Investigation of cosmic ray anisotropy based on Tsumeb neutron monitor data

Abstract

We investigate sidereal daily variations of galactic cosmic rays (CR) by using Tsumeb neutron monitor’s (NM) hourly count during 1977–2000. For that purpose a special data analysis is applied to remove solar induced daily variations and improve sidereal variations. Then, the declination of protons in the Earth’s magnetosphere is analyzed for the proper determination of particles’ asymptotic directions. As a result we found ∼0.07% anisotropy with an excess flux from the direction of the right ascension (RA) ∼6 h. The reversal of solar magnetic field from the negative polarity state (in the time interval 1977–1989) to the positive state (in the interval 1990–2000) does not decrease the amplitude of anisotropy, which means that the origin of anisotropy found is “heliospheric” and not “galactic”. We conclude that the observed anisotropy is a tail-in anisotropy, which has been discovered earlier for higher energy (150 GeV–10 TeV) particles in several experiments with underground muon monitors and air-shower detectors. It is believed that the tail-in anisotropy is caused by heliotail boundary, located in the direction RA ∼6 h, where the interaction between galactic and solar magnetic fields produces CR acceleration