Spectral energy density (J m^-3) along the Langmuir wave dispersion curve for Run 1 (top) and Run 2 (bottom) for subsequent time windows (centred about the indicated time). Note the development of a broad spectrum in negative k_x in Run 1 with energy densities several orders of magntude larger than the negative k_x spectrum of Run 2. The dotted lines show the expected initial k_x value of the initially growing electrostatic modes.