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Magnetosonic (MS) waves are dominant plasma waves causing severe Martian ionospheric erosion. They are generally considered to originate upstream of Martian bow shock with frequencies near the upstream proton gyrofrequency. However, whether MS waves can be locally excited lacks theoretical analysis. Here we present an event of MS waves with frequencies above and closely related to the local proton gyrofrequency in the Martian ionosphere. Concurrently, ring beam hot proton distributions are observed due to the penetration of magnetosheath protons. By employing the observed plasma and magnetic field data, the calculated linear growth rates for MS waves agree well with the observed wave power spectra, demonstrating that they can be locally excited by unstable ring beam hot protons at Mars. Our results could be of great help in understanding the excitation of MS waves in a heavy ion‐rich environment around unmagnetized planets. Plain Language Summary Magnetosonic (MS) wave is one of the most important plasma waves contributing to the Martian atmospheric loss. They are generally considered to originate upstream of the Martian bow shock. Recent observations at Mars have shown that some MS waves with frequencies near the local proton gyrofrequency were accompanied by ring/shell‐like hot proton distributions. However, whether these waves can be locally excited by such protons lacks the support of the theoretical analysis. In this letter, on the basis of linear instability analysis, we show that MS waves with frequencies well above the local proton gyrofrequency can be locally excited by unstable ring beam hot protons in the Martian ionosphere. These results advance our knowledge of the MS wave excitation in a heavy ion‐rich environment around planets without an intrinsic magnetic field. Key Points Magnetosonic waves above the local proton gyrofrequency are observed in the Martian ionosphere Ring beam hot proton distribution associated with magnetosonic waves is formed by the penetration of solar wind protons simultaneously Magnetosonic waves are locally generated by the ring beam hot protons

Electromagnetic ion cyclotron (EMIC) waves and mirror modes (MMs), both driven by ion temperature anisotropy, are commonly observed in planetary magnetosheaths. Conventional explanations for their co‐occurrence are largely based on linear instability theory in proton–electron plasmas, which requires comparable growth rates for the EMIC and MM instabilities. Magnetosheath plasmas, however, contain a fraction of heavy ions, and how such composition affects the coexistence of EMIC waves and MMs has been less explored. Using kinetic hybrid simulations with typical magnetosheath parameters, we show that although the presence of heavy ions suppresses the initial linear EMIC instability, EMIC waves arise as MMs develop. The evolving MMs generate flat‐top proton velocity distributions with enhanced resonant populations, which in turn excite EMIC waves. These results extend the conventional coexistence scenario of MMs and EMIC waves and reveal a new pathway for energy transfer among MMs, EMIC waves, and particles in magnetosheath plasmas.

Mirror-mode waves driven by the large temperature anisotropy of T ⊥/T ∣∣ > 1 have been widely observed in the solar wind, planetary magnetosheaths, heliosheath, etc. Recent studies have shown that the phase relations and thermodynamics of the mirror waves observed in the terrestrial magnetosheath may well be interpreted by the linear mixed kinetic–MHD theory of proton mirror instability. In particular, the energy laws possess the form of double-polytropic closures with the thermodynamic exponents being functions of β ⊥,∣∣ = p ⊥,∣∣/(B 2/2μ 0). In this study, we examine the time evolution of proton mirror instability based on the hybrid particle simulations for twenty sets of β ⊥,∣∣ values. Quantitative comparisons between the kinetic simulations, linear Vlasov theory, and observations are made in terms of the growth rates, phase relations, thermodynamic conditions, etc., which show high agreements. In particular, the dependences of various compressibility and thermodynamic exponents on β ⊥,∣∣ are confirmed by the kinetic simulations, which show that the polytropic exponents are in the ranges of γ ⊥ = 0.64 ± 0.21, and γ ∣∣ = 1.07 ± 0.12 consistent with the theoretical predictions and mirror observations of γ ⊥ < 1 and γ ∣∣ ≳ 1. It is shown that the observed features, including the various perturbations and wavelengths, may indeed be reproduced by the nonlinear simulations. The saturated temperature anisotropy β ⊥/β ∣∣ and plasma β ∣∣ show an anticorrelation, which may well be fitted by the modified mirror instability threshold of γ∣∣β∣∣=β⊥2/2+γ⊥β⊥ with γ ⊥ ≈ 0.8, γ ∣∣ ≈ 1.3, and the saturated magnetic field of δ B/B ≈ 0.26 ∼ 0.97 increases with increasing values of β ≈ 1.6 ∼ 8.3.

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 magnetosheath When the ion densities of both plasmas become comparable the mini induced magnetospheres are built These Magnetospheres possess all typical features of the classical induced magnetospheres

Mars directly interacts with the solar wind and hosts a complex plasma‐wave environment. Using MAVEN observations, we identify an event in the dusk‐side Martian magnetosheath where strongly compressive sub‐cyclotron kinetic‐scale fluctuations coexist with second‐harmonic proton cyclotron waves (PCWs). The low‐frequency fluctuations are quasi‐perpendicular and linearly polarized, whereas the harmonic PCWs are quasi‐parallel and left‐hand polarized. Their large magnetic compressibility indicates that slow‐/mirror‐like compressive contributions are important. An ion Alfvén ratio analysis shows that the low‐frequency fluctuations are inconsistent with both pure kinetic Alfvén wave and pure kinetic slow wave interpretations, favoring mixed kinetic‐scale contributions rather than a single pure mode. Ion velocity distributions reveal strong field‐aligned beams, suggesting a common free‐energy source. During intervals of enhanced low‐frequency activity, the fundamental PCW weakens while second harmonics persist, implying mode competition or background modification.

Magnetosonic (MS) waves can be generated by hot Maxwellian ring protons locally within the Martian upper ionosphere, characterized by a weak ambient magnetic field and the presence of cold plasma abundant in heavier ions. A comprehensive study on the resonant instabilities of MS waves making use of the derived growth rates show that ring proton population with energies around 100 eV is optimal for wave generation. Highly oblique propagation leads to sharp harmonic structures at frequencies closer to local proton gyrofrequency. An increase in ring energy, ωpe/Ωce${\\omega }_{pe}/{{\\Omega }}_{ce}$ratio, and heavier ion concentration decreases the wave growth rates. When the background ions have very low temperatures, O2+${\\mathrm{O}}_{2}^{+}$ions lower the growth rate more as compared to O+${\\mathrm{O}}^{+}$ions. Additionally, the increase in temperatures of cold electrons and ions have opposite effects, with the former increasing the growth rate and the latter decreasing it. Plain Language Summary Magnetosonic (MS) waves with frequencies near the local proton gyrofrequency are frequently observed in the Martian upper ionosphere. Recent observations confirm the presence of hot ring proton distributions coinciding with these wave detections. While instability of MS waves generated by ring protons have been studied in the context of Earth's magnetosphere, the Martian environment is notably different due to weaker ambient magnetic field and the presence of heavier ions in the background plasma. The current study on MS wave generation in Martian upper ionosphere reveals that an increase in ring energy, ωpe/Ωce${\\omega }_{pe}/{{\\Omega }}_{ce}$ratio, and background ion temperature all contribute to a decrease in wave growth rates, whereas a higher background electron temperature enhances the growth rate. Heavier ions at low temperature in the background plasma also suppress the growth rate, with O2+${\\mathrm{O}}_{2}^{+}$ions having predominant effect. Maxwellian ring protons with energies around 100 eV, likely penetrating from the magnetosheath, can locally generate lower harmonics of highly oblique MS waves in the Martian upper ionosphere. Key Points Hot Maxwellian ring protons of energy around 100 eV are favorable for the generation of Magnetosonic waves in the Martian upper ionosphere O2+${\\mathrm{O}}_{2}^{+}$ions are the primary contributors to the growth rate decrease when the heavier ions are at very low temperatures The growth rate is significantly affected by the ratio of electron plasma to cyclotron frequency and temperature of background species

As incident solar wind encounters the martian upper atmosphere, it undergoes deflection particularly in the magnetosheath. However, the plasma flow exhibits asymmetrical distribution features within this transition region, which is investigated by employing a three‐dimensional Hall magnetohydrodynamic (MHD) model from an energy transfer perspective in this study. Simulation results reveal that solar wind protons transfer momentum to ionospheric heavy ions through motional electric field in the hemisphere where the motional electric field points outward from the planet. In the opposite hemisphere, solar wind flow tends to be effectively accelerated by ambipolar and Hall electric fields. The distinct dynamics of solar wind protons in both hemispheres result in the asymmetrical deflection. Furthermore, the extent of asymmetry grows as the cross‐flow component of the upstream interplanetary magnetic field increases, but diminishes as the density of the solar wind proton increases, contingent upon the energy effectively acquired from ambipolar and Hall electric fields. Plain Language Summary Due to the lack of a global intrinsic magnetic field at Mars, the solar wind has a direct interaction with the upper atmosphere of the planet. During this interaction, heavy ions from the martian ionosphere can be accelerated by the motional electric field of the solar wind, resulting in an excess of momentum in the martian system that necessitates the deflection of solar wind protons in the opposite direction to maintain balance. In this study, we utilize a Hall‐MHD model to study the asymmetrical deflection of the solar wind in the martian magnetosheath from an energy transfer perspective. Simulation results indicate that solar wind protons tend to effectively acquire energy from the ambipolar and Hall electric fields in the hemisphere opposite to the direction of the motional electric field and transfer its energy to heavy ions through the motional electric field in the opposite hemisphere, leading to an asymmetrical deflection of the solar wind. Furthermore, the degree of asymmetry is impacted by external solar wind conditions, including the strength of interplanetary magnetic field cross‐flow component and the density of solar wind protons. These findings provide valuable insights into the flow asymmetries that arise during the interaction between Mars and solar wind. Key Points The multi‐fluid MHD model effectively reproduces the asymmetrical deflection of solar wind flow within the magnetosheath The asymmetrical deflection of solar wind is a consequence of the discrepancy in energy transfer patterns between the two hemispheres The impact of the strength of interplanetary magnetic field By and solar wind density on asymmetrical deflection is individually examined

The interplanetary magnetic field (IMF) is one of the primary factors influencing the Martian plasma environment. In this study, a multifluid magnetohydrodynamic model is adopted to investigate how variations in IMF affect planetary ion escape, particularly the tailward escape flux. Our results reveal that for nominal IMF direction ( 56° Parker spiral), as IMF intensity increases, the ion escape rate decreases considerably. This reduction is primarily due to the decrease in planetary ion density in the plume and the magnetotail, which is caused by the lower ion production rate through the charge exchange process under high IMF conditions. With high IMF conditions, the dynamo at the bow shock is significantly enhanced, leading to a more severe deceleration of solar wind protons and fewer protons entering the magnetosheath. Consequently, intensified electromagnetic fields create a stronger induced magnetosphere, which shields the Martian ionosphere and atmosphere. Although the enhanced loading process for planetary ions results in higher ion escape velocities, the overall ion escape fluxes decrease due to the significant reduction in planetary ion density.

An important aspect of energy dissipation in weakly collisional plasmas is that of energy partitioning between different species (e.g., protons and electrons) and between different energy channels. Here we analyse pressure–strain interaction to quantify the fractions of isotropic compressive, gyrotropic, and nongyrotropic heating for each species. An analysis of kinetic turbulence simulations is compared and contrasted with corresponding observational results from Magnetospheric Multiscale Mission data in the magnetosheath. In assessing how protons and electrons respond to different ingredients of the pressure–strain interaction, we find that compressive heating is stronger than incompressive heating in the magnetosheath for both electrons and protons, while incompressive heating is stronger in kinetic plasma turbulence simulations. Concerning incompressive heating, the gyrotropic contribution for electrons is dominant over the nongyrotropic contribution, while for protons nongyrotropic heating is enhanced in both simulations and observations. Variations with plasma β are also discussed, and protons tend to gain more heating with increasing β.

Plasma waves can initiate, regulate, or reflect magnetic reconnection efficiently converting magnetic energy into plasma energy. While waves ranging from below the ion cyclotron frequency to above the electron plasma frequency are commonly observed near reconnection sites, electromagnetic ion cyclotron (EMIC) waves—frequent in other plasma environments—have been rarely observed in the reconnection region. Here, we report the first detection of EMIC waves in a magnetic reconnection exhaust at Earth's magnetopause. The free energy required for EMIC wave growth was supplied by the strong perpendicular‐to‐parallel temperature anisotropy of hot proton beams. This proton temperature anisotropy was generated by magnetopause reconnection, rather than inherited from the magnetosheath. Our findings differ from previous reports of parallel‐preferential proton heating during magnetopause reconnection, calling for revised theoretical frameworks to reconcile observed perpendicular‐preferential heating with established reconnection paradigms.