Simultaneous Compressive Kinetic‐Scale Fluctuations and Second‐Harmonic Proton Cyclotron Waves Inside a Martian Magnetosheath Structure

Abstract 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.