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A Durable Reduction of Cosmic Ray Intensity in the Outer HeliosphereThis paper reports Pioneer 10 (P10) and Pioneer 11 (P11) observations of the intensity J(E(sub p) greater than 80 MeV) of galactic cosmic rays in the heliosphere near the heliographic equator during the 24-year period 1972-1996 and out to a heliocentric radial distance of 65 AU. It updates previous P10/P11 determinations of the time dependence of the radial gradient of intensity and emphasizes the recent 10-year period, especially the consequences of the great Forbush decrease in 1991. A fresh analysis compares P10 and P11 data with comparable data from IMP 8 at 1.0 AU. For this purpose, we have made a critical study of the data from three different instruments on IMP 8 and have developed a new time-dependent reference level of intensity at 1.0 AU for the period 1974-1996. Using this reference, we find that as of late 1996, recovery of intensity following the 1991 Forbush decrease has been markedly less complete in the outer heliosphere than at 1.0 AU. As a consequence, the mean radial gradient between 4 and 65 AU is now only about +0.3% 1/AU. Our findings favor the latitudinal wedge model of the heliosphere (Van Allen and Mihalov, 1990) and suggest that the modulation boundary of the heliosphere is far beyond 65 AU. Generally concordant, but less decisive, evidence of a similar nature has been reported previously by Van Allen (1993), Van Allen (1996), and Webber and Lockwood (1995b).
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
VanAllen, James A. (Iowa Univ. Iowa City, IA United States)
Randall, Bruce A. (Iowa Univ. Iowa City, IA United States)
Date Acquired
September 6, 2013
Publication Date
March 1, 1997
Publication Information
Publication: Journal of Geophysical Research
Volume: 102
Issue: A3
ISSN: 0148-0227
Subject Category
Solar Physics
Report/Patent Number
NAS 1.15:112519
Distribution Limits
Public Use Permitted.

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