S A Simulation Study of Spacecraft Electrostatic Sheath Changes with the Heliocentric Distances from 0 . 044 to 1 AU

spacecraft potential could interfere with the low energy (a few tens of eV) plasma measurements, by biasing the particle distribution functions measured by the detectors. If the spacecraft charges to large negative potentials, the problem will be more severe as low energy electrons will not be seen at all. The Solar Probe Plus and Solar Orbiter cases will be presented in details and extended to other distances through a parametric study, to investigate the influence of the heliocentric distance to spacecraft. Our main result is that for our spacecraft model the floating potential is a few volts positive from 1 AU to about 0.3 AU while below 0.3 AU the space charge of the photoelectrons and secondary electrons create a potential barrier that drives the spacecraft potential negative.