Evidence for a Latitudinal Gradient of the Cosmic Ray Intensity Associated with a Change in the Tilt of the Heliospheric Current Sheet

Since mid-1985, the average flux of >70 MeV/nucleon cosmic rays at Voyager 2 (r_2 ∼ 17 AU, Θ_2 ∼ 0°) has been ∼3-5% greater than that at Voyager 1 (r_1 ∼ 24 AU, Θ_1 ∼ 26°N). This is the first direct observation over such a large radial range in which the galactic cosmic ray flux closer to the sun is higher than the flux farther from the sun for an extended period of time. This observation is consistent with the presence of a negative latitudinal gradient G_Θ=−0.36 ± 0.05 (or −0.60 ± 0.08) %/deg, assuming a coexistent radial gradient G_r of 1 (or 2) %/AU. We suggest that the appearance of this persistent negative latitudinal gradient may be due to the abrupt, large decrease of the heliospheric current sheet tilt to ∼20° in early 1985