Observed solar modulation of galactic cosmic ray intensity in the outer heliosphere, 1997-2001

[1] Instruments on Pioneer 10, Voyager 1 and Voyager 2 measure galactic cosmic ray intensity (> 80 MeV/nucleon) as the spacecraft move along their solar system escape trajectories. As of 1 June 2001, their respective heliocentric radial distances were 78 AU (in approximately the antapex direction from the Sun), 81 AU and 64 AU (the latter two in approximately the apex direction). The principal finding of this paper is that all three spacecraft were still under the delayed influence of increasing solar activity during cycle 23 and were, therefore, inside the modulation boundary of the heliosphere. Also the combined evidence favors essential antapexapex symmetry in the large scale modulation medium. The contents of this paper are generally concordant with studies by other groups of authors but are unique in the inclusion of the recent four years of antapex data from Pioneer 10.