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Mercury has a large loss cone difference in its two hemispheres due to the northward shifted magnetic dipole. The precipitation difference of energetic electrons in both hemispheres is poorly understood. We show that the northern precipitation is 2.5‐times higher than for a symmetric loss cone due to the effects of the enhanced whistler instability at the southern hemisphere with the larger loss cone. Simulations including nonlinear pitch angle scattering by the whistler‐mode waves show rapid (tens of milliseconds) electron flux modulation related to the wave subpacket structures by repeated interactions within a discrete wave element. The difference in the nonlinear whistler instability in the two hemispheres should enhance the electron precipitation, which, along with the direct impact effects of solar wind, contributes to Mercury's surface–magnetosphere coupling. Electrons hitting the planet's surface may be a possible factor in the formation of water through the formation of hydroxyl groups. Plain Language Summary: Mercury, the first planet from the Sun, has north–south asymmetric magnetic fields due to the northward shifted magnetic dipole from the planet's center. Computer simulations of plasma waves and electrons, taking into account Mercury's magnetic dipole offset, show that the northward precipitation of electrons is 2.5‐times higher than in the case of no magnetic dipole offset, which is the case like the Earth. This difference in electron precipitation fraction arises from a difference in the characteristics of wave growth due to the spatial characteristics of the planet's magnetic field, because plasma waves can efficiently push trapped electrons in Mercury's magnetosphere toward the planet's surface. This study contributes to the understanding of the electron precipitation fraction on Mercury and may help to estimate the production of water ice through the formation of hydroxyl groups not only by the direct impact of solar wind but also by the electron precipitation caused by plasma waves. Key Points: Whistler‐driven electrons at Mercury are simulated, as the electron precipitation on the planet surface may contribute to water productionFor an asymmetric (southward wider) loss cone, the northward precipitation fraction is 2.5‐times higher than for a symmetric loss coneRapid (tens of milliseconds) electron flux modulations can be observed as a signature of repeated interactions with wave subpackets [ABSTRACT FROM AUTHOR]

Nonlinear gyroresonant acceleration is an essential mechanism for the formation of relativistic electrons in Earth's radiation belts, yet we still have only limited knowledge of how electron motion interacts nonlinearly with plasma waves. Here we demonstrate the compelling suppression of whistler‐mode chorus emissions as a resonant wave with energetic electrons by tuning an external transmitter signal in space plasmas. Through forcible modification of electron motions via phase trapping by the external waves, the nonlinear gyroresonant current is suppressed along with the electron hole deformation. Moreover, this can decrease the population of relativistic electrons due to the lack of coherent resonant waves seen by the phase‐organized electrons. Understanding such nonlinear wave suppression effects is crucial for a potential active control of Earth's plasma environment. Plain Language Summary: Since the strong radiation in space can cause health risks for astronauts and satellite anomalies of electronics, understanding toward the active control of the particle populations is in increasing demand for sustainable space industry growth. The gyroresonant interactions between energetic electrons and whistler‐mode waves play important roles in the acceleration mechanism of the Earth's radiation belt electrons. We find that transmitter signals as an external wave source in space plasmas can utilize the changing of nonlinear electron motion by stimulating electrons in a phase space. We show that nonlinear whistler mode chorus wave excitation, which is the resonant wave source for energetic electrons, can effectively be suppressed by deforming the shape of electron holes to control resonant currents in the phase space. This mechanism is crucial for understanding the active control of Earth's plasma environments. Key Points: Chorus waves can be suppressed by inhibition of coherent interactions with resonant electrons due to adding external transmitter wavesThe external waves having a different frequency from the maximum linear growth can become incoherent from the point of view of electronsThe external waves can precipitate the seed electrons, then this reduces the anisotropy of particles and hence inhibits wave growth [ABSTRACT FROM AUTHOR]

The terrestrial bow shock is the boundary where supersonic solar wind slows down abruptly near the magnetopause. The shock front geometry could be modulated by surface waves to form rippled structures, which impact the acceleration process of the solar wind particles. However, the rippled structures are hard to be identified unambiguously due to the similar signatures in single‐spacecraft observations between rippled shocks and reforming shocks. Here, we utilize the four‐spacecraft observations from the MMS mission to investigate an event of quasi‐perpendicular bow shock crossing. The periodic oscillations of shock normal directions and normal velocities support the scenario of surface wave propagation in the tangential direction. We also reconstruct the shock profile along the normal direction, and its monotonic shape further excludes the occurrence of shock reformation. These ripples are found to modulate the reflected ions and whistler wave packets, which adds to the complexity of the bow shock plasma environments. Plain Language Summary: Collisionless shocks are ubiquitous structures in the space and astrophysical plasma environments where significant energy conversion occurs between electromagnetic energy, kinetic energy, and thermal energy. These structures often exhibit nonstationary signatures, from which important information on the associated plasma dynamics can be extracted. In this paper, we report the spacecraft observations of a bow shock crossing with periodic enhancements of the magnetic field strength. Such signatures have been previously attributed either to the rippled shock structures or to the shock reformation process. These two potential processes are examined via simultaneous observations at four different locations, which shows the periodic oscillations of the local shock normal directions indicative of rippled rather than reforming shocks. We also show that the rippled shocks could spatially modulate the reflected ions and whistler wave packets, so they can also be detected periodically by the spacecraft. This phenomenon has been reported on other planets across a wide range of Mach numbers, indicating that the shock ripples, universal in the plasma universe, play a critical role in impacting particle dynamics and shock evolution processes. Key Points: Periodic magnetic strength enhancements are observed during the bow shock crossing of the MMS spacecraftMulti‐spacecraft analysis supports the scenario of shock ripples rather than the shock reformation processThe surface waves alter the shock geometry, modulating the reflected ions and whistler wave packets [ABSTRACT FROM AUTHOR]

The 1 Hz whistler wave precursors attached to shock‐like structures are often observed in the foreshock. Using observations from the Magnetospheric Multiscale mission, we investigate the interactions between 1 Hz waves and ions. Incoming solar wind ions do not cyclotron‐resonate with the wave, since typically the wave is right‐handed in their frame. We demonstrate that solar wind ions commonly exhibit 180° gyro‐phase bunching from the wave magnetic field, understanding it with a reconciled linear theory and non‐linear trapping theory for non‐cyclotron‐resonant modulations. Along the longitudinal direction, solar wind ions experience Landau resonance, exhibiting either modulations at small wave potentials or trapping in phase‐space holes at large potentials. The results also improve our understanding of foreshock structure evolution and 1 Hz wave excitation. Shock‐like structures start with having incoming solar wind and remotely reflected ions from further downstream. The ion‐scale 1 Hz waves can already appear during this stage. The excitation may be due to shock‐like dispersive radiation or kinetic instabilities resonant with these remotely reflected ions. Ions reflected by local shock‐like structures occur later, so they are not always necessary for generating 1 Hz waves. The wave leads to ion reflection further upstream, which may cause reformation of shock‐like structures. In one event, locally reflected ions exhibit non‐cyclotron‐resonant modulation in the early stage, and later approach the anomalous cyclotron resonant condition with gyro‐phases ∼270°. The latter is possibly due to nonlinear trapping in regions with an upstream‐pointing magnetic field gradient, linked to reformation. Some additional special features, such as frequency dispersions, are observed, encouraging further investigations. Key Points: Solar wind ions do not gyro‐resonate with 1 Hz whistlers, while they commonly exhibit gyro‐phase bunching and Landau resonance with the waveSLAMS reflected ions show hints of gyro‐resonance with 1 Hz whistlers and losing energy to the fields, with aid of background B gradientWhistler‐ion interactions can cause ion reflections upstream of the main ramp, which in turn modifies shock‐like structures [ABSTRACT FROM AUTHOR]

Ultra‐low frequency (ULF) waves contribute significantly to the dynamic evolution of Earth's magnetosphere by accelerating and transporting charged particles within a wide energy range. A substantial excitation mechanism of these waves is their drift‐bounce resonant interactions with magnetospheric particles. Here, we extend the conventional drift‐bounce resonance theory to formulate the nonlinear particle trapping in the ULF wave‐carried potential well, which can be approximately described by a pendulum equation. We also predict the observable signatures of the nonlinear drift‐bounce resonance, and compare them with spacecraft observations. We further discuss potential drivers of the pendulum including the convection electric field and the magnetospheric dayside compression, which lead to additional particle acceleration or deceleration depending on magnetic longitude. These drivers indicate preferred regions for nonlinear ULF wave growth, which are consistent with previous statistical studies. Plain Language Summary: In Earth's magnetosphere, electromagnetic oscillations in the mHz frequency range referred to as ultra‐low frequency (ULF) waves have been frequently reported in space and ground‐based observations. These waves have been known to interact resonantly with magnetospheric particles during their drift and bounce motions, which plays a key role in the acceleration and transport of charged particles in the Van Allen radiation belts. Drift‐bounce resonance occurs when a resonant particle experiences repetitive patterns of the wave field over consecutive bounce cycles along its drift orbit, thus enabling a cumulative energy exchange between waves and particles. In previous studies of drift‐bounce resonance, a linearization approach is applied for simplicity. Here, we extend the conventional theory into the nonlinear regime, to formulate the nonlinear trapping of particles based on a pendulum equation and to predict the characteristic signatures of drift‐bounce resonance. Moreover, we discuss the potential drivers of the pendulum, including the magnetospheric convection and the dayside compression, which break the symmetry in the phase portrait of the particle trajectories and therefore, lead to continuous particle acceleration or deceleration depending on magnetic longitude. Our study on nonlinear drift‐bounce resonance is helpful to the understanding of the ULF wave‐particle interactions and the wave excitation processes. Key Points: A nonlinear framework for drift‐bounce resonance is developed to describe the particle behavior in Ultra‐low frequency (ULF) wavesNonlinear drift‐bounce resonance is characterized observationally by rolled‐up structures in particle pitch angle spectrumOur theory predicts that dayside and dusk sectors are preferred regions for nonlinear ULF wave growth [ABSTRACT FROM AUTHOR]

Chorus waves are intense electromagnetic emissions critical in modulating electron dynamics. In this study, we perform two‐dimensional particle‐in‐cell simulations to investigate self‐consistent wave‐particle interactions with oblique chorus waves. We first analyze the electron dynamics sampled from cyclotron and Landau resonances with waves, and then quantify the advection and diffusion coefficients through statistical studies. It is found that phase‐trapped cyclotron resonant electrons satisfy the second‐order resonance condition and gain energy from waves. While phase‐bunched cyclotron resonant electrons cannot remain in resonance for long periods. They transfer energy to waves and are scattered to smaller pitch angles. Landau resonant electrons are primarily energized by waves. For both types of resonances, advection coefficients are greater than diffusion coefficients when the wave amplitude is large. Our study highlights the important role of advection in electron dynamics modulation resulting from nonlinear wave‐particle interactions. Plain Language Summary: Wave‐particle interactions can modulate electron distributions through advection and diffusion. Previous studies focusing on advection and diffusion primarily relied on test particle simulations, which uses a simplified model of wave evolution. In this study, we perform self‐consistent simulations to investigate the wave‐particle interactions with chorus waves and quantify the advection and diffusion coefficients of resonant electrons. It is found that advection coefficients are greater than diffusion coefficients in both cyclotron and Landau resonances, indicating the significant role of nonlinear wave‐particle interactions. The quantification of advection and diffusion coefficients in a self‐consistent system is important for understanding and predicting the loss and energization processes in radiation belt electrons. This study complements previous diffusion models that regarded the evolution of electron dynamics in wave‐particle interactions as a slow diffusive process. Key Points: Electron advection and diffusion in wave‐particle interactions with chorus waves are investigated through self‐consistent simulationsThe second‐order time derivative of gyrophase angle is nearly zero for phase‐trapped electrons but is negative for phase‐bunched electronsThe advection and diffusion coefficients for cyclotron and Landau resonant electrons interacting with chorus waves are quantified [ABSTRACT FROM AUTHOR]

The present study provides an evidence for the generation of harmonics of magnetosonic waves in the Martian magnetosheath region. The wave signatures are manifested in the magnetic field measurements recorded by the fluxgate magnetometer instrument onboard the Mars Atmosphere and Volatile Evolution missioN (MAVEN) spacecraft in the dawn sector around 5–10 LT at an altitude of 4,000–6,000 kms. The wave that is observed continuously from 19.1 to 20.7 UT below the proton cyclotron frequency (fci ≈ 46 mHz) is identified as fundamental mode of the magnetosonic wave. Whereas harmonics of the magnetosonic wave are observed during 19.7–20.3 UT at frequencies that are multiple of fci. The ambient solar wind proton density and plasma flow velocity are found to vary with a fundamental mode frequency of 46 mHz. It is noticed that the fundamental mode is mainly associated with the left‐hand (LH), and higher frequency harmonics are associated with the right‐hand (RH) circular polarizations. A clear difference in the polarization and ellipticity is noticed during the time of occurrence of harmonics. The magnetosonic wave harmonics are found to propagate in the quasi‐perpendicular directions to the ambient magnetic field. The results of linear theory and Particle‐In‐Cell simulation performed here are in agreement with the observations. The present study provides a conclusive evidence for the occurrence of harmonics of magnetosonic wave in the close vicinity of the magnetosheath region of the unmagnetized planet Mars. Plain Language Summary: Mars is an unmagnetized planet. The solar wind particles that bombard Mars continuously, are responsible for the loss of its atmosphere. This scenario is opposite to that of the Earth, as its strong intrinsic magnetic field forms a protective shield around the planet, called the magnetosphere. Various electrostatic/electromagnetic waves are often generated in the Earth's magnetosphere, which are potential candidates for controlling the transfer of energy and momentum from one region to another. In case of Mars, it possesses a weakly induced magnetosphere, which is dynamic due to its continuous interactions with solar wind. The field measurements from the MAVEN spacecraft can be used to understand plasma waves in the vicinity of Mars. Here, we report the first observations of the harmonics of magnetosonic wave in the Martian magnetosheath region. The magnetosonic waves are low‐frequency compressive waves driven by the ions in the presence of magnetic field. These waves are known to play a role in the particle heating process in the Earth's magnetosphere. Therefore, its observation in the Martian plasma environment is of interest to the scientific community to understand their role in plasma heating in the Martian ionosphere‐magnetosphere system. Key Points: Harmonics of Magnetosonic wave are observed in the Martian magnetosphere in dawn sector (5–10 LT)Magnetosonic harmonics dominantly have right‐handed polarizationAmbient proton density and velocity found to vary with fundamental mode frequency of 46 mHz [ABSTRACT FROM AUTHOR]

We present a physical mechanism for generating ∼GeV ions in the Jovian radiation belts. The mechanism is called relativistic turning acceleration (RTA) and involves a special form of nonlinear wave trapping by electromagnetic ion cyclotron (EMIC) waves. Necessary conditions for RTA include a near‐equatorial source of EMIC waves, strong wave amplitudes (of the order of a few percent of the background magnetic field strength), and a source of ions of sufficiently high energy. RTA occurs when a fraction of equator‐ward moving ions encounters pole‐ward moving waves, and, in so doing, becomes entrapped and undergoes a turning motion. The trapped ions then move poleward in the same direction as the waves and eventually become detrapped, but during the turning motion the ions undergo significant acceleration. We rigorously verify this process by providing the theory of nonlinear interactions between relativistic protons and coherent EMIC waves. The RTA process has been previously established for the analogous whistler mode wave‐electron interaction. We carry out particle simulations for protons at R = 2RJ (where RJ = Jovian radius) interacting with EMIC waves of amplitude Bw = 0.02B0eq (where B0eq = background magnetic field strength at the equator). We confirm that a large portion of test protons experience RTA and that some protons of critical energy 240 MeV can be accelerated to 10 GeV in a period of 5 s. The nonlinear acceleration process is crucially controlled by the trapping condition 0 < S < 1 where S is the inhomogeneity factor. Plain Language Summary: We present a mechanism for energetic proton acceleration to relativistic energies with parameters representing the inner Jovian magnetosphere. We assume an electromagnetic ion cyclotron (EMIC) wave generated around the equator by anisotropic protons transported into the inner magnetosphere. After wave‐particle resonance occurs, energetic protons are trapped by the nonlinear potential of the wave. Energetic protons moving initially toward the equator change their direction of motion, and are accelerated by the wave electric field moving in the same direction of the wave propagating away from the equator. For an EMIC wave with amplitude 2% of the background magnetic field, protons ∼240 MeV are accelerated to 10 GeV in about 5 s through the relativistic turning motion near the equator. EMIC waves may contribute to the rapid formation of proton radiation belts in the inner Jovian magnetosphere. Key Points: We have conducted test particle simulations of relativistic proton acceleration by EMIC waves in the Jovian magnetosphereProtons with initial energy 240 MeV are accelerated to 10 GeV in 5 s through relativistic turning acceleration at R = 2RJEffective acceleration of protons is enhanced by anomalous trapping at low pitch angles, while it is limited in the parameter space [ABSTRACT FROM AUTHOR]