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 – 180 MHz could be described by df / dt = -0.0077 f ^ { 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 .