Magnetic field and plasma wave observations in a plasma cloud at Venus

Pioneer Venus magnetic field and plasma wave data are examined in a particularly clear example of a plasma cloud above the Venus ionosphere. The magnetic configuration is suggestive of acceleration of the plasma cloud by magnetic tension. If the plasma is at rest at the subsolar point, it could be accelerated to approx.90 km/sec by the observed stress at the location of the measurement. This far exceeds the escape velocity and suggests that plasma clouds do form a significant loss mechanism for the Venus ionosphere but do not necessarily indicate that the plasma cloud is detached from the ionosphere proper. The plasma cloud is accompanied by strong plasma wave activity and is significantly hotter than the ionospheric plasma encountered later on the same pass. We estimate a loss rate of the order of 2 x 1025 ions per second during this event. The geometry suggested by these observations is one of a ridge of dense cold plasma starting in the sub solar regions and flowing over the poles of the planet. Thus, these plasma clouds may be the planetary analogue of cometary tail rays