Observation of photoelectrons and secondary electrons reflected from a potential barrier in the vicinity of ATS 6

Abstract

The electron detectors on ATS 6 frequently have peaks in the counting rate at low energies from 4 to 10 eV in addition to other peaks at much higher energies. It is shown from the shape of the corresponding velocity distribution functions, the inferred electron densities, and the behavior of the counting rate versus angle that the low-energy electrons must at times be photoelectrons or secondary electrons emitted from the spacecraft. The data indicate the presence of a potential barrier in the spacecraft environment which is as much as 50 V negative with respect to the spacecraft. The barrier turns back the low-energy spacecraft-emitted electrons and prevents the low-energy ambient electrons from reaching the detector. It is argued that the magnitude of the observed barrier is too large to be explained in terms of a simple photoelectron or secondary electron sheath around a uniformly charged spacecraft. The most likely explanation is the presence of differential charging of the spacecraft surfaces.