MAVEN Observations of the Martian Magnetospheric Response to Simultaneous Quasi‐Radial Interplanetary Magnetic Field and Low Dynamic Pressure

Overview & Synopsis

The state of Mars' induced magnetosphere is highly dependent on upstream solar wind conditions. Changes in dynamic pressure alter the locations of magnetospheric boundaries such as the bow shock and induced magnetosphere boundary, while the orientation of the interplanetary magnetic field (IMF) impacts the formation of the bow shock. We present Mars Atmosphere and Volatile EvolutioN observations from a period when the solar wind dynamic pressure was low and the IMF was nearly radial. The quasi‐radial IMF appeared to prevent the formation of the usual quasi‐perpendicular bow shock, instead producing a spatially extended quasi‐parallel shock in the observed region. The ambipolar potential in the magnetosheath/shock region enabled planetary heavy ions to leak through the shock and flow upstream. These ions were first accelerated by the solar wind motional electric field, which remained an important ion energization mechanism even though the solar wind flow and IMF were roughly aligned throughout most of the system. The ionosphere had expanded out to very high altitudes as the result of low dynamic pressure. The ionosphere was also unmagnetized, perhaps because of the formation of an ionopause‐like boundary. Finally, we compare two adjacent orbits and suggest that Mars' remanent crustal fields may fundamentally alter bow shock formation during radial IMF conditions. Studying unusual combinations of solar wind conditions like the case presented here is critical for understanding the fundamental physics driving induced magnetospheres.