We present suprathermal electron data recorded by the Pioneer Venus (PV) orbiter retarding potential analyzer for two orbits with periapsis in the subsolar region of Venus. In the first of the two orbits, the PV spacecraft passed entirely through the Venus bow shock transition region, both inbound and outbound from periapsis, and in the second it only grazed the upstream border of the transition region. In both orbits, two electron populations are observed, a low-temperature population interpreted as an electron sheath population trapped in the positive spacecraft potential well and a high-temperature population interpreted as the ambient solar wind electron population. The density of the sheath (low temperature) electron population is observed to increase and to decrease almost identically in both orbits by a factor of about 5 as the spacecraft approaches and recedes from a location near periapsis. The density evidently depends only on the coordinates of the spacecraft and not on the location of the bow shock. We interpret the increase and decrease in sheath electron density as responsible for the increase and decrease in negative current recorded by the PV Langmuir probe experiment as the PV spacecraft passes through the subsolar region. Brace et al. (1985) have called the region of increased Langmuir probe current the Venus precursor region and have suggested that the cause is mass loading of the solar wind upstream of the bow shock.

The observed gradual increase and decrease in density of the solar wind (high temperature) electron population, previously interpreted as evidence for upstream mass loading of the solar wind (Knudsen, 1985), is not interpreted as having resulted from the near-grazing passage of the spacecraft into or through the bow shock transition region. The increase in density of the cold sheath electrons as the spacecraft approaches periapsis in the subsolar region is interpreted as having been caused by an increase in production rate of secondary electrons from the spacecraft surfaces. The increased production rate is caused by an as yet unestablished process, or processes, operating in the vicinity of Venus. We see in the behavior of the two suprathermal electron populations no evidence for an increase in upstream solar wind plasma density produced by mass loading. An increase in density as small as a few percent would not be detectable. ¿ American Geophysical Union 1989