvén waves which are electromagnetic waves with field vectors perpendicular to the background magnetic field that have common analogy to waves on a string. Specifically, the energization is associated with inertial Alfvén waves (IAWs) which are the limit of Alfvén waves when the perpendicular scale length of the wave is on the order of the electron skin depth whereby the waves can support significant parallel electric fields to accelerate electrons that lead to aurora. This energization is described as broadband since the range of energies extends from tens of eV to MeV levels. At Earth, IAWs are associated with electron energization to keV levels and so it has been a puzzle to explain the very high electron energies evident at Jupiter. In this work, we illustrate that for observed parameters, IAWs can energize electrons to much higher energies at Jupiter than in the terrestrial analogue. This is because the very low plasma densities evident in the high latitude Jupiter magnetosphere necessitates the IAW to generate a sufficiently large parallel electric field to accelerate a limited supply of electrons to correspondingly higher energies in order to carry the parallel current. Key Points: In Jupiter's density depleted flux tubes, inertial Alfvén waves can accelerate electrons to very high energies for modest parallel currentsDue to the high energies, this energization can be manifest in primarily unidirectional electron beamsIt is expected that inertial Alfvén waves can explain observed electron energizations to 105−106 eV levels [ABSTRACT FROM AUTHOR]

We report on observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft at Mars, in the region of the ion plume. We observe that in some cases, when the number density of oxygen ions is comparable to the density of the solar wind protons interaction between both plasmas leads to formation in the magnetosheath of mini induced magnetospheres possessing all typical features of induced magnetospheres typically observed at Mars or Venus: a pileup of the magnetic field at the head of the ion cloud, magnetospheric cavity, partially void of solar wind protons, draping of the interplanetary magnetic field around the mini obstacle, formation of a magnetic tail with a current sheet, in which protons are accelerated by the magnetic field tensions. These new observations may shed a light on the mechanism of formation of induced magnetospheres. Plain Language Summary: There is a class of the induced planetary magnetospheres when the absence of intrinsic magnetic field allows a direct interaction of solar wind with planetary atmospheres/ionospheres. We have shown the existence of mini‐induced magnetospheres at Mars. When the density of the extracted from the ionosphere oxygen ions becomes comparable with the proton density in solar wind mini‐induced magnetospheres with all typical features of the planetary induced magnetospheres arise. Key Points: Oxygen ions extracted from the Martian ionosphere interact with shocked solar wind in the magnetosheathWhen the ion densities of both plasmas become comparable the mini induced magnetospheres are builtThese Magnetospheres possess all typical features of the classical induced magnetospheres [ABSTRACT FROM AUTHOR]

Magnetosonic waves (MSWs) can be discrete or continuous in a time‐frequency spectrogram. For discrete MSWs that satisfy the Chirikov resonance overlap criterion (CROC), the spectrum can be regarded as continuous and hence the quasi‐linear theory can be employed to calculate the corresponding electron diffusion coefficient; otherwise, the contribution of each harmonic emission to the diffusion coefficient should be calculated separately. From the Van Allen Probe‐A (RBSP‐A) burst mode observations taken between January 2014 to December 2017, a total of 21,766 discrete MSW events were selected for statistical analysis, among which 314 events (1.44%) are found to not satisfy the CROC. The MSW events not satisfying the CROC are mainly distributed in the range between L ∼ 4 and 6 outside the plasmapause, they all have a frequency gap (e.g., a frequency difference ≥2 fcH between two discrete bands). Plain Language Summary: Radiation belt electrons resonate with magnetosonic waves (MSWs) through Landau and bounce resonances. Quasi‐linear diffusion theory is widely used in describing the interactions between charged particles and small amplitude, sufficiently broadband waves. However, the diffusion coefficients incorporated into numerical codes aiming to describe the inner magnetosphere dynamics are typically computed under the assumption that the MSW spectrum is continuous in frequency space. In this letter, based on high‐resolution Van Allen Probe‐A (RBSP‐A) burst mode data, two typical events in which discrete spectrum MSWs do not satisfy the Chirikov resonance overlap criterion (CROC) are reported. To analyze the global distribution and harmonic characteristics of MSWs that do not satisfy the CROC, 314 MSW events that do not satisfy the CROC are screened for statistical study. Key Points: Among 21,766 discrete magnetosonic waves (MSW) events, 314 events (1.44%) do not satisfy the Chirikov resonance overlap criterion (CROC) while 21,452 events (98.56%) satisfy the CROCOf 1,057 discrete MSW events with a frequency gap greater than 7 fcH, 281 events (26.58%) do not satisfy the CROCQuasi‐linear diffusion theory can be applied to all discrete spectrum MSWs with continuous harmonics [ABSTRACT FROM AUTHOR]