Cosmic ray intensity gradients in the solar system

The measurement of the heliocentric intensity gradient of galactic cosmic rays provides a crucial test for our understanding of the modulation of the cosmic radiation by the irregular magnetic fields carried by the solar wind. The basic model for the modulation involving diffusion of energetic particles by scattering from magnetic irregularities, adiabatic deceleration, and convection by the solar wind has not been much in dispute since the equations governing propagation of cosmic rays in the solar wind were first written down by Parker [1963, 1965]. However, varying assumptions concerning, for example, the location and shape of the boundary of the modulation region and the form and spatial dependence of the diffusion coefficients led to widely differing predictions concerning the distribution of cosmic rays in the heliosphere and the nature of the interstellar cosmic ray spectrum at low energies. Accurate measurements of the gradients offer hope of distinguishing among the various detailed models proposed. It is an indication of the significance of such a measurement that since the first observations establishing the heliocentric nature of the modulation [ Simpson et al. , 1962], more than 30 attempts to determine the intensity gradients of cosmic rays of various energies and nuclear species have been made. O'Gallagher [1972 a ] has reviewed the status of these measurements as of 1971. At that time the majority of measurements indicated a moderate (several percent per astronomical unit) positive radial gradient for the integral intensity of cosmic rays above threshold energies of the order of 100 MeV/nucleon, but measurements in differential energy windows below ∼100 MeV/nucleon were in ‘irreconcilable disagreement.’ In this paper, progress in the determination of intensity gradients reported in the years 1971–1974 will be reviewed, with concentration on efforts by U.S. workers. Several areas which are related to gradient measurements, such as the determination of the location of the modulation boundary, and solutions of the modulation equations aimed solely at the fitting of observed cosmic ray spectra at the orbit of earth have been deliberately excluded.