IONOSPHERE OF VENUS - 1ST OBSERVATIONS OF THE EFFECTS OF DYNAMICS ON THE DAYSIDE ION COMPOSITION

Bennett radio-frequency ion mass spectrometers have returned the first in situ measurements of the Venus dayside ion composition, including evidence of pronounced structural variability resulting from a dynamic interaction with the solar wind. The ionospheric envelope, dominated above 200 kilometers by O +, responds dramatically to variations in the solar wind pressure, which is observed to compress the thermal ion distributions from heights as great as 1800 kilometers inward to 280 kilometers. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, such that a zone of accelerated suprathermal plasma is encountered. At higher altitudes, extending outward on some orbits for thousands of kilometers to the bow shock, energetic ion currents are detected, apparently originating from the shocked solar wind plasma. Within the ionosphere, observations of pass-to-pass differences in the ion scale heights are indicative of the effects of ion convection stimulated by the solar wind interaction.