The Venus precursor - An environmental effect on the Pioneer Venus spacecraft

Data obtained by the Pioneer Venus Orbiter's retarding potential analyzer during two orbits with periapsis in the subpolar region of Venus are presented. Two electron populations are observed: a low-temperature population that is interpreted to be an electron sheath population trapped in the positive spacecraft potential well, and a high-temperature population representing the ambient solar wind electron population. The sheath population's increase and decrease by a factor of 5, as the spacecraft approaches and recedes from a position near periapsis (independently of the bow shock's position relative to the spacecraft), is interpreted as being due to an increase and decrease in the density of cold electrons trapped in the spacecraft potential well.