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Precise time-dependent measurements of the Z = 2 component in the cosmic radiation provide crucial information about the propagation of charged particles through the heliosphere. The PAMELA experiment, with its long flight duration (2006 June 15–2016 January 23) and the low energy threshold (80 MeV/n) is an ideal detector for cosmic-ray solar modulation studies. In this paper, the helium nuclei spectra measured by the PAMELA instrument from 2006 July to 2009 December over a Carrington rotation time basis are presented. A state-of-the-art three-dimensional model for cosmic-ray propagation inside the heliosphere was used to interpret the time-dependent measured fluxes. Proton-to-helium flux ratio time profiles at various rigidities are also presented in order to study any features that could result from the different masses and local interstellar spectra shapes

The satellite-borne PAMELA experiment was launched on the 15th June 2006 from the Baikonur cosmodrome. Since then and until January 2016 PAMELA made high-precision measurements of the charged component of cosmic-rays over a wide energy range. Because of its long-duration operation, PAMELA represents an ideal detector for cosmic-ray solar modulation studies. The PAMELA collaboration already published time-dependent proton, helium and electron spectra as well as the positron to electron ratio over ten years of data taking. These results are fundamental to improve and refine propagation models the propagation models of cosmic rays through the heliosphere. Here, the yearly average spectra over the 23rd solar minimum (July 2006 - January 2009) until the middle of the 24rd solar maximum (December 2015) for 1H, 2H, 3He and 4He are presented. The isotopic composition was measured between 0.1 and 1.1 GeV/n using two different detector systems. These spectra show an increasing trend during the solar minimum period with a decrease as the solar maximum is approached. The time-dependent ratio of these isotopes is also presented. From a solar modulation point of view, a non constant ratio is typically caused by their different masses (related to their rigidity) and also the different shapes of the respective local interstellar spectra. Apart from these effects, it is worthwhile to look deeper for more fundamental causes related to their propagation and modulation mechanisms

Precision measurements of the Z = 2 component in the cosmic radiation provide crucial information about both the origin and propagation of the helium nuclei in cosmic rays inside the Galaxy. Moreover, the precise measurement of the time-dependent helium nuclei fluxes allows an in-depth study of the solar modulation of such nuclei. These results can be compared with the widely studied modulation of protons, to possibly spot dependencies on particle’s charge, mass or local interstellar spectra shape. The helium differential fluxes together with comparison between helium and proton energy spectra, measured by the PAMELA experiment down to 80 MeV, has been studied in the unusually long minimum of solar cycle 23 (July 2006 December 2009) over a 1-month time basis.