On the role of the magnetic field in the solar wind interaction with venus: Expectations versus observations

Abstract

Prior to Pioneer Venus Orbiter encounter in December 1978, it was generally believed that the solar wind interaction with the Venus atmosphere would fall into one of three categories: (1) a direct interaction model, in which solar wind plasma flows directly into the ionosphere; (2) a tangential discontinuity model, in which a boundary is formed between the solar wind and ionospheric plasmas, across which no plasma flows; and (3) a magnetic barrier model, in which the flowing solar wind is held off from the ionospheric plasma by an intermediate region of high magnetic field, due either to “piled up” interplanetary field lines or currents induced in the ionosphere by the solar wind motional electric field. Observations of the magnetic field near Venus suggest that some elements of each of these models is in fact appropriate. A bow shock occurs at an altitude of about 0.3 Venus radii at the subsolar point. Behind the bow shock, the compression of the decelerated solar wind plasma causes interplanetary field lines to “pile up”, as anticipated. The magnetic field inside of the bow shock increases from about twice the IMF strength at the bow shock to values in the range ∼40–110γ at altitudes between about 200 and 1200 km. The maximum value of the piled up field, which is correlated with the dynamic pressure of the solar wind outside of the bow shock, occurs at lower altitudes for larger field strengths. Just Venus-ward of the maximum field, the pressure of the cold plasma rises to a level balancing the pressure of the external magnetic field. Thus, to a first approximation the ionosphere has a diamagnetic response excluding the magnetosheath field. However at times strong magnetic fields are found throughout the ionosphere.