ANALYSES OF SIDEREAL AND SOLAR ANISOTROPIES IN COSMIC-RAYS

The Sun's interplanetary magnetic field and the solar wind modulate th e distribution of galactic cosmic-ray particles in the heliosphere. Th e particles diffuse inward, convect outward and have drifts in the mot ion of their gyro-centres. Irregularities in the IMF also scatter part icles from their gyro-orbits. These processes are the components of so lar modulation and produce streaming (and higher-order anisotropies) o f particles in the heliosphere. The anisotropies can be investigated a t the Earth by examining the count rates of cosmic-ray detectors. The anisotropic streams appear as diurnal variations in solar and sidereal time in the count rates. Higher-order anisotropies produce generally much smaller semi-diurnal and higher-order variations. Theoretical mod els of solar modulation predict effects that depend on the polarity of the Sun's magnetic dipole. The solar diurnal and north-south anisotro pies can be used to test these predictions. This paper is a short revi ew of analyses of 60 years of cosmic-ray data collected at the Earth f or the solar and sidereal diurnal variations present. Past analyses ha ve yielded interesting and controversial results regarding the rigidit y spectra and components of these anisotropies. Some of the controvers y remains today. Analyses of these anisotropies have also yielded quan titative information about parameters important to solar modulation, s uch as latitudinal and radial density gradients. The relatively new te chniques used for these determinations are explained here. Calculation s of these modulation parameters from Earth-based cosmic-ray detectors are reviewed and compared to spaceprobe measurements and theoretical predictions of their values. Recently, investigations of the sidereal and solar diurnal anisotropies have been combined to calculate mean-fr ee-paths of cosmic rays in the heliosphere. The latest conclusions fro m these analyses are that the parallel mean-free-paths of cosmic rays may depend on the polarity of the Sun's magnetic field. The results of these investigations are included in this paper to indicate the prese nt state of knowledge concerning this facet of cosmic-ray research.