Title: Observations of the magnetosheath-solar wind boundary
Abstract: By the use of data from the satellite Explorer 34, it is shown that discontinuous transitions between the magnetosheath and the interplanetary medium occur simultaneously in the proton spectrum, the flow direction, and the magnetic field intensity, during both geomagnetically quiet and disturbed times. A few boundary crossings were observed for which the plasma spectra appear to indicate a diffuse transition; a detailed study of an extreme example of such a crossing, based on simultaneous measurements of the spectra, the flow direction, and the magnetic field intensity, shows that the apparent diffuseness requires no new hypotheses for its explanation, but can be the result of motions of a bow shock. Observations of the velocity field in the neighborhood of the outer boundary of the magnetosheath are presented and are compared with results of the fluid theory, which predicts a standing shock. It is shown that, along the boundary on the flank of the magnetosheath, the ratios of the magnetosheath flow speed to the interplanetary flow speed are in close quantitative agreement with a prediction of the fluid theory, which is essentially independent of the Mach number and the adiabatic exponent. We observe a highly directional flow in the region 30° ≲ ϕ ≲ 100°, where ϕ is the sun-earth-satellite angle. The direction of this flow is consistent with the bow shock theory.