Local Generation of Mirror Modes by Pickup Protons at Mars

Abstract

Mirror mode structures are born from a plasma instability driven by a large temperature anisotropy and appear downstream of planetary and interplanetary shocks, in their magnetosheath. As so-called “magnetic bottles” imprisoning dense and hot plasma, they are usually observed downstream of their region of formation, where the anisotropy is large and free energy is available, implying that they are advected with the plasma flow to the detection region. 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, which boasts an extended exosphere, an additional source of temperature anisotropy exists, through unstable ring-beam velocity distributions, that is, through ions locally ionized and subsequently picked up by the local electric fields. We report here for the first time an example of near locally-generated mirror mode structures due to pickup protons at Mars using the full plasma instrument suite on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We present events with mirror modes in quasi-perpendicular and quasi-parallel shock conditions, discuss the locality of their generation and show that, in addition to the classic quasi-perpendicular source of anisotropy, another source exists, that is, unstable pickup protons. The existence at Mars of this extra ion anisotropy-generating mechanism is reminiscent of comets.