Second Harmonic Electromagnetic Emissions by an Electron Beam in Solar Wind Plasmas with Density Fluctuations

Two-dimensional particle-in-cell simulations are performed to study the electromagnetic radiation emitted at the second harmonic 2ωp of the plasma frequency by a weak electron beam propagating in a background plasma with random density fluctuations, in solar wind conditions relevant to Type III solar radio bursts. The dynamics of the waves, the beam, and the plasma are calculated over several thousands of plasma periods. For relevant comparisons, simulations with and without applied density fluctuations are performed for the same parameters. This Letter evidences for the first time the impact of density fluctuations on the physical mechanisms driving the generation of electromagnetic waves emitted at 2ωp. Results obtained show that (i) the beam radiates electromagnetic waves at 2ωp as a result of nonlinear processes of Langmuir waves' coalescence, despite wave scattering on the density fluctuations that strongly affect the Langmuir spectra; (ii) the fraction of initial beam energy transferred asymptotically to the electromagnetic waves at 2ωp is by one order of magnitude smaller when the plasma involves density fluctuations of average level around 5%; (iii) compared to the homogeneous case, the ratio of electromagnetic energy radiated at 2ωp to the energy carried by the Langmuir wave turbulence is significantly larger during all the nonlinear stage; (iv) asymptotically, when the plasma is inhomogeneous, electromagnetic emissions at 2ωp present isotropized spectra whereas quadrupolar radiation occurs for the homogeneous plasma case.