The modulation of galactic cosmic rays over a solar cycle

Abstract

This work studied the modulation of galactic cosmic rays in the heliosphere by using a well-established, time dependent numerical modulation model to calculate cosmic ray transport inside the heliosphere over a solar cycle. Results were compared to observations from the Voyager 1 and 2 spacecraft. It was shown that, when using the modified compound approach of Manuel et al. [2014] to scale the transport coefficients over a solar cycle, the model resulted in compatibility with spacecraft observations on a global scale. However, for certain periods, e.g. 1985 - 1990 and 1992 - 2001, the model did not agree with observations. For instance, for the period 1985 - 1990, the Voyager 1 spacecraft observed a plateau-like intensity profile while the model computed a peak-like intensity profile along the Voyager 1 trajectory. Voyager 2 on the other hand measured a peak-like intensity profile as expected from a traditional drift description of cosmic ray intensities around solar minimum. It was shown that, for this period, the Voyager 1 spacecraft was above the heliospheric current sheet region while Voyager 2 was inside the heliospheric current sheet region close to the equatorial region. It was shown that the time-dependent function that scales drifts up or down depending on the level of solar activity over a solar cycle, resulted in the peak-like intensity profile compatible with Voyager 2 observations but not Voyager 1 observations. It was proposed that this time-dependent function has a latitude dependence with different values inside and outside of the heliospheric current sheet, i.e. a different dependence above compared to in the heliospheric current sheet region. This improved compatibility between the model and Voyager 1 observations. However, the full implementation of such an additional spatial dependence in the 2D model is beyond the scope of this study. For the period 1992 - 2001, it was shown that the tilt angle increases much faster towards solar maximum than the corresponding decrease in observed cosmic ray intensities. This resulted in the model computing intensities decreasing faster than the observed intensities as a function of increasing solar activity and therefore causing incompatibility between the model and the observations. It was shown for this period that modifying the tilt angle by using an averaged tilt angle resulted in improved compatibility with observations.