Scaler rates from the Pierre Auger Observatory: a new proxy of solar activity

TL;DR

The paper investigates how solar activity modulates the flux of low-energy galactic cosmic rays and proposes scaler rates from the Pierre Auger Observatory as a high-precision solar activity proxy. By constructing a long, uniformly sampled scaler time series from 2006–2022 and applying Monte Carlo Singular Spectrum Analysis to extract significant oscillations, the authors identify a dominant decadal modulation tied to the solar cycle, along with annual, 28-day, and mid-term (≈9, 6, 20, and 14 days) components. Hemispheric sunspot-area analysis clarifies the origins of the 6- and 9-month periodicities, while the 14-day component relates to active longitudes and tilted dipole structures. Overall, the results demonstrate that Auger scalers provide a low-noise, multi-timescale tracer of heliospheric GCR modulation, offering a robust solar-variability proxy extendable with AugerPrime.

Abstract

The modulation of low-energy galactic cosmic rays reflects interplanetary magnetic field variations and can provide useful information on solar activity. An array of ground-surface detectors can reveal the secondary particles, which originate from the interaction of cosmic rays with the atmosphere. In this work, we present an investigation of the low-threshold rate (scaler) time series recorded in 16 years of operation by the Pierre Auger Observatory surface detectors in Malargue, Argentina. Through an advanced spectral analysis, we detected highly statistically significant variations in the time series with periods ranging from the decadal to the daily scale. We investigate their origin, revealing a direct connection with solar variability. Thanks to their intrinsic very low noise level, the Auger scalers allow a thorough and detailed investigation of the galactic cosmic-ray flux variations in the heliosphere at different timescales and can, therefore, be considered a new proxy of solar variability.

Figures (6)

• Figure 1: Relative scaler rates series from 01 January 2006 to 19 March 2022. The rate incorporates all the corrections detailed in Schimassek:2020soa and the text. Panel (a) displays the series sampled at 12-hour intervals. Some gaps in the series have been filled through a gap-filling process relying on an Auto-Regressive model (see text). The most substantial gaps are marked in green. The scaler rate in panel (b) was obtained by resampling the original series every 6 days after applying the gap-filling procedure to the series in panel (a).
• Figure 2: MC-SSA spectrum of the relative scaler rate. The Monte Carlo ensemble size is 10 000. The gray bars, which bracket 99% of the power values obtained from the ensemble, represent the Monte Carlo band. The significant spectral components are indicated by the red squares, while the black dots represent the spectral components that can be parameterized as red noise. The significant components with the same period specified in blue are grouped with blue boundaries.
• Figure 3: The significant components identified through MC-SSA analysis in the relative scaler rate series (a), with decadal scale (b), annual (c), ${\sim}9$ month (d), ${\sim}6$ month (e), ${\sim}28$ d (f), ${\sim}20$ d (g), and ${\sim}14$ d (h) periods. The variance described by each component is also indicated in per cent inside each panel. The sunspot number (SN) series sampled every 6 days is also shown in the bottom panel.
• Figure 4: (a) Comparison between the decadal trend revealed in the Auger scaler rate (black curve) and the SN series sampled every 6 d (shaded red curve), superimposed by the decadal modulation revealed in the latter by SSA (red curve). An anticorrelation among the decadal trends is visible. The shaded gray bar represents the total time interval required for the polar field reversal in both hemispheres from June 2012 to November 2014. Panel (b) shows the 28 d oscillation revealed in the SN time series.
• Figure 5: Comparison between the full Sun (b), Northern N- (a) and Southern S- (c) Hemispheres sunspot area time series (black curves) sampled every 30 d, to better distinguish the main peaks by reducing the noise in the signal. The NH and SH data are superimposed by the reconstructed ${\sim}6$ m (red curve in panel a) and ${\sim}9$ m (blue curve in panel c) components, respectively, obtained by applying the MC-SSA to the 6 d sampled series. The shaded red and blue bars highlight the main peaks in solar activity linked to higher variability in the 6 and 9 m components.