Anisotropies of galactic cosmic rays in the solar system

Abstract

At least four processes appear capable of contributing to the creation of an anisotropy of galactic cosmic rays in interplanetary space. All need not necessarily be active on a particular day to the same extent. The processes are (1) azimuthal streaming, (2) streaming due to non-uniform diffusion in a longitudinal sector structure, (3) the scattering at irregularities along the interplanetary magnetic field, and (4) latitudinal gradients in a relatively smooth magnetic field. The characteristics of the daily variation which must be expected from each process are quantitatively evaluated. Observational evidence derived from measurements of the daily variation from a network of super neutron monitors is used to derive, with precision, the spectrum of variation, the amplitude and the principal direction of maximum and minimum intensity in interplanetary space, on each day during 1964–65. The experimental results are compared with predictions to derive conclusions as to the process which operates on individual days. It is demonstrated that the diurnal and the semi-diurnal components of the anisotropy have characteristically different energy spectra of variation. The predominant process responsible for the diurnal component is the azimuthal streaming while the semidiurnal component appears to be due to scattering at magnetic field irregularities and latitudinal gradients. The manner in which the results can be used to derive the threshold energy below which isotropic diffusion seems to occur on individual days in the inner solar system is demonstrated. The results of the study strongly support the postulate of a gradient of galactic cosmic rays in the solar system perpendicular to the ecliptic.