Perpendicular Heating of Solar Wind Protons Driven by Proton Cyclotron Waves Upstream of Mars

Proton cyclotron waves (PCWs) generated by newborn pickup ions are ubiquitous in the Martian upstream region, but their interaction with the solar wind remains poorly understood. Utilizing MAVEN observations, we report perpendicular heating of solar wind protons coincident with intense PCWs. During a 90 hr stream interaction region (SIR) passage, detailed observational and theoretical analyses of a representative event show that the measured large-amplitude ( B w / B 0 up to ∼47%) quasi-parallel PCWs are intrinsically right-hand polarized and propagate sunward in the plasma frame, enabling Landau and multiple low-order anomalous Doppler cyclotron resonances with solar wind protons. Test particle simulations indicate a strong wave-induced pitch angle and energy diffusion of solar wind protons over moderate-to-large folded pitch angles, with diffusion coefficients reaching 10 −4 –10 −3 s −1 . Our findings highlight a fundamental wave-particle interaction process that can explain the observed perpendicular heating of solar wind protons upstream of Mars. In terms of a cross-population energy transfer from planetary pickup ions to the incident solar wind, Martian PCWs act as an important agent to actively reshape the upstream plasma environment and alter the solar wind thermodynamics.