Subsonic magnetosheath observations from Explorer 33

Data are presented from one orbit of Explorer 33 which entered the magnetosphere near the stagnation point. Magnetic activity as measured at the earth was relatively quiet during the orbit, although minor substorm activity began 2 hours prior to the boundary crossing. Well before entry the magnetosheath observations show flow patterns consistent with hydrodynamic models. Near the stagnation point close to the boundary the observations show rapid variations between virtually isotropic flux (stagnated flow) and flows tangent to the boundary in alternately opposite directions. Movement of the stagnation point of the order of 1 RE is inferred from these observations. Multiple boundary crossings are observed in the magnetic field covering a thickness of about 500 km. Prior to the magnetic crossings the higher-energy proton fluxes begin to disappear. The bulk of the proton flux in the lower energy ranges and also the electron flux decrease rapidly at the last magnetic crossing. No boundary is apparent in the highest energy range measured by the detector. Further results await analysis of the 20 remaining orbits of Explorer 33 which traverse the magnetosheath in the subsonic region.