  Time-dependent energy spectra of galactic cosmic rays (GCRs) carry
fundamental information regarding their origin and propagation. When observed
at the Earth, these spectra are significantly affected by the solar wind and
the embedded solar magnetic field that permeates the heliosphere, changing
significantly over an 11-year solar cycle. Energy spectra of GCRs measured
during different epochs of solar activity provide crucial information for a
thorough understanding of solar and heliospheric phenomena. The PAMELA
experiment had collected data for almost ten years (15th June 2006 - 23rd
January 2016), including the minimum phase of solar cycle 23 and the maximum
phase of solar cycle 24. In this paper, we present new spectra for helium
nuclei measured by the PAMELA instrument from January 2010 to September 2014
over a three Carrington rotation time basis. These data are compared to the
PAMELA spectra measured during the previous solar minimum providing a picture
of the time dependence of the helium nuclei fluxes over a nearly full solar
cycle. Time and rigidity dependencies are observed in the proton-to-helium flux
ratios. The force-field approximation of the solar modulation was used to
relate these dependencies to the shapes of the local interstellar proton and
helium-nuclei spectra.
