  A scenario based on the electron cyclotron maser emission is proposed for the
fine structures of solar radio emission in the present discussion. It is
suggested that under certain conditions modulation of the ratio between the
plasma frequency and electron gyro-frequency by ultra low frequency waves,
which is a key parameter for excitation of the electron cyclotron maser
instability, may lead to the intermittent emission of radio waves. As an
example, the explanation of the observed fine-structure components in the solar
type IIIb burst is discussed in detail. Three primary issues of the type IIIb
bursts are addressed: 1) what is the physical mechanism that results in the
intermittent emission elements that form a chain in the dynamic spectrum of
type IIIb bursts, 2) what causes the split pair (or double stria) and the
triple stria, 3) why in the events of fundamental-harmonic pair emission there
is only IIIb-III, but IIIb-IIIb or III-IIIb cases are very rarely observed.
