Type-lIIb, l lid, and III solar decametric radio bursts, being distinguished by the typical negative drift rate of their dynamic spectra, are compared. Observational data were obtained with a UTR-2 antenna during the period 1973-1982. During the analysis of the bursts of all these spectral varieties, the frequency drift time (drift delay) was measured in the ranges 25 to 12.5 MHz, 25 to 20 Mliz, and 12.5 to 10 MHz. Durations of type-lll bursts were determined at the harmonically-related frequencies of 25 and 12.5 Mttz; radio source locations were also used.
It is shown that these decametric bursts arc distinctly divided into two groups: (1)type-lllb chains of simple stria bursts and also normal type-Ill storm bursts observed at central regions constitute a group of events with a fast drifting spectrum; (2)type-Ill bursts from type-Illb-lll pairs and the limb variant of normal III bursts, as well as peculiar type-llld chains of diffuse striae and related chains with an echo component, constitute a second group of events with comparatively slow drift rates.
The first group of the phenomena is associated with the fundamental F frequency and the second one, with the harmonic H of the coronal plasma frequency. The results of the present investigation agree well with earlier conclusions on the harmonic origin of decametric chains and type-Ill bursts. Measurements of drift delays in narrow fi'equency ranges, an octave apart, as well as type-I I I burst durations at harmonically-related fi'equencies confirm the existence of both F and H components in the solar radiation. The essential result of 10 years ofdecametric observations is that the ti-cquency drift rates and durations arc rather stable parameters for the various type-III bursts and stria-burst chains. The stability characterizes some unspecified conditions of burst generation in the middle corona.