  Galactic cosmic rays (GCRs) entering the heliosphere are disturbed by the
magnetic field of the Sun, which varies the shape and intensity of their local
interstellar spectrum. The force-field approximation is a popular way of
dealing with solar modulation, especially for studies focused on galactic
transport of cosmic rays. The validity of this approach to reproduce the
modulated GCR fluxes at Earth is tested using monthly proton fluxes measured by
PAMELA between July 2006 and January 2014 and monthly proton and helium fluxes
measured by AMS-02 between May 2011 and May 2017. We show that the precision of
the new AMS-02 data requires a rigidity-dependent modification of the
force-field approximation.
