  Cubic gold-platinum free-falling test masses (TMs) constitute the mirrors of
future LISA and LISA-like interferometers for low-frequency gravitational wave
detection in space. High-energy particles of Galactic and solar origin charge
the TMs and thus induce spurious electrostatic and magnetic forces that limit
the sensitivity of these interferometers. Prelaunch Monte Carlo simulations of
the TM charging were carried out for the LISA Pathfinder (LPF) mission, that
was planned to test the LISA instrumentation. Measurements and simulations were
compared during the mission operations. The measured net TM charging agreed
with simulation estimates, while the charging noise was three to four times
higher. We aim to bridge the gap between LPF TM charging noise simulations and
observations. New Monte Carlo simulations of the LPF TM charging due to both
Galactic and solar particles were carried out with the FLUKA/LEI toolkit. This
allowed propagating low-energy electrons down to a few electronvolt. These
improved FLUKA/LEI simulations agree with observations gathered during the
mission operations within statistical and Monte Carlo errors. The charging
noise induced by Galactic cosmic rays is about one thousand charges per second.
This value increases to tens of thousands charges per second during solar
energetic particle events. Similar results are expected for the LISA TM
charging.
