The distant interplanetary wake of Venus: Plasma observations from Pioneer Venus

In June 1979 the Pioneer Venus orbiter made its first series of passes through the distant solar wind wake of Venus at distances of 8–12 Rv behind the planet. During this period the plasma analyzer aboard the spacecraft detected disturbed magnetosheath plasma that intermittently disappeared and reappeared, suggesting a tattered, filamentary cavity trailing behind the planet. The magnetosheath dropouts almost always occurred inside the region of ‘magnetotail’ observed by Russell et al. Sporadic bursts of energetic ions (E/q ≳ 4 kV) are detected inside and, occasionally, outside the magnetotail; all such bursts are consistent with identification of the ion as O+ of planetary origin moving at the local magnetosheath flow speed. The morphology of the plasma dropouts and of the O+ bursts is analyzed in detail. The cavity appears to contract at times of high solar wind dynamic pressure. The intensity of the O+ component is highly variable, and appears not to be strongly correlated with solar wind dynamic pressure. The most intense bursts correspond to a flux ≲107 ions cm−2 s−1. This maximum flux, if steady and filling a cylinder 1 Rv in radius would correspond to a mass loss rate of ≲1025 ions s−1; the intermittency and variability of the flux suggest that the true mean loss rate is very much lower. The kinetic temperature of the O+ component is estimated as 105–106 K in order of magnitude.