The cosmic radiation in the heliosphere at successive solar minima

Abstract

Comparative cosmic ray studies at the time of successive solar minima are of special importance in establishing the relative role of large‐scale drift effects in the modulation process and in estimating the size of the modulation region and the local interstellar spectra of low‐ and medium‐energy cosmic rays. In this paper the 1 AU cosmic ray observations are compared for the last three solar minimum periods along with the 1977/1978 and 1987 Pioneer 10 and Voyager 1 and 2 data from the outer heliosphere. There is good agreement between the 1965 and 1987 galactic cosmic ray H and He spectra at 1 AU. However, there are significant and complex differences between the 1977/1978 and 1987 measurements of the galactic and anomalous cosmic ray components at 1 and 15 AU. In the outer heliosphere there are large negative latitudinal gradients that reach their maximum magnitude when the inclination of the outer heliosphere current sheet is at a minimum. The radial gradients decrease with heliocentric distance as ∼1/r0.7 and do not differ significantly at the successive solar minima. While these effects, along with the shift in the intensity maximum of anomalous helium (He+), are in the direction expected from large‐scale gradient and curvature drifts in the heliosphere, they are about an order of magnitude smaller than those calculated using the standard Parker model of the interplanetary magnetic field. The measured radial and latitudinal gradients are used to estimate the particle transport parameters in the outer heliosphere. The slope of the H spectra measured at Pioneer 10 for both solar minimum periods is given by γH ≈ 0.5 down to energies of 40 MeV. Using the local interstellar He spectrum of Webber et al. (1987), it is estimated that the modulation boundary is of the order of 160 AU.