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

摘要

We analyze the temporal variation of the diurnal anisotropy of sub-TeV cosmicray intensity observed with the Matsushiro (Japan) underground muon detectorover two full solar activity cycles in 1985-2008. The average siderealamplitude over the entire period is 0.034+-0.003 %, which is roughly one thirdof the amplitude reported from AS and deep-underground muon experimentsmonitoring multi-TeV GCR intensity suggesting a significant attenuation of theanisotropy due to the solar modulation. We find, on the other hand, only weakcorrelations either with the solar activity- or magnetic-cycles. We examine thetemporal variation of the "single-band valley depth" (SBVD) quoted by theMilagro experiment and, by contrast with recent Milagro reports, we find nosteady increase in the Matsushiro observations in a 7-year period between 2000and 2007. We suggest, therefore, that the steady increase of the SBVD reportedby the Milagro experiment is not caused by the decreasing solar modulation inthe declining phase of the 23rd solar activity cycle.