The diurnal anisotropy and field-aligned diffusion of cosmic rays

Abstract

From the hourly counting rate of the Deep River neutron monitor we have calculated the magnitude and phase of the diurnal anisotropy, and from them we have inferred the magnitude and direction of cosmic ray diffusion on a daily basis. These data are compared with observed interplanetary magnetic field data, and we conclude that the diffusion is field aligned (within observational uncertainties) on essentially all days which are separated from sector boundaries. Days in which diffusion does not appear field aligned are characterized either by sector boundaries or by poor statistical accuracy.

Cosmic-ray anisotropy in interplanetary space

“…The cosmic-ray variation with a period of a day and an amplitude of ~ 0.4-0.6~ is well known (Krymsky, 1964;Parker, 1964;Krymsky, 1969;Dorman, 1975a;Alaniya and Shatashvili, 1974). The cosmic-ray flux giving rise to the diurnal variation usually includes the azimuthal and radial components.…”

“…It has been known (Parker, 1964;Krymsky, 1969;Owens and Kash, 1976) that the field-aligned cosmic-ray diffusion coefficient is much in excess of the field-perpendicular diffusion coefficient. If interplanetary magnetic field can be described in terms of the Parker model (Parker, 1964), the distance along force line to the modulation region boundary near the helio-equator is much in excess of the same distance in the regions located far from the helio-equator.…”

“…The radial component of the cosmic-ray flux is probably of the same order of magnitude as the azimuthal component (Owens and Kash, 1976;Mori etal., 1977;Gavrilyev et al, 1977) and is directed from the Sun for the particles of sufficiently high energies detected on the Earth. It will be noted that the above-mentioned properties of cosmic-ray anisotropy which give rise to the diurnal variation take place during the relatively quiet periods (the absence of shock waves and high-velocity fluxes of solarwind plasma, traverse of sectorial boundaries).…”

“…The cosmic-ray flux giving rise to the diurnal variation usually includes the azimuthal and radial components. The direction of the azimuthal component corresponds to the maximum cosmic-ray intensity at 18 : 00 LT, i.e., coincides with the direction of the Earth's orbital movement and the Sun's rotation (Krymsky, 1964;Axford, 1965;Parker, 1964).…”

“…It has been known (Parker, 1964;Krymsky, 1969;Owens and Kash, 1976) that the field-aligned cosmic-ray diffusion coefficient is much in excess of the field-perpendicular diffusion coefficient. If interplanetary magnetic field can be described in terms of the Parker model (Parker, 1964), the distance along force line to the modulation region boundary near the helio-equator is much in excess of the same distance in the regions located far from the helio-equator. Therefore, the effective modulation parameter #(0) = uro/Z, where u is solar wind velocity; to, the distance from the modulation region center in a spherical coordinate system with origin at the Sun's center to the modulation region boundary; 0, the azimuthal angle measured from the Sun's northern semi-axis; and z(0) is the cosmic-ray diffusion coefficient) which is maximum at 0 ~ ~/2 (i.e., at the helio-equator); so that the cosmic-ray density near the Astrophysics and Space Science 93 (1983) 97-126.…”