ARGO-YBJ Observation of the Large-scale Anisotropy During the Solar Minimum between Cycles 23 and 24

TL;DR

This study uses ARGO-YBJ data collected during a solar minimum to analyze large-scale cosmic-ray anisotropy, revealing energy-dependent variations in intensity and phase over a broad energy range.

Contribution

It provides detailed measurements of cosmic-ray anisotropy during solar minimum, highlighting the energy dependence and phase shift of the anisotropy spectrum with unprecedented statistical precision.

Findings

Anisotropy amplitude increases with energy up to 10 TeV.

Large-scale anisotropy features are characterized by excess and deficit regions.

Phase of anisotropy shifts toward lower R.A. with increasing energy.

Abstract

This paper reports on the measurement of the large-scale anisotropy in the distribution of cosmic-ray arrival directions using the data collected by the air shower detector ARGO-YBJ from 2008 January to 2009 December,during the minimum of solar activity between cycles 23 and 24. In this period, more than 200 billion showers were recorded with energies between 1 and 30 TeV. The observed two-dimensional distribution of cosmic rays is characterized by two wide regions of excess and deficit, respectively, both of relative intensity 0.001 with respect to a uniform flux, superimposed on smaller size structures. The harmonic analysis shows that the large-scale cosmic-ray relative intensity as a function of R.A. can be described by the first and second terms of a Fouries series. The high event statistics allow the study of the energy dependence of the anistropy, showing that the amplitude increases with energy, with a maximum intensity at 10 TeV, and then decreases while the phase slowly shifts toward lower values of R.A. with increasing energy. The ARGO-YBJ data provide accurate observations over more than a decade of energy around this feature of the anisotropy spectrum.