The solar wind interaction with Venus: Pioneer Venus observations of bow shock location and structure

Pioneer Venus observations are used in conducting a study of the location and structure of the Venus bow shock. The trace of the shock in the solar wind aberrated terminator plane is nearly circular at an altitude of 1.38 Rv independent of interplanetary magnetic field orientation with an extrapolated sub solar height of 0.38 R-v. Gas dynamic relations and scaling of the terrestrial analogue are used to determine the effective impenetrable obstacle altitude from the mean shock surface with the conclusion that it lies beneath the observed height of the ionopause. The short-term variability in shock position is similar to that found at the earth, while over the long-term bow shock, altitude varies by up to similar to 35% in phase with the solar cycle owing to causes other than changing solar wind Mach number. In contrast to ionopause position, which is shown to be well determined by external pressure measurements, bow shock altitude is found to be only weakly dependent upon ionopause height and solar wind dynamic pressure. These results are interpreted in terms of interactions with exospheric neutrals and/or lack of complete deflection of the incident solar wind by currents induced in the ionosphere modifying the flow about Venus from that associated with a tangential discontinuity obstacle of nearly constant radius. The downstream bow shock is smaller in diameter than that of terrestrial case despite the larger Mach cone angle at 0.72 AU most probably due to the smaller relative size of the Venus magnetotail. A brief survey of shock structure with Pioneer Venus instrumentation shows general agreement as to the time and location of the shock crossings with a transition layer thickness of the order of the ion inertial length scale. The observed variation in bow shock structure and the foreshock with upstream parameters was similar to that seen at the earth.