Modelization of galactic cosmic-ray short-term variations for LISA

The European Space Agency Laser Interferometer Space Antenna (LISA) will be the first mission dedicated to the detection of low-frequency gravitational waves in space. Particles of galactic and solar origin above tens of MeV will penetrate the spacecraft and charge the metal free-falling test masses (TMs) playing the role of mirrors of the interferometer. The poissonian fluctuations of the charging process and associated spurious Coulomb forces acting on the TMs limit the sensitivity of LISA mainly below 1 mHz. Moreover, galactic cosmic-ray (GCR) flux short-term variations will modulate differently the TM charging on the three satellites of the LISA constellation. Without a proper GCR flux monitoring, the LISA TM charging estimates will be carried out on the basis of the long-term solar modulation only. In this work we report about models of galactic cosmic-ray short-term variations to investigate to which extent the galactic cosmic-ray depressions can be also used as a proxy of the increase of interplanetary magnetic field and solar wind speed observed at the passage of high-speed solar wind streams and interplanetary coronal mass ejections. Our final aim is to study the optimum characteristics of particle detectors for both TM charging estimate and interplanetary medium monitoring for LISA.