LISA test-mass charging, Particle flux modeling, Monte Carlo simulations, and induced effects on the sensitivity of the observatory

Context. The Laser Interferometer Space Antenna (LISA) will probe the subhertz spectrum of gravitational-wave emission from the Universe. The space environment causes charge accumulation on its free-falling test masses (TMs) through galactic cosmic rays (GCRs) and solar energetic particles (SEPs) that impinge on the spacecraft. Primary and secondary particles that are produced in the spacecraft material eventually reach the TMs, where they deposit a net positive charge that fluctuates in time. This work is relevant for any current or future space missions that, like LISA, use free-falling TMs as inertial references. Aims. The coupling of the TM charge with the native stray electrostatic field produces noisy forces on the TM that can limit the performance of the LISA mission. Precise knowledge of the charging process allows us to predict the intensity of these charge-induced disturbances and to design specific countermeasures. Methods. We present a comprehensive toolkit that allowed us to calculate the TM charging time-series in a geometry representative of the LISA mission, and the associated induced forces under different conditions of the space environment by considering the effects of short and long GCR flux modulations and SEPs. Results. We studied the impact of spurious forces associated with the TM charging process on the sensitivity of the mission for the gravitational-wave detection for each of the conditions described above.