  A model is presented that explains the charging rate of the LISA Pathfinder
test masses by the interplanetary cosmic ray environment. The model
incorporates particle-tracking from TeV to eV energies using a combination of
GEANT4 and a custom low-energy particle generation and tracking code. The
electrostatic environment of the test mass is simulated allowing for a
comparison of the test-mass charging-rate dependence on local electric fields
with observations made in orbit. The model is able to reproduce the observed
charging behavior with good accuracy using gold surface properties compatible
with literature values. The results of the model confirm that a significant
fraction of the net charging current is caused by a population of low-energy
($\sim$eV) electrons produced by electron- and ion-induced kinetic emission
from the test mass and surrounding metal surfaces. Assuming a gold work
function of 4.2 eV, the unbalanced flow of these electrons to and from the
unbiased test mass contributes $\sim$10% of the overall test mass charging
rate. Their contribution to the charging-current shot noise is
disproportionately higher and it adds $\sim$40% to the overall predicted noise.
However, even with this increased noise contribution the overall
charging-current noise is still only 40% of that measured in-orbit, and this
remains an unsolved issue.
