INITIAL PIONEER VENUS MAGNETIC-FIELD RESULTS - DAYSIDE OBSERVATIONS

Initial observations by the Pioneer Venus magnetometer in the sunlit ionosphere reveal a dynamic ionosphere, very responsive to external solar wind conditions. The locations of the how shock and ionosphere are variable. The strength of the magnetic field just outside the ionopause is in approximate pressure balance with the thermal plasma of the ionosphere and changes markedly from day to day in response to changes in solar wind pressure. The field strength in the ionosphere is also variable from day to day. The field is often weak, at most a few gammas, but reaching many tens of gammas for periods of the order of seconds. These field enchantments are interpreted as due to the passage of spacecraft through flux ropes consisting of bundles of twisted field lines surrounded by the ionospheric plasma. The helicity of the flux varies through the flux tube, with low pitch angles on the inside and very large angles in the low-field outer edges of the ropes. These ropes may have external or internal sources. Consistent with previous results, the average position of the bow shock is much closer to the planet than would be expected if the solar wind were completely deflected by the planet. In total, these observations indicate that the solar wind plays a significant role in the physics of the Venus ionosphere. Copyright © 1979 AAAS.