Time-dependent cosmic ray modulation in the outer heliosphere: Signatures of a heliospheric asymmetry and model predictions along Voyager 1 and 2 trajectories

Abstract

A two-dimensional, time-dependent numerical model is used to calculate the modulation of cosmic rays in the heliosphere. Computations are compared to spacecraft observations in the inner and outer heliosphere. It is shown that the model produces cosmic ray proton intensities compatible to different spacecraft observations on a global scale, at Earth and along both Voyager spacecraft trajectories. The study reveals that when the same modulation parameters, which resulted in compatible intensities along Voyager 1, were assumed along the Voyager 2 trajectory, the model failed to reproduce the observations. The study also found that any change in diffusion parameters alone could not reproduce the cosmic ray observations along Voyager 2 so that changes to the heliospheric geometry were necessary i.e the computed intensities along both Voyager trajectories suggest that the heliosphere is asymmetric. Furthermore, $E >$ 70 MeV and 133-242 MeV proton intensities along Voyager 1 and 2 trajectories are predicted from end of 2012 onwards. It is shown that the computed intensities along Voyager 1 increase with an almost constant rate up to the heliopause. However, the model shows that Voyager 2 is still under the influence of temporal solar activity changes because of its relatively large distance to the heliopause. Along the Voyager 2 trajectory, the intensities remained generally constant for some time and should soon start to increase steadily.