Investigation of the diurnal variation of cosmic rays at large latitudes by the detectors of the Extreme Energy Events Project

The diurnal modulation in galactic cosmic ray flux observed on the Earth is a widely studied phenomenon. Most of the analyses of this effect have been undertaken with data obtained by neutron monitor stations distributed over the world. The effect may also be seen in muon detectors, although muons are affected in a different way than neutrons. Particularly interesting is a comparison between the data obtained from neutron stations and muon telescopes located in the same region. In this paper we report an investigation of the diurnal modulation of cosmic rays at large latitudes, based on the data collected by three scintillator telescopes of the EEE (Extreme Energy Events) Project, installed at Ny-Ålesund, in the Svalbard archipelago, at 79° N, during the period 2019–2025. Amplitudes and phases of the observed diurnal maximum in the cosmic ray flux were extracted in various periods by a harmonic fit of the averaged daily trend. Muon results were also compared to data collected in the period 2003–2025 by the Barentsburg Neutron Monitor Station, located not far from our detectors. The data, integrated along periods of one year, with a moving average in six months steps, show evidence of a diurnal modulation, with amplitudes of (0.05−0.1) %, smaller, roughly by a factor 2, than for neutrons, and a phase generally close to 12–16 h local time. No sensible differences were observed in the anisotropy parameters between solar nights and days in the polar region. The possible contribution of a second harmonic in the observed diurnal trend was also investigated.