Latitudinal gradient of energetic particles in the outer heliosphere during 1985–1986

Abstract

We report a measurement of a sustained latitudinal gradient of 70‐MeV galactic cosmic ray protons using data from the interplanetary probes Voyager 1 and 2 and the earth‐orbiting satellite IMP 8 during a 1‐year period from mid‐1985 to mid‐1986. Starting in early 1985 the intensity of cosmic rays at Voyager 2 began increasing faster than that at Voyager 1. By mid‐1985 the intensity at Voyager 2 (helioradius 17 AU, heliolatitude ∼0°) exceeded and remained higher than that at Voyager 1 (helioradius 24 AU, heliolatitude ∼26°) for at least 14 solar rotations. Using the Voyager 2‐IMP 8 data to correct for the radial gradient, we determine an average latitudinal gradient during this period of (−0.53 ± 0.10)% deg−1 or (−0.38 ± 0.11)% deg−1 (i.e., intensity decreasing northward of the solar equatorial plane), assuming either a constant radial gradient or one that decreased at an average rate of −0.025% AU−1 AU−1, as was the case during 1984, respectively. In addition, we present Voyager data at very low (≳30 keV) ion energies which are associated with acceleration at corotating shocks. These ions also exhibit a latitudinal gradient (∼ −3% deg−1), with lower intensities at higher latitudes, and serve to highlight changes that occurred in the interplanetary medium in early 1985.