Study of the solar modulation for the cosmic ray isotopes with the PAMELA experiment

The space-borne PAMELA experiment was launched on the 15th June 2006 on board the Russian satellite Resurs-DK1 from the Baikonur cosmodrome. From the beginning PAMELA performed high-precision measurements of cosmic rays over a wide energy range until January 2016. Owing to its long-duration operation, PAMELA had turned out to be an optimal detecting apparatus for studies of the solar modulation of cosmic rays over time. The PAMELA collaboration has already published time-dependent proton, helium and electron spectra as well as the positron to electron ratio over ten years of data. These results are fundamentally important in the fine-tuning of propagation and modulation models of cosmic rays through the Heliosphere. In this talk, the yearly average spectra for proton, deuteron, Helium-3 and Helium-4 nuclei are presented for the 23rd solar minimum (July 2006 - January 2009) and the first part of the 24th solar maximum (until September 2014). The isotopic composition was measured between 0.1 and 1.1 GeV/n using two different detector systems. As expected, the measured spectra display a rising trend towards solar minimum followed by a decreasing trend, which has continued as solar maximum approached. The subsequent time-dependent ratio of these isotopes is also presented. According to solar modulation studies, a non-constant ratio is expected due to the different charge-to-mass ratios (and therefore the appropriate rigidities) and the different shapes of the respective local interstellar spectra. Additionally, it is of interest to analyze the observed spectra with state-of-the-art solar modulation models to obtain a deeper understanding of the relative importance of the mechanisms responsible for the propagation of cosmic rays in the Heliosphere over time.