Solar activity variations strongly impact the modulation of the flux of low-energy Galactic Cosmic
Rays (GCRs) reaching the Earth. The secondary particles, which originate from the interaction
of GCRs with the atmosphere, can be revealed by an array of ground detectors. We show that the
low-threshold rate (scaler) time series recorded over 16 years of operation by the surface detectors
of the Pierre Auger Observatory in Malargüe (Argentina) strongly reflects solar activity and can be
considered as a new proxy of solar variability. To achieve this result, we apply advanced spectral
methods to this time series and to the classical solar sunspot number and sunspot area series. We
detect and compare highly significant variations with periods ranging from the decadal to the daily
scale and identify the origin of each variability mode. In conclusion, we show that the Auger
scaler data, thanks to the very low noise level and high statistical significance related to the very
high count rates (∼ 10^^6 counts per second), allow for a thorough and detailed investigation of the
GCR flux variations in the heliosphere.