A DURABLE REDUCTION OF COSMIC-RAY INTENSITY IN THE OUTER HELIOSPHERE

This paper reports Pioneer 10 (P10) and Pioneer 11 (P11) observations of the intensity J(E(p) > 80 MeV) of galactic cosmic rays in the helio sphere near the heliographic equator during the 24-year period 1972-19 96 and out to a heliocentric radial distance of 65 AU. It updates prev ious P10/P11 determinations of the time dependence of the radial gradi ent of intensity and emphasizes the recent 10-year period, especially the consequences of the great Forbush decrease in 1991. A fresh analys is 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 thr ee different instruments on IMP 8 and have developed a new time-depend ent reference level of intensity at 1.0 AU for the period 1974-1996. U sing this reference, we find that as of late 1996, recovery of intensi ty following the 1991 Forbush decrease has been markedly less complete in the outer heliosphere than at 1.0 AU. As a consequence, the mean r adial gradient between 4 and 65 AU is now only about +0.3% AU(-1). Our findings favor the latitudinal wedge model of the heliosphere [Van Al ien and Mihalov, 1990] and suggest that the modulation boundary of the heliosphere is far beyond 65 AU. Generally concordant; but less decis ive, evidence of a similar nature has been reported previously by Van Alien. [1993], Van Alien [1996], and Webber and Lockwood [1995b].