Top: radio emission time profiles at frequencies of 200, 150, 100, 75, 50, 25, 10, and 5 MHz (from dark to light blue) for a 1 s rise time of electron injection. Middle/Bottom: half-width-half-maximum of emission (black line), the collisional Langmuir wave decay time (Eq. (28), red dotted line), the timescale for Langmuir wave evolution due to plasma density inhomogeneity (Eq. (29), blue dot-dashed line) and the empirical result given by Eq. (27) (black dashed line), as a function of both background plasma frequency and distance from injection site.