For the purpose of astroparticle physics the influence of the Sun’s and the Earth’s magnetic field is a perturbation, which complicates a search for the sources of cosmic rays. The solar activity produces an additional magnetic field, which prevents part of galactic cosmic rays from reaching Earth. Figure 7.1, however, shows that the influence of the Sun is limited to primary particles with energies below 10 GeV. The flux of low-energy primary cosmic-ray particles is anti-correlated to the solar activity.