  Wakes behind spacecraft caused by supersonic drifting positive ions are
common in plasmas and disturb in situ measurements. We review the impact of
wakes on observations by the Electric Field and Wave double-probe instruments
on the Cluster satellites. In the solar wind, the equivalent spacecraft
charging is small compared to the ion drift energy and the wake effects are
caused by the spacecraft body and can be compensated for. We present statistics
of the direction, width, and electrostatic potential of wakes, and we compare
with an analytical model. In the low-density magnetospheric lobes, the
equivalent positive spacecraft charging is large compared to the ion drift
energy and an enhanced wake forms. In this case observations of the geophysical
electric field with the double-probe technique becomes extremely challenging.
Rather, the wake can be used to estimate the flux of cold (eV) positive ions.
For an intermediate range of parameters, when the equivalent charging of the
spacecraft is similar to the drift energy of the ions, also the charged wire
booms of a double-probe instrument must be taken into account. We discuss an
example of these effects from the MMS spacecraft near the magnetopause. We find
that many observed wake characteristics provide information that can be used
for scientific studies. An important example is the enhanced wakes used to
estimate the outflow of ionospheric origin in the magnetospheric lobes to about
${10}^{26}$ cold (eV) ions/s, constituting a large fraction of the mass outflow
from planet Earth.
