  As ASTROD I travels through space, its test mass will accrue charge due to
galactic cosmic-rays and solar energetic particles incident on the spacecraft.
This test mass charge will result in Coulomb forces between the test mass and
the surrounding electrodes. In earlier work using the GEANT4 toolkit, we
predicted a net charging rate of nearly 9.0 +e/s from cosmic-ray protons
between 0.1 and 1000 GeV at solar maximum, and rising to 26.5 +e/s at solar
minimum. Here we use GEANT4 to simulate the charging process due to solar
energetic particle events and to estimate the magnitude of acceleration noise
due to this charging. The predicted charging rates range from 2840 to 64300
+e/s, at peak intensity, for the 4 largest SEP events in September and October
1989. For the 2 larger events, the acceleration disturbances due to charging
exceeds the ASTROD I acceleration noise budget requirement. Continuous
discharge should be considered for suppressing this charging noise. The
acceleration noise during the 2 small events is well below the design target,
although during these events, the net charging rate will be dominated by these
solar fluxes.
