Physics – Plasma Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sm52a18l&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SM52A-18
Physics
Plasma Physics
7800 Space Plasma Physics, 7807 Charged Particle Motion And Acceleration, 7815 Electrostatic Structures, 7853 Spacecraft/Atmosphere Interactions, 7855 Spacecraft Sheaths, Wakes, Charging
Scientific paper
It is an open question why photoelectron currents emitted from negatively charged spacecraft surfaces are often lower than expected. At equilibrium, spacecraft potentials are determined by current balance. The important currents are those of the ambient electrons and ions, the secondary and backscattered electrons, and the photoelectrons. The ambient electron and ion currents can be measured in space; the rest can be deduced from laboratory measurements. At geosynchronous environment, plasma collection by a charged spacecraft is well-described by the Mott-Langmuir formulation. Such calculations of the spacecraft potential in eclipse are often reasonably accurate. In sunlight, however, the net photoelectron currents are often found to be much lower than the values deduced from laboratory measurements. As a result, common charging algorithms often give unsatisfactory sunlit charging results unless the photoelectron current input is reduced substantially. The elusive causes must be identified and accounted for. We advocate four causes: (1) The monopole-dipole potential difference between the sunlit and dark sides can form a local potential well which can efficiently trap much of the photoelectron current. (2) The angular dependence of the photoelectron yield has been commonly neglected. If included, it can reduce the photoelectron current by a good percentage. (3) Because the ground surfaces are conducting and connected, those on the dark side can conduct electrons to those on the sunlit side. While those on the sunlit side are emitting photoelectrons, those on the dark side are receiving not only ambient electrons and also the secondary electrons from nearby negatively charged (dielectric or isolated conductor) surfaces on the dark side. Such an incoming secondary electron current from one surface to another has also been commonly neglected. If included, the secondary electrons received by the ground surfaces on the dark side can substantially replenish the photoelectrons emitted from the sunlit ground surfaces. (4) The sunlit area is often less than assumed. At most, only half of the satellite surface area is in sunlight. The shadows of the antennae and panels should also be taken into account if a satellite rotates.
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