(a) The dynamic spectrum of a type IIIb–type III burst pair in the frequency range of 30–80 MHz observed by LOFAR on 2015 April 16. F and H indicate the fundamental and second harmonic branches, respectively. The solid rectangular boxes show the time and frequency ranges investigated in the analysis. (b) An enlarged version of the harmonic components from 55 to 65 MHz. (c) The detailed spectrum between 30 and 32.25 MHz. The dashed horizontal lines represent the different frequencies in Figure 1(d). (d) The fluxes of the F and H components selected from the striae structure in (c).