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A three-dimensional (3-D), self-consistent code is employed to solve for thestatic potential structure surrounding a spacecraft in a high photoelectronenvironment. The numerical solutions show that, under certain conditions, aspacecraft can take on a negative potential in spite of strong photoelectroncurrents. The negative potential is due to an electrostatic barrier near thesurface of the spacecraft that can reflect a large fraction of thephotoelectron flux back to the spacecraft. This electrostatic barrier forms if(1) the photoelectron density at the surface of the spacecraft greatly exceedsthe ambient plasma density, (2) the spacecraft size is significantly largerthan local Debye length of the photoelectrons, and (3) the thermal electronenergy is much larger than the characteristic energy of the escapingphotoelectrons. All of these conditions are present near the Sun. The numericalsolutions also show that the spacecraft's negative potential can be amplifiedby an ion wake. The negative potential of the ion wake prevents secondaryelectrons from escaping the part of spacecraft in contact with the wake. Thesefindings may be important for future spacecraft missions that go nearer to theSun, such as Solar Orbiter and Solar Probe Plus.

Spacecraft charging and ion wake formation in the near-Sun environment