The role of interplanetary electrons at the time of the LISA missions

LISA (Laser Interferometer Space Antenna) is the first space interferometer devoted to the detection of gravitational waves in the frequency range 10^(-4) to 10^(-1) Hz. Free-fall gold-platinum test masses constitute the mirrors of the interferometer. Solar and galactic particles charging the test masses induce spurious forces that might mimic genuine gravitational wave signals. Proton and helium nuclei are more than 98% in composition of both galactic and energetic solar particles. The charging due to these ions was carefully studied. However, highly penetrating interplanetary electrons play a role similar to helium nuclei at solar minimum and balance more than half of the net charge induced by galactic protons at solar maximum. In this paper, we report the study of LISA test-mass charging and radiation monitor countrate due to interplanetary electrons under different conditions of solar modulation and global solar magnetic field (GSMF) polarity. The radiation monitors designed for the LISA precursor mission, LISA Pathfinder (LISA-PF), were considered. Solar electrons do not produce any detectable signal in the radiation monitors. No relevant increase in the test-mass charging is generated by solar electrons with respect to protons as well. However, we point out that the detection of electrons of solar origin on-board LISA will allow us to short-forecast incoming, intense solar ion fluxes. An optimized environmental survey would lead us to further improve the test-mass discharging process, reduce the overall noise and, possibly, extend the mission lifetime. Important contributions to solar physics and space-weather investigations will be provided as well.