Kaijun Liu

Magnetospheric chorus waves often accompany a gap in intensity near half of the electron cyclotron frequency, , whose generation process is still debated. One mechanism involving a nonlinear damping process proposed by Omura et al. (2009, https://doi.org/10.1029/2009ja014206 ) predicts a characteristic spectral signature where the lower cutoff of upper band waves, , follows local . Here, we statistically analyze spectral features of banded chorus waves from the Van Allen Probes and Arase spacecraft. Observationally, is almost one‐to‐one correlated with up to magnetic latitude for the banded chorus events in the midnight‐to‐dawn sector, but becomes nearly independent of for the events found on the dayside. By contrast, the upper cutoff of lower band waves, , depends only weakly on magnetic latitude and remains at approximately (where is the equatorial ) near the equatorial region, meaning that the power gap should have been already formed there. These results indicate that while the good correlation between and lends strong support for the nonlinear damping mechanism as a power gap enhancer during poleward propagation of chorus waves, its formation near the equatorial source region and/or on the dayside likely needs more concrete explanation.

In the induced magnetotail of Mars, the neutral sheet is an important channel for atmospheric ion escape, yet the role of heavy planetary pickup ions in shaping its geometry has been unclear. Using three‐dimensional hybrid simulations, we demonstrate that these ions induce a global neutral sheet asymmetry via mass loading. Ambient plasma flow deceleration bends draped interplanetary magnetic field (IMF) lines, extending the sheet along the direction at large IMF Parker spiral angles and tilting its ‐hemisphere segment toward the plane in the Mars–Solar–Electric coordinate system at small angles. The asymmetry's magnitude is modulated by both the planetary ion production rate and solar wind dynamic pressure. These results show that Mars' magnetotail is not solely governed by IMF draping and crustal fields, but also dynamically reshaped by pickup ions.

The interaction between the solar wind and the Martian upper atmosphere involves mass loading by newly created pickup ions (PUIs). Accordingly, these PUIs can modify the solar wind flow upstream of the Martian bow shock through momentum exchange. While the relevant solar wind deceleration and a deflection opposite to the solar wind convective electric field direction have been identified in previous studies, the present study reveals an extra solar wind deflection transverse to both the solar wind flow and the convective electric field by statistically analyzing the Solar Wind Ion Analyzer and magnetometer data of Mars Atmosphere and Volatile EvolutioN between 2015 and 2019. This transverse deflection demonstrates opposite shifts under two groups of interplanetary magnetic field orientations, with mode values of approximately +2.3 and −2.1 km s ⁻¹ . Both Student’s t-test and the Mann–Whitney U-test confirm the statistical significance of the transverse deflection reversal. An estimate based on momentum conservation between the solar wind and PUIs indicates that the observed transverse deflection is consistent with the expectation from PUI mass loading. This study provides the first direct observational evidence of the transverse solar wind deflection upstream of the Martian bow shock and demonstrates that the impact of PUI mass loading on the solar wind velocity is inherently three-dimensional, involving simultaneous a deceleration along the solar wind flow direction, a deflection opposite to the convective electric field direction, as well as an extra deflection perpendicular to both the solar wind flow and the convective electric field.

Linear instability analysis is performed to investigate the influence of solar wind parameters on instabilities driven by a cool pickup ion beam distribution, which is believed to excite the proton cyclotron waves upstream of Mars. Our analysis reveals that both parallel and oblique waves are excited, with parallel waves showing right-hand polarization and oblique waves exhibiting quasi-perpendicular, quasi-electrostatic characteristics at higher solar wind velocities. The growth rates of both types of waves increase with solar wind velocity, while solar wind plasma beta primarily enhances oblique wave growth, leaving parallel waves unaffected. Quasi-linear theory indicates that both parallel and oblique waves induce pitch-angle scattering of pickup ions, facilitating their escape from Mars. These findings advance our understanding of wave–particle interactions and their role in atmospheric escape at Mars.

Oxygen ion cyclotron harmonic waves, observed in the Earth's magnetosphere as discrete spectral peaks near harmonics of the oxygen ion cyclotron frequency, are widely attributed to the oxygen ion Bernstein instability (OIBI) driven by hot oxygen ions of shell‐ or ring‐like velocity distributions. In this study, we conducted a systematic parametric analysis of OIBI driven by shell distributions using linear kinetic theory to examine how key plasma ion parameters affect the instability. Our results reveal that increasing the hot O ⁺ concentration enhances the growth rate and broadens the harmonic range of unstable modes. A higher shell velocity of hot O ⁺ broadens the harmonic range of unstable modes and shifts them to smaller wave numbers. In contrast, increasing the hot O ⁺ thermal spread tends to reduce the growth rate. Moreover, cool heavy ions affect instability growth differently depending on the harmonics: Cool He ⁺ and O ⁺ preferentially dampen the fourth and first harmonics, respectively.

This study investigates upstream wave generation at quasi-parallel shocks using large-scale one-dimensional hybrid simulations. The results reveal a spatial evolution of ultra-low-frequency waves from the far upstream region to the shock front. In the far upstream region, the waves are quasi-monochromatic, incompressible, and exhibit left-hand polarization in the downstream frame. Closer to the shock front, these waves transition into shorter-wavelength compressional fluctuations characterized by both left- and right-hand polarizations. Linear kinetic instability analysis based on simulated upstream proton velocity distributions confirms that the observed waves are primarily excited by local instabilities driven by backstreaming proton populations throughout the foreshock region. Near the shock front, the wave activity arises from a combination of locally excited waves from hot diffuse ions and upstream-generated waves convected downstream, jointly producing the chaotic polarization and compressional signatures observed. The parallel shock simulation also captures an early splitting of backstreaming protons into two beams. Trajectory analysis identifies the low-energy beam as an artifact resulting from proton reflection prior to full shock formation, underscoring the need for adequate simulation time to achieve a more realistic foreshock. A comparison of simulations of shock normal angles ranging from 0° to 40° indicates that while the overall pattern of wave evolution remains consistent across the quasi-parallel regime, increasing shock normal angle enhances wave compressibility and induces a polarization transition from circular to elliptical in the far upstream region.

The generation mechanism of banded chorus is still debated. One hypothesis invokes the frequency‐dependent ducting of whistler waves by a localized density enhancement, where the lower band propagates ducted while the upper band is unguided. The present study extracts the background density structures during banded chorus activities and investigates the correlations between them. We find that (a) half of the banded chorus events are associated with small peak‐to‐base density irregularities (the ratio ); (b) only 28% of the banded chorus events occur with localized enhancements (by contrast, 14% for no‐gap chorus), with radial extents 10–600 km and peak‐to‐base ratios ; and (c) for these 28% events occurring with localized enhancements, no strong correlation between lower band chorus and the parameters of the density enhancements is found. We also find that most of the lower‐band waves in our banded chorus events can be considered being ducted by these localized moderate‐to‐large density structures and that the use of geometric optic theory can be justified for them. We conclude that although the banded chorus occurrence is somewhat favored with moderate‐to‐large density ducts, the lack of strong interpretable correlation between them calls for a more comprehensive theoretical framework aided by improved plasma density measurements.

High-speed electron flows (HSEFs) are widely regarded as a significant source of various plasma waves and instabilities, which can subsequently interact with electrons and significantly impact electron dynamics. Using high-resolution data from the Magnetospheric Multiscale (MMS) mission, Liu et al. (2025) collected HSEFs in the Earth's magnetotail from 2017 to 2021, proving an excellent basis for a statistical investigation of associated plasma waves. Here, we perform a statistical investigation of the plasma waves in and out of the plasma sheet, respectively. In the plasma sheet, the observed fluctuations, including upper-hybrid waves, broadband electrostatic waves (BEWs), and low-frequency electrostatic waves, are mainly associated with perpendicular-moving electrons. Out of the plasma sheet, MMS observed Langmuir waves, BEWs, and low-frequency electrostatic fluctuations, primarily related to field-aligned electrons. The association of the observed plasma waves with magnetic reconnection is also discussed.

The present study employs self-consistent three-dimensional global hybrid simulations of Mars–solar wind interactions to investigate how intrinsic magnetic fields regulate the escape of planetary ions with different masses, with escaping ion trajectories traced. Present-day Martian crustal magnetic fields modify ionospheric ion escape primarily by restructuring local electric and magnetic field configurations. First, they alter the magnetic topology (closed, open, or draped), inclination and intensity of magnetic field lines, thereby increasing local ion density and facilitating outward transport along open field channels. Second, they reduce the radial component of the local electric field, which directly influences ion acceleration.The combined effects preferentially enhance the escape of heavy oxygen ions while suppressing the escape of light hydrogen ions, mainly because light ions are more effectively trapped within strong closed crustal magnetic loops. Finally, we extend our investigation to ancient Mars conditions and compare how intrinsic magnetic fields in early and present epochs differently regulate planetary ion escape, providing insight into the long-term evolution of the Martian atmosphere.

An one-dimensional fluid model is presented which describes the generation of type III radiation as an antenna problem at which the triggering current pulse imitates the temporal evolution of the beam instability. The mechanism works without the involvement of the classical plasma emission via the parametric processes and the coalescence of waves. After linearization of the Maxwell-fluid equations and Fourier transform in space, the system of nine differential equations describing the temporal evolution of the fluid and electromagnetic quantities is solved numerically. It is shown that the commonly observed beating structure of the electromagnetic radiation in form of a double-peak in their spectra, commonly explained by parametric decay of the beam-excited Langmuir wave, is caused by the superposition of two wave modes of mixed polarisation (Langmuir/z wave) which belong the wave number of optimum mode coupling. Within in the same formalism the generation of the second harmonic of the electromagnetic radiation is calculated by taking into account the nonlinear currents as product of the first-order terms. Satellite observations of beam-excited Langmuir waves and solar type III radiation are discussed in the light of the presented antenna model.