Simulation Study of Spacecraft Electrostatic Sheath Changes With the Heliocentric Distances From 0.044 to 1 AU

In this paper, the electrostatic sheath of a simplified spacecraft is investigated for heliocentric distances varying from 0.044 to 1 AU, using the 3-D Particle in Cell software Satellite-Plasma Interaction System. The baseline context is the prediction of sheath effects on solar wind measurements for various missions, including the Solar Probe Plus mission (perihelion at 0.044 AU from the sun) and Solar Orbiter (SO) (perihelion at 0.28 AU). The electrostatic sheath and the 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 SO 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.