  The Interstellar Neutral Helium (ISN He) is an important source of
information on the physical state of the Local Interstellar Medium. Radiation
pressure acting on the neutral helium atoms in the heliosphere has always been
neglected, its effect has been considered insignificant compared to
gravitational force. The most advanced numerical models of ISN He take into
account more and more subtle effects, therefore it is important to check if the
effect of radiation pressure is still negligible. In this paper, we use the
most up-to-date version of the Warsaw Test Particle Model (WTPM) to calculate
the expected helium distribution in the heliosphere, and simulate the flux of
ISN He observed by the Interstellar Boundary Explorer (IBEX) and in the future
by the Interstellar Mapping and Acceleration Probe (IMAP). We compare results
calculated with and without radiation pressure during low and high solar
activity. The results show that in the analysis of IBEX-Lo observations the
radiation pressure acting on typical helium causes flux differences at a level
of 1-4% and is comparable to the observational errors. For the more sensitive
IMAP-Lo instrument, there are some regions in the considered observations
configurations where radiation pressure causes potentially statistically
significant changes in the calculated fluxes. The effect can be up to 9% for
the indirect beam and is likely to be higher than the estimated errors.
Therefore, we claim that in the future analysis of the IMAP-Lo observations
radiation pressure acting on ISN He should be considered.
