Intensity variations in the interplanetary magnetic field measured by Voyager 2 and the 11‐year solar cycle modulation of galactic cosmic rays

Abstract

We present new evidence to support the hypothesis that the 11‐year solar cycle modulation of galactic cosmic rays is caused by strong particle diffusion inside long‐lived, merged interaction regions. These regions are represented by local enhancements in the heliospheric magnetic field strength. To test this hypothesis, we solved the one‐dimensional, force field approximation of the cosmic ray modulation equation. The only variables were the strength of the local magnetic field and the position of the spacecraft, both taken directly from Voyager 2 data. We assume that a constant solar wind speed convects magnetic field compressions and rarefactions unchanged through a model heliosphere. The result is a reasonable simulation of the integrated, high‐energy cosmic ray intensity profile from about 1982 to mid‐1989. This period encompasses both the full recovery portion of the last 11‐year cosmic ray cycle and the first year and a half of the new cycle. In particular, this model responds to the Voyager 2 magnetic field data by correctly timing the beginning of the new modulation cycle in late 1987. We conclude that our hypothesis is consistent with the results of this simulation.