  We simulated the modulation of the interstellar neutral (ISN) He, Ne, and O
density and pick-up ion (PUI) production rate and count rate along the Earth's
orbit over the solar cycle from 2002 to 2013 to verify if solar cycle-related
effects may modify the inferred ecliptic longitude of the ISN inflow direction.
We adopted the classical PUI model with isotropic distribution function and
adiabatic cooling, modified by time- and heliolatitude-dependent ionization
rates and non-zero injection speed of PUIs. We found that the ionization losses
have a noticeable effect on the derivation of the ISN inflow longitude based on
the Gaussian fit to the crescent and cone peak locations. We conclude that the
non-zero radial velocity of the ISN flow and the energy range of the PUI
distribution function that is accumulated are of importance for a precise
reproduction of the PUI count rate along the Earth orbit. However, the temporal
and latitudinal variations of the ionization in the heliosphere, and
particularly their variation on the solar cycle time-scale, may significantly
modify the shape of PUI cone and crescent and also their peak positions from
year to year and thus bias by a few degrees the derived longitude of the ISN
gas inflow direction.
