We have made a numerical investigation of the wake potential and density perturbations caused by the presence of a large negative voltage on a sector situated on the wake side of a spacecraft moving at orbital speed through the ionosphere. The spacecraft is represented as an infinite cylinder moving in an unmagnetized plasma. The rest of the cylinder's surface is biased to a moderate negative voltage representing floating potential. It is shown that in the case of large Debye ratio (spacecraft size much larger than Debye length) the geometrical wake is almost ion-empty for a high differential bias voltage on the sector, and that in the case of low Debye ratio and low Mach number, ion focusing effects can lead to the formation of a high-ion-density strip parallel to the surface when the differential bias is made very large. In the latter case, the results show evidence of an ionic emptying effect in the near wake and in the midwake. We propose a mechanism to explain this result. Ion collection on the biased sector is also addressed. ¿ American Geophysical Union 1991