Time‐dependent modulation of galactic cosmic rays by merged interaction regions

Abstract

Models that solve the one‐dimensional, solar modulation equation have reproduced the 11‐year galactic cosmic ray cycle using functional representations of global merged interaction regions (MIRs). This study extends those results to the solution of the modulation equation with explicit time dependence. The magnetometers on Voyagers 1 and 2 provide local magnetic field intensities at regular intervals, from which one calculates the ratio of the field intensity to the average local field. These ratios in turn are inverted to form diffusion coefficients. Strung together in radius and time, these coefficients then fall and rise with the strength of the interplanetary magnetic field, becoming representations of MIRs. These diffusion coefficients, calculated locally, propagate unchanged from ∼10 AU to the outer boundary (120 AU). Inside 10 AU, all parameters, including the diffusion coefficient are assumed constant in time and space. The model reproduces the time‐intensity profiles of Voyager 2 and Pioneer 10. Radial gradient data from 1982‐1990 between Pioneer 10 and Voyager 2 are about the same magnitude as those calculated in the model. It also shows agreement in rough magnitude with the radial gradient between Pioneer 10 and 1 AU. When coupled with enhanced, time‐dependent solar wind speed at the probe's high latitude, as measured by independent observers, the model also follows Voyager 1's time‐intensity profile reasonably well, providing a natural source for the observed negative latitudinal gradients. The model exhibits the 11‐year cyclical cosmic ray intensity behavior at all radii, including 1 AU, not just at the location of the spacecraft where the magnetic fields are measured. In addition, the model's point of cosmic ray maximum correctly travels at the solar wind speed, illustrating the well‐known propagation of modulation. Finally, at least in the inner heliosphere this model accounts for the delay experienced by lower‐rigidity protons in reaching their time‐intensity peak. The actual delays in this model, however, are somewhat smaller than the data. In the outer heliosphere the model sees no delays, and the data are ambiguous as to their existence. It appears that strong magnetic field compression regions (merged interaction regions) that are 3‐4 times the average field strength can, at least in a helioequatorial band, disrupt effects, such as drifts, that could dominate in quieter magnetic fields. The question remains: Is the heliosphere ever quiet enough to allow such effects to be unambiguously measured, at least in the midlatitudes?