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

Cummings, A. C. and Stone, E. C. (1999) Radial Interplanetary Mean Free Paths Inferred from Anomalous Cosmic Ray Observations in the Outer Heliosphere. In: Proceedings of the 26th International Cosmic Ray Conference. Vol.7. International Union of Pure and Applied Physics , pp. 496-499.

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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.

Item Type:Book Section
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Cummings, A. C.0000-0002-3840-7696
Stone, E. C.0000-0002-2010-5462
Additional Information:Copyright University of Utah. Provided by the NASA Astrophysics Data System. We thank E. Christian for some of the data processing algorithms used in the analysis. We thank J. Richardson for provid ing the V2 solar wind velocites and J. T. Hoeksema for providing the tilt angles of the heliospheric current sheet (Hoeksema 1992). This work was supported by NASA under contract NAS7-918.
Group:Space Radiation Laboratory
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Space Radiation Laboratory1999-04
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ID Code:54693
Deposited By: Deborah Miles
Deposited On:13 Feb 2015 14:36
Last Modified:20 Feb 2020 14:33

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