A global view of the modulation of cosmic ray protons in the heliosphere for the solar minimum period up to 2009

Abstract

The heliosphere was in a state of ideal solar minimum conditions for at least three years up to the end of 2009. During this period the highest ever recorded cosmic ray spectra were observed at the Earth. Fortunately, the PAMELA and Ulysses KET instruments simultaneously observed proton intensities for most of the period between July 2006 and June 2009, while Voyager 1 made observations in the outer heliosphere. This provides a good opportunity to compare the basic features of a comprehensive numerical model for the global modulation of cosmic rays in the heliosphere with these observations. Global gradients for protons are computed with the model for this prolonged solar minimum of cycle 23/24. This is done for both radial and latitudinal gradients, with the latter possible because Ulysses changed its position significantly in the heliocentric meridional plane during this period. The modulation model is set up for the conditions that prevailed during this unusual solar minimum period so that insight is gained on what role particle drifts played in establishing the observed gradients for this period. Good agreement is found between computed and observed gradients so that we conclude that the model gives a most reasonable representation of modulation conditions from the Earth to the heliopause for the mentioned period. These results can be used to refine the theory for diffusion, particle drifts and turbulence in the heliosphere