  Full disk observations of the solar chromosphere in the Ca II K line
represent a valuable dataset for studies of solar magnetic activity. The
S-index is widely used to investigate the magnetic activity of stars, however,
its connection to the coverage of stellar magnetic structure is still poorly
understood. We use the archives of full disk Ca II K images taken by the Royal
Observatory of Belgium with the USET to derive the area fraction of the
brightest chromospheric structures over the last decade. These data allowed us
to study the end of the solar cycle 24 and the beginning of solar cycle 25. The
brightest regions of the solar surface were then segmented using an algorithm
based on an intensity threshold. We computed the area fraction over the solar
disk and compared it with the S-index from TIGRE. For the detection of periodic
modulations, we applied a discrete Fourier power spectrum method to both
datasets. A tight linear relationship was found between the USET area fraction
and the TIGRE S-index, with an improved correlation obtained in the
low-activity regime by considering the enhanced network. In both time series,
we detected the modulation caused by the rotation of bright structures on the
solar disk. However, this detection is constrained in the case of TIGRE due to
its observation strategy. We studied the correlation between the disk coverage
with chromospheric structures and the variability of the S-index on an
overlapping period of ten years. We concluded that the disk coverage index is a
good proxy for the S-index and will be useful in future studies of the magnetic
activity of solar-type stars. The USET area fraction dataset is most
appropriate for evaluating the solar rotation period and will be used in future
works to analyze the impact of the inclination of the stellar rotation axis on
the detectability of such periodic modulations in solar-type stars.
