Simulation of the charging process of the LISA test masses due to solar particles

Cosmic-ray and solar particles above 100 MeV penetrate the LISA experiment test masses. Consequently, electric charges accumulating there generate spurious Coulomb forces between the masses and the surrounding electrodes. This process increments the noise level of the experiment. We have estimated the amount of charge deposited per second on the LISA test masses by primary cosmic-ray protons at solar minimum and solar maximum and by solar energetic particle (SEP) events. This simulation has been carried out with the Fluka Monte Carlo program. A simplified geometry for the experiment has been considered. We have found an effective charge rate of 110 e s−1 for primary protons at solar maximum and 150 e s−1 at solar minimum between 0.1 and 1000 GeV. The amount of charge released by a medium intensity gradual event (7 May 1978) varies from 206 e s−1 in the first few minutes to 4575 e s−1 at the peak of the event. At the occurrence of medium or strong solar events, the LISA sensitivity curve at frequencies lower than 3 × 10−4 Hz is dominated by the noise due to the test-mass charging process.