Estimating Characteristics of the Heliosphere Using Anomalous Cosmic Ray Observations at Solar Maximum

Abstract

Observations of the energy spectra of anomalous cosmic ray (ACR) oxygen were acquired in the outer heliosphere during the last solar maximum period in 1990 when the particle distributions are expected to be more nearly spherically symmetric than at solar minimum. Hence, we use a simple one-dimensional model of ACR acceleration and transport to fit the observed energy spectra at Voyagers 1 and 2 and Pioneer 10. The inferred interplanetary radial diffusion coefficient is remarkably consistent with recent theoretical estimates. The next solar maximum is expected to occur in the outer heliosphere in early 2001 when Voyager 1 will be ∼80 AU from the Sun. We predict that the intensity of ACR oxygen and helium at Voyagers 1 and 2 will be sufficiently large, and the model uncertainties sufficiently small, that it should be possible in 2001 to determine the remaining distance to the shock to within ±1 AU if the shock is within ∼5 AU, −3 to +6 AU if the shock is within ∼15 AU, and −5 to +14 AU if the shock is within ∼25 AU.