The cosmic radiation in the heliosphere at successive solar minima: Steady state no‐drift solutions of the transport equation

Abstract

Cosmic ray intensities and density gradients, as observed by the Pioneer 10 and 11, Voyager 1 and 2, and IMP spacecraft during the 1977 and 1987 solar minimum periods are interpreted. This is done with a two‐dimensional, no‐drift version of the cosmic ray transport equation. While this oversimplified model obviously cannot fit all the observations satisfactorily, it does produce useful insight into spectral shapes in the outer heliosphere, the question of anomalous hydrogen, and the properties of radial gradients. These effects are discussed systematically, and it is shown that many of them are due to adiabatic losses inherent in the modulation process. It is also pointed out which effects need more sophisticated models, including drifts, time‐dependent effects and acceleration at the termination shock. In particular, it is demonstrated that there is probably no physically realistic way to reproduce the observed negative latitudinal gradients without resorting to drift models of the modulation. The best fit to the anomalous oxygen data is achieved with either a charge state of 2+, or with a source function that changes between solar minimum periods with a factor of 2.8.