  Solar S-bursts are short duration ($<1$ s at decameter wavelengths) radio
bursts that have been observed during periods of moderate solar activity, where
S stands for short. The frequency drift of S-bursts can reflect the density
variation and the motion state of the electron beams. In this work, we
investigate the frequency drift and the fine structure of the S-bursts with the
LOw Frequency ARray (LOFAR). We find that the average frequency drift rate of
the S-bursts within 110-180MHz could be described by $df/dt=-0.0077f^{1.59}$.
With the high time and frequency resolution of LOFAR, we can resolve the fine
structures of the observed solar S-bursts. A fine drift variation pattern was
found in the structure of S-bursts (referred to as solar Sb-bursts in this
paper) during the type-III storm on 2019 April 13, in the frequency band of
120-240 MHz. The Sb-bursts have a quasi-periodic segmented pattern, and the
relative flux intensity tends to be large when the frequency drift rate is
relatively large. This kind of structure exists in about 20\% of the solar
S-burst events within the observed frequency range. We propose that the fine
structure is due to the density fluctuations of the background coronal density.
We performed a simulation based on this theory which can reproduce the shape
and relative flux intensity of the Sb-bursts. This work shows that the fine
structure of solar radio bursts can be used to diagnose the coronal plasma.

Solar and Stellar Astrophysics;Space Physics