Observations of the eleven-year cosmic-ray modulation cycle

Abstract

This paper reviews the available observations and the present state of knowledge of the long-term modulation of the intensity of galactic cosmic rays. This modulation occurs in response to the 11-year cycle of solar activity. The propagation of solar cosmic rays, as well as short-term modulation changes, are not explicitly discussed. The main theme of the paper is to interpret the observations in terms of the existing modulation theories. The theoretical model describes the propagation of cosmic rays in the interplanetary medium in terms of diffusion, convection, and energy changes. The topic is introduced with a brief description of how the cosmic-ray diffusion tensor is calculated from the fluctuations in the interplanetary magnetic field (Section 2). The cosmic-ray transport equations are discussed in Section 3. Observations of cosmic-ray intensity spectra, density gradients and anisotropies, and the evolution of the 11-year modulation cycle, are described in Sections 4, 5, and 6 respectively. It is shown that the principles of the modulation are understood in terms of the theoretical models. However, measurements of low-energy cosmic rays (kinetic energies ≲ 50 MeV nucleon−1) during the last few years, indicate that the models may be incomplete. On the other hand, it is also pointed out that the available observations are not always sufficient to evaluate the full consequences of the theoretical models. In particular, we need observations out of the ecliptic plane, as well as an increased effort to measure the density gradients and anisotropies at a single position (e.g. Earth) and as function of time.