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Structure and Variability of the Martian Ion Composition Boundary Layer
Author(s) -
Halekas J. S.,
McFadden J. P.,
Brain D. A.,
Luhmann J. G.,
DiBraccio G. A.,
Connerney J. E. P.,
Mitchell D. L.,
Jakosky B. M.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2018ja025866
Subject(s) - solar wind , martian , polar wind , boundary layer , magnetosheath , magnetopause , geophysics , atmospheric sciences , physics , bow shock (aerodynamics) , mars exploration program , computational physics , plasma , astrobiology , mechanics , shock wave , quantum mechanics
A 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.