  The foreshock region of a CME shock front, where shock accelerated electrons
form a beam population in the otherwise quiescent plasma is generally assumed
to be the source region of type II radio bursts. Nonlinear wave interaction of
electrostatic waves excited by the beamed electrons are the prime candidates
for the radio waves' emission.
  To address the question whether a single, or two counterpropagating beam
populations are a requirement for this process, we have conducted 2.5D particle
in cell simulations using the fully relativistic ACRONYM code.
  Results show indications of three wave interaction leading to electromagnetic
emission at the fundamental and harmonic frequency for the two-beam case. For
the single-beam case, no such signatures were detectable.
