Solar influence on galactic cosmic ray anistropy measurements

Abstract

COSMIC ray particles of energies ≳ 1012 eV are virtually unaffected by the interplanetary magnetic field, and thus any galactic anisotropy manifests itself in sidereal intensity variation in Earth-based measurements. Below 1011 eV, on the other hand, solar modulation is of importance. Galactic anisotropy would be smeared out at this energy, and an anisotropy is introduced by the solar modulation effects. In the intermediate 1011–1012 eV region sidereal variation may originate either in genuine galactic anisotropy or in solar modulation1. The London–Torino groups found2, in accordance with other measurements3, that, in this intermediate energy range, the phase of the sidereal variation shifted from ∼ 1800 to ∼ 330 local sidereal time (LST) in 1969 when the polarity of the solar magnetic field reversed. The authors suggested that the change of phase was a result of the change of ‘optics’: different regions of the sky were scanned before and after 1969, respectively. Here we consider the alternative possibility— that the observed variation was a result of the electromagnetic field embedded in the solar wind.