The Pierre Auger Observatory is the largest cosmic-ray observatory in existence and covers an area of about 3000 km$^2$ with its 1660 surface detectors. Each of these detector stations records signals of relativistic particles in 12 tonnes of ultra-pure water, measured with three 9-inch photomultipliers. With a collecting area of about 10 m$^2$, each surface detector station is sensitive to single particles down to MeV energies. Such particles are recorded in counters and histograms for each detector and transmitted to the central data acquisition system at regular time intervals.
The background flux of particles is measured with very high statistics by combining all surface detector stations and therefore enables investigations of the source of variations in this flux.
We present an updated analysis of the particle-counter data and highlight the long-term stability of the corrected rates.
With these corrected rates, we present the frequency spectrum of the observed variations and highlight the expected solar contributions,
show an in-depth analysis of the diurnal variation, and update the observation of the eleven-year solar-cycle from 2006 until the end of 2018.