Evidence for a Mixture of Mirror Mode and Electromagnetic Ion Cyclotron Waves in the Martian Magnetosheath

Abstract Electromagnetic ion cyclotron (EMIC) and mirror-mode (MM) waves are widely observed in terrestrial and other planetary magnetosheaths. Even though linear theory supports that EMIC and MM waves may grow at comparable rates under suitable plasma conditions, their coexistence is rarely reported within magnetosheaths, primarily due to their similar free energy source from anisotropic plasmas. Using MAVEN spacecraft data, we present the first direct in situ observations of mixed MM and EMIC waves in the Martian magnetosheath. Our observations reveal the concurrent presence of EMIC waves and MM waves, both generated in the Martian magnetosheath. Unlike in the Earth’s magnetosheath, where strong plasma compressions at the quasi-perpendicular bow shock can drive the growth of EMIC or MM waves, our results suggest that, in the Martian magnetosheath, the substantial ion anisotropy to generate EMIC and MM waves is provided by both upstream bow shock compressions and ubiquitous ion pickup processes of newborn ions. This study offers new insights into the role of ion pickup processes in the excitation and growth of EMIC and MM waves within planetary magnetosheaths, particularly in magnetosheaths where both shock heating and newborn ion pickup processes provide the prevailing anisotropic plasma environment, as seen in the magnetosheaths of Venus, Mars, and comets.