Emission of Type II Radio Bursts - Single-Beam versus Two-Beam Scenario
  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. 
Urs Ganse, Patrick Kilian, Rami Vainio, Felix Spanier
astro-ph.SR