  The wake behind a large object (such as the moon) moving rapidly through a
plasma (such as the solar wind) contains a region of depleted density, into
which the plasma expands along the magnetic field, transverse to the flow. It
is shown here that (in addition to any ion instability) a bump-on-tail which is
unstable appears on the electrons' parallel velocity distribution function
because of the convective non-conservation of parallel energy. It arises
regardless of any non-thermal features on the external electron velocity
distribution. The detailed electron distribution function throughout the wake
is calculated by integration along orbits; and the substantial energy level of
resulting electron plasma (Langmuir) turbulence is evaluated quasilinearly. It
peaks near the wake axis. If the mass of the electrons is artificially
enhanced, for example in order to make numerical simulation feasible, then much
more unstable electron distributions arise; but these are caused by the
unphysical mass ratio.
