SIMBAD references

Backward propagating and second harmonic Langmuir waves (LWs) play an important role in the second harmonic emission of solar radio type III bursts and they are numerically studied in detail with a new method. We paid particular attention to the excitation mechanism of the backward propagating LWs, as well as the second harmonic LWs with two-dimensional (2D) particle-in-cell (PIC) simulations. We performed simulations with two different types of initial conditions, i.e., the beam-exited and the wave-excited cases. In the second case, the beam-excited LWs were considered to represent the free energy instead of the beam itself. It is firstly found that the backward propagating LWs are suppressed by about 20% by the beam electrons. Hence, the beam electrons just have a negative contribution to the backward LWs, which can be explained in a preliminary way by the Landau damping of the beam electrons. On the other hand, the second harmonic LWs are contributed mainly (about 80%) by the beam electrons, only 20% originating from beam-excited forward LWs, which is quite different from earlier results. Therefore, we found that the backward propagating LWs can't be amplified directly by the electron beam, and we must assume that they are generated by the beam-excited forward LWs scattered by ion sound waves. The direct amplification of beam-plasma instability may play an important role in producing the second harmonic LWs.