Persistent pressure anisotropy in the subsonic magnetosheath region

Observations of proton fluxes in the subsonic flow region of the magnetosheath show double peaks as a function of angle in the equatorial plane of the spacecraft. The peaks are separated by a wide angle, usually more than 90 deg. Suggestions of double flux peaks are present in 95 per cent of the data. They are interpreted as the effect of a persistent pressure anisotropy. The clearest cases were analyzed to determine the orientation of the flux peaks relative to the magnetic field and direction of a model hydrodynamic flow. The peaks are shown to be consistent with a greater pressure perpendicular to the field. Possible sources of the pressure anisotropy in the magnetosheath are discussed.