On the role of pickup protons in the generation of mirror modes at Mars

In the magnetosheath of planets, mirror modes triggered by the mirror mode instability form as large magnetic structures imprisoning dense and hot plasma in their midst. The free energy created from a large pressure anisotropy at their origin can come from several sources. At Earth and other planets, the quasi-perpendicular shock provides the plasma with the necessary heating along the perpendicular direction to the local magnetic field. At Mars, the extended exosphere theoretically provides another source of temperature anisotropy, with exospheric neutrals locally ionised and subsequently picked up by local electric fields creating unstable ring-beam velocity distribution functions. Using the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission plasma instrumentation, we show for the first time at Mars the unmistakable signature of near locally-generated mirror mode structures due to pickup protons. The pickup ion mechanism is reminiscent of temperature anisotropy-generating mechanisms found at comets, the outgassing moons of Jupiter, and in other heliospheric scenarios.