Does spacecraft potential depend on the ambient electron density?

Tuesday, 16 December 2014
Shu Tim Lai1,2, Manuel Martinez-Sanchez1, Kerri Cahoy1, Michelle F Thomsen3, Yuri Shprits4, Whitney Quinne Lohmeyer4 and Frankie Wong5, (1)Massachusetts Institute of Technology, Space Propulsion Laboratory, Cambridge, MA, United States, (2)Boston College, Institute for Scientific Research, Chestnut Hill, MA, United States, (3)Planetary Science Institute, Los Alamos, NM, United States, (4)Massachusetts Institute of Technology, Cambridge, MA, United States, (5)Space Systems Loral, Palo Alto, CA, United States
In a Maxwellian space plasma model, the onset of spacecraft charging at geosynchronous altitudes is due to the ambient electron, ambient ions, and secondary electrons. By using current balance, one can show that the onset of spacecraft charging depends not on the ambient electron density but instead on the critical temperature of the ambient electrons. If the ambient plasma deviates significantly from equilibrium, a non-Maxwellian electron distribution results. For a kappa distribution, the onset of spacecraft charging remains independent of ambient electron density. However, for double Maxwellian distributions, the densities do have a role in the onset of spacecraft charging. For a dielectric spacecraft in sunlight, the trapping of photoelectrons on the sunlit side enhances the local electron density. Using the coordinated environmental satellite data from the Los Alamos National Laboratory geosynchronous satellites, we have obtained results that confirm that the observed spacecraft potential is independent of the ambient electron density during eclipse and that in sunlight charging the low-energy population around the sunlit side of the spacecraft is enhanced by photoelectrons trapped inside the potential barrier.