Structure and Variability of the Martian Ion Composition Boundary Layer

摘要

abstract_textpA complex boundary layer with a variety of charged particle and electromagnetic field signatures, including a transition between plasma predominantly of solar wind origin and plasma of planetary origin, lies between the Martian bow shock and the ionosphere. In this paper, we develop and utilize algorithms to autonomously identify and characterize this ion composition boundary (ICB), using data from the Mars Atmosphere and Volatile EvolutioN mission. We find an asymmetric ICB with a larger average thickness, lower altitude, and lower velocity shear in the hemisphere where the solar wind motional electric field points outward, as a result of the asymmetry of the mass loading process. The ICB thickness scales with the magnetosheath proton gyroradius at the top of the boundary layer but does not clearly vary with external drivers. The ICB location varies with solar wind ram pressure and crustal magnetic field strength, but does not clearly respond to solar wind Mach number or extreme ultraviolet irradiance. The ICB represents a distinct boundary for ion density and flow speed, but the magnetic field strength and direction typically do not vary significantly across the ICB. The plasma density and flow speed at the ICB vary seasonally, likely in response to variations in the neutral exosphere and/or atmosphere. However, the ICB on average remains at or below the altitude where pressure balance is achieved between the piled up magnetic field and the solar wind ram pressure, regardless of season or crustal magnetic field strength./ppPlain Language Summary Planets without global intrinsic magnetic fields like that of the Earth interact in a distinctly different manner with the solar wind, a hot ionized gas (known as a plasma) that flows out through our solar system from the Sun. The interaction region includes a complex boundary layer where plasma dominated by the light ions of the solar wind transitions to plasma mainly composed of heavy ions derived from the planetary atmosphere. This boundary layer forms an interface between solar and planetary matter, and processes that occur in this unique region can transfer energy from the fast solar wind particles to the initially slower atmospheric particles, leading to the escape of atmospheric species. We investigate this compositional transition at Mars, utilizing data from the Mars Atmosphere and Volatile EvolutioN mission, to determine its location, structure, and response to external and internal influences./p/abstract_text