Study of Backward-Propagating Langmuir Waves with Numerical Simulations Based on the Particle-in-Cell Method

The conversion of Langmuir waves into electromagnetic radiations is an important mechanism of Type III solar bursts. Langmuir Waves (LW) can be easily excited by electron-beam instabilities, and they can become backward propagating throuth the wave-wave interaction. Generally, the backward-propagating LW are very important for the second-harmonic waves of Type III solar bursts. In this paper we pay particular attention to the mechanism of the backward-propagating LW as revealed through numerical simulations based on the Partical-in-Cell (PIC) method. We found that ions play a key role to excite the backward-propagating LW, with draw much of their energy from the scattering of the forward-propagating LW. Moreover, an electron beam can hardly generate backward-propagating LW directly, but may directly amplify second-harmonic forward-propagating LW.