Solar cycle dependence of the diurnal anisotropy of 0.6 TeV cosmic ray intensity observed with the Matsushiro underground muon detector

TL;DR

This study investigates the diurnal anisotropy of 0.6 TeV cosmic rays over two solar cycles, revealing attenuation due to solar modulation and weak correlation with solar activity, challenging previous Milagro findings.

Contribution

It provides a long-term analysis of cosmic ray anisotropy with underground muon detectors, highlighting differences from previous high-altitude experiments and questioning earlier reported trends.

Findings

Average sidereal amplitude is about one third of higher-energy measurements.

Weak correlation between anisotropy and solar activity or magnetic cycles.

No steady increase in SBVD during 2000-2007, contrary to Milagro reports.

Abstract

We analyze the temporal variation of the diurnal anisotropy of sub-TeV cosmic ray intensity observed with the Matsushiro (Japan) underground muon detector over two full solar activity cycles in 1985-2008. The average sidereal amplitude over the entire period is 0.034+-0.003 %, which is roughly one third of the amplitude reported from AS and deep-underground muon experiments monitoring multi-TeV GCR intensity suggesting a significant attenuation of the anisotropy due to the solar modulation. We find, on the other hand, only weak correlations either with the solar activity- or magnetic-cycles. We examine the temporal variation of the "single-band valley depth" (SBVD) quoted by the Milagro experiment and, by contrast with recent Milagro reports, we find no steady increase in the Matsushiro observations in a 7-year period between 2000 and 2007. We suggest, therefore, that the steady increase of the SBVD reported by the Milagro experiment is not caused by the decreasing solar modulation in the declining phase of the 23rd solar activity cycle.