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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?1for primary protons at solar maximum and 150 e s?1at 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?1in the first few minutes to 4575 e s?1at the peak of the event. At the occurrence of medium or strong solar events, the LISA sensitivity curve at frequencies lower than 3 × 10?4Hz is dominated by the noise due to the test-mass charging process.

Solar energetic particles and galactic cosmic rays with energies larger than 100 MeV cause progressive charging of the LISA experiment test masses. Consequently, Coulomb forces occur between the test masses and the surrounding conducting surfaces generating spurious signals that might be mistaken for gravitational wave signals. We have parametrized the energy spectra of galactic cosmic-ray nuclei and electrons near the LISA orbit in order to evaluate their role in the test-mass charging relative to the most abundant proton component. This work has been carried out using the FLUKA Monte Carlo program.