tokens: indicates that the generation of different emission bands occurs in different parts of the source with a non-uniform magnetic field and plasma density. From Fig. 1 , it is clear that each emission band has a fine structure: It consists of narrower (in frequency) elements that show a characteristic evolution in time. Each emission band begins with the appearance of a low-frequency element, which exists for a shorter time than the burst as a whole. Then there appears the next element, which is shifted towards the higher frequencies, etc. Each band contains 3–5 individual elements of the fine structure. Schematically, this evolution is more clearly shown in Fig. 2 , where the bright elements in the four emission bands from Fig. 1 are represented as dark horizontal lines. Figure 2. Schematic representation of the evolution of the dynamic spectrum (Fig. 1 ) with a fine structure of the emission bands. A fragment of the dynamic spectrum enclosed in a rectangle is analysed in Section 3 (see Fig. 7 ). A careful examination of the dynamic spectrum in Fig. 1 and similar spectra published by Hankins and Eilek leads to the conclusion that the elements of the fine structure are not superimposed on the quasi-harmonic emission bands. They actually form these bands, and the observed frequency drift of the emission bands towards higher frequencies is due to the successive change of one bright element to another, higher frequency element at a later instance of time. This fact is also indicated by Hankins et al. ( 2016 ), who analysed a large number of pulses and concluded that the drift of the emission bands towards higher frequencies is due to the appearance of new elements in the band that are delayed in time and start at slightly higher frequencies. Fig. 3 shows another, more rare type of the dynamic spectrum (Hankins, Rankin Eilek 2009 ). In the observed frequency range 8–10 GHz, the dynamic spectrum exhibits a continuous fine structure of 10 elements, rather than individual emission elements grouped into quasi-harmonic bands. Note that an insufficiently high frequency resolution does not allow an unambiguous conclusion to be drawn whether a finer frequency structure similar to that observed in Fig. 1 exists inside the observed emission bands. We also note that the interval 8–10 GHz, in which radiation with the above-mentioned structure was observed, is too narrow to definitely judge the change in the emission band spacing. It is quite possible that it remains constant. The latter will be taken into account in Section 3 when explaining the dynamic spectrum depicted in Fig. 3 . Figure 3. Rare type of the dynamic spectrum with the emission bands. This figure is adapted from Hankins et al. ( 2009 , fig. 5). Note that in both types of the dynamic spectrum, the signal lags as the radiation frequency is decreased. Hankins et al. ( 2016 ) relate this effect with the group delay of the signal directly in the local radiation source with a zebra pattern. It is
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