Radial Interplanetary Mean Free Paths Inferred from Anomalous Cosmic Ray Observations in the Outer Heliosphere

Abstract

We use two independent techniques to estimate the rigidity dependence of the interplanetary mean free pathlength in the outer heliosphere in 1998: I) inferences from gradients of anomalous cosmic rays (ACRs) between Voyagers 1 and 2 and 2) inferences from the shape of the ACR energy spectra. Both techniques indicate that the mean free path increases rapidly between ""0.25 GV and I GV but is independent of rigidity above "'J GV, in agreement with the dependence used by Steenberg et al. (I 999) to fit the 1998 ACR energy spectra using a full-drift, two-dimensional solution to the transport equation. The gradient method also yields an estimate of the magnitude of the mean free path, which is "'2 ± 0.4 AU at 1.5 GV, the same as the value of 2.0 AU used by Steenberg et al. (1999). In addition, we find that the mean free path at 0.42 GV is 1.4 ± 0.8 from anisotropy measurements of 4-7.8 MeV/nuc ACR He from Voyager 1. This value is consistent with the estimate from the gradient method and from the Steenberg et al. fits. At 1.5 GV, the mean free path during the 1998 solar minimum is "'I 0 times larger than that estimated for solar maximum in 1990, suggesting that for A>O, when particles drift from high to low latitudes, the radial gradient at low latitudes reflects the much smaller radial gradients theoretically expected (Zank et al. 1998) at higher latitudes.