Spectral intensity of Langmuir waves at four different times along time evolution, showing the forward L-mode excited by bump-on-tail instability (panel a) undergoing nonlinear scattering to produce backward L-mode as well (panel b; see Fig. 6 in Ziebell et al. (2015) for comparison). At later times (panels c and d), multiple scattering processes further spreads the L-mode spectrum in wave number space.