Spectral analysis of the type III and type IIIb bursts observed on 19 September 2011. (a) Type IIIb bursts with stria substructures observed by the STEREO A spacecraft. The structured type III burst is followed by a relatively intense regular type III burst. (b) Intensity-time profiles of the type IIIb envelope and regular type III burst at selected frequencies. (c) Intensity-time profiles along the stria substructures and regular type III burst. The selected frequencies (at panels b and c) are indicated by the arrows of corresponding colors in panel a. The time interval plotted in panels (b) and (c) is within the orange box in panel (a).